For information about client authentication with certificates, see the following sections:
Figure 2–2 shows how certificates and the SSL protocol are used together for authentication. To authenticate a user to a server, a client digitally signs a randomly generated piece of data and sends both the certificate and the signed data across the network. For the purposes of this discussion, the digital signature associated with some data can be thought of as evidence provided by the client to the server. The server authenticates the user’s identity on the strength of this evidence.
Like for password-based authentication illustrated in Figure 2–1, Figure 2–2 assumes that the user has already decided to trust the server and has requested a resource. The server has requested client authentication in the process of evaluating whether to grant access to the requested resource.
Unlike for password-based authentication illustrated in Figure 2–1, Figure 2–2 requires the use of SSL. In Figure 2–2 it is assumed that the client has a valid certificate that can be used to identify the client to the server.
Certificate-based authentication is generally considered preferable to password-based authentication because it is based on what the user has, the private key, as well as what the user knows, the password that protects the private key. However, it’s important to note that these two assumptions are true only if unauthorized personnel have not gained access to the user’s machine or password, the password for the client software’s private key database has been set, and the software is set up to request the password at reasonably frequent intervals.
Neither password-based authentication nor certificate-based authentication address security issues related to physical access to individual machines or passwords. Public-key cryptography can only verify that a private key used to sign some data corresponds to the public key in a certificate. It is the user’s responsibility to protect a machine’s physical security and to keep the private-key password secret.
Certificates replace the authentication portion of the interaction between the client and the server. Instead of requiring a user to send passwords across the network throughout the day, single sign-on requires the user to enter the private-key database password just once, without sending it across the network. For the rest of the session, the client presents the user’s certificate to authenticate the user to each new server it encounters. Existing authorization mechanisms based on the authenticated user identity are not affected.
In Figure 2–2, certificate-based authentication is set in the following steps.
The client software maintains a database of the private keys that correspond to the public keys published in any certificates issued for that client. The client asks for the password to this database the first time the client needs to access it during a given session—for example, the first time the user attempts to access an SSL-enabled server that requires certificate-based client authentication. After entering this password once, the user doesn’t need to enter it again for the rest of the session, even when accessing other SSL-enabled servers.
The client unlocks the private-key database, retrieves the private key for the user’s certificate, and uses that private key to digitally sign some data that has been randomly generated for this purpose on the basis of input from both the client and the server. This data and the digital signature constitute “evidence” of the private key’s validity. The digital signature can be created only with that private key and can be validated with the corresponding public key against the signed data, which is unique to the SSL session.
The client sends both the user’s certificate and the evidence, the randomly generated piece of data that has been digitally signed, across the network.
The server uses the certificate and the evidence to authenticate the user’s identity.
At this point the server may optionally perform other authentication tasks, such as checking that the certificate presented by the client is stored in the user’s entry in an LDAP directory. The server then continues to evaluate whether the identified user is permitted to access the requested resource. This evaluation process can employ a variety of standard authorization mechanisms, potentially using additional information in an LDAP directory, company databases, and so on. If the result of the evaluation is positive, the server allows the client to access the requested resource.
A certificate is an electronic document that identifies an individual, a server, a company, or some other entity. A certificate also associates that identity with a public key. Like a driver’s license, a passport, or other commonly used personal IDs, a certificate provides generally recognized proof of someone's or something's identity.
Certificate authorities, CAs, validate identities and issue certificates. CAs can be independent third parties or organizations that run their own certificate-issuing server software. The methods used to validate an identity vary depending on the policies of a given CA. In general, before issuing a certificate, the CA must use its published verification procedures for that type of certificate to ensure that an entity requesting a certificate is in fact who it claims to be.
A certificate issued by a CA binds a particular public key to the name of the entity the certificate identifies, such as the name of an employee or a server. Certificates help prevent the use of fake public keys for impersonation. Only the public key certified by the certificate works with the corresponding private key possessed by the entity identified by the certificate.
In addition to a public key, a certificate always includes the name of the entity it identifies, an expiration date, the name of the CA that issued the certificate, a serial number, and other information. Most importantly, a certificate always includes the digital signature of the issuing CA. The CA’s digital signature allows the certificate to function as a “letter of introduction” for users who know and trust the CA but don’t know the entity identified by the certificate.
Any client or server software that supports certificates maintains a collection of trusted CA certificates. These CA certificates determine which other certificates the software can validate, in other words, which issuers of certificates the software can trust. In the simplest case, the software can validate only certificates issued by one of the CAs for which it has a certificate. It’s also possible for a trusted CA certificate to be part of a chain of CA certificates, each issued by the CA above it in a certificate hierarchy.
For information about CAs, see the following sections:
In large organizations, it may be appropriate to delegate the responsibility for issuing certificates to several different certificate authorities. For example, the number of certificates required may be too large for a single CA to maintain; different organizational units may have different policy requirements; or it may be important for a CA to be physically located in the same geographic area as the people to whom it is issuing certificates.
It’s possible to delegate certificate-issuing responsibilities to subordinate CAs. The X.509 standard includes a model for setting up a hierarchy of CAs.
In this model, the root CA is at the top of the hierarchy. The root CA’s certificate is a self-signed certificate. That is, the certificate is digitally signed by the same entity, the root CA, that the certificate identifies. The CAs that are directly subordinate to the root CA have CA certificates signed by the root CA. CAs under the subordinate CAs in the hierarchy have their CA certificates signed by the higher-level subordinate CAs.
Organizations have a great deal of flexibility in terms of the way they set up their CA hierarchies. Figure 2–3 shows just one example; many other arrangements are possible.
CA hierarchies are reflected in certificate chains. A certificate chain is a series of certificates issued by successive CAs. Figure 2–4 shows a certificate chain leading from a certificate that identifies some entity through two subordinate CA certificates to the CA certificate for the root CA (based on the CA hierarchy shown in the following figure).
A certificate chain traces a path of certificates from a branch in the hierarchy to the root of the hierarchy. In a certificate chain, the following occur:
Each certificate is followed by the certificate of its issuer.
In Figure 2–4, the Engineering CA certificate contains the DN of the CA (that is, USA CA), that issued that certificate. USA CA’s DN is also the subject name of the next certificate in the chain.
Each certificate is signed with the private key of its issuer. The signature can be verified with the public key in the issuer’s certificate, which is the next certificate in the chain.
In Figure 2–4, the public key in the certificate for the USA CA can be used to verify the USA CA’s digital signature on the certificate for the Engineering CA.
Certificate chain verification is the process of making sure a given certificate chain is well-formed, valid, properly signed, and trustworthy. Directory Server software uses the following steps to form and verify a certificate chain, starting with the certificate being presented for authentication:
The certificate validity period is checked against the current time provided by the verifier’s system clock.
The issuer’s certificate is located. The source can be either the verifier’s local certificate database (on that client or server) or the certificate chain provided by the subject (for example, over an SSL connection).
The certificate signature is verified using the public key in the issuer certificate.
If the issuer’s certificate is trusted by the verifier in the verifier’s certificate database, verification stops successfully here. Otherwise, the issuer’s certificate is checked to make sure it contains the appropriate subordinate CA indication in the Directory Server certificate type extension, and chain verification returns to step 1 to start again, but with this new certificate.
Figure 2–5 shows what happens when only Root CA is included in the verifier’s local database. If a certificate for one of the intermediate CAs shown in Figure 2–6, such as Engineering CA, is found in the verifier’s local database, verification stops with that certificate, as shown in the following figure.
Expired validity dates, an invalid signature, or the absence of a certificate for the issuing CA at any point in the certificate chain causes authentication to fail. For example, the following figure shows how verification fails if neither the Root CA certificate nor any of the intermediate CA certificates are included in the verifier’s local database.
For general information about the way digital signatures work, see Digital Signatures.
Directory Server uses the following types of certificate:
Client SSL certificates are used to identify clients to servers via SSL (client authentication). Typically, the identity of the client is assumed to be the same as the identity of a human being, such as an employee in an enterprise. Client SSL certificates can also be used for form signing and as part of a single sign-on solution.
For example, a bank gives a customer a client SSL certificate that allows the bank’s servers to identify that customer and authorize access to the customer’s accounts. A company might give a new employee a client SSL certificate that allows the company’s servers to identify that employee and authorize access to the company’s servers.
Server SSL certificates are used to identify servers to clients via SSL (server authentication). Server authentication may be used with or without client authentication. Server authentication is a requirement for an encrypted SSL session.
For example, internet sites that engage in electronic commerce usually support certificate-based server authentication, at a minimum, to establish an encrypted SSL session and to assure customers that they are dealing with a web site identified with a particular company. The encrypted SSL session ensures that personal information sent over the network, such as credit card numbers, cannot easily be intercepted.
S/MIME certificates are used for signed and encrypted email. As with client SSL certificates, the identity of the client is typically assumed to be the same as the identity of a human being, such as an employee in an enterprise. A single certificate may be used as both an S/MIME certificate and an SSL certificate. S/MIME certificates can also be used for form signing and as part of a single sign-on solution.
For example, a company deploys combined S/MIME and SSL certificates solely for the purpose of authenticating employee identities, thus permitting signed email and client SSL authentication but not encrypted email. Another company issues S/MIME certificates solely for the purpose of both signing and encrypting email that deals with sensitive financial or legal matters.
For example, a software company signs software distributed over the Internet to provide users with some assurance that the software is a legitimate product of that company. Using certificates and digital signatures in this manner can also make it possible for users to identify and control the kind of access downloaded software has to their computers.
CA certificates are used to identify CAs. Client and server software use CA certificates to determine what other certificates can be trusted.
For example, the CA certificates stored in client software determine what other certificates that client can authenticate. An administrator can implement some aspects of corporate security policies by controlling the CA certificates stored in each user’s client.
The contents of certificates supported by Directory Server and many other software companies are organized according to the X.509 v3 certificate specification, which has been recommended by the International Telecommunications Union (ITU), an international standards body, since 1988. Examples in this section show samples of the data and signature sections of a certificate.
Every X.509 certificate consists of the following sections.
A data section, including the following information.
The version number of the X.509 standard supported by the certificate.
The certificate’s serial number. Every certificate issued by a CA has a serial number that is unique among the certificates issued by that CA.
Information about the user’s public key, including the algorithm used and a representation of the key itself.
The DN of the CA that issued the certificate.
The period during which the certificate is valid (for example, between 1:00 p.m. on November 15, 2003 and 1:00 p.m. November 15, 2004).
The DN of the certificate subject (for example, in a client SSL certificate this would be the user’s DN), also called the subject name.
Optional certificate extensions, which may provide additional data used by the client or server. For example, the certificate type extension indicates the type of certificate—that is, whether it is a client SSL certificate, a server SSL certificate, a certificate for signing email, and so on. Certificate extensions can also be used for a variety of other purposes.
A signature section, includes the following information.
The cryptographic algorithm, or cipher, used by the issuing CA to create its own digital signature.
The CA’s digital signature, obtained by hashing all of the data in the certificate together and encrypting it with the CA's private key.
Certificate: Data: Version: v3 (0x2) Serial Number: 3 (0x3) Signature Algorithm: PKCS #1 MD5 With RSA Encryption Issuer: OU=Certificate Authority, O=Example Industry, C=US Validity: Not Before: Fri Oct 17 18:36:25 2003 Not After: Sun Oct 17 18:36:25 2004 Subject: CN=Jane Doe, OU=Finance, O=Example Industry, C=US Subject Public Key Info: Algorithm: PKCS #1 RSA Encryption Public Key: Modulus: 00:ca:fa:79:98:8f:19:f8:d7:de:e4:49:80:48:e6:2a:2a:86: ed:27:40:4d:86:b3:05:c0:01:bb:50:15:c9:de:dc:85:19:22: 43:7d:45:6d:71:4e:17:3d:f0:36:4b:5b:7f:a8:51:a3:a1:00: 98:ce:7f:47:50:2c:93:36:7c:01:6e:cb:89:06:41:72:b5:e9: 73:49:38:76:ef:b6:8f:ac:49:bb:63:0f:9b:ff:16:2a:e3:0e: 9d:3b:af:ce:9a:3e:48:65:de:96:61:d5:0a:11:2a:a2:80:b0: 7d:d8:99:cb:0c:99:34:c9:ab:25:06:a8:31:ad:8c:4b:aa:54: 91:f4:15 Public Exponent: 65537 (0x10001) Extensions: Identifier: Certificate Type Critical: no Certified Usage: SSL Client Identifier: Authority Key Identifier Critical: no Key Identifier: f2:f2:06:59:90:18:47:51:f5:89:33:5a:31:7a:e6:5c:fb:36: 26:c9 Signature: Algorithm: PKCS #1 MD5 With RSA Encryption Signature: 6d:23:af:f3:d3:b6:7a:df:90:df:cd:7e:18:6c:01:69:8e:54:65:fc:06: 30:43:34:d1:63:1f:06:7d:c3:40:a8:2a:82:c1:a4:83:2a:fb:2e:8f:fb: f0:6d:ff:75:a3:78:f7:52:47:46:62:97:1d:d9:c6:11:0a:02:a2:e0:cc: 2a:75:6c:8b:b6:9b:87:00:7d:7c:84:76:79:ba:f8:b4:d2:62:58:c3:c5: b6:c1:43:ac:63:44:42:fd:af:c8:0f:2f:38:85:6d:d6:59:e8:41:42:a5: 4a:e5:26:38:ff:32:78:a1:38:f1:ed:dc:0d:31:d1:b0:6d:67:e9:46:a8: d:c4
-----BEGIN CERTIFICATE----- MIICKzCCAZSgAwIBAgIBAzANBgkqhkiG9w0BAQQFADA3MQswCQYDVQQGEwJVUzER MA8GA1UEChMITmV0c2NhcGUxFTATBgNVBAsTDFN1cHJpeWEncyBDQTAeFw05NzEw MTgwMTM2MjVaFw05OTEwMTgwMTM2MjVaMEgxCzAJBgNVBAYTAlVTMREwDwYDVQQK EwhOZXRzY2FwZTENMAsGA1UECxMEUHViczEXMBUGA1UEAxMOU3Vwcml5YSBTaGV0 dHkwgZ8wDQYJKoZIhvcNAQEFBQADgY0AMIGJAoGBAMr6eZiPGfjX3uRJgEjmKiqG 7SdATYazBcABu1AVyd7chRkiQ31FbXFOGD3wNktbf6hRo6EAmM5/R1AskzZ8AW7L iQZBcrXpc0k4du+2Q6xJu2MPm/8WKuMOnTuvzpo+SGXelmHVChEqooCwfdiZywyZ NMmrJgaoMa2MS6pUkfQVAgMBAAGjNjA0MBEGCWCGSAGG+EIBAQQEAwIAgDAfBgNV HSMEGDAWgBTy8gZZkBhHUfWJM1oxeuZc+zYmyTANBgkqhkiG9w0BAQQFAAOBgQBt I6/z07Z635DfzX4XbAFpjlRl/AYwQzTSYx8GfcNAqCqCwaSDKvsuj/vwbf91o3j3 UkdGYpcd2cYRCgKi4MwqdWyLtpuHAH18hHZ5uvi00mJYw8W2wUOsY0RC/a/IDy84 hW3WWehBUqVK5SY4/zJ4oTjx7dwNMdGwbWfpRqjd1A== -----END CERTIFICATE-----
The set of standards and services that facilitate the use of public-key cryptography and X.509 v3 certificates in a network environment is called thepublic key infrastructure (PKI). For information about the certificate management issues addressed by Directory Server, see the following sections:
The process for issuing a certificate depends on the certificate authority that issues it and the purpose for which it is used. The process for issuing non-digital forms of identification varies in similar ways. For example, if you want to get a generic ID card (not a driver’s license) from the Department of Motor Vehicles in California, the requirements are straightforward: you need to present some evidence of your identity, such as a utility bill with your address on it and a student identity card. If you want to get a regular driving license, you also need to take a test — a driving test when you first get the license, and a written test when you renew it. If you want to get a commercial license for an eighteen-wheeler, the requirements are much more stringent. If you live in some other state or country, the requirements for various kinds of licenses differ.
Similarly, different CAs have different procedures for issuing different kinds of certificates. In some cases the only requirement may be your mail address. In other cases, your UNIX login and password may be sufficient. At the other end of the scale, for certificates that identify people who can authorize large expenditures or make other sensitive decisions, the issuing process may require notarized documents, a background check, and a personal interview.
Depending on an organization’s policies, the process of issuing certificates can range from being completely transparent for the user to requiring significant user participation and complex procedures. In general, processes for issuing certificates should be highly flexible, so organizations can tailor them to their changing needs.
Issuing certificates is one of several management tasks that can be handled by separate Registration Authorities.
The Lightweight Directory Access Protocol (LDAP) for accessing directory services supports great flexibility in the management of certificates within an organization. System administrators can store much of the information required to manage certificates in an LDAP-compliant directory. For example, a CA can use information in a directory to pre-populate a certificate with a new employee’s legal name and other information. The CA can leverage directory information in other ways to issue certificates one at a time or in bulk, using a range of different identification techniques depending on the security policies of a given organization. Other routine management tasks, such as key management and renewing and revoking certificates, can be partially or fully automated with the aid of the directory.
Information stored in the directory can also be used with certificates to control access to various network resources by different users or groups. Issuing certificates and other certificate management tasks can thus be an integral part of user and group management.
Before a certificate can be issued, the public key it contains and the corresponding private key must be generated. Sometimes it may be useful to issue a single person one certificate and key pair for signing operations, and another certificate and key pair for encryption operations. Separate signing and encryption certificates make it possible to keep the private signing key on the local machine only, thus providing maximum nonrepudiation, and to back up the private encryption key in some central location where it can be retrieved in case the user loses the original key or leaves the company.
Keys can be generated by client software or generated centrally by the CA and distributed to users via an LDAP directory. There are trade-offs involved in choosing between local and centralized key generation. For example, local key generation provides maximum nonrepudiation, but may involve more participation by the user in the issuing process. Flexible key management capabilities are essential for most organizations.
Key recovery, or the ability to retrieve backups of encryption keys under carefully defined conditions, can be a crucial part of certificate management (depending on how an organization uses certificates). Key recovery schemes usually involve an m of n mechanism: for example, m of n managers within an organization might have to agree, and each contribute a special code or key of their own, before a particular person’s encryption key can be recovered. This kind of mechanism ensures that several authorized personnel must agree before an encryption key can be recovered.
Like a driver’s license, a certificate specifies a period of time during which it is valid. Attempts to use a certificate for authentication before or after its validity period fails. Therefore, mechanisms for managing certificate renewal are essential for any certificate management strategy. For example, an administrator may wish to be notified automatically when a certificate is about to expire, so that an appropriate renewal process can be completed in plenty of time without causing the certificate’s subject any inconvenience. The renewal process may involve reusing the same public-private key pair or issuing a new one.
A driver’s license can be suspended even if it has not expired—for example, as punishment for a serious driving offense. Similarly, it’s sometimes necessary to revoke a certificate before it has expired—for example, if an employee leaves a company or moves to a new job within the company.
Certificate revocation can be handled in several different ways. For some organizations, it may be sufficient to set up servers so that the authentication process includes checking the directory for the presence of the certificate being presented. When an administrator revokes a certificate, the certificate can be automatically removed from the directory, and subsequent authentication attempts with that certificate fails even though the certificate remains valid in every other respect. Another approach involves publishing a certificate revocation list (CRL)—that is, a list of revoked certificates—to the directory at regular intervals and checking the list as part of the authentication process. For some organizations, it may be preferable to check directly with the issuing CA each time a certificate is presented for authentication. This procedure is sometimes called real-time status checking.
Interactions between entities identified by certificates (sometimes called end entities) and CAs are an essential part of certificate management. These interactions include operations such as registration for certification, certificate retrieval, certificate renewal, certificate revocation, and key backup and recovery. In general, a CA must be able to authenticate the identities of end entities before responding to the requests. In addition, some requests need to be approved by authorized administrators or managers before being serviced.
As previously discussed, the means used by different CAs to verify an identity before issuing a certificate can vary widely, depending on the organization and the purpose for which the certificate is used. To provide maximum operational flexibility, interactions with end entities can be separated from the other functions of a CA and handled by a separate service called a Registration Authority RA.
An RA acts as a front end to a CA by receiving end entity requests, authenticating them, and forwarding them to the CA. After receiving a response from the CA, the RA notifies the end entity of the results. RAs can be helpful in scaling a PKI across different departments, geographical areas, or other operational units with varying policies and authentication requirements.