2 Java Cryptography Architecture (JCA) Reference Guide
The Java Cryptography Architecture (JCA) is a major piece of the platform, and contains a "provider" architecture and a set of APIs for digital signatures, message digests (hashes), certificates and certificate validation, encryption (symmetric/asymmetric block/stream ciphers), key generation and management, and secure random number generation, to name a few.
Introduction to Java Cryptography Architecture
The Java platform strongly emphasizes security, including language safety, cryptography, public key infrastructure, authentication, secure communication, and access control.
The JCA is a major piece of the platform, and contains a "provider" architecture and a set of APIs for digital signatures, message digests (hashes), certificates and certificate validation, encryption (symmetric/asymmetric block/stream ciphers), key generation and management, and secure random number generation, to name a few. These APIs allow developers to easily integrate security into their application code. The architecture was designed around the following principles:
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Implementation independence: Applications do not need to implement security algorithms. Rather, they can request security services from the Java platform. Security services are implemented in providers (see Cryptographic Service Providers), which are plugged into the Java platform via a standard interface. An application may rely on multiple independent providers for security functionality.
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Implementation interoperability: Providers are interoperable across applications. Specifically, an application is not bound to a specific provider, and a provider is not bound to a specific application.
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Algorithm extensibility: The Java platform includes a number of built-in providers that implement a basic set of security services that are widely used today. However, some applications may rely on emerging standards not yet implemented, or on proprietary services. The Java platform supports the installation of custom providers that implement such services.
Other cryptographic communication libraries available in the JDK use the JCA provider architecture, but are described elsewhere. The JSSE components provides access to Secure Socket Layer (SSL), Transport Layer Security (TLS), and Datagram Transport Layer Security (DTLS) implementations; see Java Secure Socket Extension (JSSE) Reference Guide. You can use Java Generic Security Services (JGSS) (via Kerberos) APIs, and Simple Authentication and Security Layer (SASL) to securely exchange messages between communicating applications; see Introduction to JAAS and Java GSS-API Tutorials and Java SASL API Programming and Deployment Guide.
Notes on Terminology
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Prior to JDK 1.4, the JCE was an unbundled product, and as such, the JCA and JCE were regularly referred to as separate, distinct components. As JCE is now bundled in the JDK, the distinction is becoming less apparent. Since the JCE uses the same architecture as the JCA, the JCE should be more properly thought of as a part of the JCA.
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The JCA within the JDK includes two software components:
- The framework that defines and supports cryptographic services for which providers supply implementations. This framework includes packages such as
java.security,javax.crypto,javax.crypto.spec, andjavax.crypto.interfaces. - The actual providers such as
Sun,SunRsaSign,SunJCE, which contain the actual cryptographic implementations.
Whenever a specific JCA provider is mentioned, it will be referred to explicitly by the provider's name.
- The framework that defines and supports cryptographic services for which providers supply implementations. This framework includes packages such as
WARNING:
The JCA makes it easy to incorporate security features into your application. However, this document does not cover the theory of security/cryptography beyond an elementary introduction to concepts necessary to discuss the APIs. This document also does not cover the strengths/weaknesses of specific algorithms, not does it cover protocol design. Cryptography is an advanced topic and one should consult a solid, preferably recent, reference in order to make best use of these tools.You should always understand what you are doing and why: DO NOT simply copy random code and expect it to fully solve your usage scenario. Many applications have been deployed that contain significant security or performance problems because the wrong tool or algorithm was selected.
JCA Design Principles
The JCA was designed around these principles:
- Implementation independence and interoperability
- Algorithm independence and extensibility
Implementation independence and algorithm independence are complementary; you can use cryptographic services, such as digital signatures and message digests, without worrying about the implementation details or even the algorithms that form the basis for these concepts. While complete algorithm-independence is not possible, the JCA provides standardized, algorithm-specific APIs. When implementation-independence is not desirable, the JCA lets developers indicate a specific implementation.
Algorithm independence is achieved by defining types of cryptographic "engines" (services), and defining classes that provide the functionality of these cryptographic engines. These classes are called engine classes, and examples are the MessageDigest, Signature, KeyFactory, KeyPairGenerator, and Cipher classes.
Implementation independence is achieved using a "provider"-based architecture. The term Cryptographic Service Provider (CSP), which is used interchangeably with the term "provider," (see Cryptographic Service Providers) refers to a package or set of packages that implement one or more cryptographic services, such as digital signature algorithms, message digest algorithms, and key conversion services. A program may simply request a particular type of object (such as a Signature object) implementing a particular service (such as the DSA signature algorithm) and get an implementation from one of the installed providers. If desired, a program may instead request an implementation from a specific provider. Providers may be updated transparently to the application, for example when faster or more secure versions are available.
Implementation interoperability means that various implementations can work with each other, use each other's keys, or verify each other's signatures. This would mean, for example, that for the same algorithms, a key generated by one provider would be usable by another, and a signature generated by one provider would be verifiable by another.
Algorithm extensibility means that new algorithms that fit in one of the supported engine classes can be added easily.
Provider Architecture
Providers contain a package (or a set of packages) that supply concrete implementations for the advertised cryptographic algorithms.
Cryptographic Service Providers
java.security.Provider is the base class for all security providers. Each CSP contains an instance of this class which contains the provider's name and lists all of the security services/algorithms it implements. When an instance of a particular algorithm is needed, the JCA framework consults the provider's database, and if a suitable match is found, the instance is created.
Providers contain a package (or a set of packages) that supply concrete implementations for the advertised cryptographic algorithms. Each JDK installation has one or more providers installed and configured by default. Additional providers may be added statically or dynamically. Clients may configure their runtime environment to specify the provider preference order. The preference order is the order in which providers are searched for requested services when no specific provider is requested.
To use the JCA, an application simply requests a particular type of object (such as a MessageDigest) and a particular algorithm or service (such as the "SHA-256" algorithm), and gets an implementation from one of the installed providers. For example, the following statement requests a SHA-256 message digest from an installed provider:
md = MessageDigest.getInstance("SHA-256");
Alternatively, the program can request the objects from a specific provider. Each provider has a name used to refer to it. For example, the following statement requests a SHA-256 message digest from the provider named ProviderC:
md = MessageDigest.getInstance("SHA-256", "ProviderC");
The following figures illustrates requesting an SHA-256 message digest implementation. They show three different providers that implement various message digest algorithms (SHA-256, SHA-384, and SHA-512). The providers are ordered by preference from left to right (1-3). In Figure 2-1, an application requests a SHA-256 algorithm implementation without specifying a provider name. The providers are searched in preference order and the implementation from the first provider supplying that particular algorithm, ProviderB, is returned. In Figure 2-2, the application requests the SHA-256 algorithm implementation from a specific provider, ProviderC. This time, the implementation from ProviderC is returned, even though a provider with a higher preference order, ProviderB, also supplies an MD5 implementation.
Figure 2-1 Request SHA-256 Message Digest Implementation Without Specifying Provider
Description of "Figure 2-1 Request SHA-256 Message Digest Implementation Without Specifying Provider"
Figure 2-2 Request SHA-256 Message Digest with ProviderC
Description of "Figure 2-2 Request SHA-256 Message Digest with ProviderC"
Cryptographic implementations in the JDK are distributed via several different providers (Sun, SunJSSE, SunJCE, SunRsaSign) primarily for historical reasons, but to a lesser extent by the type of functionality and algorithms they provide. Other Java runtime environments may not necessarily contain these providers, so applications should not request a provider-specific implementation unless it is known that a particular provider will be available.
The JCA offers a set of APIs that allow users to query which providers are installed and what services they support.
This architecture also makes it easy for end-users to add additional providers. Many third party provider implementations are already available. See The Provider Class for more information on how providers are written, installed, and registered.
How Providers Are Actually Implemented
Algorithm independence is achieved by defining a generic high-level Application Programming Interface (API) that all applications use to access a service type. Implementation independence is achieved by having all provider implementations conform to well-defined interfaces. Instances of engine classes are thus "backed" by implementation classes which have the same method signatures. Application calls are routed through the engine class and are delivered to the underlying backing implementation. The implementation handles the request and return the proper results.
The application API methods in each engine class are routed to the provider's implementations through classes that implement the corresponding Service Provider Interface (SPI). That is, for each engine class, there is a corresponding abstract SPI class which defines the methods that each cryptographic service provider's algorithm must implement. The name of each SPI class is the same as that of the corresponding engine class, followed by Spi. For example, the Signature engine class provides access to the functionality of a digital signature algorithm. The actual provider implementation is supplied in a subclass of SignatureSpi. Applications call the engine class' API methods, which in turn call the SPI methods in the actual implementation.
Each SPI class is abstract. To supply the implementation of a particular type of service for a specific algorithm, a provider must subclass the corresponding SPI class and provide implementations for all the abstract methods.
For each engine class in the API, implementation instances are requested and instantiated by calling the getInstance() factory method in the engine class. A factory method is a static method that returns an instance of a class. The engine classes use the framework provider selection mechanism described previously to obtain the actual backing implementation (SPI), and then creates the actual engine object. Each instance of the engine class encapsulates (as a private field) the instance of the corresponding SPI class, known as the SPI object. All API methods of an API object are declared final and their implementations invoke the corresponding SPI methods of the encapsulated SPI object.
To make this clearer, review Example 2-1 and Figure 2-3:
Example 2-1 Sample Code for Getting an Instance of an Engine Class
import javax.crypto.*;
Cipher c = Cipher.getInstance("AES");
c.init(ENCRYPT_MODE, key);
Figure 2-3 Application Retrieves “AES” Cipher Instance
Description of "Figure 2-3 Application Retrieves “AES” Cipher Instance"
Here an application wants an "AES" javax.crypto.Cipher instance, and doesn't care which provider is used. The application calls the getInstance() factory methods of the Cipher engine class, which in turn asks the JCA framework to find the first provider instance that supports "AES". The framework consults each installed provider, and obtains the provider's instance of the Provider class. (Recall that the Provider class is a database of available algorithms.) The framework searches each provider, finally finding a suitable entry in CSP3. This database entry points to the implementation class com.foo.AESCipher which extends CipherSpi, and is thus suitable for use by the Cipher engine class. An instance of com.foo.AESCipher is created, and is encapsulated in a newly-created instance of javax.crypto.Cipher, which is returned to the application. When the application now does the init() operation on the Cipher instance, the Cipher engine class routes the request into the corresponding engineInit() backing method in the com.foo.AESCipher class.
Keystores
A database called a "keystore" can be used to manage a repository of keys and certificates. Keystores are available to applications that need data for authentication, encryption, or signing purposes.
Applications can access a keystore via an implementation of the KeyStore class, which is in the java.security package. As of JDK 9, the default and recommended keystore type (format) is "pkcs12", which is based on the RSA PKCS12 Personal Information Exchange Syntax Standard. Previously, the default keystore type was "jks", which is a proprietary format. Other keystore formats are available, such as "jceks", which is an alternate proprietary keystore format, and "pkcs11", which is based on the RSA PKCS11 Standard and supports access to cryptographic tokens such as hardware security modules and smartcards.
Applications can choose different keystore implementations from different providers, using the same provider mechanism described previously. See Key Management.
Engine Classes and Algorithms
An engine class provides the interface to a specific type of cryptographic service, independent of a particular cryptographic algorithm or provider.
The engines provides one of the following:
- cryptographic operations (encryption, digital signatures, message digests, etc.),
- generators or converters of cryptographic material (keys and algorithm parameters), or
- objects (keystores or certificates) that encapsulate the cryptographic data and can be used at higher layers of abstraction.
The following engine classes are available:
SecureRandom: used to generate random or pseudo-random numbers.MessageDigest: used to calculate the message digest (hash) of specified data.Signature: initialized with keys, these are used to sign data and verify digital signatures.Cipher: initialized with keys, these used for encrypting/decrypting data. There are various types of algorithms: symmetric bulk encryption (e.g. AES), asymmetric encryption (e.g. RSA), and password-based encryption (e.g. PBE).-
Mac: LikeMessageDigests, Message Authentication Codes (MACs) also generate hash values, but are first initialized with keys to protect the integrity of messages. KeyFactory: used to convert existing opaque cryptographic keys of typeKeyinto key specifications (transparent representations of the underlying key material), and vice versa.SecretKeyFactory: used to convert existing opaque cryptographic keys of typeSecretKeyinto key specifications (transparent representations of the underlying key material), and vice versa.SecretKeyFactorys are specializedKeyFactorys that create secret (symmetric) keys only.KeyPairGenerator: used to generate a new pair of public and private keys suitable for use with a specified algorithm.KeyGenerator: used to generate new secret keys for use with a specified algorithm.KeyAgreement: used by two or more parties to agree upon and establish a specific key to use for a particular cryptographic operation.AlgorithmParameters: used to store the parameters for a particular algorithm, including parameter encoding and decoding.AlgorithmParameterGenerator: used to generate a set of AlgorithmParameters suitable for a specified algorithm.KeyStore: used to create and manage a keystore. A keystore is a database of keys. Private keys in a keystore have a certificate chain associated with them, which authenticates the corresponding public key. A keystore also contains certificates from trusted entities.CertificateFactory: used to create public key certificates and Certificate Revocation Lists (CRLs).CertPathBuilder: used to build certificate chains (also known as certification paths).CertPathValidator: used to validate certificate chains.CertStore: used to retrieveCertificates andCRLs from a repository.
Note:
A generator creates objects with brand-new contents, whereas a factory creates objects from existing material (for example, an encoding).Core Classes and Interfaces
The following are the core classes and interfaces provided in the JCA.
ProviderandSecuritySecureRandom,MessageDigest,Signature,Cipher,Mac,KeyFactory,SecretKeyFactory,KeyPairGenerator,KeyGenerator,KeyAgreement,AlgorithmParameter,AlgorithmParameterGenerator,KeyStore,CertificateFactory, and engineKey Interface,KeyPairAlgorithmParameterSpec Interface,AlgorithmParameters,AlgorithmParameterGenerator, and algorithm parameter specification interfaces and classes in thejava.security.specandjavax.crypto.specpackages.KeySpec Interface,EncodedKeySpec,PKCS8EncodedKeySpec, andX509EncodedKeySpec.SecretKeyFactory,KeyFactory,KeyPairGenerator,KeyGenerator,KeyAgreement, andKeyStore.
Note:
SeeCertPathBuilder, CertPathValidator, and CertStoreengine classes in the Java PKI Programmer's Guide.
The guide will cover the most useful high-level classes first (Provider, Security, SecureRandom, MessageDigest, Signature, Cipher, and Mac), then delve into the various support classes. For now, it is sufficient to simply say that Keys (public, private, and secret) are generated and represented by the various JCA classes, and are used by the high-level classes as part of their operation.
This section shows the signatures of the main methods in each class and interface. Examples for some of these classes (MessageDigest, Signature, KeyPairGenerator, SecureRandom, KeyFactory, and key specification classes) are supplied in the corresponding Code Examples sections.
The complete reference documentation for the relevant Security API packages can be found in the package summaries:
The Provider Class
The term "Cryptographic Service Provider" (used interchangeably with "provider" in this document) refers to a package or set of packages that supply a concrete implementation of a subset of the JDK Security API cryptography features. The Provider class is the interface to such a package or set of packages. It has methods for accessing the provider name, version number, and other information. Please note that in addition to registering implementations of cryptographic services, the Provider class can also be used to register implementations of other security services that might get defined as part of the JDK Security API or one of its extensions.
To supply implementations of cryptographic services, an entity (e.g., a development group) writes the implementation code and creates a subclass of the Provider class. The constructor of the Provider subclass sets the values of various properties; the JDK Security API uses these values to look up the services that the provider implements. In other words, the subclass specifies the names of the classes implementing the services.
There are several types of services that can be implemented by provider packages; See Engine Classes and Algorithms.
The different implementations may have different characteristics. Some may be software-based, while others may be hardware-based. Some may be platform-independent, while others may be platform-specific. Some provider source code may be available for review and evaluation, while some may not. The JCA lets both end-users and developers decide what their needs are.
You can find information about how end-users install the cryptography implementations that fit their needs, and how developers request the implementations that fit theirs.
Note:
To implement a provider, see Steps to Implement and Integrate a Provider.How Provider Implementations Are Requested and Supplied
For each engine class (see Engine Classes and Algorithms) in the API, a implementation instance is requested and instantiated by calling one of the getInstance methods on the engine class, specifying the name of the desired algorithm and, optionally, the name of the provider (or the Provider class) whose implementation is desired.
static EngineClassName getInstance(String algorithm)
throws NoSuchAlgorithmException
static EngineClassName getInstance(String algorithm, String provider)
throws NoSuchAlgorithmException, NoSuchProviderException
static EngineClassName getInstance(String algorithm, Provider provider)
throws NoSuchAlgorithmException
where
EngineClassName
is the desired engine type (MessageDigest/Cipher/etc). For example:
MessageDigest md = MessageDigest.getInstance("SHA-256");
KeyAgreement ka = KeyAgreement.getInstance("DH", "SunJCE");
return an instance of the "SHA-256" MessageDigest and "DH" KeyAgreement objects, respectively.
Java Security Standard Algorithm Names contains the list of names that have been standardized for use with the Java environment. Some providers may choose to also include alias names that also refer to the same algorithm. For example, the "SHA256" algorithm might be referred to as "SHA-256". Applications should use standard names instead of an alias, as not all providers may alias algorithm names in the same way.
Note:
The algorithm name is not case-sensitive. For example, all the following calls are equivalent:
MessageDigest.getInstance("SHA256")
MessageDigest.getInstance("sha256")
MessageDigest.getInstance("sHa256")
If no provider is specified, getInstance searches the registered providers for an implementation of the requested cryptographic service associated with the named algorithm. In any given Java Virtual Machine (JVM), providers are installed in a given preference order, the order in which the provider list is searched if a specific provider is not requested. (See Installing Providers.) For example, suppose there are two providers installed in a JVM, PROVIDER_1 and PROVIDER_2. Assume that:
PROVIDER_1implements SHA-256 and DESede.PROVIDER_1has preference order 1 (the highest priority).PROVIDER_2implements SHA256withDSA, SHA-256, RC5, and RSA.PROVIDER_2has preference order 2.
Now let's look at three scenarios:
- If we are looking for an SHA-256 implementation. Both providers supply such an implementation. The
PROVIDER_1implementation is returned sincePROVIDER_1has the highest priority and is searched first. - If we are looking for an SHA256withDSA signature algorithm,
PROVIDER_1is first searched for it. No implementation is found, soPROVIDER_2is searched. Since an implementation is found, it is returned. - Suppose we are looking for a SHA256withRSA signature algorithm. Since no installed provider implements it, a
NoSuchAlgorithmExceptionis thrown.
The getInstance methods that include a provider argument are for developers who want to specify which provider they want an algorithm from. A federal agency, for example, will want to use a provider implementation that has received federal certification. Let's assume that the SHA256withDSA implementation from PROVIDER_1 has not received such certification, while the DSA implementation of PROVIDER_2 has received it.
A federal agency program would then have the following call, specifying PROVIDER_2 since it has the certified implementation:
Signature dsa = Signature.getInstance("SHA256withDSA", "PROVIDER_2");
In this case, if PROVIDER_2 was not installed, a NoSuchProviderException would be thrown, even if another installed provider implements the algorithm requested.
A program also has the option of getting a list of all the installed providers (using the getProviders method in The Security Class class) and choosing one from the list.
Note:
General purpose applications SHOULD NOT request cryptographic services from specific providers. Otherwise, applications are tied to specific providers which may not be available on other Java implementations. They also might not be able to take advantage of available optimized providers (for example hardware accelerators via PKCS11 or native OS implementations such as Microsoft's MSCAPI) that have a higher preference order than the specific requested provider.Installing Providers
In order to be used, a cryptographic provider must first be installed, then registered either statically or dynamically. There are a variety of Sun providers shipped with this release (SUN, SunJCE, SunJSSE, SunRsaSign, etc.) that are already installed and registered. The following sections describe how to install and register additional providers.
All JDK providers are already installed and registered. However, if you require any third-party providers, see Step 8: Prepare for Testing from Steps to Implement and Integrate a Provider for information about how to add providers to the class or module path, register providers (statically or dynamically), and add any required permissions.
Provider Class Methods
Each Provider class instance has a (currently case-sensitive) name, a version number, and a string description of the provider and its services.
You can query the Provider instance for this information by calling the following methods:
public String getName()
public double getVersion()
public String getInfo()
The Security Class
The Security class manages installed providers and
security-wide properties. It only contains static methods and is never instantiated. The
methods for adding or removing providers, and for setting Security
properties, can only be executed by a trusted program. Currently, a "trusted program"
one of the following:
- A local application not running under a security manager
- An applet or application with permission to execute the specified method
WARNING:
The Security Manager and APIs related to it have been deprecated and are subject to removal in a future release. There is no replacement for the Security Manager. See JEP 411 for discussion and alternatives.The determination that code is considered trusted to perform an attempted action (such as adding a provider) requires that the applet is granted the proper permission(s) for that particular action. The policy configuration file(s) for a JDK installation specify what permissions (which types of system resource accesses) are allowed by code from specified code sources. See Default Policy Implementation and Policy File Syntax and Java SE Platform Security Architecture.)
Code being executed is always considered to come from a particular "code source". The code source includes not only the location (URL) where the code originated from, but also a reference to any public key(s) corresponding to the private key(s) that may have been used to sign the code. Public keys in a code source are referenced by (symbolic) alias names from the user's .
In a policy configuration file, a code source is represented by two components: a code base (URL), and an alias name (preceded by signedBy), where the alias name identifies the keystore entry containing the public key that must be used to verify the code's signature.
Each "grant" statement in such a file grants a specified code source a set of permissions, specifying which actions are allowed.
Here is a sample policy configuration file:
grant codeBase "file:/home/sysadmin/", signedBy "sysadmin" {
permission java.security.SecurityPermission "insertProvider";
permission java.security.SecurityPermission "removeProvider";
permission java.security.SecurityPermission "putProviderProperty.*";
};This configuration file specifies that code loaded from a signed JAR file in the /home/sysadmin/ directory on the local file system can add or remove providers or set provider properties. (Note that the signature of the JAR file can be verified using the public key referenced by the alias name sysadmin in the user's keystore.).
Either component of the code source (or both) may be missing. Here's an example of a configuration file where the codeBase is omitted:
grant signedBy "sysadmin" {
permission java.security.SecurityPermission "insertProvider.*";
permission java.security.SecurityPermission "removeProvider.*";
}; If this policy is in effect, code that comes in a JAR File signed by /home/sysadmin/ directory on the local filesystem can add or remove providers. The code does not need to be signed.
An example where neither codeBase nor signedBy is included is:
grant {
permission java.security.SecurityPermission "insertProvider.*";
permission java.security.SecurityPermission "removeProvider.*";
}; Here, with both code source components missing, any code (regardless of where it originates, or whether or not it is signed, or who signed it) can add/remove providers. Obviously, this is definitely not recommended, as this grant could open a security hole. Untrusted code could install a Provider, thus affecting later code that is depending on a properly functioning implementation. (For example, a rogue Cipher object might capture and store the sensitive information it receives.)
Managing Providers
The following tables summarize the methods in the Security class you can use to query which Providers are installed, as well as to install or remove providers at runtime.
Querying Providers
| Method | Description |
|---|---|
static Provider[] getProviders() |
Returns an array containing all the installed providers (technically, the Provider subclass for each package provider). The order of the Providers in the array is their preference order.
|
static Provider getProvider (String providerName) |
Returns the Provider named providerName. It returns null if the Provider is not found.
|
Adding Providers
| Method | Description |
|---|---|
static int addProvider(Provider provider) |
Adds a Provider to the end of the list of installed Providers. It returns the preference position in which the Provider was added, or -1 if the Provider was not added because it was already installed.
|
static int insertProviderAt (Provider provider, int position) |
Adds a new Provider at a specified position. If the given provider is installed at the requested position, the provider formerly at that position and all providers with a position greater than position are shifted up one position (towards the end of the list). This method returns the preference position in which the Provider was added, or -1 if the Provider was not added because it was already installed.
|
Removing Providers
| Method | Description |
|---|---|
static void removeProvider(String name) |
Removes the Provider with the specified name. It returns silently if the provider is not installed. When the specified provider is removed, all providers located at a position greater than where the specified provider was are shifted down one position (towards the head of the list of installed providers).
|
Note:
If you want to change the preference position of a provider, you must first remove it, and then insert it back in at the new preference position.Security Properties
The Security class maintains a list of system-wide Security Properties. These properties are similar to the System properties, but are security-related. These properties can be set statically (through the <java-home>/conf/security/java.security file) or dynamically (using an API). See Step 8.1: Configure the Provider from Steps to Implement and Integrate a Provider. for an example of registering a provider statically with the security.provider.n Security Property. If you want to set properties dynamically, trusted programs can use the following methods:
static String getProperty(String key)
static void setProperty(String key, String datum)Note:
The list of security providers is established during VM startup; therefore, the methods described previously must be used to alter the provider list.The SecureRandom Class
The SecureRandom class is an engine class (see Engine Classes and Algorithms) that provides cryptographically strong random numbers, either by accessing a pseudo-random number generator (PRNG), a deterministic algorithm that produces a pseudo-random sequence from an initial seed value, or by reading a native source of randomness (for example, /dev/random or a true random number generator). One example of a PRNG is the Deterministic Random Bits Generator (DRBG) as specified in NIST SP 800-90Ar1. Other implementations may produce true random numbers, and yet others may use a combination of both techniques. A cryptographically strong random number minimally complies with the statistical random number generator tests specified in FIPS 140-2, Security Requirements for Cryptographic Modules, section 4.9.1.
All Java SE implementations must indicate the strongest (most random) implementation of SecureRandom that they provide in the securerandom.strongAlgorithms property of the java.security.Security class. This implementation can be used when a particularly strong random value is required.
The securerandom.drbg.config property is used to specify the DRBG SecureRandom configuration and implementations in the SUN provider. The securerandom.drbg.config is a property of the java.security.Security class. Other DRBG implementations can also use the securerandom.drbg.config property.
Creating a SecureRandom Object
There are several ways to obtain an instance of SecureRandom:
-
All Java SE implementations provide a default
SecureRandomusing the no-argument constructor:new SecureRandom(). This constructor traverses the list of registered security providers, starting with the most preferred provider, then returns a new SecureRandom object from the first provider that supports aSecureRandomrandom number generator (RNG) algorithm. If none of the providers support a RNG algorithm, then it returns a SecureRandom object that uses SHA1PRNG from the SUN provider. -
To get a specific implementation of
SecureRandom, use one of the How Provider Implementations Are Requested and Supplied. -
Use the
getInstanceStrong()method to obtain a strongSecureRandomimplementation as defined by thesecurerandom.strongAlgorithmsproperty of thejava.security.Securityclass. This property lists platform implementations that are suitable for generating important values.
Seeding or Re-Seeding the SecureRandom Object
The SecureRandom object is initialized with a random seed unless the call to getInstance() is followed by a call to one of the following setSeed methods.
void setSeed(byte[] seed)
void setSeed(long seed)
You must call setSeed before the first nextBytes call to prevent any environmental randomness.
The randomness of the bits produced by the SecureRandom object depends on the randomness of the seed bits
At any time a SecureRandom object may be re-seeded using one of the setSeed or reseed methods. The given seed for setSeed supplements, rather than replaces, the existing seed; therefore, repeated calls are guaranteed never to reduce randomness.
The MessageDigest Class
The MessageDigest class is an engine class (see Engine Classes and Algorithms) designed to provide the functionality of cryptographically secure message digests such as SHA-256 or SHA-512. A cryptographically secure message digest takes arbitrary-sized input (a byte array), and generates a fixed-size output, called a digest or hash.
For example, the SHA-256 algorithm produces a 32-byte digest, and SHA-512's is 64 bytes.
A digest has two properties:
- It should be computationally infeasible to find two messages that hash to the same value.
- The digest should not reveal anything about the input that was used to generate it.
Message digests are used to produce unique and reliable identifiers of data. They are sometimes called "checksums" or the "digital fingerprints" of the data. Changes to just one bit of the message should produce a different digest value.
Message digests have many uses and can determine when data has been modified, intentionally or not. Recently, there has been considerable effort to determine if there are any weaknesses in popular algorithms, with mixed results. When selecting a digest algorithm, one should always consult a recent reference to determine its status and appropriateness for the task at hand.
Computing the Digest
Procedure to compute the digest using different types of digest() methods.
update method. See Updating a Message Digest Object.
The Signature Class
The Signature class is an engine class (see Engine Classes and Algorithms( designed to provide the functionality of a cryptographic digital signature algorithm such as SHA256withDSA or SHA512withRSA. A cryptographically secure signature algorithm takes arbitrary-sized input and a private key and generates a relatively short (often fixed-size) string of bytes, called the signature, with the following properties:
- Only the owner of a private/public key pair is able to create a signature. It should be computationally infeasible for anyone having only the public key and a number of signatures to recover the private key.
- Given the public key corresponding to the private key used to generate the signature, it should be possible to verify the authenticity and integrity of the input.
A Signature object is initialized for signing with a Private Key and is given the data to be signed. The resulting signature bytes are typically kept with the signed data. When verification is needed, another Signature object is created and initialized for verification and given the corresponding Public Key. The data and the signature bytes are fed to the signature object, and if the data and signature match, the Signature object reports success.
Even though a signature seems similar to a message digest, they have very different purposes in the type of protection they provide. In fact, algorithms such as "SHA256WithRSA" use the message digest "SHA256" to initially "compress" the large data sets into a more manageable form, then sign the resulting 32 byte message digest with the "RSA" algorithm.
For an example for signing and verifying data, see Generating and Verifying a Signature Using Generated Keys.
Signature Object States
Signature objects are modal objects. This means that a Signature object is always in a given state, where it may only do one type of operation.
States are represented as final integer constants defined in their respective classes.
The three states a Signature object may have are:
UNINITIALIZEDSIGNVERIFY
When it is first created, a Signature object is in the UNINITIALIZED state. The Signature class defines two initialization methods, initSign and initVerify, which change the state to SIGN and VERIFY , respectively.
Creating a Signature Object
The first step for signing or verifying a signature is to create a Signature instance.
Signature objects are obtained by using one of the Signature getInstance() static factory methods. See How Provider Implementations Are Requested and Supplied.
Initializing a Signature Object
A Signature object must be initialized before it is used. The initialization method depends on whether the object is going to be used for signing or for verification.
If it is going to be used for signing, the object must first be initialized with the private key of the entity whose signature is going to be generated. This initialization is done by calling the method:
final void initSign(PrivateKey privateKey)
This method puts the Signature object in the SIGN state. If instead the Signature object is going to be used for verification, it must first be initialized with the public key of the entity whose signature is going to be verified. This initialization is done by calling either of these methods:
final void initVerify(PublicKey publicKey)
final void initVerify(Certificate certificate)
This method puts the Signature object in the VERIFY state.
Signing with a Signature Object
If the Signature object has been initialized for signing (if it is in the SIGN state), the data to be signed can then be supplied to the object. This is done by making one or more calls to one of the update methods:
final void update(byte b)
final void update(byte[] data)
final void update(byte[] data, int off, int len)
Calls to the update method(s) should be made until all the data to be signed has been supplied to the Signature object.
To generate the signature, simply call one of the sign methods:
final byte[] sign()
final int sign(byte[] outbuf, int offset, int len)
The first method returns the signature result in a byte array. The second stores the signature result in the provided buffer outbuf, starting at offset. len is the number of bytes in outbuf allotted for the signature. The method returns the number of bytes actually stored.
Signature encoding is algorithm specific. See Java Security Standard Algorithm Names to know more about the use of ASN.1 encoding in the Java Cryptography Architecture.
A call to a sign method resets the signature object to the state it was in when previously initialized for signing via a call to initSign. That is, the object is reset and available to generate another signature with the same private key, if desired, via new calls to update and sign.
Alternatively, a new call can be made to initSign specifying a different private key, or to initVerify (to initialize the Signature object to verify a signature).
Verifying with a Signature Object
If the Signature object has been initialized for verification (if it is in the VERIFY state), it can then verify if an alleged signature is in fact the authentic signature of the data associated with it. To start the process, the data to be verified (as opposed to the signature itself) is supplied to the object. The data is passed to the object by calling one of the update methods:
final void update(byte b)
final void update(byte[] data)
final void update(byte[] data, int off, int len)
Calls to the update method(s) should be made until all the data to be verified has been supplied to the Signature object. The signature can now be verified by calling one of the verify methods:
final boolean verify(byte[] signature)
final boolean verify(byte[] signature, int offset, int length)
The argument must be a byte array containing the signature. This byte array would hold the signature bytes which were returned by a previous call to one of the sign methods.
The verify method returns a boolean indicating whether or not the encoded signature is the authentic signature of the data supplied to the update method(s).
A call to the verify method resets the signature object to its state when it was initialized for verification via a call to initVerify. That is, the object is reset and available to verify another signature from the identity whose public key was specified in the call to initVerify.
Alternatively, a new call can be made to initVerify specifying a different public key (to initialize the Signature object for verifying a signature from a different entity), or to initSign (to initialize the Signature object for generating a signature).
The Cipher Class
The Cipher class provides the functionality of a cryptographic cipher used for encryption and decryption. Encryption is the process of taking data (called cleartext) and a key, and producing data (ciphertext) meaningless to a third-party who does not know the key. Decryption is the inverse process: that of taking ciphertext and a key and producing cleartext.
Symmetric vs. Asymmetric Cryptography
There are two major types of encryption: symmetric (also known as secret key), and asymmetric (or public key cryptography). In symmetric cryptography, the same secret key to both encrypt and decrypt the data. Keeping the key private is critical to keeping the data confidential. On the other hand, asymmetric cryptography uses a public/private key pair to encrypt data. Data encrypted with one key is decrypted with the other. A user first generates a public/private key pair, and then publishes the public key in a trusted database that anyone can access. A user who wishes to communicate securely with that user encrypts the data using the retrieved public key. Only the holder of the private key will be able to decrypt. Keeping the private key confidential is critical to this scheme.
Asymmetric algorithms (such as RSA) are generally much slower than symmetric ones. These algorithms are not designed for efficiently protecting large amounts of data. In practice, asymmetric algorithms are used to exchange smaller secret keys which are used to initialize symmetric algorithms.
Stream versus Block Ciphers
There are two major types of ciphers: block and stream. Block ciphers process entire blocks at a time, usually many bytes in length. If there is not enough data to make a complete input block, the data must be padded: that is, before encryption, dummy bytes must be added to make a multiple of the cipher's block size. These bytes are then stripped off during the decryption phase. The padding can either be done by the application, or by initializing a cipher to use a padding type such as "PKCS5PADDING". In contrast, stream ciphers process incoming data one small unit (typically a byte or even a bit) at a time. This allows for ciphers to process an arbitrary amount of data without padding.
Modes Of Operation
When encrypting using a simple block cipher, two identical blocks of plaintext will always produce an identical block of cipher text. Cryptanalysts trying to break the ciphertext will have an easier job if they note blocks of repeating text. A cipher mode of operation makes the ciphertext less predictable with output block alterations based on block position or the values of other ciphertext blocks. The first block will need an initial value, and this value is called the initialization vector (IV). Since the IV simply alters the data before any encryption, the IV should be random but does not necessarily need to be kept secret. There are a variety of modes, such as CBC (Cipher Block Chaining), CFB (Cipher Feedback Mode), and OFB (Output Feedback Mode). ECB (Electronic Codebook Mode) is a mode in which there is no influence from block position or other ciphertext blocks. Because ECB ciphertexts are the same if they use the same plaintext/key, this mode is not typically suitable for cryptographic applications and should not be used.
Some algorithms such as AES and RSA allow for keys of different lengths, but others are fixed, such as 3DES. Encryption using a longer key generally implies a stronger resistance to message recovery. As usual, there is a trade off between security and time, so choose the key length appropriately.
Most algorithms use binary keys. Most humans do not have the ability to remember long sequences of binary numbers, even when represented in hexadecimal. Character passwords are much easier to recall. Because character passwords are generally chosen from a small number of characters (for example, [a-zA-Z0-9]), protocols such as "Password-Based Encryption" (PBE) have been defined which take character passwords and generate strong binary keys. In order to make the task of getting from password to key very time-consuming for an attacker (via so-called "rainbow table attacks" or "precomputed dictionary attacks" where common dictionary word->value mappings are precomputed), most PBE implementations will mix in a random number, known as a salt, to reduce the usefulness of precomputed tables.
Newer cipher modes such as Authenticated Encryption with Associated Data (AEAD) (for example, Galois/Counter Mode (GCM)) encrypt data and authenticate the resulting message simultaneously. Additional Associated Data (AAD) can be used during the calculation of the resulting AEAD tag (MAC), but this AAD data is not output as ciphertext. (For example, some data might not need to be kept confidential, but should figure into the tag calculation to detect modifications.) The Cipher.updateAAD() methods can be used to include AAD in the tag calculations.
Using an AES Cipher with GCM Mode
AES Cipher with GCM is an AEAD Cipher which has different usage patterns than the non-AEAD ciphers. Apart from the regular data, it also takes AAD which is optional for encryption/decryption but AAD must be supplied before data for encryption/decryption. In addition, in order to use GCM securely, callers should not re-use key and IV combinations for encryption. This means that the cipher object should be explicitly re-initialized with a different set of parameters every time for each encryption operation.
Example 2-2 Sample Code for Using an AES Cipher with GCM Mode
SecretKey myKey = ...
byte[] myAAD = ...
byte[] plainText = ...
int myTLen = ...
byte[] myIv = ...
GCMParameterSpec myParams = new GCMParameterSpec(myTLen, myIv);
Cipher c = Cipher.getInstance("AES/GCM/NoPadding");
c.init(Cipher.ENCRYPT_MODE, myKey, myParams);
// AAD is optional, if present, it must be supplied before any update/doFinal calls.
c.updateAAD(myAAD); // if AAD is non-null
byte[] cipherText = new byte[c.getOutputSize(plainText.length)];
// conclusion of encryption operation
int actualOutputLen = c.doFinal(plainText, 0, plainText.length, cipherText);
// To decrypt, same AAD and GCM parameters must be supplied
c.init(Cipher.DECRYPT_MODE, myKey, myParams);
c.updateAAD(myAAD);
byte[] recoveredText = c.doFinal(cipherText, 0, actualOutputLen);
// MUST CHANGE IV VALUE if the same key were to be used again for encryption
byte[] newIv = ...;
myParams = new GCMParameterSpec(myTLen, newIv);Creating a Cipher Object
Cipher objects are obtained by using one of the Cipher getInstance() static factory methods. See How Provider Implementations Are Requested and Supplied. Here, the algorithm name is slightly different than with other engine classes, in that it specifies not just an algorithm name, but a "transformation". A transformation is a string that describes the operation (or set of operations) to be performed on the given input to produce some output. A transformation always includes the name of a cryptographic algorithm (e.g., AES), and may be followed by a mode and padding scheme.
A transformation is of the form:
- "algorithm/mode/padding" or
- "algorithm"
For example, the following are valid transformations:
"AES/CBC/PKCS5Padding"
"AES"If just a transformation name is specified, the system will determine if there is an implementation of the requested transformation available in the environment, and if there is more than one, returns there is a preferred one.
If both a transformation name and a package provider are specified, the system will determine if there is an implementation of the requested transformation in the package requested, and throw an exception if there is not.
It is recommended to use a transformation that fully specifies the algorithm, mode, and padding. By not doing so, the provider will use a default. For example, the SunJCE and SunPKCS11 providers use ECB as the default mode, and PKCS5Padding as the default padding for many symmetric ciphers.
This means that in the case of the SunJCE provider:
Cipher c1 = Cipher.getInstance("AES/ECB/PKCS5Padding");and
Cipher c1 = Cipher.getInstance("AES");are equivalent statements.
Note:
ECB mode is the easiest block cipher mode to use and is the default cipher mode. ECB works well for single blocks of data and can be parallelized but generally should not be used for encrypting multiple data blocks due to characteristics of the mode. This could result in trivial and full disclosure of confidential data. While this mode is available for use, it should only be used with an understanding of the cryptographic risks involved.Using modes such as CFB and OFB, block ciphers can encrypt data in units smaller than the cipher's actual block size. When requesting such a mode, you may optionally specify the number of bits to be processed at a time by appending this number to the mode name as shown in the "AES/CFB8/NoPadding" and "AES/OFB32/PKCS5Padding" transformations. If no such number is specified, a provider-specific default is used. (For example, the SunJCE provider uses a default of 128 bits for AES.) Thus, block ciphers can be turned into byte-oriented stream ciphers by using an 8 bit mode such as CFB8 or OFB8.
Java Security Standard Algorithm Names contains a list of standard names that can be used to specify the algorithm name, mode, and padding scheme components of a transformation.
The objects returned by factory methods are uninitialized, and must be initialized before they become usable.
Initializing a Cipher Object
A Cipher object obtained via getInstance must be initialized for
one of four modes, which are defined as final integer constants in the
Cipher class. The modes can be referenced by their symbolic
names:
- ENCRYPT_MODE
- Encryption of data.
- DECRYPT_MODE
- Decryption of data.
- WRAP_MODE
- Wrapping a
java.security.Keyinto bytes so that the key can be securely transported. - UNWRAP_MODE
- Unwrapping of a previously wrapped key into a
java.security.Keyobject.
Each of the Cipher initialization methods takes an operational mode parameter (opmode), and initializes the Cipher object for that mode. Other parameters include the key (key) or certificate containing the key (certificate), algorithm parameters (params), and a source of randomness (random).
To initialize a Cipher object, call one of the following init methods:
public void init(int opmode, Key key);
public void init(int opmode, Certificate certificate);
public void init(int opmode, Key key, SecureRandom random);
public void init(int opmode, Certificate certificate,
SecureRandom random);
public void init(int opmode, Key key,
AlgorithmParameterSpec params);
public void init(int opmode, Key key,
AlgorithmParameterSpec params, SecureRandom random);
public void init(int opmode, Key key,
AlgorithmParameters params);
public void init(int opmode, Key key,
AlgorithmParameters params, SecureRandom random);If a Cipher object that requires parameters (e.g., an initialization vector) is initialized for encryption, and no parameters are supplied to the init method, the underlying cipher implementation is supposed to supply the required parameters itself, either by generating random parameters or by using a default, provider-specific set of parameters.
However, if a Cipher object that requires parameters is initialized for decryption, and no parameters are supplied to the init method, an InvalidKeyException or InvalidAlgorithmParameterException exception will be raised, depending on the init method that has been used.
See Managing Algorithm Parameters.
The same parameters that were used for encryption must be used for decryption.
Note that when a Cipher object is initialized, it loses all previously-acquired state. In other words, initializing a Cipher is equivalent to creating a new instance of that Cipher, and initializing it. For example, if a Cipher is first initialized for decryption with a given key, and then initialized for encryption, it will lose any state acquired while in decryption mode.
Encrypting and Decrypting Data
Data can be encrypted or decrypted in one step (single-part operation) or in multiple steps (multiple-part operation). A multiple-part operation is useful if you do not know in advance how long the data is going to be, or if the data is too long to be stored in memory all at once.
To encrypt or decrypt data in a single step, call one of the doFinal methods:
public byte[] doFinal(byte[] input);
public byte[] doFinal(byte[] input, int inputOffset, int inputLen);
public int doFinal(byte[] input, int inputOffset,
int inputLen, byte[] output);
public int doFinal(byte[] input, int inputOffset,
int inputLen, byte[] output, int outputOffset)To encrypt or decrypt data in multiple steps, call one of the update methods:
public byte[] update(byte[] input);
public byte[] update(byte[] input, int inputOffset, int inputLen);
public int update(byte[] input, int inputOffset, int inputLen,
byte[] output);
public int update(byte[] input, int inputOffset, int inputLen,
byte[] output, int outputOffset)A multiple-part operation must be terminated by one of the these
doFinal methods (if there is still some input data left for the
last step), or by one of the following doFinal methods (if there is
no input data left for the last step):
public byte[] doFinal();
public int doFinal(byte[] output, int outputOffset);All the doFinal methods take care of any necessary padding (or unpadding), if padding (or unpadding) has been requested as part of the specified transformation.
A call to doFinal resets the Cipher object to the state it was in when initialized via a call to init. That is, the Cipher object is reset and available to encrypt or decrypt (depending on the operation mode that was specified in the call to init) more data.
Wrapping and Unwrapping Keys
Wrapping a key enables secure transfer of the key from one place to another.
The wrap/unwrap API makes it more convenient to write code since it works with key objects directly. These methods also enable the possibility of secure transfer of hardware-based keys.
To wrap a Key, first initialize the Cipher object for WRAP_MODE, and then call the following:
public final byte[] wrap(Key key);If you are supplying the wrapped key bytes (the result of calling wrap) to someone else who will unwrap them, be sure to also send additional information the recipient will need in order to do the unwrap:
- The name of the key algorithm.
- The type of the wrapped key (one of
Cipher.SECRET_KEY,Cipher.PRIVATE_KEY, orCipher.PUBLIC_KEY).
The key algorithm name can be determined by calling the getAlgorithm method from the Key interface:
public String getAlgorithm();To unwrap the bytes returned by a previous call to wrap, first initialize a Cipher object for UNWRAP_MODE, then call the following:
public final Key unwrap(byte[] wrappedKey,
String wrappedKeyAlgorithm,
int wrappedKeyType));Here, wrappedKey is the bytes returned from the previous call to wrap, wrappedKeyAlgorithm is the algorithm associated with the wrapped key, and wrappedKeyType is the type of the wrapped key. This must be one of Cipher.SECRET_KEY, Cipher.PRIVATE_KEY, or Cipher.PUBLIC_KEY.
Managing Algorithm Parameters
The parameters being used by the underlying Cipher implementation, which were either explicitly passed to the init method by the application or generated by the underlying implementation itself, can be retrieved from the Cipher object by calling its getParameters method, which returns the parameters as a java.security.AlgorithmParameters object (or null if no parameters are being used). If the parameter is an initialization vector (IV), it can also be retrieved by calling the getIV method.
In the following example, a Cipher object implementing password-based encryption (PBE) is initialized with just a key and no parameters. However, the selected algorithm for password-based encryption requires two parameters - a salt and an iteration count. Those will be generated by the underlying algorithm implementation itself. The application can retrieve the generated parameters from the Cipher object, see Example 2-3.
The same parameters that were used for encryption must be used for decryption. They can be instantiated from their encoding and used to initialize the corresponding Cipher object for decryption, see Example 2-4.
If you did not specify any parameters when you initialized a Cipher object, and you are not sure whether or not the underlying implementation uses any parameters, you can find out by simply calling the getParameters method of your Cipher object and checking the value returned. A return value of null indicates that no parameters were used.
The following cipher algorithms implemented by the SunJCE provider use parameters:
- AES, DES-EDE, and Blowfish, when used in feedback (i.e., CBC, CFB, OFB, or PCBC) mode, use an initialization vector (IV). The
javax.crypto.spec.IvParameterSpecclass can be used to initialize a Cipher object with a given IV. In addition, CTR and GCM modes require an IV. - PBE Cipher algorithms use a set of parameters, comprising a salt and an iteration count. The
javax.crypto.spec.PBEParameterSpecclass can be used to initialize a Cipher object implementing a PBE algorithm (for example: PBEWithHmacSHA256AndAES_256) with a given salt and iteration count.
Note that you do not have to worry about storing or transferring any algorithm parameters for use by the decryption operation if you use the The SealedObject Class class. This class attaches the parameters used for sealing (encryption) to the encrypted object contents, and uses the same parameters for unsealing (decryption).
Example 2-3 Sample Code for Retrieving Parameters from the Cipher Object
The application can retrieve the generated parameters for encryption from the Cipher object as follows:
import javax.crypto.*;
import java.security.AlgorithmParameters;
// get cipher object for password-based encryption
Cipher c = Cipher.getInstance("PBEWithHmacSHA256AndAES_256");
// initialize cipher for encryption, without supplying
// any parameters. Here, "myKey" is assumed to refer
// to an already-generated key.
c.init(Cipher.ENCRYPT_MODE, myKey);
// encrypt some data and store away ciphertext
// for later decryption
byte[] cipherText = c.doFinal("This is just an example".getBytes());
// retrieve parameters generated by underlying cipher
// implementation
AlgorithmParameters algParams = c.getParameters();
// get parameter encoding and store it away
byte[] encodedAlgParams = algParams.getEncoded();Example 2-4 Sample Code for Initializing the Cipher Object for Decryption
The same parameters that were used for encryption must be used for decryption. They can be instantiated from their encoding and used to initialize the corresponding Cipher object for decryption as follows:
import javax.crypto.*;
import java.security.AlgorithmParameters;
// get parameter object for password-based encryption
AlgorithmParameters algParams;
algParams = AlgorithmParameters.getInstance("PBEWithHmacSHA256AndAES_256");
// initialize with parameter encoding from the previous example
algParams.init(encodedAlgParams);
// get cipher object for password-based encryption
Cipher c = Cipher.getInstance("PBEWithHmacSHA256AndAES_256");
// initialize cipher for decryption, using one of the
// init() methods that takes an AlgorithmParameters
// object, and pass it the algParams object from the previous example
c.init(Cipher.DECRYPT_MODE, myKey, algParams);Cipher Output Considerations
Some of the update and doFinal methods of Cipher allow the caller to specify the output buffer into which to encrypt or decrypt the data. In these cases, it is important to pass a buffer that is large enough to hold the result of the encryption or decryption operation.
The following method in Cipher can be used to determine how big the output buffer should be:
public int getOutputSize(int inputLen)Other Cipher-based Classes
There are some helper classes which internally use Ciphers to provide easy access to common cipher uses.
The Cipher Stream Classes
The CipherInputStream and CipherOutputStream classes are Cipher stream classes.
The CipherInputStream Class
This class is a FilterInputStream that encrypts or decrypts the data passing through it. It is composed of an InputStream. CipherInputStream represents a secure input stream into which a Cipher object has been interposed. The read methods of CipherInputStream return data that are read from the underlying InputStream but have additionally been processed by the embedded Cipher object. The Cipher object must be fully initialized before being used by a CipherInputStream.
For example, if the embedded Cipher has been initialized for decryption, the CipherInputStream will attempt to decrypt the data it reads from the underlying InputStream before returning them to the application.
This class adheres strictly to the semantics, especially the failure semantics, of its ancestor classes java.io.FilterInputStream and java.io.InputStream. This class has exactly those methods specified in its ancestor classes, and overrides them all, so that the data are additionally processed by the embedded cipher. Moreover, this class catches all exceptions that are not thrown by its ancestor classes. In particular, the skip(long) method skips only data that has been processed by the Cipher.
It is crucial for a programmer using this class not to use methods that are not defined or overridden in this class (such as a new method or constructor that is later added to one of the super classes), because the design and implementation of those methods are unlikely to have considered security impact with regard to CipherInputStream. See Example 2-5 for its usage, suppose cipher1 has been initialized for encryption. The program reads and encrypts the content from the file /tmp/a.txt and then stores the result (the encrypted bytes) in /tmp/b.txt.
Example 2-6 demonstrates how to easily connect several instances of CipherInputStream and FileInputStream. In this example, assume that cipher1 and cipher2 have been initialized for encryption and decryption (with corresponding keys), respectively. The program copies the content from file /tmp/a.txt to /tmp/b.txt, except that the content is first encrypted and then decrypted back when it is read from /tmp/a.txt. Of course since this program simply encrypts text and decrypts it back right away, it's actually not very useful except as a simple way of illustrating chaining of CipherInputStreams.
Note that the read methods of the CipherInputStream will block until data is returned from the underlying cipher. If a block cipher is used, a full block of cipher text will have to be obtained from the underlying InputStream.
Example 2-5 Sample Code for Using CipherInputStream and FileInputStream
The following code demonstrates how to use a CipherInputStream
containing that cipher and a FileInputStream in order to encrypt
input stream data:
try (FileInputStream fis = new FileInputStream("/tmp/a.txt");
CipherInputStream cis = new CipherInputStream(fis, cipher1);
FileOutputStream fos = new FileOutputStream("/tmp/b.txt")) {
byte[] b = new byte[8];
int i = cis.read(b);
while (i != -1) {
fos.write(b, 0, i);
i = cis.read(b);
}
}Example 2-6 Sample Code for Connecting CipherInputStream and FileInputStream
The following example demonstrates how to easily connect several instances of CipherInputStream and FileInputStream. In this example, assume that cipher1 and cipher2 have been initialized for encryption and decryption (with corresponding keys), respectively:
try (FileInputStream fis = new FileInputStream("/tmp/a.txt");
CipherInputStream cis1 = new CipherInputStream(fis, cipher1);
CipherInputStream cis2 = new CipherInputStream(cis1, cipher2);
FileOutputStream fos = new FileOutputStream("/tmp/b.txt")) {
byte[] b = new byte[8];
int i = cis2.read(b);
while (i != -1) {
fos.write(b, 0, i);
i = cis2.read(b);
}
} The CipherOutputStream Class
This class is a FilterOutputStream that encrypts or decrypts the data passing through it. It is composed of an OutputStream, or one of its subclasses, and a Cipher. CipherOutputStream represents a secure output stream into which a Cipher object has been interposed. The write methods of CipherOutputStream first process the data with the embedded Cipher object before writing them out to the underlying OutputStream. The Cipher object must be fully initialized before being used by a CipherOutputStream.
For example, if the embedded Cipher has been initialized for encryption, the CipherOutputStream will encrypt its data, before writing them out to the underlying output stream.
This class adheres strictly to the semantics, especially the failure semantics, of its ancestor classes java.io.OutputStream and java.io.FilterOutputStream. This class has exactly those methods specified in its ancestor classes, and overrides them all, so that all data are additionally processed by the embedded cipher. Moreover, this class catches all exceptions that are not thrown by its ancestor classes.
It is crucial for a programmer using this class not to use methods that are not defined or overridden in this class (such as a new method or constructor that is later added to one of the super classes), because the design and implementation of those methods are unlikely to have considered security impact with regard to CipherOutputStream.
See Example 2-7 , for its usage, suppose cipher1 has been initialized for encryption. The program reads the content from the file /tmp/a.txt, then encrypts and stores the result (the encrypted bytes) in /tmp/b.txt.
Example 2-7 demonstrates how to easily connect several instances of CipherOutputStream and FileOutputStream. In this example, assume that cipher1 and cipher2 have been initialized for decryption and encryption (with corresponding keys), respectively. The program copies the content from file /tmp/a.txt to /tmp/b.txt, except that the content is first encrypted and then decrypted back before it is written to /tmp/b.txt.
One thing to keep in mind when using block cipher algorithms is that a full block of plaintext data must be given to the CipherOutputStream before the data will be encrypted and sent to the underlying output stream.
There is one other important difference between the flush and close methods of this class, which becomes even more relevant if the encapsulated Cipher object implements a block cipher algorithm with padding turned on:
flushflushes the underlying OutputStream by forcing any buffered output bytes that have already been processed by the encapsulated Cipher object to be written out. Any bytes buffered by the encapsulated Cipher object and waiting to be processed by it will not be written out.closecloses the underlying OutputStream and releases any system resources associated with it. It invokes thedoFinalmethod of the encapsulated Cipher object, causing any bytes buffered by it to be processed and written out to the underlying stream by calling itsflushmethod.
Example 2-7 Sample Code for Using CipherOutputStream and FileOutputStream
The code demonstrates how to use aCipherOutputStream containing that cipher and a FileOutputStream in order to encrypt data to be written to an output stream:try (FileInputStream fis = new FileInputStream("/tmp/a.txt");
FileOutputStream fos = new FileOutputStream("/tmp/b.txt");
CipherOutputStream cos = new CipherOutputStream(fos, cipher1)) {
byte[] b = new byte[8];
int i = fis.read(b);
while (i != -1) {
cos.write(b, 0, i);
i = fis.read(b);
}
cos.flush();
}Example 2-8 Sample Code for Connecting CipherOutputStream and FileOutputStream
The code demonstrates how to easily connect several instances of CipherOutputStream and FileOutputStream. In this example, assume thatcipher1 and cipher2 have been initialized for decryption and encryption (with corresponding keys), respectively:try (FileInputStream fis = new FileInputStream("/tmp/a.txt");
FileOutputStream fos = new FileOutputStream("/tmp/b.txt");
CipherOutputStream cos1 = new CipherOutputStream(fos, cipher1);
CipherOutputStream cos2 = new CipherOutputStream(cos1, cipher2)) {
byte[] b = new byte[8];
int i = fis.read(b);
while (i != -1) {
cos2.write(b, 0, i);
i = fis.read(b);
}
cos2.flush();
}The SealedObject Class
This class enables a programmer to create an object and protect its confidentiality with a cryptographic algorithm.
Given any object that implements the java.io.Serializable interface, one can create a SealedObject that encapsulates the original object, in serialized format (i.e., a "deep copy"), and seals (encrypts) its serialized contents, using a cryptographic algorithm such as AES, to protect its confidentiality. The encrypted content can later be decrypted (with the corresponding algorithm using the correct decryption key) and de-serialized, yielding the original object.
A typical usage is illustrated in the following code segment: In order to seal an object, you create a SealedObject from the object to be sealed and a fully initialized Cipher object that will encrypt the serialized object contents. In this example, the String "This is a secret" is sealed using the AES algorithm. Note that any algorithm parameters that may be used in the sealing operation are stored inside of SealedObject:
// create Cipher object
// NOTE: sKey is assumed to refer to an already-generated
// secret AES key.
Cipher c = Cipher.getInstance("AES");
c.init(Cipher.ENCRYPT_MODE, sKey);
// do the sealing
SealedObject so = new SealedObject("This is a secret", c);
The original object that was sealed can be recovered in two different ways:
- by using a
Cipherobject that has been initialized with the exact same algorithm, key, padding scheme, etc., that were used to seal the object:c.init(Cipher.DECRYPT_MODE, sKey); try { String s = (String)so.getObject(c); } catch (Exception e) { // do something };This approach has the advantage that the party who unseals the sealed object does not require knowledge of the decryption key. For example, after one party has initialized the cipher object with the required decryption key, it could hand over the cipher object to another party who then unseals the sealed object.
- by using the appropriate decryption key (since AES is a symmetric encryption algorithm, we use the same key for sealing and unsealing):
try { String s = (String)so.getObject(sKey); } catch (Exception e) { // do something };In this approach, the
getObjectmethod creates a cipher object for the appropriate decryption algorithm and initializes it with the given decryption key and the algorithm parameters (if any) that were stored in the sealed object. This approach has the advantage that the party who unseals the object does not need to keep track of the parameters (e.g., the IV) that were used to seal the object.
The Mac Class
Similar to a MessageDigest, a Message Authentication Code (MAC) provides a way to check the integrity of information transmitted over or stored in an unreliable medium, but includes a secret key in the calculation.
Only someone with the proper key will be able to verify the received message. Typically, message authentication codes are used between two parties that share a secret key in order to validate information transmitted between these parties.
A MAC mechanism that is based on cryptographic hash functions is referred to as HMAC. HMAC can be used with any cryptographic hash function, e.g., SHA-256, in combination with a secret shared key.
The Mac class provides the functionality of a Message Authentication Code (MAC). See HMAC-SHA256 Example.
Creating a Mac Object
Mac objects are obtained by using one of the Mac getInstance() static factory methods. See How Provider Implementations Are Requested and Supplied.
Initializing a Mac Object
A Mac object is always initialized with a (secret) key and may optionally be initialized with a set of parameters, depending on the underlying MAC algorithm.
To initialize a Mac object, call one of its init methods:
public void init(Key key);
public void init(Key key, AlgorithmParameterSpec params);You can initialize your Mac object with any (secret-)key object that implements the javax.crypto.SecretKey interface. This could be an object returned by javax.crypto.KeyGenerator.generateKey(), or one that is the result of a key agreement protocol, as returned by javax.crypto.KeyAgreement.generateSecret(), or an instance of javax.crypto.spec.SecretKeySpec.
With some MAC algorithms, the (secret-)key algorithm associated with the (secret-)key object used to initialize the Mac object does not matter (this is the case with the HMAC-MD5 and HMAC-SHA1 implementations of the SunJCE provider). With others, however, the (secret-)key algorithm does matter, and an InvalidKeyException is thrown if a (secret-)key object with an inappropriate (secret-)key algorithm is used.
Computing a MAC
A MAC can be computed in one step (single-part operation) or in multiple steps (multiple-part operation). A multiple-part operation is useful if you do not know in advance how long the data is going to be, or if the data is too long to be stored in memory all at once.
To compute the MAC of some data in a single step, call the following doFinal method:
public byte[] doFinal(byte[] input);To compute the MAC of some data in multiple steps, call one of the update methods:
public void update(byte input);
public void update(byte[] input);
public void update(byte[] input, int inputOffset, int inputLen);A multiple-part operation must be terminated by the doFinal method
(if there is still some input data left for the last step), or by one of the
following doFinal methods (if there is no input data left for the
last step):
public byte[] doFinal();
public void doFinal(byte[] output, int outOffset);Key Interfaces
The java.security.Key interface is the top-level interface for all opaque keys. It defines the functionality shared by all opaque key objects.
To this point, we have focused the high-level uses of the JCA without getting lost in the details of what keys are and how they are generated/represented. It is now time to turn our attention to keys.
An opaque key representation is one in which you have
no direct access to the key material that constitutes a key. In other words: "opaque"
gives you limited access to the key--just the three methods defined by the
Key interface: getAlgorithm,
getFormat, and getEncoded.
This is in contrast to a transparent representation, in which you can access each key material value individually, through one of the get methods defined in the corresponding KeySpec interface (see The KeySpec Interface).
All opaque keys have three characteristics:
- An Algorithm
- The key algorithm for that key. The key algorithm is usually an encryption or asymmetric operation algorithm (such as
AES,DSAorRSA), which will work with those algorithms and with related algorithms (such asSHA256withRSA). The name of the algorithm of a key is obtained using this method:String getAlgorithm() - An Encoded Form
- The external encoded form for the key used when a standard representation of the key is needed outside the Java Virtual Machine, as when transmitting the key to some other party. The key is encoded according to a standard format (such as X.509 or PKCS8), and is returned using the method:
byte[] getEncoded() - A Format
- The name of the format of the encoded key. It is returned by the method:
String getFormat()
Keys are generally obtained through key generators such as the KeyGenerator class and the KeyPairGenerator class, certificates, key specifications (see the The KeySpec Interface) using a KeyFactory, or a Keystore implementation accessing a keystore database used to manage keys. It is possible to parse encoded keys, in an algorithm-dependent manner, using a KeyFactory.
It is also possible to parse certificates, using a CertificateFactory.
Here is a list of interfaces which extend the Key interface in the java.security.interfaces and javax.crypto.interfaces packages:
The PublicKey and PrivateKey Interfaces
The PublicKey and PrivateKey interfaces (which both extend the Key interface) are methodless interfaces, used for type-safety and type-identification.
The KeyPair Class
The KeyPair class is a simple holder for a key pair (a public key and a private key).
It has two public methods, one for returning the private key, and the other for returning the public key:
PrivateKey getPrivate()
PublicKey getPublic()
Key Specification Interfaces and Classes
Key objects and key specifications (KeySpecs) are two different representations of key data. Ciphers use Key objects to initialize their encryption algorithms, but keys may need to be converted into a more portable format for transmission or storage.
A transparent representation of keys means that you can access each key material value individually, through one of the get methods defined in the corresponding specification class. For example, DSAPrivateKeySpec defines getX, getP, getQ, and getG methods, to access the private key x, and the DSA algorithm parameters used to calculate the key: the prime p, the sub-prime q, and the base g. If the key is stored on a hardware device, its specification may contain information that helps identify the key on the device.
This representation is contrasted with an opaque representation, as defined by the Key Interfaces interface, in which you have no direct access to the key material fields. In other words, an "opaque" representation gives you limited access to the key--just the three methods defined by the Key interface: getAlgorithm, getFormat, and getEncoded.
A key may be specified in an algorithm-specific way, or in an algorithm-independent encoding format (such as ASN.1). For example, a DSA private key may be specified by its components x, p, q, and g (see DSAPrivateKeySpec), or it may be specified using its DER encoding (see PKCS8EncodedKeySpec).
The The KeyFactory Class and The SecretKeyFactory Class classes can be used to convert between opaque and transparent key representations (that is, between Keys and KeySpecs, assuming that the operation is possible. (For example, private keys on smart cards might not be able leave the card. Such Keys are not convertible.)
In the following sections, we discuss the key specification interfaces and classes in the java.security.spec package.
The KeySpec Interface
This interface contains no methods or constants. Its only purpose is to group and provide type safety for all key specifications. All key specifications must implement this interface.
The EncodedKeySpec Class
This abstract class (which implements the The KeySpec Interface interface) represents a public or private key in encoded format. Its getEncoded method returns the encoded key:
abstract byte[] getEncoded();
and its getFormat method returns the name of the encoding format:
abstract String getFormat();
See the next sections for the concrete implementations PKCS8EncodedKeySpec and X509EncodedKeySpec.
The PKCS8EncodedKeySpec Class
This class, which is a subclass of EncodedKeySpec, represents the DER encoding of a private key, according to the format specified in the PKCS8 standard.
Its getEncoded method returns the key bytes, encoded according to the PKCS8 standard. Its getFormat method returns the string "PKCS#8".
The X509EncodedKeySpec Class
This class, which is a subclass of EncodedKeySpec, represents the DER encoding of a public key, according to the format specified in the X.509 standard.
getEncoded method returns the key bytes, encoded according to the X.509 standard. Its getFormat method returns the string "X.509".
Generators and Factories
Newcomers to Java and the JCA APIs in particular sometimes do not grasp the distinction between generators and factories.
Generators are used to generate brand new objects. Generators can be initialized in either an algorithm-dependent or algorithm-independent way. For example, to create a Diffie-Hellman (DH) keypair, an application could specify the necessary P and G values, or the generator could simply be initialized with the appropriate key length, and the generator will select appropriate P and G values. In both cases, the generator will produce brand new keys based on the parameters.
On the other hand, factories are used to convert data from one existing object type to another. For example, an application might have available the components of a DH private key and can package them as a The KeySpec Interface, but needs to convert them into a PrivateKey object that can be used by a KeyAgreement object, or vice-versa. Or they might have the byte array of a certificate, but need to use a CertificateFactory to convert it into a X509Certificate object. Applications use factory objects to do the conversion.
The KeyFactory Class
The KeyFactory class is an Engine Classes and Algorithms designed to perform conversions between opaque cryptographic Key Interfaces and Key Specification Interfaces and Classes (transparent representations of the underlying key material).
Key factories are bi-directional. They allow you to build an opaque key object from a given key specification (key material), or to retrieve the underlying key material of a key object in a suitable format.
Multiple compatible key specifications can exist for the same key. For example, a DSA public key may be specified by its components y, p, q, and g (see java.security.spec.DSAPublicKeySpec), or it may be specified using its DER encoding according to the X.509 standard (see The X509EncodedKeySpec Class).
A key factory can be used to translate between compatible key specifications. Key parsing can be achieved through translation between compatible key specifications, e.g., when you translate from X509EncodedKeySpec to DSAPublicKeySpec, you basically parse the encoded key into its components. For an example, see the end of the Generating/Verifying Signatures Using Key Specifications and KeyFactory section.
Creating a KeyFactory Object
KeyFactory objects are obtained by using one of the KeyFactorygetInstance() static factory methods. See How Provider Implementations Are Requested and Supplied.
Converting Between a Key Specification and a Key Object
If you have a key specification for a public key, you can obtain an opaque PublicKey object from the specification by using the generatePublic method:
PublicKey generatePublic(KeySpec keySpec)
Similarly, if you have a key specification for a private key, you can obtain an opaque PrivateKey object from the specification by using the generatePrivate method:
PrivateKey generatePrivate(KeySpec keySpec)
Converting Between a Key Object and a Key Specification
If you have a Key object, you can get a corresponding key specification object by calling the getKeySpec method:
KeySpec getKeySpec(Key key, Class keySpec)
keySpec identifies the specification class in which the key material should be returned. It could, for example, be DSAPublicKeySpec.class , to indicate that the key material should be returned in an instance of the DSAPublicKeySpec class. See Generating/Verifying Signatures Using Key Specifications and KeyFactory.
The SecretKeyFactory Class
The SecretKeyFactory class represents a factory for secret keys. Unlike the KeyFactory class (see The KeyFactory Class), a javax.crypto.SecretKeyFactory object operates only on secret (symmetric) keys, whereas a java.security.KeyFactory object processes the public and private key components of a key pair.
Key factories are used to convert Key Interfaces (opaque cryptographic keys of type java.security.Key) into Key Specification Interfaces and Classes (transparent representations of the underlying key material in a suitable format), and vice versa.
Objects of type java.security.Key, of which java.security.PublicKey, java.security.PrivateKey, and javax.crypto.SecretKey are subclasses, are opaque key objects, because you cannot tell how they are implemented. The underlying implementation is provider-dependent, and may be software or hardware based. Key factories allow providers to supply their own implementations of cryptographic keys.
For example, if you have a key specification for a Diffie-Hellman public key, consisting of the public value y, the prime modulus p, and the base g, and you feed the same specification to Diffie-Hellman key factories from different providers, the resulting PublicKey objects will most likely have different underlying implementations.
A provider should document the key specifications supported by its secret key factory. For example, the SecretKeyFactory for DES keys supplied by the SunJCE provider supports DESKeySpec as a transparent representation of DES keys, the SecretKeyFactory for DES-EDE keys supports DESedeKeySpec as a transparent representation of DES-EDE keys, and the SecretKeyFactory for PBE supports PBEKeySpec as a transparent representation of the underlying password.
The following is an example of how to use a SecretKeyFactory to convert secret key data into a SecretKey object, which can be used for a subsequent Cipher operation:
// Note the following bytes are not realistic secret key data
// bytes but are simply supplied as an illustration of using data
// bytes (key material) you already have to build a DESedeKeySpec.
byte[] desEdeKeyData = getKeyData();
DESedeKeySpec desEdeKeySpec = new DESedeKeySpec(desEdeKeyData);
SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("DESede");
SecretKey secretKey = keyFactory.generateSecret(desEdeKeySpec);
In this case, the underlying implementation of SecretKey is based on the provider of KeyFactory.
An alternative, provider-independent way of creating a functionally equivalent SecretKey object from the same key material is to use the javax.crypto.spec.SecretKeySpec class, which implements the javax.crypto.SecretKey interface:
byte[] aesKeyData = getKeyData();
SecretKeySpec secretKey = new SecretKeySpec(aesKeyData, "AES");Creating a SecretKeyFactory Object
SecretKeyFactory objects are obtained by using one of the SecretKeyFactory getInstance() static factory methods. See How Provider Implementations Are Requested and Supplied.
Converting Between a Key Specification and a Secret Key Object
If you have a key specification for a secret key, you can obtain an opaque SecretKey object from the specification by using the generateSecret method:
SecretKey generateSecret(KeySpec keySpec)Converting Between a Secret Key Object and a Key Specification
If you have a SecretKey object, you can get a corresponding key specification object by calling the getKeySpec method:
KeySpec getKeySpec(Key key, Class keySpec)keySpec identifies the specification class in which the key material should be returned. It could, for example, be DESKeySpec.class, to indicate that the key material should be returned in an instance of the DESKeySpec class.
The KeyPairGenerator Class
The KeyPairGenerator class is an engine class (see Engine Classes and Algorithms) used to generate pairs of public and private keys.
There are two ways to generate a key pair: in an algorithm-independent manner, and in an algorithm-specific manner. The only difference between the two is the initialization of the object.
See Generating a Pair
of Keys for examples of calls to the
methods of KeyPairGenerator.
Creating a KeyPairGenerator
All key pair generation starts with a KeyPairGenerator. KeyPairGenerator objects are obtained by using one of the KeyPairGenerator getInstance() static factory methods. See How Provider Implementations Are Requested and Supplied.
Initializing a KeyPairGenerator
A key pair generator for a particular algorithm creates a public/private key pair that can be used with this algorithm. It also associates algorithm-specific parameters with each of the generated keys.
A key pair generator needs to be initialized before it can generate keys. In most cases, algorithm-independent initialization is sufficient. But in other cases, algorithm-specific initialization can be used.
Algorithm-Independent Initialization
All key pair generators share the concepts of a keysize and a source of randomness. The keysize is interpreted differently for different algorithms. For example, in the case of the DSA algorithm, the keysize corresponds to the length of the modulus. (See Java Security Standard Algorithm Names for information about the keysizes for specific algorithms.)
An initialize method takes two universally shared types of arguments:
void initialize(int keysize, SecureRandom random) Another initialize method takes only a keysize argument; it uses a system-provided source of randomness:
void initialize(int keysize)Since no other parameters are specified when you call these algorithm-independent
initialize methods, it is up to the provider what to do about
the algorithm-specific parameters (if any) to be associated with each of the
keys.
If the algorithm is a "DSA" algorithm, and the modulus size (keysize) is 512, 768,
1024, 2048, or 3072, then the SUN provider uses a set of
precomputed values for the p, q, and
g parameters. If the modulus size is not one of these values,
the SUN provider creates a new set of parameters. Other providers
might have precomputed parameter sets for more than just the three modulus sizes
mentioned previously. Still others might not have a list of precomputed parameters
at all and instead always create new parameter sets.
Algorithm-Specific Initialization
For situations where a set of algorithm-specific parameters already exists (such as "community parameters" in DSA), there are two initialize methods that have an The AlgorithmParameterSpec Interface argument. One also has a SecureRandom argument, while the source of randomness is system-provided for the other:
void initialize(AlgorithmParameterSpec params,
SecureRandom random)
void initialize(AlgorithmParameterSpec params)Generating a Key Pair
The procedure for generating a key pair is always the same, regardless of initialization (and of the algorithm). You always call the following method from KeyPairGenerator:
KeyPair generateKeyPair()Multiple calls to generateKeyPair will yield different key pairs.
The KeyGenerator Class
A key generator is used to generate secret keys for symmetric algorithms.
Creating a KeyGenerator
KeyGenerator objects are obtained by using one of the KeyGenerator getInstance() static factory methods. See How Provider Implementations Are Requested and Supplied.
Initializing a KeyGenerator Object
A key generator for a particular symmetric-key algorithm creates a symmetric key that can be used with that algorithm. It also associates algorithm-specific parameters (if any) with the generated key.
There are two ways to generate a key: in an algorithm-independent manner, and in an algorithm-specific manner. The only difference between the two is the initialization of the object:
- Algorithm-Independent Initialization
All key generators share the concepts of a keysize and a source of randomness. There is an
initmethod that takes these two universally shared types of arguments. There is also one that takes just akeysizeargument, and uses a system-provided source of randomness, and one that takes just a source of randomness:public void init(SecureRandom random); public void init(int keysize); public void init(int keysize, SecureRandom random);Since no other parameters are specified when you call these algorithm-independent
initmethods, it is up to the provider what to do about the algorithm-specific parameters (if any) to be associated with the generated key. - Algorithm-Specific Initialization
For situations where a set of algorithm-specific parameters already exists, there are two
initmethods that have anAlgorithmParameterSpecargument. One also has aSecureRandomargument, while the source of randomness is system-provided for the other:public void init(AlgorithmParameterSpec params); public void init(AlgorithmParameterSpec params, SecureRandom random);
In case the client does not explicitly initialize the KeyGenerator (via a call to an init method), each provider must supply (and document) a default initialization.
Creating a Key
The following method generates a secret key:
public SecretKey generateKey();The KeyAgreement Class
Key agreement is a protocol by which 2 or more parties can establish the same cryptographic keys, without having to exchange any secret information.
Each party initializes their key agreement object with their private key, and then enters the public keys for each party that will participate in the communication. In most cases, there are just two parties, but algorithms such as Diffie-Hellman allow for multiple parties (3 or more) to participate. When all the public keys have been entered, each KeyAgreement object will generate (agree upon) the same key.
The KeyAgreement class provides the functionality of a key agreement protocol. The keys involved in establishing a shared secret are created by one of the key generators (KeyPairGenerator or KeyGenerator), a KeyFactory, or as a result from an intermediate phase of the key agreement protocol.
Creating a KeyAgreement Object
Each party involved in the key agreement has to create a KeyAgreement object. KeyAgreement objects are obtained by using one of the KeyAgreement getInstance() static factory methods. See How Provider Implementations Are Requested and Supplied.
Initializing a KeyAgreement Object
You initialize a KeyAgreement object with your private information. In the case of Diffie-Hellman, you initialize it with your Diffie-Hellman private key. Additional initialization information may contain a source of randomness and/or a set of algorithm parameters. Note that if the requested key agreement algorithm requires the specification of algorithm parameters, and only a key, but no parameters are provided to initialize the KeyAgreement object, the key must contain the required algorithm parameters. (For example, the Diffie-Hellman algorithm uses a prime modulus p and a base generator g as its parameters.)
To initialize a KeyAgreement object, call one of its init methods:
public void init(Key key);
public void init(Key key, SecureRandom random);
public void init(Key key, AlgorithmParameterSpec params);
public void init(Key key, AlgorithmParameterSpec params,
SecureRandom random);
Executing a KeyAgreement Phase
Every key agreement protocol consists of a number of phases that need to be executed by each party involved in the key agreement.
To execute the next phase in the key agreement, call the doPhase method:
public Key doPhase(Key key, boolean lastPhase);
The key parameter contains the key to be processed by that phase. In most cases, this is the public key of one of the other parties involved in the key agreement, or an intermediate key that was generated by a previous phase. doPhase may return an intermediate key that you may have to send to the other parties of this key agreement, so they can process it in a subsequent phase.
The lastPhase parameter specifies whether or not the phase to be executed is the last one in the key agreement: A value of FALSE indicates that this is not the last phase of the key agreement (there are more phases to follow), and a value of TRUE indicates that this is the last phase of the key agreement and the key agreement is completed, i.e., generateSecret can be called next.
In the example of Diffie-Hellman Key Exchange between Two Parties , you call doPhase once, with lastPhase set to TRUE. In the example of Diffie-Hellman between three parties, you call doPhase twice: the first time with lastPhase set to FALSE, the 2nd time with lastPhase set to TRUE.
Generating the Shared Secret
After each party has executed all the required key agreement phases, it can compute the shared secret by calling one of the generateSecret methods:
public byte[] generateSecret();
public int generateSecret(byte[] sharedSecret, int offset);
public SecretKey generateSecret(String algorithm);
Key Management
A database called a "keystore" can be used to manage a repository of keys and certificates. (A certificate is a digitally signed statement from one entity, saying that the public key of some other entity has a particular value.)
Keystore Location
The user keystore is by default stored in a file named
.keystore in the user's home directory, as determined by the
user.home system property whose default value depends on the
operating system:
- Linux and macOS:
/home/username/ - Windows:
C:\Users\username\
Of course, keystore files can be located as desired. In some environments, it may make sense for multiple keystores to exist. For example, one keystore might hold a user's private keys, and another might hold certificates used to establish trust relationships.
In addition to the user's
keystore, the JDK also maintains a system-wide keystore which is used to store
trusted certificates from a variety of Certificate Authorities (CA's). These CA
certificates can be used to help make trust decisions. For example, in SSL/TLS/DTLS
when the SunJSSE provider is presented with certificates from a
remote peer, the default trustmanager will consult one of the following files to
determine if the connection is to be trusted:
- Linux and macOS:
<java-home>/lib/security/cacerts - Windows:
<java-home>\lib\security\cacerts
Instead of using the system-wide cacerts keystore, applications can
set up and use their own keystores, or even use the user keystore described
previously.
Keystore Implementation
The KeyStore class supplies well-defined interfaces to
access and modify the information in a keystore. It is possible for there to be
multiple different concrete implementations, where each implementation is that for a
particular type of keystore.
Currently, there are two command-line tools that make use of
KeyStore: keytool and
jarsigner. It is also used by the
Policy reference implementation when it processes policy files
specifying the permissions (allowed accesses to system resources) to be granted to
code from various sources. Since KeyStore is publicly available,
JDK users can write additional security applications that use it.
Applications can choose different types of keystore
implementations from different providers, using the getInstance
factory method in the KeyStore class. A keystore type defines the
storage and data format of the keystore information, and the algorithms used to
protect private keys in the keystore and the integrity of the keystore itself.
Keystore implementations of different types are not compatible.
The default keystore implementation is "pkcs12". This is a cross-platform keystore based on the RSA PKCS12 Personal Information Exchange Syntax Standard. This standard is primarily meant for storing or transporting a user's private keys, certificates, and miscellaneous secrets. Arbitrary attributes can be associated with individual entries in a PKCS12 keystore.
keystore.type=pkcs12To have tools and other applications use a different default keystore implementation, you can change that line to specify a different type.
Some applications, such as keytool, also let you
override the default keystore type (via the -storetype command-line
parameter).
Note:
Keystore type designations are case-insensitive. For example, "jks" would be considered the same as "JKS".PKCS12 is the default and recommened keystore type. However, there are three other types of keystores that come with the JDK implementation.
- "jceks" is an alternate
proprietary keystore format to "jks" that uses Password-Based Encryption with
Triple-DES.
The "jceks" implementation can parse and convert a "jks" keystore file to the "jceks" format. You may upgrade your keystore of type "jks" to a keystore of type "jceks" by changing the password of a private-key entry in your keystore and specifying
"-storetype jceks"as the keystore type. To apply the cryptographically strong(er) key protection supplied to a private key named "signkey" in your default keystore, use the following command, which will prompt you for the old and new key passwords:
Seekeytool -keypass -alias signkey -storetype jcekskeytoolin Java Development Kit Tool Specifications . - "jks" is another option. It implements the keystore as a file, utilizing a proprietary keystore type (format). It protects each private key with its own individual password, and also protects the integrity of the entire keystore with a (possibly different) password.
- "dks" is a domain keystore. It is a collection of keystores
presented as a single logical keystore. The keystores that comprise a given
domain are specified by configuration data whose syntax is described in
DomainLoadStoreParameter.
Keystore implementations are provider-based. If you want to write your own KeyStore implementations, see How to Implement a Provider in the Java Cryptography Architecture.
The KeyStore Class
The KeyStore class supplies well-defined interfaces to access and modify the information in a keystore.
The KeyStore class is an Engine Classes and Algorithms.
This class represents an in-memory collection of keys and certificates. KeyStore manages two types of entries:
-
Key Entry: This type of keystore entry holds very sensitive cryptographic key information, which must be protected from unauthorized access. Typically, a key stored in this type of entry is a secret key, or a private key accompanied by the certificate chain authenticating the corresponding public key.
Private keys and certificate chains are used by a given entity for self-authentication using digital signatures. For example, software distribution organizations digitally sign JAR files as part of releasing and/or licensing software.
-
Trusted Certificate Entry: This type of entry contains a single public key certificate belonging to another party. It is called a trusted certificate because the keystore owner trusts that the public key in the certificate indeed belongs to the identity identified by the subject (owner) of the certificate.
This type of entry can be used to authenticate other parties.
Each entry in a keystore is identified by an "alias" string. In the case of private keys and their associated certificate chains, these strings distinguish among the different ways in which the entity may authenticate itself. For example, the entity may authenticate itself using different certificate authorities, or using different public key algorithms.
Whether keystores are persistent, and the mechanisms used by the keystore if it is persistent, are not specified here. This convention allows use of a variety of techniques for protecting sensitive (e.g., private or secret) keys. Smart cards or other integrated cryptographic engines (SafeKeyper) are one option, and simpler mechanisms such as files may also be used (in a variety of formats).
The following describes the main KeyStore methods.
Creating a KeyStore Object
KeyStore objects are obtained by using one of the KeyStore getInstance() method. See How Provider Implementations Are Requested and Supplied.
Loading a Particular Keystore into Memory
Before a KeyStore object can be used, the actual keystore data must be loaded into memory via the load method:
final void load(InputStream stream, char[] password)The optional password is used to check the integrity of the keystore data. If no password is supplied, no integrity check is performed.
To create an empty keystore, you pass null as the InputStream argument to the load method.
A DKS keystore is loaded by passing a DomainLoadStoreParameter to the alternative load method:
final void load(KeyStore.LoadStoreParameter param)Getting a List of the Keystore Aliases
All keystore entries are accessed via unique aliases. The aliases method returns an enumeration of the alias names in the keystore:
final Enumeration aliases()Determining Keystore Entry Types
As stated in the KeyStore class, there are two different types of entries in a keystore. The following methods determine whether the entry specified by the given alias is a key/certificate or a trusted certificate entry, respectively:
final boolean isKeyEntry(String alias)
final boolean isCertificateEntry(String alias)Adding/Setting/Deleting Keystore Entries
The setCertificateEntry method assigns a certificate to a specified alias:
final void setCertificateEntry(String alias, Certificate cert)If alias doesn't exist, a trusted certificate entry with that alias is created. If alias exists and identifies a trusted certificate entry, the certificate associated with it is replaced by cert.
The setKeyEntry methods add (if alias doesn't yet exist) or set key entries:
final void setKeyEntry(String alias,
Key key,
char[] password,
Certificate[] chain)
final void setKeyEntry(String alias,
byte[] key,
Certificate[] chain)In the method with key as a byte array, it is the bytes for a key in protected format. For example, in the keystore implementation supplied by the SUN provider, the key byte array is expected to contain a protected private key, encoded as an EncryptedPrivateKeyInfo as defined in the PKCS8 standard. In the other method, the password is the password used to protect the key.
The deleteEntry method deletes an entry:
final void deleteEntry(String alias)PKCS #12 keystores support entries containing arbitrary attributes. Use the PKCS12Attribute class to create the attributes. When creating the new keystore entry use a constructor method that accepts attributes. Finally, use the following method to add the entry to the keystore:
final void setEntry(String alias, Entry entry,
ProtectionParameter protParam)Getting Information from the Keystore
The getKey method returns the key associated with the given alias. The key is recovered using the given password:
final Key getKey(String alias, char[] password)The following methods return the certificate, or certificate chain, respectively, associated with the given alias:
final Certificate getCertificate(String alias)
final Certificate[] getCertificateChain(String alias)You can determine the name (alias) of the first entry whose certificate matches a given certificate via the following:
final String getCertificateAlias(Certificate cert)PKCS #12 keystores support entries containing arbitrary attributes. Use the following method to retrieve an entry that may contain attributes:
final Entry getEntry(String alias, ProtectionParameter protParam)and then use the KeyStore.Entry.getAttributes method to extract such attributes and use the methods of the KeyStore.Entry.Attribute interface to examine them.
Saving the KeyStore
The in-memory keystore can be saved via the store method:
final void store(OutputStream stream, char[] password)The password is used to calculate an integrity checksum of the keystore data, which is appended to the keystore data.
A DKS keystore is stored by passing a DomainLoadStoreParameter to the alternative store method:
final void store(KeyStore.LoadStoreParameter param)Algorithm Parameters Classes
Like Keys and Keyspecs, an algorithm's initialization parameters are represented by either AlgorithmParameters or AlgorithmParameterSpecs.
Depending on the use situation, algorithms can use the parameters directly, or the parameters might need to be converted into a more portable format for transmission or storage.
A transparent representation of a set of parameters (via AlgorithmParameterSpec) means that you can access each parameter value in the set individually. You can access these values through one of the get methods defined in the corresponding specification class (e.g., DSAParameterSpec defines getP, getQ, and getG methods, to access p, q, and g, respectively).
In contrast, the The AlgorithmParameters Class class supplies an opaque representation, in which you have no direct access to the parameter fields. You can only get the name of the algorithm associated with the parameter set (via getAlgorithm) and some kind of encoding for the parameter set (via getEncoded).
The AlgorithmParameterSpec Interface
AlgorithmParameterSpec is an interface to a transparent specification of cryptographic parameters. This interface contains no methods or constants. Its only purpose is to group (and provide type safety for) all parameter specifications. All parameter specifications must implement this interface.
The following are the algorithm parameter specification interfaces and classes in the java.security.spec and javax.crypto.spec packages:
DHParameterSpecDHGenParameterSpecDSAParameterSpecECGenParameterSpecECParameterSpecGCMParameterSpecIvParameterSpecMGF1ParameterSpecOAEPParameterSpecPBEParameterSpecPSSParameterSpecRC2ParameterSpecRC5ParameterSpecRSAKeyGenParameterSpec
The following algorithm parameter specs are used specifically for XML digital signatures.
The AlgorithmParameters Class
The AlgorithmParameters class provides an opaque representation of cryptographic parameters.
The AlgorithmParameters Class
The AlgorithmParameters class is an Engine Classes and Algorithms .You can initialize the AlgorithmParameters class using a specific AlgorithmParameterSpec object, or by encoding the parameters in a recognized format. You can retrieve the resulting specification with the getParameterSpec method (see the following section).
Creating an AlgorithmParameters Object
AlgorithmParameters objects are obtained by using one of the AlgorithmParameters getInstance() static factory methods. For more information, see How Provider Implementations Are Requested and Supplied.
Initializing an AlgorithmParameters Object
Once an AlgorithmParameters object is instantiated, it must be initialized via a call to init, using an appropriate parameter specification or parameter encoding:
void init(AlgorithmParameterSpec paramSpec)
void init(byte[] params)
void init(byte[] params, String format)
In these init methods, params is an array containing the encoded parameters, and format is the name of the decoding format. In the init method with a params argument but no format argument, the primary decoding format for parameters is used. The primary decoding format is ASN.1, if an ASN.1 specification for the parameters exists.
Obtaining the Encoded Parameters
A byte encoding of the parameters represented in an AlgorithmParameters object may be obtained via a call to getEncoded:
byte[] getEncoded()
This method returns the parameters in their primary encoding format. The primary encoding format for parameters is ASN.1, if an ASN.1 specification for this type of parameters exists.
If you want the parameters returned in a specified encoding format, use
byte[] getEncoded(String format)
If format is null, the primary encoding format for parameters is used, as in the other getEncoded method.
Converting an AlgorithmParameters Object to a Transparent Specification
A transparent parameter specification for the algorithm parameters may be obtained from an AlgorithmParameters object via a call to getParameterSpec:
AlgorithmParameterSpec getParameterSpec(Class paramSpec)
paramSpec identifies the specification class in which the parameters should be returned. The specification class could be, for example, DSAParameterSpec.class to indicate that the parameters should be returned in an instance of the DSAParameterSpec class. (This class is in the java.security.spec package.)
The AlgorithmParameterGenerator Class
The AlgorithmParameterGenerator class is an Engine Classes and Algorithms used to generate a set of brand-new parameters suitable for a certain algorithm (the algorithm is specified when an AlgorithmParameterGenerator instance is created). This object is used when you do not have an existing set of algorithm parameters, and want to generate one from scratch.
Creating an AlgorithmParameterGenerator Object
AlgorithmParameterGenerator objects are obtained by using one of the AlgorithmParameterGenerator getInstance() static factory methods. See How Provider Implementations Are Requested and Supplied.
Initializing an AlgorithmParameterGenerator Object
The AlgorithmParameterGenerator object can be initialized in two different ways: an algorithm-independent manner or an algorithm-specific manner.
The algorithm-independent approach uses the fact that all parameter generators share the concept of a "size" and a source of randomness. The measure of size is universally shared by all algorithm parameters, though it is interpreted differently for different algorithms. For example, in the case of parameters for the DSA algorithm, "size" corresponds to the size of the prime modulus, in bits. (See Java Security Standard Algorithm Names to know more about the sizes for specific algorithms.) When using this approach, algorithm-specific parameter generation values--if any--default to some standard values. One init method that takes these two universally shared types of arguments:
void init(int size, SecureRandom random);
Another init method takes only a size argument and uses a system-provided source of randomness:
void init(int size)
A third approach initializes a parameter generator object using algorithm-specific semantics, which are represented by a set of algorithm-specific parameter generation values supplied in an AlgorithmParameterSpec object:
void init(AlgorithmParameterSpec genParamSpec,
SecureRandom random)
void init(AlgorithmParameterSpec genParamSpec)
To generate Diffie-Hellman system parameters, for example, the parameter generation values usually consist of the size of the prime modulus and the size of the random exponent, both specified in number of bits.
Generating Algorithm Parameters
Once you have created and initialized an AlgorithmParameterGenerator object, you can use the generateParameters method to generate the algorithm parameters:
AlgorithmParameters generateParameters()The CertificateFactory Class
The CertificateFactory class defines the functionality of a certificate factory, which is used to generate certificate and certificate revocation list (CRL) objects from their encoding.
The CertificateFactory class is an Engine Classes and Algorithms.
A certificate factory for X.509 must return certificates that are an instance of java.security.cert.X509Certificate, and CRLs that are an instance of java.security.cert.X509CRL.
Creating a CertificateFactory Object
CertificateFactory objects are obtained by using one of the getInstance() static factory methods. For more information, see How Provider Implementations Are Requested and Supplied.
Generating Certificate Objects
To generate a certificate object and initialize it with the data read from an input stream, use the generateCertificate method:
final Certificate generateCertificate(InputStream inStream)
To return a (possibly empty) collection view of the certificates read from a given input stream, use the generateCertificates method:
final Collection generateCertificates(InputStream inStream)
Generating CRL Objects
To generate a certificate revocation list (CRL) object and initialize it with the data read from an input stream, use the generateCRL method:
final CRL generateCRL(InputStream inStream)
To return a (possibly empty) collection view of the CRLs read from a given input stream, use the generateCRLs method:
final Collection generateCRLs(InputStream inStream)
Generating CertPath Objects
The certificate path builder and validator for PKIX is defined by the Internet X.509 Public Key Infrastructure Certificate and CRL Profile, RFC 5280.
A certificate store implementation for retrieving certificates and CRLs from Collection and LDAP directories, using the PKIX LDAP V2 Schema is also available from the IETF as RFC 2587.
To generate a CertPath object and initialize it with data read from an input stream, use one of the following generateCertPath methods (with or without specifying the encoding to be used for the data):
final CertPath generateCertPath(InputStream inStream)
final CertPath generateCertPath(InputStream inStream,
String encoding)
To generate a CertPath object and initialize it with a list of certificates, use the following method:
final CertPath generateCertPath(List certificates)
To retrieve a list of the CertPath encoding supported by this certificate factory, you can call the getCertPathEncodings method:
final Iterator getCertPathEncodings()
The default encoding will be listed first.
How the JCA Might Be Used in a SSL/TLS Implementation
With an understanding of the JCA classes, consider how these classes might be combined to implement an advanced network protocol like SSL/TLS.
The SSL/TLS Overview section in the TLS and DTLS Protocols describes at a high level how the protocols work. As asymmetric (public key) cipher operations are much slower than symmetric operations (secret key), public key cryptography is used to establish secret keys which are then used to protect the actual application data. Vastly simplified, the SSL/TLS handshake involves exchanging initialization data, performing some public key operations to arrive at a secret key, and then using that key to encrypt further traffic.
Note:
The details presented here simply show how some of these classes might be employed. This section will not present sufficient information for building a SSL/TLS implementation. For more information, see Java Secure Socket Extension (JSSE) Reference Guide and RFC 5246: The Transport Layer Security (TLS) Protocol, Version 1.2.Assume that this SSL/TLS implementation will be made available as a JSSE provider. A concrete implementation of the Provider class is first written that will eventually be registered in the Security class' list of providers. This provider mainly provides a mapping from algorithm names to actual implementation classes. (that is: "SSLContext.TLS"->"com.foo.TLSImpl") When an application requests an "TLS" instance (via SSLContext.getInstance("TLS")), the provider's list is consulted for the requested algorithm, and an appropriate instance is created.
Before discussing details of the actual handshake, a quick review of some of the JSSE's architecture is needed. The heart of the JSSE architecture is the SSLContext. The context eventually creates end objects (SSLSocket and SSLEngine) which actually implement the SSL/TLS protocol. SSLContexts are initialized with two callback classes, KeyManager and TrustManager, which allow applications to first select authentication material to send and second to verify credentials sent by a peer.
A JSSE KeyManager is responsible for choosing which credentials to present to a peer. Many algorithms are possible, but a common strategy is to maintain a RSA or DSA public/private key pair along with a X509Certificate in a KeyStore backed by a disk file. When a KeyStore object is initialized and loaded from the file, the file's raw bytes are converted into PublicKey and PrivateKey objects using a KeyFactory, and a certificate chain's bytes are converted using a CertificateFactory. When a credential is needed, the KeyManager simply consults this KeyStore object and determines which credentials to present.
A KeyStore's contents might have originally been created using a utility such as keytool. keytool creates a RSA or DSA KeyPairGenerator and initializes it with an appropriate keysize. This generator is then used to create a KeyPair which keytool would store along with the newly-created certificate in the KeyStore, which is eventually written to disk.
A JSSE TrustManager is responsible for verifying the credentials received from a peer. There are many ways to verify credentials: one of them is to create a CertPath object, and let the JDK's built-in Public Key Infrastructure (PKI) framework handle the validation. Internally, the CertPath implementation might create a Signature object, and use that to verify that the each of the signatures in the certificate chain.
With this basic understanding of the architecture, we can look at some of the steps in the SSL/TLS handshake. The client begins by sending a ClientHello message to the server. The server selects a ciphersuite to use, and sends that back in a ServerHello message, and begins creating JCA objects based on the suite selection. We'll use server-only authentication in the following examples.
Server-only authentication is described in the following examples. The examples are vastly simplified, but gives an idea of how the JSSE classes might be combined to create a higher level protocol:
Example 2-9 SSL/TLS Server Uses a RSA-based ciphersuite Such as TLS_RSA_WITH_AES_128_CBC_SHA
The server'sKeyManager is queried, and returns an appropriate RSA entry. The server's credentials (that is: certificate/public key) are sent in the server's Certificate message. The client's TrustManager verifies the server's certificate, and if accepted, the client generates some random bytes using a SecureRandom object. This is then encrypted using an encrypting asymmetric RSA Cipher object that has been initialized with the PublicKey found in the server's certificate. This encrypted data is sent in a Client Key Exchange message. The server would use its corresponding PrivateKey to recover the bytes using a similar Cipher in decrypt mode. These bytes are then used to establish the actual encryption keys.
Example 2-10 Choose an Ephemeral Diffie-Hellman Key Agreement Algorithm Along with the DSA Signature Algorithm such as TLS_DHE_DSS_WITH_AES_128_CBC_SHA
The two sides must each establish a new temporary DH public/private keypair using aKeyPairGenerator. Each generator creates DH keys which can then be further converted into pieces using the KeyFactory and DHPublicKeySpec classes. Each side then creates a KeyAgreement object and initializes it with their respective DH PrivateKeys. The server sends its public key pieces in a ServerKeyExchange message (protected by the DSA signature algorithm, and the client sends its public key in a ClientKeyExchange message. When the public keys are reassembled using another KeyFactory, they are fed into the agreement objects. The KeyAgreement objects then generate agreed-upon bytes that are then used to establish the actual encryption keys.
Once the actual encryption keys have been established, the secret key is used to initialize a symmetric Cipher object, and this cipher is used to protect all data in transit. To help determine if the data has been modified, a MessageDigest is created and receives a copy of the data destined for the network. When the packet is complete, the digest (hash) is appended to data, and the entire packet is encrypted by the Cipher. If a block cipher such as AES is used, the data must be padded to make a complete block. On the remote side, the steps are simply reversed.
Cryptographic Strength Configuration
You can configure the cryptographic strength of the Java Cryptography Extension (JCE) architecture using jurisdiction policy files (see Jurisdiction Policy File Format) and the security properties file.
Prior to Oracle Java JDK 9, the default cryptographic strength allowed by Oracle implementations was “strong but limited” (for example AES keys limited to 128 bits). To remove this restriction, administrators could download and install a separate “unlimited strength” Jurisdiction Policy Files bundle. The Jurisdiction Policy File mechanism was reworked for JDK 9. It now allows for much more flexible configuration. The Oracle JDK now ships with a default value of “unlimited” rather than “limited”. As always, administrators and users must still continue to follow all import/export guidelines for their geographical locations. The active cryptographic strength is now determined using a Security Property (typically set in the java.security properties file), in combination with the jurisdiction policy files found in the configuration directory.
All the necessary JCE policy files to provide either unlimited cryptographic strength or strong but limited cryptographic strength are bundled with the JDK.
Cryptographic Strength Settings
Each directory under <java_home>/conf/security/policy represents a set of policy configurations defined by the jurisdiction policy files that they contain. You activate a particular cryptographic strength setting represented by the policy files in a directory by setting the crypto.policy Security Property (configured in the file <java_home>/conf/security/java.security) to point to that directory.
Note:
Properties in thejava.security file are typically parsed only once. If you
have modified any property in this file, restart your applications to ensure that
the changes are properly reflected.limited and unlimited, each containing a number of policy files. By default, the crypto.policy Security Property is set to: crypto.policy = unlimitedThe overall value is the intersection of the files contained within the directory. These policy files settings are VM-wide, and affect all applications running on this VM. If you want to override cryptographic strength at the application level, see How to Make Applications Exempt from Cryptographic Restrictions.
Unlimited Directory Contents
The unlimited directory contains the following policy files:
-
<java_home>/conf/security/unlimited/default_US_export.policy// Default US Export policy file. grant { // There is no restriction to any algorithms. permission javax.crypto.CryptoAllPermission; };Note:
As there are no current restrictions on export of cryptography from the United States, thedefault_US_export.policyfile is set with no restrictions. -
<java_home>/conf/security/unlimited/default_local.policy// Country specific policy file for countries with no limits on crypto strength. grant { // There is no restriction to any algorithms. permission javax.crypto.CryptoAllPermission; };Note:
Depending on the country, there may be local restrictions, but as this policy file is located in theunlimiteddirectory, there are no restrictions listed here.
To select unlimited cryptographic strength as defined in these two files set crypto.policy = unlimited in the file <java_home>/conf/security/java.security.
Limited Directory Contents
The limited directory currently contains the following policy files:
-
<java_home>/conf/security/limited/default_US_export.policy// Default US Export policy file. grant { // There is no restriction to any algorithms. permission javax.crypto.CryptoAllPermission; };Note:
Even though this is in thelimiteddirectory, as there are no current restrictions on export of cryptography from the United States, thedefault_US_export.policyfile is set with no restrictions. -
<java_home>/conf/security/limited/default_local.policy// Some countries have import limits on crypto strength. This policy file // is worldwide importable. grant { permission javax.crypto.CryptoPermission "DES", 64; permission javax.crypto.CryptoPermission "DESede", *; permission javax.crypto.CryptoPermission "RC2", 128, "javax.crypto.spec.RC2ParameterSpec", 128; permission javax.crypto.CryptoPermission "RC4", 128; permission javax.crypto.CryptoPermission "RC5", 128, "javax.crypto.spec.RC5ParameterSpec", *, 12, *; permission javax.crypto.CryptoPermission "RSA", *; permission javax.crypto.CryptoPermission *, 128; };Note:
This local policy file shows the default restrictions. It should be allowed by any country, including those that have import restrictions, but please obtain legal guidance. -
<java_home>/conf/security/limited/exempt_local.policy// Some countries have import limits on crypto strength, but may allow for // these exemptions if the exemption mechanism is used. grant { // There is no restriction to any algorithms if KeyRecovery is enforced. permission javax.crypto.CryptoPermission *, "KeyRecovery"; // There is no restriction to any algorithms if KeyEscrow is enforced. permission javax.crypto.CryptoPermission *, "KeyEscrow"; // There is no restriction to any algorithms if KeyWeakening is enforced. permission javax.crypto.CryptoPermission *, "KeyWeakening"; };Note:
Countries that have import restrictions should use “limited”, but these restrictions could be relaxed if the exemption mechanism can be employed. See How to Make Applications Exempt from Cryptographic Restrictions. Please obtain legal guidance for your situation.
Custom Cryptographic Strength Settings
To set up restrictions to cryptographic strength that are different than the settings in the policy files in the limited or unlimited directory, you can create a new directory, parallel with limited and unlimited, and place your policy files there. For example, you may create a directory called custom. In this custom directory you include the files default_*export.policy and/or exempt_*local.policy.
To select cryptographic strength as defined in the files in the custom directory, set crypto.policy = custom in the file <java_home>/conf/security/java.security.
Jurisdiction Policy File Format
JCA represents its jurisdiction policy files as Java-style policy files with corresponding permission statements. As described in Cryptographic Strength Configuration, a Java policy file specifies what permissions are allowed for code from specified code sources. A permission represents access to a system resource. In the case of JCA, the "resources" are cryptography algorithms, and code sources do not need to be specified, because the cryptographic restrictions apply to all code.
A jurisdiction policy file consists of a very basic "grant entry" containing one or more "permission entries."
grant {
<permission entries>;
};
The format of a permission entry in a jurisdiction policy file is:
permission <crypto permission class name>
[<alg_name>
[
[, <exemption mechanism name>]
[, <maxKeySize>
[, <AlgorithmParameterSpec class name>,
<parameters for constructing an AlgorithmParameterSpec object>
]
]
]
];
A sample jurisdiction policy file that includes restricting the AES algorithm to maximum key sizes of 128 bits is:
grant {
permission javax.crypto.CryptoPermission "AES", 128;
// ...
};
A permission entry must begin with the word permission. Items that appear in a permission entry must appear in the specified order. An entry is terminated with a semicolon. Case is unimportant for the identifiers (grant, permission) but is significant for the <crypto permission class name> or for any string that is passed in as a value. An asterisk (*) can be used as a wildcard for any permission entry option. For example, an asterisk for an <alg_name> option means "all algorithms."
The following table describes a permission entry's options:
Table 2-1 Permission Entry Options
| Option | Description |
|---|---|
<crypto permission class name> |
Specific permission class name, such as A crypto permission class reflects the ability of an application to use certain algorithms with certain key sizes in certain environments. There are two crypto permission classes: |
<alg_name> |
Quoted string specifying the standard name of a cryptography algorithm, such as "AES" or "RSA". Optional. |
<exemption mechanism name> |
Quoted string indicating an exemption mechanism which, if enforced, enables a reduction in cryptographic restrictions. Optional. Exemption mechanism names that can be used include "KeyRecovery" "KeyEscrow", and "KeyWeakening". |
<maxKeySize> |
Integer specifying the maximum key size (in bits) allowed for the specified algorithm. Optional. |
<AlgorithmParameterSpec class name> |
Class name that specifies the strength of the algorithm. Optional. For some algorithms, it may not be sufficient to specify the algorithm strength in terms of just a key size. For example, in the case of the "RC5" algorithm, the number of rounds must also be considered. For algorithms whose strength needs to be expressed as more than a key size, use this option to specify the |
<parameters for constructing an AlgorithmParameterSpec object> |
List of parameters for constructing the specified AlgorithmParameterSpec object. Required if <AlgorithmParameterSpec class name> has been specified and requires parameters.
|
How to Make Applications Exempt from Cryptographic Restrictions
Attention:
This section should be ignored by most application developers. It is only for people whose applications may be exported to those few countries whose governments mandate cryptographic restrictions, if it is desired that such applications have fewer cryptographic restrictions than those mandated.By default, an application can use cryptographic algorithms of any strength. However, due to import control restrictions by the governments of a few countries, you may have to limit those algorithms' strength. The JCA framework includes an ability to enforce restrictions regarding the maximum strengths of cryptographic algorithms available to applications in different jurisdiction contexts (locations). You specify these restrictions in jurisdiction policy files. For more information about jurisdiction policy files and how to create and configure them, see Cryptographic Strength Configuration.
It is possible that the governments of some or all such countries may allow certain applications to become exempt from some or all cryptographic restrictions. For example, they may consider certain types of applications as "special" and thus exempt. Or they may exempt any application that utilizes an "exemption mechanism," such as key recovery. Applications deemed to be exempt could get access to stronger cryptography than that allowed for non-exempt applications in such countries.
In order for an application to be recognized as "exempt" at runtime, it must meet the following conditions:
- It must have a permission policy file bundled with it in a JAR file. The permission policy file specifies what cryptography-related permissions the application has, and under what conditions (if any).
- The JAR file containing the application and the permission policy file must have been signed using a code-signing certificate issued after the application was accepted as exempt.
The following are sample steps required in order to make an application exempt from some cryptographic restrictions. This is a basic outline that includes information about what is required by JCA in order to recognize and treat applications as being exempt. You will need to know the exemption requirements of the particular country or countries in which you would like your application to be able to be run but whose governments require cryptographic restrictions. You will also need to know the requirements of a JCA framework vendor that has a process in place for handling exempt applications. Consult such a vendor for further information.
Note:
TheSunJCE provider does not supply an implementation of the ExemptionMechanismSpi class
- Write and Compile Your Application Code
- Create a Permission Policy File Granting Appropriate Cryptographic Permissions
- Prepare for Testing
- Apply for Government Approval From the Government Mandating Restrictions.
- Get a Code-Signing Certificate
- Bundle the Application and Permission Policy File into a JAR file
- Step 7.1: Get a Code-Signing Certificate
- Set Up Your Environment Like That of a User in a Restricted Country
- (only for applications using exemption mechanisms) Install a Provider Implementing the Exemption Mechanism Specified by the entry in the Permission Policy File
- Test Your Application
- Apply for U.S. Government Export Approval If Required
- Deploy Your Application
Special Code Requirements for Applications that Use Exemption Mechanisms
When an application has a permission policy file associated with it (in the same JAR file) and that permission policy file specifies an exemption mechanism, then when the Cipher getInstance method is called to instantiate a Cipher, the JCA code searches the installed providers for one that implements the specified exemption mechanism. If it finds such a provider, JCA instantiates an ExemptionMechanism API object associated with the provider's implementation, and then associates the ExemptionMechanism object with the Cipher returned by getInstance.
After instantiating a Cipher, and prior to initializing it (via a call to the Cipher init method), your code must call the following Cipher method:
public ExemptionMechanism getExemptionMechanism()
This call returns the ExemptionMechanism object associated with the Cipher. You must then initialize the exemption mechanism implementation by calling the following method on the returned ExemptionMechanism:
public final void init(Key key)
The argument you supply should be the same as the argument of the same types that you will subsequently supply to a Cipher init method.
Once you have initialized the ExemptionMechanism, you can proceed as usual to initialize and use the Cipher.
Permission Policy Files
In order for an application to be recognized at runtime as being "exempt" from some or all cryptographic restrictions, it must have a permission policy file bundled with it in a JAR file. The permission policy file specifies what cryptography-related permissions the application has, and under what conditions (if any).
The format of a permission entry in a permission policy file that accompanies an exempt application is the same as the format for a jurisdiction policy file downloaded with the JDK, which is:
permission <crypto permission class name>
[<alg_name>
[
[, <exemption mechanism name>]
[, <maxKeySize>
[, <AlgorithmParameterSpec class name>,
<parameters for constructing an AlgorithmParameterSpec object>
]
]
]
];
Permission Policy Files for Exempt Applications
Some applications may be allowed to be completely unrestricted. Thus, the permission policy file that accompanies such an application usually just needs to contain the following:
grant {
// There are no restrictions to any algorithms.
permission javax.crypto.CryptoAllPermission;
};
If an application just uses a single algorithm (or several specific algorithms), then the permission policy file could simply mention that algorithm (or algorithms) explicitly, rather than granting CryptoAllPermission.
For example, if an application just uses the Blowfish algorithm, the permission policy file doesn't have to grant CryptoAllPermission to all algorithms. It could just specify that there is no cryptographic restriction if the Blowfish algorithm is used. In order to do this, the permission policy file would look like the following:
grant {
permission javax.crypto.CryptoPermission "Blowfish";
};
Permission Policy Files for Applications Exempt Due to Exemption Mechanisms
If an application is considered "exempt" if an exemption mechanism is enforced, then the permission policy file that accompanies the application must specify one or more exemption mechanisms. At run time, the application will be considered exempt if any of those exemption mechanisms is enforced. Each exemption mechanism must be specified in a permission entry that looks like the following:
// No algorithm restrictions if specified
// exemption mechanism is enforced.
permission javax.crypto.CryptoPermission *,
"<ExemptionMechanismName>";
where <ExemptionMechanismName> specifies the name of an exemption mechanism. The list of possible exemption mechanism names includes:
KeyRecoveryKeyEscrowKeyWeakening
As an example, suppose your application is exempt if either key recovery or key escrow is enforced. Then your permission policy file should contain the following:
grant {
// No algorithm restrictions if KeyRecovery is enforced.
permission javax.crypto.CryptoPermission *, "KeyRecovery";
// No algorithm restrictions if KeyEscrow is enforced.
permission javax.crypto.CryptoPermission *, "KeyEscrow";
};
Note:
Permission entries that specify exemption mechanisms should not also specify maximum key sizes. The allowed key sizes are actually determined from the installed exempt jurisdiction policy files, as described in the next section.How Bundled Permission Policy Files Affect Cryptographic Permissions
At runtime, when an application instantiates a Cipher (via a call to its getInstance method) and that application has an associated permission policy file, JCA checks to see whether the permission policy file has an entry that applies to the algorithm specified in the getInstance call. If it does, and the entry grants CryptoAllPermission or does not specify that an exemption mechanism must be enforced, it means there is no cryptographic restriction for this particular algorithm.
If the permission policy file has an entry that applies to the algorithm specified in the getInstance call and the entry does specify that an exemption mechanism must be enforced, then the exempt jurisdiction policy file(s) are examined. If the exempt permissions include an entry for the relevant algorithm and exemption mechanism, and that entry is implied by the permissions in the permission policy file bundled with the application, and if there is an implementation of the specified exemption mechanism available from one of the registered providers, then the maximum key size and algorithm parameter values for the Cipher are determined from the exempt permission entry.
If there is no exempt permission entry implied by the relevant entry in the permission policy file bundled with the application, or if there is no implementation of the specified exemption mechanism available from any of the registered providers, then the application is only allowed the standard default cryptographic permissions.
Standard Names
The Standard Names document contains information about the algorithm specifications.
Java Security Standard Algorithm Names describes the standard names for algorithms, certificate and keystore types that the JDK Security API requires and uses. It also contains more information about the algorithm specifications. Specific provider information can be found in JDK Providers Documentation.
Cryptographic implementations in the JDK are distributed through several different providers primarily for historical reasons (Sun, SunJSSE, SunJCE, SunRsaSign). Note these providers may not be available on all JDK implementations, and therefore, truly portable applications should call getInstance() without specifying specific providers. Applications specifying a particular provider may not be able to take advantage of native providers tuned for an underlying operating environment (such as PKCS or Microsoft's CAPI).
The SunPKCS11 provider itself does not contain any cryptographic algorithms, but instead, directs requests into an underlying PKCS11 implementation. Consult PKCS#11 Reference Guide and the underlying PKCS11 implementation to determine if a desired algorithm will be available through the PKCS11 provider. Likewise, on Windows systems, the SunMSCAPI provider does not provide any cryptographic functionality, but instead routes requests to the underlying Operating System for handling.
Packaging Your Application
You can package an application in three different kinds of modules:
-
Named or explicit module: A module that appears on the module path and contains module configuration information in the
module-info.classfile. -
Automatic module: A module that appears on the module path, but does not contain module configuration information in a
module-info.classfile (essentially a "regular" JAR file). -
Unnamed module: A module that appears on the class path. It may or may not have a
module-info.classfile; this file is ignored.
It is recommended that you package your applications in named modules as they provide better performance, stronger encapsulation, and simpler configuration. They also offer greater flexibility; you can use them with non-modular JDKs or even as unnamed modules by specifying them in a modular JDK's class path.
For more information about modules, see The State of the Module System and JEP 261: Module System
Additional JCA Code Samples
These examples illustrate use of several JCA mechanisms. See also Sample Programs for Diffie-Hellman Key Exchange, AES/GCM, and HMAC-SHA256
Computing a MessageDigest Object
These steps describe the procedure to compute a MessageDigest object.
Example 2-11 Hash Implementations Through Cloning
Some hash implementations may support intermediate hashes through cloning. Suppose we want to calculate separate hashes for:
i1i1 and i2i1, i2, and i3
The following is one way to calculate these hashes; however, this code works only if the SHA-256 implementation is cloneable:
/* compute the hash for i1 */
sha.update(i1);
byte[] i1Hash = sha.clone().digest();
/* compute the hash for i1 and i2 */
sha.update(i2);
byte[] i12Hash = sha.clone().digest();
/* compute the hash for i1, i2 and i3 */
sha.update(i3);
byte[] i123hash = sha.digest();Example 2-12 Determine if the Hash Implementation is Cloneable or not Cloneable
Some implementations of message digests are cloneable, others are not. To determine whether or not cloning is possible, attempt to clone theMessageDigest object and catch the potential exception as follows:
try {
// try and clone it
/* compute the hash for i1 */
sha.update(i1);
byte[] i1Hash = sha.clone().digest();
// ...
byte[] i123hash = sha.digest();
} catch (CloneNotSupportedException cnse) {
// do something else, such as the code in the section
// "Compute Intermediate Digests if the Hash Implementation is not Cloneable"
}Example 2-13 Compute Intermediate Digests if the Hash Implementation is not Cloneable
If a message digest is not cloneable, the other, less elegant way to compute intermediate digests is to create several digests. In this case, the number of intermediate digests to be computed must be known in advance:MessageDigest md1 = MessageDigest.getInstance("SHA-256");
MessageDigest md2 = MessageDigest.getInstance("SHA-256");
MessageDigest md3 = MessageDigest.getInstance("SHA-256");
byte[] i1Hash = md1.digest(i1);
md2.update(i1);
byte[] i12Hash = md2.digest(i2);
md3.update(i1);
md3.update(i2);
byte[] i123Hash = md3.digest(i3);Generating a Pair of Keys
In this example we will generate a public-private key pair for the algorithm named "DSA" (Digital Signature Algorithm), and use this keypair in future examples. We will generate keys with a 2048-bit modulus. We don't care which provider supplies the algorithm implementation.
Creating the Key Pair Generator
The first step is to get a key pair generator object for generating keys for the DSA algorithm:
KeyPairGenerator keyGen = KeyPairGenerator.getInstance("DSA");Initializing the Key Pair Generator
The next step is to initialize the key pair generator. In most cases, algorithm-independent initialization is sufficient, but in some cases, algorithm-specific initialization is used.
Algorithm-Independent Initialization
All key pair generators share the concepts of a keysize and a source of randomness. The KeyPairGenerator class initialization methods at a minimum needs a keysize. If the source of randomness is not explicitly provided, a SecureRandom implementation of the highest-priority installed provider will be used. Thus to generate keys with a keysize of 2048, simply call:
keyGen.initialize(2048);The following code illustrates how to use a specific, additionally seeded SecureRandom object:
SecureRandom random = SecureRandom.getInstance("DRBG", "SUN");
random.setSeed(userSeed);
keyGen.initialize(2048, random);Since no other parameters are specified when you call these algorithm-independent initialize method, it is up to the provider what to do about the algorithm-specific parameters (if any) to be associated with each of the keys. The provider may use precomputed parameter values or may generate new values.
Algorithm-Specific Initialization
For situations where a set of algorithm-specific parameters already exists (such as "community parameters" in DSA), there are two initialize methods that have an AlgorithmParameterSpec argument. Suppose your key pair generator is for the "DSA" algorithm, and you have a set of DSA-specific parameters, p, q, and g, that you would like to use to generate your key pair. You could execute the following code to initialize your key pair generator (recall that DSAParameterSpec is an AlgorithmParameterSpec):
DSAParameterSpec dsaSpec = new DSAParameterSpec(p, q, g);
keyGen.initialize(dsaSpec);Generating the Pair of Keys
The final step is actually generating the key pair. No matter which type of initialization was used (algorithm-independent or algorithm-specific), the same code is used to generate the KeyPair:
KeyPair pair = keyGen.generateKeyPair();Generating and Verifying a Signature Using Generated Keys
Examples of generating and verifying a signature using generated keys.
KeyPair generated in the Generating a Pair of Keys .
Generating a Signature
We first create a Signature Class object:
Signature dsa = Signature.getInstance("SHA256withDSA");
Next, using the key pair generated in the key pair example, we initialize the object with the private key, then sign a byte array called data.
/* Initializing the object with a private key */
PrivateKey priv = pair.getPrivate();
dsa.initSign(priv);
/* Update and sign the data */
dsa.update(data);
byte[] sig = dsa.sign();
Verifying a Signature
Verifying the signature is straightforward. (Note that here we also use the key pair generated in the key pair example.)
/* Initializing the object with the public key */
PublicKey pub = pair.getPublic();
dsa.initVerify(pub);
/* Update and verify the data */
dsa.update(data);
boolean verifies = dsa.verify(sig);
System.out.println("signature verifies: " + verifies);
Generating/Verifying Signatures Using Key Specifications and KeyFactory
Suppose that, rather than having a public/private key pair (as, for example, was
generated in the section Generating a Pair of Keys), you simply have the
components of your DSA private key: x (the private key),
p (the prime), q (the sub-prime), and
g (the base).
Furthermore, suppose you want to use your private key to digitally sign some data,
which is in a byte array named someData. You would do the following
steps, which also illustrate creating a key specification and using a key factory to
obtain a PrivateKey from the key specification
(initSign requires a PrivateKey):
DSAPrivateKeySpec dsaPrivKeySpec = new DSAPrivateKeySpec(x, p, q, g);
KeyFactory keyFactory = KeyFactory.getInstance("DSA");
PrivateKey privKey = keyFactory.generatePrivate(dsaPrivKeySpec);
Signature sig = Signature.getInstance("SHA256withDSA");
sig.initSign(privKey);
sig.update(someData);
byte[] signature = sig.sign();
Suppose Alice wants to use the data you signed. In order for her to do so, and to verify your signature, you need to send her three things:
- The data
- The signature
- The public key corresponding to the private key you used to sign the data
You can store the someData bytes in one file, and the signature bytes in another, and send those to Alice.
For the public key, assume, as in the previous signing example, you have the
components of the DSA public key corresponding to the DSA private key used to sign
the data. Then you can create a DSAPublicKeySpec from those
components:
DSAPublicKeySpec dsaPubKeySpec = new DSAPublicKeySpec(y, p, q, g);
You still need to extract the key bytes so that you can put them in a file. To do so,
you can first call the generatePublic method on the DSA key factory
already created in the previous example:
PublicKey pubKey = keyFactory.generatePublic(dsaPubKeySpec);
Then you can extract the (encoded) key bytes via the following:
byte[] encKey = pubKey.getEncoded();
You can now store these bytes in a file, and send it to Alice along with the files containing the data and the signature.
Now, assume Alice has received these files, and she copied the data bytes from the data file to a byte array named data, the signature bytes from the signature file to a byte array named signature, and the encoded public key bytes from the public key file to a byte array named encodedPubKey.
Alice can now execute the following code to verify the signature. The code also illustrates how to use a key factory in order to instantiate a DSA public key from its encoding (initVerify requires a PublicKey).
X509EncodedKeySpec pubKeySpec = new X509EncodedKeySpec(encodedPubKey);
KeyFactory keyFactory = KeyFactory.getInstance("DSA");
PublicKey pubKey = keyFactory.generatePublic(pubKeySpec);
Signature sig = Signature.getInstance("SHA256withDSA");
sig.initVerify(pubKey);
sig.update(data);
sig.verify(signature);
Note:
In the previous example, Alice needed to generate aPublicKey from the encoded key bits, since
initVerify requires a PublicKey . Once she
has a PublicKey, she could also use the
KeyFactorygetKeySpec method to convert it
to a DSAPublicKeySpec so that she can access the components, if
desired, as in:
DSAPublicKeySpec dsaPubKeySpec =
(DSAPublicKeySpec)keyFactory.getKeySpec(pubKey, DSAPublicKeySpec.class);Now she can access the DSA public key components y, p, q, and g through the corresponding "get" methods on the DSAPublicKeySpec class (getY, getP, getQ, and getG).
Generating Random Numbers
The following code sample illustrates generating random numbers configured with different security strengths using a DRBG implementation of the SecureRandom class:
SecureRandom drbg;
byte[] buffer = new byte[32];
// Any DRBG can be provided
drbg = SecureRandom.getInstance("DRBG");
drbg.nextBytes(buffer);
SecureRandomParameters params = drbg.getParameters();
if (params instanceof DrbgParameters.Instantiation) {
DrbgParameters.Instantiation ins = (DrbgParameters.Instantiation) params;
if (ins.getCapability().supportsReseeding()) {
drbg.reseed();
}
}
// The following call requests a weak DRBG instance. It is only
// guaranteed to support 112 bits of security strength.
drbg = SecureRandom.getInstance("DRBG",
DrbgParameters.instantiation(112, NONE, null));
// Both the next two calls will likely fail, because drbg could be
// instantiated with a smaller strength with no prediction resistance
// support.
drbg.nextBytes(buffer,
DrbgParameters.nextBytes(256, false, "more".getBytes()));
drbg.nextBytes(buffer,
DrbgParameters.nextBytes(112, true, "more".getBytes()));
// The following call requests a strong DRBG instance, with a
// personalization string. If it successfully returns an instance,
// that instance is guaranteed to support 256 bits of security strength
// with prediction resistance available.
drbg = SecureRandom.getInstance("DRBG", DrbgParameters.instantiation(
256, PR_AND_RESEED, "hello".getBytes()));
// Prediction resistance is not requested in this single call,
// but an additional input is used.
drbg.nextBytes(buffer,
DrbgParameters.nextBytes(-1, false, "more".getBytes()));
// Same for this call.
drbg.reseed(DrbgParameters.reseed(false, "extra".getBytes()));Determining If Two Keys Are Equal
Example code for determining if two keys are equal.
In many cases you would like to know if two keys are equal; however, the default method java.lang.Object.equals may not give the desired result. The most provider-independent approach is to compare the encoded keys. If this comparison isn't appropriate (for example, when comparing an RSAPrivateKey and an RSAPrivateCrtKey), you should compare each component.
The following code demonstrates this idea:
static boolean keysEqual(Key key1, Key key2) {
if (key1.equals(key2)) {
return true;
}
if (Arrays.equals(key1.getEncoded(), key2.getEncoded())) {
return true;
}
// More code for different types of keys here.
// For example, the following code can check if
// an RSAPrivateKey and an RSAPrivateCrtKey are equal:
// if ((key1 instanceof RSAPrivateKey) &&
// (key2 instanceof RSAPrivateKey)) {
// if ((key1.getModulus().equals(key2.getModulus())) &&
// (key1.getPrivateExponent().equals(
// key2.getPrivateExponent()))) {
// return true;
// }
// }
return false;
}
Reading Base64-Encoded Certificates
The following example reads a file with Base64-encoded certificates, which are each bounded at the beginning by
-----BEGIN CERTIFICATE-----
and at the end by
-----END CERTIFICATE-----
We convert the FileInputStream (which does not support mark and reset ) to a ByteArrayInputStream (which supports those methods), so that each call to generateCertificate consumes only one certificate, and the read position of the input stream is positioned to the next certificate in the file:
try (FileInputStream fis = new FileInputStream(filename);
BufferedInputStream bis = new BufferedInputStream(fis)) {
CertificateFactory cf = CertificateFactory.getInstance("X.509");
while (bis.available() > 0) {
Certificate cert = cf.generateCertificate(bis);
System.out.println(cert.toString());
}
}Parsing a Certificate Reply
The following example parses a PKCS7-formatted certificate reply stored in a file and extracts all the certificates from it:
try (FileInputStream fis = new FileInputStream(filename)) {
CertificateFactory cf = CertificateFactory.getInstance("X.509");
Collection<? extends Certificate> c = cf.generateCertificates(fis);
for (Certificate cert : c) {
System.out.println(cert);
}
// Alternatively, use this aggregate operation instead of a for-loop:
// c.stream().forEach(e -> System.out.println(e));
}Using Encryption
This section takes the user through the process of generating a key, creating and initializing a cipher object, encrypting a file, and then decrypting it. Throughout this example, we use the Advanced Encryption Standard (AES).
Generating a Key
To create an AES key, we have to instantiate a KeyGenerator for AES. We do not specify a provider, because we do not care about a particular AES key generation implementation. Since we do not initialize the KeyGenerator, a system-provided source of randomness and a default keysize will be used to create the AES key:
KeyGenerator keygen = KeyGenerator.getInstance("AES");
keygen.init(128);
SecretKey aesKey = keygen.generateKey();After the key has been generated, the same KeyGenerator object can be re-used to create further keys.
Creating a Cipher
The next step is to create a Cipher instance. To do this, we use one of the getInstance factory methods of the Cipher class. We must specify the name of the requested transformation, which includes the following components, separated by slashes (/):
- the algorithm name
- the mode (optional)
- the padding scheme (optional)
In this example, we create an AES cipher in Cipher Block Chaining mode, with PKCS5-style padding. We do not specify a provider, because we do not care about a particular implementation of the requested transformation.
The standard algorithm name for AES is "AES", the standard name for the Cipher Block Chaining mode is "CBC", and the standard name for PKCS5-style padding is "PKCS5Padding":
Cipher aesCipher;
// Create the cipher
aesCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");We use the aesKey generated previously to initialize the Cipher object for encryption:
// Initialize the cipher for encryption
aesCipher.init(Cipher.ENCRYPT_MODE, aesKey);
// Our cleartext
byte[] cleartext = "This is just an example".getBytes();
// Encrypt the cleartext
byte[] ciphertext = aesCipher.doFinal(cleartext);
// Retrieve the parameters used during encryption to properly
// initialize the cipher for decryption
AlgorithmParameters params = aesCipher.getParameters();
// Initialize the same cipher for decryption
aesCipher.init(Cipher.DECRYPT_MODE, aesKey, params);
// Decrypt the ciphertext
byte[] cleartext1 = aesCipher.doFinal(ciphertext);cleartext and cleartext1 are identical.
Using Password-Based Encryption
In this example, we prompt the user for a password from which we derive an encryption key.
It would seem logical to collect and store the password in an object of type java.lang.String. However, here's the caveat: Objects of type String are immutable, i.e., there are no methods defined that allow you to change (overwrite) or zero out the contents of a String after usage. This feature makes String objects unsuitable for storing security sensitive information such as user passwords. You should always collect and store security sensitive information in a char array instead. For that reason, the javax.crypto.spec.PBEKeySpec class takes (and returns) a password as a char array.
In order to use Password-Based Encryption (PBE) as defined in PKCS5, we have to specify a salt and an iteration count. The same salt and iteration count that are used for encryption must be used for decryption. Newer PBE algorithms use an iteration count of at least 1000.
PBEKeySpec pbeKeySpec;
PBEParameterSpec pbeParamSpec;
SecretKeyFactory keyFac;
// Salt
byte[] salt = new SecureRandom().nextBytes(salt);
// Iteration count
int count = 1000;
// Create PBE parameter set
pbeParamSpec = new PBEParameterSpec(salt, count);
// Prompt user for encryption password.
// Collect user password as char array, and convert
// it into a SecretKey object, using a PBE key
// factory.
char[] password = System.console.readPassword("Enter encryption password: ");
pbeKeySpec = new PBEKeySpec(password);
keyFac = SecretKeyFactory.getInstance("PBEWithHmacSHA256AndAES_256");
SecretKey pbeKey = keyFac.generateSecret(pbeKeySpec);
// Create PBE Cipher
Cipher pbeCipher = Cipher.getInstance("PBEWithHmacSHA256AndAES_256");
// Initialize PBE Cipher with key and parameters
pbeCipher.init(Cipher.ENCRYPT_MODE, pbeKey, pbeParamSpec);
// Our cleartext
byte[] cleartext = "This is another example".getBytes();
// Encrypt the cleartext
byte[] ciphertext = pbeCipher.doFinal(cleartext);
Sample Programs for Diffie-Hellman Key Exchange, AES/GCM, and HMAC-SHA256
The following are sample programs for Diffie-Hellman key exchange, AES/GCM, and HMAC-SHA256.
Diffie-Hellman Key Exchange between Two Parties
The program runs the Diffie-Hellman key agreement protocol between two parties.
import java.io.*;
import java.math.BigInteger;
import java.security.*;
import java.security.spec.*;
import java.security.interfaces.*;
import javax.crypto.*;
import javax.crypto.spec.*;
import javax.crypto.interfaces.*;
import com.sun.crypto.provider.SunJCE;
public class DHKeyAgreement2 {
private DHKeyAgreement2() {}
public static void main(String argv[]) throws Exception {
/*
* Alice creates her own DH key pair with 2048-bit key size
*/
System.out.println("ALICE: Generate DH keypair ...");
KeyPairGenerator aliceKpairGen = KeyPairGenerator.getInstance("DH");
aliceKpairGen.initialize(2048);
KeyPair aliceKpair = aliceKpairGen.generateKeyPair();
// Alice creates and initializes her DH KeyAgreement object
System.out.println("ALICE: Initialization ...");
KeyAgreement aliceKeyAgree = KeyAgreement.getInstance("DH");
aliceKeyAgree.init(aliceKpair.getPrivate());
// Alice encodes her public key, and sends it over to Bob.
byte[] alicePubKeyEnc = aliceKpair.getPublic().getEncoded();
/*
* Let's turn over to Bob. Bob has received Alice's public key
* in encoded format.
* He instantiates a DH public key from the encoded key material.
*/
KeyFactory bobKeyFac = KeyFactory.getInstance("DH");
X509EncodedKeySpec x509KeySpec = new X509EncodedKeySpec(alicePubKeyEnc);
PublicKey alicePubKey = bobKeyFac.generatePublic(x509KeySpec);
/*
* Bob gets the DH parameters associated with Alice's public key.
* He must use the same parameters when he generates his own key
* pair.
*/
DHParameterSpec dhParamFromAlicePubKey = ((DHPublicKey)alicePubKey).getParams();
// Bob creates his own DH key pair
System.out.println("BOB: Generate DH keypair ...");
KeyPairGenerator bobKpairGen = KeyPairGenerator.getInstance("DH");
bobKpairGen.initialize(dhParamFromAlicePubKey);
KeyPair bobKpair = bobKpairGen.generateKeyPair();
// Bob creates and initializes his DH KeyAgreement object
System.out.println("BOB: Initialization ...");
KeyAgreement bobKeyAgree = KeyAgreement.getInstance("DH");
bobKeyAgree.init(bobKpair.getPrivate());
// Bob encodes his public key, and sends it over to Alice.
byte[] bobPubKeyEnc = bobKpair.getPublic().getEncoded();
/*
* Alice uses Bob's public key for the first (and only) phase
* of her version of the DH
* protocol.
* Before she can do so, she has to instantiate a DH public key
* from Bob's encoded key material.
*/
KeyFactory aliceKeyFac = KeyFactory.getInstance("DH");
x509KeySpec = new X509EncodedKeySpec(bobPubKeyEnc);
PublicKey bobPubKey = aliceKeyFac.generatePublic(x509KeySpec);
System.out.println("ALICE: Execute PHASE1 ...");
aliceKeyAgree.doPhase(bobPubKey, true);
/*
* Bob uses Alice's public key for the first (and only) phase
* of his version of the DH
* protocol.
*/
System.out.println("BOB: Execute PHASE1 ...");
bobKeyAgree.doPhase(alicePubKey, true);
/*
* At this stage, both Alice and Bob have completed the DH key
* agreement protocol.
* Both generate the (same) shared secret.
*/
try {
byte[] aliceSharedSecret = aliceKeyAgree.generateSecret();
int aliceLen = aliceSharedSecret.length;
byte[] bobSharedSecret = new byte[aliceLen];
int bobLen;
} catch (ShortBufferException e) {
System.out.println(e.getMessage());
} // provide output buffer of required size
bobLen = bobKeyAgree.generateSecret(bobSharedSecret, 0);
System.out.println("Alice secret: " +
toHexString(aliceSharedSecret));
System.out.println("Bob secret: " +
toHexString(bobSharedSecret));
if (!java.util.Arrays.equals(aliceSharedSecret, bobSharedSecret))
throw new Exception("Shared secrets differ");
System.out.println("Shared secrets are the same");
/*
* Now let's create a SecretKey object using the shared secret
* and use it for encryption. First, we generate SecretKeys for the
* "AES" algorithm (based on the raw shared secret data) and
* Then we use AES in CBC mode, which requires an initialization
* vector (IV) parameter. Note that you have to use the same IV
* for encryption and decryption: If you use a different IV for
* decryption than you used for encryption, decryption will fail.
*
* If you do not specify an IV when you initialize the Cipher
* object for encryption, the underlying implementation will generate
* a random one, which you have to retrieve using the
* javax.crypto.Cipher.getParameters() method, which returns an
* instance of java.security.AlgorithmParameters. You need to transfer
* the contents of that object (e.g., in encoded format, obtained via
* the AlgorithmParameters.getEncoded() method) to the party who will
* do the decryption. When initializing the Cipher for decryption,
* the (reinstantiated) AlgorithmParameters object must be explicitly
* passed to the Cipher.init() method.
*/
System.out.println("Use shared secret as SecretKey object ...");
SecretKeySpec bobAesKey = new SecretKeySpec(bobSharedSecret, 0, 16, "AES");
SecretKeySpec aliceAesKey = new SecretKeySpec(aliceSharedSecret, 0, 16, "AES");
/*
* Bob encrypts, using AES in CBC mode
*/
Cipher bobCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
bobCipher.init(Cipher.ENCRYPT_MODE, bobAesKey);
byte[] cleartext = "This is just an example".getBytes();
byte[] ciphertext = bobCipher.doFinal(cleartext);
// Retrieve the parameter that was used, and transfer it to Alice in
// encoded format
byte[] encodedParams = bobCipher.getParameters().getEncoded();
/*
* Alice decrypts, using AES in CBC mode
*/
// Instantiate AlgorithmParameters object from parameter encoding
// obtained from Bob
AlgorithmParameters aesParams = AlgorithmParameters.getInstance("AES");
aesParams.init(encodedParams);
Cipher aliceCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
aliceCipher.init(Cipher.DECRYPT_MODE, aliceAesKey, aesParams);
byte[] recovered = aliceCipher.doFinal(ciphertext);
if (!java.util.Arrays.equals(cleartext, recovered))
throw new Exception("AES in CBC mode recovered text is " +
"different from cleartext");
System.out.println("AES in CBC mode recovered text is "
"same as cleartext");
}
/*
* Converts a byte to hex digit and writes to the supplied buffer
*/
private static void byte2hex(byte b, StringBuffer buf) {
char[] hexChars = { '0', '1', '2', '3', '4', '5', '6', '7', '8',
'9', 'A', 'B', 'C', 'D', 'E', 'F' };
int high = ((b & 0xf0) >> 4);
int low = (b & 0x0f);
buf.append(hexChars[high]);
buf.append(hexChars[low]);
}
/*
* Converts a byte array to hex string
*/
private static String toHexString(byte[] block) {
StringBuffer buf = new StringBuffer();
int len = block.length;
for (int i = 0; i < len; i++) {
byte2hex(block[i], buf);
if (i < len-1) {
buf.append(":");
}
}
return buf.toString();
}
}Diffie-Hellman Key Exchange between Three Parties
The program runs the Diffie-Hellman key agreement protocol between 3 parties.
import java.security.*;
import java.security.spec.*;
import javax.crypto.*;
import javax.crypto.spec.*;
import javax.crypto.interfaces.*;
/*
* This program executes the Diffie-Hellman key agreement protocol between
* 3 parties: Alice, Bob, and Carol using a shared 2048-bit DH parameter.
*/
public class DHKeyAgreement3 {
private DHKeyAgreement3() {}
public static void main(String argv[]) throws Exception {
// Alice creates her own DH key pair with 2048-bit key size
System.out.println("ALICE: Generate DH keypair ...");
KeyPairGenerator aliceKpairGen = KeyPairGenerator.getInstance("DH");
aliceKpairGen.initialize(2048);
KeyPair aliceKpair = aliceKpairGen.generateKeyPair();
// This DH parameters can also be constructed by creating a
// DHParameterSpec object using agreed-upon values
DHParameterSpec dhParamShared = ((DHPublicKey)aliceKpair.getPublic()).getParams();
// Bob creates his own DH key pair using the same params
System.out.println("BOB: Generate DH keypair ...");
KeyPairGenerator bobKpairGen = KeyPairGenerator.getInstance("DH");
bobKpairGen.initialize(dhParamShared);
KeyPair bobKpair = bobKpairGen.generateKeyPair();
// Carol creates her own DH key pair using the same params
System.out.println("CAROL: Generate DH keypair ...");
KeyPairGenerator carolKpairGen = KeyPairGenerator.getInstance("DH");
carolKpairGen.initialize(dhParamShared);
KeyPair carolKpair = carolKpairGen.generateKeyPair();
// Alice initialize
System.out.println("ALICE: Initialize ...");
KeyAgreement aliceKeyAgree = KeyAgreement.getInstance("DH");
aliceKeyAgree.init(aliceKpair.getPrivate());
// Bob initialize
System.out.println("BOB: Initialize ...");
KeyAgreement bobKeyAgree = KeyAgreement.getInstance("DH");
bobKeyAgree.init(bobKpair.getPrivate());
// Carol initialize
System.out.println("CAROL: Initialize ...");
KeyAgreement carolKeyAgree = KeyAgreement.getInstance("DH");
carolKeyAgree.init(carolKpair.getPrivate());
// Alice uses Carol's public key
Key ac = aliceKeyAgree.doPhase(carolKpair.getPublic(), false);
// Bob uses Alice's public key
Key ba = bobKeyAgree.doPhase(aliceKpair.getPublic(), false);
// Carol uses Bob's public key
Key cb = carolKeyAgree.doPhase(bobKpair.getPublic(), false);
// Alice uses Carol's result, cb
aliceKeyAgree.doPhase(cb, true);
// Bob uses Alice's result, ac
bobKeyAgree.doPhase(ac, true);
// Carol uses Bob's result, ba
carolKeyAgree.doPhase(ba, true);
// Alice, Bob and Carol compute their secrets
byte[] aliceSharedSecret = aliceKeyAgree.generateSecret();
System.out.println("Alice secret: " + toHexString(aliceSharedSecret));
byte[] bobSharedSecret = bobKeyAgree.generateSecret();
System.out.println("Bob secret: " + toHexString(bobSharedSecret));
byte[] carolSharedSecret = carolKeyAgree.generateSecret();
System.out.println("Carol secret: " + toHexString(carolSharedSecret));
// Compare Alice and Bob
if (!java.util.Arrays.equals(aliceSharedSecret, bobSharedSecret))
throw new Exception("Alice and Bob differ");
System.out.println("Alice and Bob are the same");
// Compare Bob and Carol
if (!java.util.Arrays.equals(bobSharedSecret, carolSharedSecret))
throw new Exception("Bob and Carol differ");
System.out.println("Bob and Carol are the same");
}
/*
* Converts a byte to hex digit and writes to the supplied buffer
*/
private static void byte2hex(byte b, StringBuffer buf) {
char[] hexChars = { '0', '1', '2', '3', '4', '5', '6', '7', '8',
'9', 'A', 'B', 'C', 'D', 'E', 'F' };
int high = ((b & 0xf0) >> 4);
int low = (b & 0x0f);
buf.append(hexChars[high]);
buf.append(hexChars[low]);
}
/*
* Converts a byte array to hex string
*/
private static String toHexString(byte[] block) {
StringBuffer buf = new StringBuffer();
int len = block.length;
for (int i = 0; i < len; i++) {
byte2hex(block[i], buf);
if (i < len-1) {
buf.append(":");
}
}
return buf.toString();
}
}
AES/GCM Example
The following is a sample program to demonstrate AES/GCM usage to encrypt/decrypt data.
import java.security.AlgorithmParameters;
import java.util.Arrays;
import javax.crypto.*;
public class AESGCMTest {
public static void main(String[] args) throws Exception {
// Slightly longer than 1 AES block (128 bits) to show PADDING
// is "handled" by GCM.
byte[] data = {
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
0x10};
// Create a 128-bit AES key.
KeyGenerator kg = KeyGenerator.getInstance("AES");
kg.init(128);
SecretKey key = kg.generateKey();
// Obtain a AES/GCM cipher to do the enciphering. Must obtain
// and use the Parameters for successful decryption.
Cipher encCipher = Cipher.getInstance("AES/GCM/NOPADDING");
encCipher.init(Cipher.ENCRYPT_MODE, key);
byte[] enc = encCipher.doFinal(data);
AlgorithmParameters ap = encCipher.getParameters();
// Obtain a similar cipher, and use the parameters.
Cipher decCipher = Cipher.getInstance("AES/GCM/NOPADDING");
decCipher.init(Cipher.DECRYPT_MODE, key, ap);
byte[] dec = decCipher.doFinal(enc);
if (Arrays.compare(data, dec) != 0) {
throw new Exception("Original data != decrypted data");
}
}
}HMAC-SHA256 Example
The following is a sample program that demonstrates how to generate a secret-key object for HMAC-SHA256, and initialize a HMAC-SHA256 object with it.
Example 2-14 Generate a Secret-key Object for HMAC-SHA256
import java.security.*;
import javax.crypto.*;
/**
* This program demonstrates how to generate a secret-key object for
* HMACSHA256, and initialize an HMACSHA256 object with it.
*/
public class initMac {
public static void main(String[] args) throws Exception {
// Generate secret key for HmacSHA256
KeyGenerator kg = KeyGenerator.getInstance("HmacSHA256");
SecretKey sk = kg.generateKey();
// Get instance of Mac object implementing HmacSHA256, and
// initialize it with the secret key, sk
Mac mac = Mac.getInstance("HmacSHA256");
mac.init(sk);
byte[] result = mac.doFinal("Hi There".getBytes());
}
}