Table 33: An overview of CRL extensions
SUPPORT REFERENCES
# EXTENSION OID SEMANTICS CRITI CAL GEN “DIRECT”/ INDIR.CRL PROC RFC5280 CO.PKI NO TES X.509BASIC EXTENSIONS
1 AuthorityKeyIdentifier {2 5 29 35} An ID identifying the public key (thus possibly several certs)
corresponding to the signing private key of the issuing CA.
--
(RFC n.a.)
++/++ + 5.2.1 T11 [1]
[2] 2 IssuerAltNames {2 5 29 18} Alternative technical names of the issuing CA:
email, DNS name, IP address, URI
- -/+-
(RFC n.a.)
+ 5.2.2 T16.#2 [2]
3 CRLNumber {2 5 29 20} Number of the CRL -- ++/++ ++ 5.2.3 T34
4 DeltaCRLIndicator {2 5 29 27} Indicates that the CRL is a delta-CRL, i.e. contains only entries of
the current complete CRL that are not present in a previous complete CRL, the base CRL.
++ +-/+- ++ 5.2.4 T35
5 IssuingDistributionPoint {2 5 29 28} Indicates whether the CRL covers revocations for end entity
certificates only, for CA certificates only or for a limited set of reason codes and whether it is an indirect CRL.
++ +-/++ + 5.2.5 T36
6 FreshestCRL {2 5 29 46} This extension (a.k.a. DeltaCRLDistributionPoint) identifies how
delta CRL information is obtained.
-- +-/+- + 5.2.6 T36a
7 AuthorityInfoAccess {id-pe 1} Access to online validation service and/or policy information of the
CA issuing this CRL.
-- +-/+- + 5.2.7 T36b
[1] Common PKI Profile: The crlSign-Flag in the KeyUsage extension MUST be set in all CA- or end-entity certificates, that correspond to CRL-signing keys. Issuers of
indirect CRLs typically posses an end-entity certificate.
[2] Common PKI Profile: As readily described in T22.[2], there are two types of CRLs:
1) “direct” CRL: the CA that issued the certificate issues the corresponding CRLs too. This situation can be recognized by relying software if the following conditions apply:
a. if the CRLDistributionPoints extension is missing from the certificate or b. it is present, but the cRLIssuer field is missing.
2) indirect CRL: the CRLs are signed with a key different from the key of the CA. This situation can be recognized by relying software if the CRLDistributionPoints extension is present in the certificate and the cRLIssuer field holds a DName (different from the subject of the CA certificate). Additionally, indirect CRLs MUST include an IssuingDistributionPoint extension with indirectCRL flag set to true.
So that relying software can locate the certificate of the issuer of an indirect CRL, AuthorityKeyIdentifier MUST and IssuerAltNames MAY be included in indirect CRLs. The IssuerAltNames extension MAY contain the LDAP-URL of the node that holds the CRL-signer’s certificate.
CRL Format
Common PKI Part 1 – Page 57 of 67
Table 34: CRLNumber
SUPPORT REFERENCES # ASN.1DEFINITION SEMANTICS GEN PROC RFC5280 CO.PKI NO TES1 id-ce-cRLNumber OBJECT IDENTIFIER ::= { id-ce 20 } OID to be used in conjunction with extension CRLNumber 5.2.3
2 CRLNumber ::= INTEGER (0..MAX) Syntax of extension CRLNumber 5.2.3 [1]
[1] [RFC5280]: CRLs MUST be assigned numbers of a monotonically increasing sequence. This extension allows easily determining whether a particular CRL supersedes
another one.
[Common PKI PROFILE]: [RFC5280] does not constrain the value or the length of this field. Similarly to CertificateSerialNumber, a maximal length of 20 octets will be
defined here, i.e. 0 ≤ CRLNumber < 2159 (MSB=0 indicates the positive sign! ). Processing components MUST be able to work with such long numbers.
Table 35: DeltaCRLIndicator
SUPPORT REFERENCES # ASN.1DEFINITION SEMANTICS GEN PROC RFC5280 CO.PKI NO TES1 id-ce-deltaCRLIndicator OBJECT IDENTIFIER ::=
{ id-ce 27 } Indicates that the CRL is a delta-CRL, i.e. contains only entries of the current complete CRL that are not present in a previous
complete CRL, the base CRL. Using a complete CRL and all subs equent delta-CRLs, the relying component is able to continuously maintain a local instance of subsequent complete CRLs.
5.2.4
2 BaseCRLNumber ::= CRLNumber Syntax of extension DeltaCRLIndicator 5.2.4 T34.#2 [1]
[1] [RFC5280]: The CRL issuer MAY also generate delta CRLs. A delta CRL only lists those certificates, within its scope, whose revocation status has changed since the
issuance of a referenced complete CRL. The referenced complete CRL is referred to as a base CRL. The scope of a delta CRL MUST be the same as the base CRL that it references. Conforming applications are not required to support processing of delta CRLs .
CRL Format
Common PKI Part 1 – Page 58 of 67
Table 36: IssuingDistributionPoint
SUPPORT REFERENCES # ASN.1DEFINITION SEMANTICS GEN PROC RFC5280 CO.PKI NO TES1 id-ce-issuingDistributionPoint OBJECT IDENTIFIER ::=
{ id-ce 28 } 5.2.5
2 IssuingDistributionPoint ::= SEQUENCE { Syntax of extension IssuingDistributionPoint. Indicates
whether the CRL covers revocations for end entity certificates only, for CA certificates only or for a limited set of reason codes.
5.2.5 [1]
distributionPoint [0] EXPLICIT DistributionPointName
OPTIONAL, If the CRL is stored in an X.500 directory, it will be
stored under the entry indicated by this field and which may be different from the directory entry of CA signing the CRL.
T22.#2 [2]
[3]
onlyContainsUserCerts [1] IMPLICIT BOOLEAN DEFAULT FALSE, Set if CRL contains only end entity certificates onlyContainsCACerts [2] IMPLICIT BOOLEAN DEFAULT FALSE, Set if CRL contains only end CA certificates
onlySomeReasons [3] IMPLICIT ReasonFlags OPTIONAL, CAs may use this flag to partition their CRL according to
the reason of revocation, e.g. on the basis of compromise or routine revocation.
T22..#9
indirectCRL [4] IMPLICIT BOOLEAN DEFAULT FALSE, Indicates that the CRL is an indirect one, i.e. the CRL
issuer is not the same entity as the issuer of (some of) the certificates listed in the CRL.
onlyContainsAttributeCerts
[5] IMPLICIT BOOLEAN DEFAULT FALSE } Indicates that the CRL only contains revoked attribute
certificates.
[1] [RFC5280]: It is the decision of the CA whether it issues delta-CRLs. When a CA issues a delta-CRL, it MUST also issue a corresponding complete CRL (the current
complete CRL). The delta-CRL and the complete CRL MUST have the same CRLNumber.
[2] CHOICE objects are always EXPLICITly tagged, independent of the default tagging modus.
[3] [RFC5280]: If an URL is given, it MUST point to the most current CRL issued by this CA. The URL schemes ftp, http [RFC1738] [RFC3986], mailto [RFC2368] and ldap
CRL Format
Common PKI Part 1 – Page 59 of 67
Table 36a: FreshestCRL
SUPPORT REFERENCES
# ASN.1DEFINITION SEMANTICS
GEN PROC RFC5280 CO.PKI NO TES
1 FreshestCRL ::= SEQUENCE SIZE (1..MAX) OF
CRLDistributionPoint This extension (a.k.a. DeltaCRLDistributionPoint) identifies
how delta CRL information is obtained.
+- + 5.2.6 T22#2 [1]
[1] [RFC5280]: This extension MUST NOT appear in delta CRLs.
The same syntax is used for this extension and the cRLDistributionPoints extension. The same conventions apply to both extensions. Each distribution point name provides the location at which a delta CRL for this complete CRL can be found.
Table 36b: AuthorityInfoAccess
SUPPORT REFERENCES # ASN.1DEFINITION SEMANTICS GEN PROC RFC5280 CO.PKI NO TES1 AuthorityInfoAccessSyntax ::= SEQUENCE SIZE (1..MAX) OF
AccessDescription Indicates how to access information and services for the subject of the certificate. +- + 5.2.7 T23#4
2 id-ad-caIssuers OBEJCT IDENTIFIER ::= {id-ad 2} An OID for the case, when the referenced information
lists CAs that have issued certificates for the issuer of this CRL.
++ + 4.2.2.1 T23#8 [1]
[1] [RFC5280]: When present in a CRL, this extension MUST include at least one AccessDescription specifying id-ad-caIssuers as the accessMethod.
The id-ad-caIssuers OID is used when the information available lists certificates that can be used to verify the signature on the CRL Access method types other than id-ad-caIssuers MUST NOT be included.