XML Digital Signature API

The Java XML Digital Signature API is a standard Java API for generating and validating XML Signatures. This API was defined under the Java Community Process as JSR 105 (see http://jcp.org/en/jsr/detail?id=105).

XML Signatures can be applied to data of any type, XML or binary (see http://www.w3.org/TR/xmldsig-core/). The resulting signature is represented in XML. An XML Signature can be used to secure your data and provide data integrity, message authentication, and signer authentication.

After providing a brief overview of XML Signatures and the XML Digital Signature API, this document presents two examples that demonstrate how to use the API to validate and generate an XML Signature. This document assumes that you have a basic knowledge of cryptography and digital signatures.

The API is designed to support all of the required or recommended features of the W3C Recommendation for XML-Signature Syntax and Processing. The API is extensible and pluggable and is based on the Java Cryptography Service Provider Architecture. The API is designed for two types of developers:

Developers who want to use the XML Digital Signature API to generate and validate XML signatures

Developers who want to create a concrete implementation of the XML Digital Signature API and register it as a cryptographic service of a JCA provider

Package Hierarchy

The six packages listed below comprise the XML Digital Signature API:

javax.xml.crypto

javax.xml.crypto.dsig

javax.xml.crypto.dsig.keyinfo

javax.xml.crypto.dsig.spec

javax.xml.crypto.dom

javax.xml.crypto.dsig.dom

The javax.xml.crypto package contains common classes that are used to perform XML cryptographic operations, such as generating an XML signature or encrypting XML data. Two notable classes in this package are the KeySelector class, which allows developers to supply implementations that locate and optionally validate keys using the information contained in a KeyInfo object, and the URIDereferencer class, which allows developers to create and specify their own URI dereferencing implementations.

The javax.xml.crypto.dsig package includes interfaces that represent the core elements defined in the W3C XML digital signature specification. Of primary significance is the XMLSignature class, which allows you to sign and validate an XML digital signature. Most of the XML signature structures or elements are represented by a corresponding interface (except for the KeyInfo structures, which are included in their own package and are discussed in the next paragraph). These interfaces include: SignedInfo, CanonicalizationMethod, SignatureMethod, Reference, Transform, DigestMethod, XMLObject, Manifest, SignatureProperty, and SignatureProperties. The XMLSignatureFactory class is an abstract factory that is used to create objects that implement these interfaces.

The javax.xml.crypto.dsig.keyinfo package contains interfaces that represent most of the KeyInfo structures defined in the W3C XML digital signature recommendation, including KeyInfo, KeyName, KeyValue, X509Data, X509IssuerSerial, RetrievalMethod, and PGPData. The KeyInfoFactory class is an abstract factory that is used to create objects that implement these interfaces.

The javax.xml.crypto.dsig.spec package contains interfaces and classes representing input parameters for the digest, signature, transform, or canonicalization algorithms used in the processing of XML signatures.

Finally, the javax.xml.crypto.dom and javax.xml.crypto.dsig.dom packages contains DOM-specific classes for the javax.xml.crypto and javax.xml.crypto.dsig packages, respectively. Only developers and users who are creating or using a DOM-based XMLSignatureFactory or KeyInfoFactory implementation will need to make direct use of these packages.

Service Providers

A JSR 105 cryptographic service is a concrete implementation of the abstract XMLSignatureFactory and KeyInfoFactory classes and is responsible for creating objects and algorithms that parse, generate and validate XML Signatures and KeyInfo structures. A concrete implementation of XMLSignatureFactory must provide support for each of the required algorithms as specified by the W3C recommendation for XML Signatures. It can optionally support other algorithms as defined by the W3C recommendation or other specifications.

JSR 105 leverages the JCA provider model for registering and loading XMLSignatureFactory and KeyInfoFactory implementations.

Each concrete XMLSignatureFactory or KeyInfoFactory implementation supports a specific XML mechanism type that identifies the XML processing mechanism that an implementation uses internally to parse and generate XML signature and KeyInfo structures. This JSR supports one standard type, DOM. The XML Digital Signature provider implementation that is bundled with Java SE supports the DOM mechanism. Support for new standard types, such as JDOM, may be added in the future.

An XML Digital Signature API implementation should use underlying JCA engine classes, such as java.security.Signature and java.security.MessageDigest, to perform cryptographic operations.

In addition to the XMLSignatureFactory and KeyInfoFactory classes, JSR 105 supports a service provider interface for transform and canonicalization algorithms. The TransformService class allows you to develop and plug in an implementation of a specific transform or canonicalization algorithm for a particular XML mechanism type. The TransformService class uses the standard JCA provider model for registering and loading implementations. Each JSR 105 implementation should use the TransformService class to find a provider that supports transform and canonicalization algorithms in XML Signatures that it is generating or validating.

Introduction to XML Signatures

You can use an XML Signature to sign any arbitrary data, whether it is XML or binary. The data is identified via URIs in one or more Reference elements. XML Signatures are described in one or more of three forms: detached, enveloping, or enveloped. A detached signature is over data that is external, or outside of the signature element itself. Enveloping signatures are signatures over data that is inside the signature element, and an enveloped signature is a signature that is contained inside the data that it is signing.

Example of an XML Signature

The easiest way to describe the contents of an XML Signature is to show an actual sample and describe each component in more detail. The following is an example of an enveloped XML Signature generated over the contents of an XML document. The contents of the document before it is signed are:

<Envelope xmlns="urn:envelope">
</Envelope>

The resulting enveloped XML Signature, indented and formatted for readability, is as follows:

<?xml version="1.0" encoding="UTF-8"?>
<Envelope xmlns="urn:envelope">
  <Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
    <SignedInfo>
      <CanonicalizationMethod 
        Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments"/>
      <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>
      <Reference URI="">
        <Transforms>
          <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
        </Transforms>
        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
        <DigestValue>uooqbWYa5VCqcJCbuymBKqm17vY=</DigestValue>
      </Reference>
    </SignedInfo>
    <SignatureValue>
      KedJuTob5gtvYx9qM3k3gm7kbLBwVbEQRl26S2tmXjqNND7MRGtoew==
    </SignatureValue>
    <KeyInfo>
      <KeyValue>
        <DSAKeyValue>
          <P>
            /KaCzo4Syrom78z3EQ5SbbB4sF7ey80etKII864WF64B81uRpH5t9jQTxe
            Eu0ImbzRMqzVDZkVG9xD7nN1kuFw==
          </P>
          <Q>li7dzDacuo67Jg7mtqEm2TRuOMU=</Q>
          <G>
            Z4Rxsnqc9E7pGknFFH2xqaryRPBaQ01khpMdLRQnG541Awtx/
            XPaF5Bpsy4pNWMOHCBiNU0NogpsQW5QvnlMpA==
          </G>
          <Y>
            qV38IqrWJG0V/mZQvRVi1OHw9Zj84nDC4jO8P0axi1gb6d+475yhMjSc/
            BrIVC58W3ydbkK+Ri4OKbaRZlYeRA==
          </Y>
        </DSAKeyValue>
      </KeyValue>
    </KeyInfo>
  </Signature>
</Envelope>

The Signature element has been inserted inside the content that it is signing, thereby making it an enveloped signature. The required SignedInfo element contains the information that is actually signed:

<SignedInfo>
  <CanonicalizationMethod 
    Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments"/>
  <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>
  <Reference URI="">
    <Transforms>
      <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
    </Transforms>
    <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
    <DigestValue>uooqbWYa5VCqcJCbuymBKqm17vY=</DigestValue>
  </Reference>
</SignedInfo>

The required CanonicalizationMethod element defines the algorithm used to canonicalize the SignedInfo element before it is signed or validated. Canonicalization is the process of converting XML content to a canonical form, to take into account changes that can invalidate a signature over that data. Canonicalization is necessary due to the nature of XML and the way it is parsed by different processors and intermediaries, which can change the data such that the signature is no longer valid but the signed data is still logically equivalent.

The required SignatureMethod element defines the digital signature algorithm used to generate the signature, in this case DSA with SHA-1.

One or more Reference elements identify the data that is digested. Each Reference element identifies the data via a URI. In this example, the value of the URI is the empty String (""), which indicates the root of the document. The optional Transforms element contains a list of one or more Transform elements, each of which describes a transformation algorithm used to transform the data before it is digested. In this example, there is one Transform element for the enveloped transform algorithm. The enveloped transform is required for enveloped signatures so that the signature element itself is removed before calculating the signature value. The required DigestMethod element defines the algorithm used to digest the data, in this case SHA1. Finally the required DigestValue element contains the actual base64-encoded digested value.

The required SignatureValue element contains the base64-encoded signature value of the signature over the SignedInfo element.

The optional KeyInfo element contains information about the key that is needed to validate the signature:

<KeyInfo>
  <KeyValue>
    <DSAKeyValue>
      <P>
        /KaCzo4Syrom78z3EQ5SbbB4sF7ey80etKII864WF64B81uRpH5t9jQTxe
        Eu0ImbzRMqzVDZkVG9xD7nN1kuFw==
      </P>
      <Q>li7dzDacuo67Jg7mtqEm2TRuOMU=</Q>
      <G>
        Z4Rxsnqc9E7pGknFFH2xqaryRPBaQ01khpMdLRQnG541Awtx/
        XPaF5Bpsy4pNWMOHCBiNU0NogpsQW5QvnlMpA==
      </G>
      <Y>
        qV38IqrWJG0V/mZQvRVi1OHw9Zj84nDC4jO8P0axi1gb6d+475yhMjSc/
        BrIVC58W3ydbkK+Ri4OKbaRZlYeRA==
      </Y>
    </DSAKeyValue>
  </KeyValue>
</KeyInfo>

This KeyInfo element contains a KeyValue element, which in turn contains a DSAKeyValue element consisting of the public key needed to validate the signature. KeyInfo can contain various content such as X.509 certificates and PGP key identifiers. See the KeyInfo section of the XML Signature Recommendation for more information on the different KeyInfo types.

XML Signature Secure Validation Mode

The XML Signature secure validation mode can protect you from XML Signatures that may contain potentially hostile constructs that can cause denial-of-service or other types of security issues.

XML Signature secure validation mode is enabled by default.

If necessary, and at your own risk, you can disable the XML Signature secure validation mode by setting the system property org.jcp.xml.dsig.secureValidation to false or by setting the org.jcp.xml.dsig.secureValidation property to Boolean.FALSE with the DOMValidateContext.setProperty() method. The system property supersedes the value of the API property.

When XML Signature secure validation mode is enabled, XML Signatures are processed more securely. Limits are set on various XML Signature constructs to avoid conditions such as denial-of-service attacks. By default, it enforces the following restrictions:

Forbids the use of XSLT transforms
Restricts the number of SignedInfo or Manifest Reference elements to 30 or less
Restricts the number of Reference transforms to 5 or less
Forbids the use of MD5 signatures or MD5 MAC algorithms
Ensures that Reference IDs are unique to help prevent signature wrapping attacks
Forbids Reference URIs of type http, https, or file
Does not allow a RetrievalMethod element to reference another RetrievalMethod element
Forbids RSA or DSA keys less than 1024 bits
Forbids EC keys less than 224 bits

In addition, you can use the jdk.xml.dsig.secureValidationPolicy Security Property to control and fine-tune the restrictions listed previously or add additional restrictions. See the definition of this Security Property in the java.security file for more information.

XML Digital Signature API Examples

The following sections describe two examples that show how to use the XML Digital Signature API:

Validate example

Signing example

validate Example

You can find the code shown in this section in the Validate.java file in the docs/technotes/guides/security/xmldsig directory. The file on which it operates, envelopedSignature.xml, is in the same directory.

To compile and run the example, execute the following commands from the docs/technotes/guides/security/xmldsig directory:

$ javac Validate.java $ java Validate signature.xml

The sample program will validate the signature in the file signature.xml in the current working directory.

Validating an XML Signature

This example shows you how to validate an XML Signature using the JSR 105 API. The example uses DOM (the Document Object Model) to parse an XML document containing a Signature element and a JSR 105 DOM implementation to validate the signature.

Instantiating the Document that Contains the Signature

First we use a JAXP DocumentBuilderFactory to parse the XML document containing the Signature. An application obtains the default implementation for DocumentBuilderFactory by calling the following line of code:

DocumentBuilderFactory dbf = 
  DocumentBuilderFactory.newInstance();

We must also make the factory namespace-aware:

dbf.setNamespaceAware(true);

Next, we use the factory to get an instance of a DocumentBuilder, which is used to parse the document:

DocumentBuilder builder = dbf.newDocumentBuilder();  
Document doc = builder.parse(new FileInputStream(argv[0]));

Specifying the Signature Element to be Validated

We need to specify the Signature element that we want to validate, since there could be more than one in the document. We use the DOM method Document.getElementsByTagNameNS, passing it the XML Signature namespace URI and the tag name of the Signature element, as shown:

NodeList nl = doc.getElementsByTagNameNS
  (XMLSignature.XMLNS, "Signature");
if (nl.getLength() == 0) {
  throw new Exception("Cannot find Signature element");
}

This returns a list of all Signature elements in the document. In this example, there is only one Signature element.

Creating a Validation Context

We create an XMLValidateContext instance containing input parameters for validating the signature. Since we are using DOM, we instantiate a DOMValidateContext instance (a subclass of XMLValidateContext), and pass it two parameters, a KeyValueKeySelector object and a reference to the Signature element to be validated (which is the first entry of the NodeList we generated earlier):

DOMValidateContext valContext = new DOMValidateContext
  (new KeyValueKeySelector(), nl.item(0));

The KeyValueKeySelector is explained in greater detail in Using KeySelectors.

Unmarshaling the XML Signature

We extract the contents of the Signature element into an XMLSignature object. This process is called unmarshaling. The Signature element is unmarshalled using an XMLSignatureFactory object. An application can obtain a DOM implementation of XMLSignatureFactory by calling the following line of code:

XMLSignatureFactory factory = 
  XMLSignatureFactory.getInstance("DOM");

We then invoke the unmarshalXMLSignature method of the factory to unmarshal an XMLSignature object, and pass it the validation context we created earlier:


XMLSignature signature = 
  factory.unmarshalXMLSignature(valContext);

Validating the XML Signature

Now we are ready to validate the signature. We do this by invoking the validate method on the XMLSignature object, and pass it the validation context as follows:

boolean coreValidity = signature.validate(valContext);

The validate method returns "true" if the signature validates successfully according to the core validation rules in the W3C XML Signature Recommendation, and false otherwise.

What If the XML Signature Fails to Validate?

If the XMLSignature.validate method returns false, we can try to narrow down the cause of the failure. There are two phases in core XML Signature validation:

Signature validation (the cryptographic verification of the signature)

Reference validation (the verification of the digest of each reference in the signature)

Each phase must be successful for the signature to be valid. To check if the signature failed to cryptographically validate, we can check the status, as follows:

boolean sv = 
  signature.getSignatureValue().validate(valContext);
System.out.println("signature validation status: " + sv);

We can also iterate over the references and check the validation status of each one, as follows:

Iterator i =
  signature.getSignedInfo().getReferences().iterator();
for (int j=0; i.hasNext(); j++) {
  boolean refValid = ((Reference) 
    i.next()).validate(valContext);
  System.out.println("ref["+j+"] validity status: " + 
    refValid);
}

Using KeySelectors

KeySelectors are used to find and select keys that are needed to validate an XMLSignature. Earlier, when we created a DOMValidateContext object, we passed a KeySelector object as the first argument:

DOMValidateContext valContext = new DOMValidateContext
  (new KeyValueKeySelector(), nl.item(0));

Alternatively, we could have passed a PublicKey as the first argument if we already knew what key is needed to validate the signature. However, we often don't know.

The KeyValueKeySelector is a concrete implementation of the abstract KeySelector class. The KeyValueKeySelector implementation tries to find an appropriate validation key using the data contained in KeyValue elements of the KeyInfo element of an XMLSignature. It does not determine if the key is trusted. This is a very simple KeySelector implementation, designed for illustration rather than real-world usage. A more practical example of a KeySelector is one that searches a KeyStore for trusted keys that match X509Data information (for example, X509SubjectName, X509IssuerSerial, X509SKI, or X509Certificate elements) contained in a KeyInfo.

The implementation of the KeyValueKeySelector is as follows:

private static class KeyValueKeySelector extends KeySelector {

  public KeySelectorResult select(KeyInfo keyInfo,
      KeySelector.Purpose purpose,
      AlgorithmMethod method,
      XMLCryptoContext context)
    throws KeySelectorException {

    if (keyInfo == null) {
      throw new KeySelectorException("Null KeyInfo object!");
    }
    SignatureMethod sm = (SignatureMethod) method;
    List list = keyInfo.getContent();

    for (int i = 0; i < list.size(); i++) {
      XMLStructure xmlStructure = (XMLStructure) list.get(i);
      if (xmlStructure instanceof KeyValue) {
        PublicKey pk = null;
        try {
          pk = ((KeyValue)xmlStructure).getPublicKey();
        } catch (KeyException ke) {
          throw new KeySelectorException(ke);
        }
        // make sure algorithm is compatible with method
        if (algEquals(sm.getAlgorithm(), 
            pk.getAlgorithm())) {
          return new SimpleKeySelectorResult(pk);
        }
      }
    }
    throw new KeySelectorException("No KeyValue element 
found!");
  }

  static boolean algEquals(String algURI, String algName) {
    if (algName.equalsIgnoreCase("DSA") &&
        algURI.equalsIgnoreCase("http://www.w3.org/2009/xmldsig11#dsa-sha256")) {
      return true;
    } else if (algName.equalsIgnoreCase("RSA") &&
        algURI.equalsIgnoreCase("http://www.w3.org/2001/04/xmldsig-more#rsa-sha256")) {
      return true;
    } else {
      return false;
    }
  }
}

genenveloped Example

The code discussed in this section is in the GenEnveloped.java file in the docs/technotes/guides/security/xmldsig directory. The file on which it operates, envelope.xml, is in the same directory. It generates the file envelopedSignature.xml.

To compile and run this sample, execute the following command from the docs/technotes/guides/security/xmldsig directory:

$ javac GenEnveloped.java

$ java GenEnveloped envelope.xml
envelopedSignature.xml

The sample program will generate an enveloped signature of the document in the file envelope.xml and store it in the file envelopedSignature.xml in the current working directory.

Generating an XML Signature

This example shows you how to generate an XML Signature using the XML Digital Signature API. More specifically, the example generates an enveloped XML Signature of an XML document. An enveloped signature is a signature that is contained inside the content that it is signing. The example uses DOM (the Document Object Model) to parse the XML document to be signed and a JSR 105 DOM implementation to generate the resulting signature.

A basic knowledge of XML Signatures and their different components is helpful for understanding this section. See http://www.w3.org/TR/xmldsig-core/ for more information.

Instantiating the Document to be Signed

First, we use a JAXP DocumentBuilderFactory to parse the XML document that we want to sign. An application obtains the default implementation for DocumentBuilderFactory by calling the following line of code:

DocumentBuilderFactory dbf =
  DocumentBuilderFactory.newInstance();

We must also make the factory namespace-aware:

dbf.setNamespaceAware(true);

Next, we use the factory to get an instance of a DocumentBuilder, which is used to parse the document:

DocumentBuilder builder = dbf.newDocumentBuilder();  
Document doc = builder.parse(new FileInputStream(argv[0]));

Creating a Public Key Pair

We generate a public key pair. Later in the example, we will use the private key to generate the signature. We create the key pair with a KeyPairGenerator. In this example, we will create a DSA KeyPair with a length of 2048 bytes :

KeyPairGenerator kpg = KeyPairGenerator.getInstance("DSA");
kpg.initialize(2048);
KeyPair kp = kpg.generateKeyPair();

In practice, the private key is usually previously generated and stored in a KeyStore file with an associated public key certificate.

Creating a Signing Context

We create an XML Digital Signature XMLSignContext containing input parameters for generating the signature. Since we are using DOM, we instantiate a DOMSignContext (a subclass of XMLSignContext), and pass it two parameters, the private key that will be used to sign the document and the root of the document to be signed:

DOMSignContext dsc = new DOMSignContext
  (kp.getPrivate(), doc.getDocumentElement());

Assembling the XML Signature

We assemble the different parts of the Signature element into an XMLSignature object. These objects are all created and assembled using an XMLSignatureFactory object. An application obtains a DOM implementation of XMLSignatureFactory by calling the following line of code:

XMLSignatureFactory fac = 
  XMLSignatureFactory.getInstance("DOM");

We then invoke various factory methods to create the different parts of the XMLSignature object as shown below. We create a Reference object, passing to it the following:

The URI of the object to be signed (We specify a URI of "", which implies the root of the document.)

The DigestMethod (we use SHA1)

A single Transform, the enveloped Transform, which is required for enveloped signatures so that the signature itself is removed before calculating the signature value

Reference ref = fac.newReference
  ("", fac.newDigestMethod(DigestMethod.SHA256, null),
    Collections.singletonList
      (fac.newTransform(Transform.ENVELOPED,
        (TransformParameterSpec) null)), null, null);

Next, we create the SignedInfo object, which is the object that is actually signed, as shown below. When creating the SignedInfo, we pass as parameters:

The CanonicalizationMethod (we use inclusive and preserve comments)

The SignatureMethod (we use DSA)

A list of References (in this case, only one)

SignedInfo si = fac.newSignedInfo
  (fac.newCanonicalizationMethod
    (CanonicalizationMethod.INCLUSIVE_WITH_COMMENTS,
      (C14NMethodParameterSpec) null),
    fac.newSignatureMethod("http://www.w3.org/2009/xmldsig11#dsa-sha256", null),
    Collections.singletonList(ref));

Next, we create the optional KeyInfo object, which contains information that enables the recipient to find the key needed to validate the signature. In this example, we add a KeyValue object containing the public key. To create KeyInfo and its various subtypes, we use a KeyInfoFactory object, which can be obtained by invoking the getKeyInfoFactory method of the XMLSignatureFactory, as follows:

KeyInfoFactory kif = fac.getKeyInfoFactory();

We then use the KeyInfoFactory to create the KeyValue object and add it to a KeyInfo object:

KeyValue kv = kif.newKeyValue(kp.getPublic());
KeyInfo ki = kif.newKeyInfo(Collections.singletonList(kv));

Finally, we create the XMLSignature object, passing as parameters the SignedInfo and KeyInfo objects that we created earlier:

XMLSignature signature = fac.newXMLSignature(si, ki);

Notice that we haven't actually generated the signature yet; we'll do that in the next step.

Generating the XML Signature

Now we are ready to generate the signature, which we do by invoking the sign method on the XMLSignature object, and pass it the signing context as follows:

signature.sign(dsc);

The resulting document now contains a signature, which has been inserted as the last child element of the root element.

Printing or Displaying the Resulting Document

You can use the following code to print the resulting signed document to a file or standard output:

OutputStream os;
if (args.length > 1) {
  os = new FileOutputStream(args[1]);
} else {
  os = System.out;
} 
TransformerFactory tf = TransformerFactory.newInstance();
Transformer trans = tf.newTransformer();
trans.transform(new DOMSource(doc), new StreamResult(os));