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Certificates are created in the belief that they will be valid and usable throughout the expected lifetime indicated in the Validity field. In some cases, however, an unexpired certificate should no longer be used. For example, the corresponding private key may have been compromised, the CA has discovered that it has made a mistake, or the holder of the key is no longer employed at a company. As a result, CAs need a way to revoke an unexpired certificate and notify relying parties of the revocation. The most common method is the use of a certificate revocation list (CRL). Simply stated, a CRL is a signed data structure containing a timestamped list of revoked certificates. The signer of the CRL is typically the same entity that originally issued it (the CA). After a CRL is created and digitally signed, it can be freely distributed across a network or stored in a directory in the same way that certificates are handled. CAs issue CRLs periodically on schedules ranging from every few hours to every few weeks. A new CRL is issued whether or not it contains any new revocations; in this way, relying parties always know that the most recently received CRL is current. A PKI s certificate policy governs its CRL time interval. Latency between CRLs is one of the major drawbacks of their use. For example, a reported revocation may not be received by the relying party until the next CRL issue, perhaps several hours or several weeks later.
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Currently, most applications (such as Web browsers and e-mail readers) do not use the various revocation mechanisms that are in place. However, this is beginning to change as PKIs are becoming more widespread.
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As stated previously, a CRL is nothing more than a time-stamped, digitally signed list of revoked certificates. The following section describes, in detail, the various fields that make up a CRL. Figure 6-3 illustrates these fields.
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Figure 6-3 The standard structure of a CRL
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Version Signature Algorithm Identifier Issuer Name This Update (Date/Time) Next Update (Date/Time) User Certificate Serial Number / Revocation Date CRL Entry Extensions
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User Certificate Serial Number / Revocation Date CRL Entry Extensions CRL Extensions Signature
Version This field indicates the version of the CRL. (This field is optional for Version 1 CRLs but must be present for Version 2.) Signature Algorithm Identifier This field contains the identifier of the algorithm used to sign the CRL. For example, if this field
Public-Key Infrastructures and the X.509 Standard
contains the object identifier for SHA-1 with RSA , it means that the digital signature is a SHA-1 hash (see 5) encrypted using RSA (see 4).
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Issuer Name This field identifies the DN, in X.500 format, of the entity that issued the CRL. This Update (Date/Time) This field contains a date/time value indicating when the CRL was issued. Next Update (Date/Time) This optional field contains a date/time value indicating when the next CRL will be issued. (Although this field is optional, RFC2459 mandates its use.) User Certificate Serial Number/Revocation Date This field contains the list of certificates that have been revoked or suspended. The list contains the certificate s serial number and the date and time it was revoked. CRL Entry Extensions section. CRL Extensions Extensions. These fields are discussed in the following
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These fields are discussed in the section CRL
Signature This field contains the CA signature.
CRL Entry Extensions
Just as an X.509 Version 3 certificate can be enhanced through the use of extensions, Version 2 CRLs are provided a set of extensions that enable CAs to convey additional information with each individual revocation. The X.509 standard defines the following four extensions for use with a Version 2 CRL:
Reason Code This extension specifies the reason for certificate revocation. Valid entries include the following: unspecified, key compromise, CA compromise, superseded, certificate hold, and others. (For valid reasons, RFC2459 recommends the use of this field.) Hold Instruction Code This noncritical extension supports the temporary suspension of a certificate. It contains an OID that describes the action to be taken if the extension exists. Certificate Issuers This extension identifies the name of the certificate issuer associated with an indirect CRL (discussed later in the section titled Indirect CRLs ). If this extension is present, RFC2459 mandates that it be marked critical.
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