RFC 5280
Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile
Pages: 151
Proposed Standard
→ Errata
Obsoletes: 3280 4325 4630
Updated by: 6818 8398 8399 9549
Proposed Standard
→ Errata
Obsoletes: 3280 4325 4630
Updated by: 6818 8398 8399 9549
Part 2 of 7 – Pages 16 to 43
4. Certificate and Certificate Extensions Profile
This section presents a profile for public key certificates that will foster interoperability and a reusable PKI. This section is based upon the X.509 v3 certificate format and the standard certificate extensions defined in [X.509]. The ISO/IEC and ITU-T documents use the 1997 version of ASN.1; while this document uses the 1988 ASN.1 syntax, the encoded certificate and standard extensions are equivalent. This section also defines private extensions required to support a PKI for the Internet community. Certificates may be used in a wide range of applications and environments covering a broad spectrum of interoperability goals and a broader spectrum of operational and assurance requirements. The goal of this document is to establish a common baseline for generic applications requiring broad interoperability and limited special purpose requirements. In particular, the emphasis will be on supporting the use of X.509 v3 certificates for informal Internet electronic mail, IPsec, and WWW applications.4.1. Basic Certificate Fields
The X.509 v3 certificate basic syntax is as follows. For signature calculation, the data that is to be signed is encoded using the ASN.1 distinguished encoding rules (DER) [X.690]. ASN.1 DER encoding is a tag, length, value encoding system for each element. Certificate ::= SEQUENCE { tbsCertificate TBSCertificate, signatureAlgorithm AlgorithmIdentifier, signatureValue BIT STRING } TBSCertificate ::= SEQUENCE { version [0] EXPLICIT Version DEFAULT v1, serialNumber CertificateSerialNumber, signature AlgorithmIdentifier, issuer Name, validity Validity, subject Name, subjectPublicKeyInfo SubjectPublicKeyInfo, issuerUniqueID [1] IMPLICIT UniqueIdentifier OPTIONAL, -- If present, version MUST be v2 or v3
subjectUniqueID [2] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version MUST be v2 or v3
extensions [3] EXPLICIT Extensions OPTIONAL
-- If present, version MUST be v3
}
Version ::= INTEGER { v1(0), v2(1), v3(2) }
CertificateSerialNumber ::= INTEGER
Validity ::= SEQUENCE {
notBefore Time,
notAfter Time }
Time ::= CHOICE {
utcTime UTCTime,
generalTime GeneralizedTime }
UniqueIdentifier ::= BIT STRING
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
Extensions ::= SEQUENCE SIZE (1..MAX) OF Extension
Extension ::= SEQUENCE {
extnID OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING
-- contains the DER encoding of an ASN.1 value
-- corresponding to the extension type identified
-- by extnID
}
The following items describe the X.509 v3 certificate for use in the
Internet.
4.1.1. Certificate Fields
The Certificate is a SEQUENCE of three required fields. The fields
are described in detail in the following subsections.
4.1.1.1. tbsCertificate
The field contains the names of the subject and issuer, a public key associated with the subject, a validity period, and other associated information. The fields are described in detail in Section 4.1.2; the tbsCertificate usually includes extensions, which are described in Section 4.2.4.1.1.2. signatureAlgorithm
The signatureAlgorithm field contains the identifier for the cryptographic algorithm used by the CA to sign this certificate. [RFC3279], [RFC4055], and [RFC4491] list supported signature algorithms, but other signature algorithms MAY also be supported. An algorithm identifier is defined by the following ASN.1 structure: AlgorithmIdentifier ::= SEQUENCE { algorithm OBJECT IDENTIFIER, parameters ANY DEFINED BY algorithm OPTIONAL } The algorithm identifier is used to identify a cryptographic algorithm. The OBJECT IDENTIFIER component identifies the algorithm (such as DSA with SHA-1). The contents of the optional parameters field will vary according to the algorithm identified. This field MUST contain the same algorithm identifier as the signature field in the sequence tbsCertificate (Section 4.1.2.3).4.1.1.3. signatureValue
The signatureValue field contains a digital signature computed upon the ASN.1 DER encoded tbsCertificate. The ASN.1 DER encoded tbsCertificate is used as the input to the signature function. This signature value is encoded as a BIT STRING and included in the signature field. The details of this process are specified for each of the algorithms listed in [RFC3279], [RFC4055], and [RFC4491]. By generating this signature, a CA certifies the validity of the information in the tbsCertificate field. In particular, the CA certifies the binding between the public key material and the subject of the certificate.4.1.2. TBSCertificate
The sequence TBSCertificate contains information associated with the subject of the certificate and the CA that issued it. Every TBSCertificate contains the names of the subject and issuer, a public
key associated with the subject, a validity period, a version number, and a serial number; some MAY contain optional unique identifier fields. The remainder of this section describes the syntax and semantics of these fields. A TBSCertificate usually includes extensions. Extensions for the Internet PKI are described in Section 4.2.4.1.2.1. Version
This field describes the version of the encoded certificate. When extensions are used, as expected in this profile, version MUST be 3 (value is 2). If no extensions are present, but a UniqueIdentifier is present, the version SHOULD be 2 (value is 1); however, the version MAY be 3. If only basic fields are present, the version SHOULD be 1 (the value is omitted from the certificate as the default value); however, the version MAY be 2 or 3. Implementations SHOULD be prepared to accept any version certificate. At a minimum, conforming implementations MUST recognize version 3 certificates. Generation of version 2 certificates is not expected by implementations based on this profile.4.1.2.2. Serial Number
The serial number MUST be a positive integer assigned by the CA to each certificate. It MUST be unique for each certificate issued by a given CA (i.e., the issuer name and serial number identify a unique certificate). CAs MUST force the serialNumber to be a non-negative integer. Given the uniqueness requirements above, serial numbers can be expected to contain long integers. Certificate users MUST be able to handle serialNumber values up to 20 octets. Conforming CAs MUST NOT use serialNumber values longer than 20 octets. Note: Non-conforming CAs may issue certificates with serial numbers that are negative or zero. Certificate users SHOULD be prepared to gracefully handle such certificates.4.1.2.3. Signature
This field contains the algorithm identifier for the algorithm used by the CA to sign the certificate. This field MUST contain the same algorithm identifier as the signatureAlgorithm field in the sequence Certificate (Section
4.1.1.2). The contents of the optional parameters field will vary according to the algorithm identified. [RFC3279], [RFC4055], and [RFC4491] list supported signature algorithms, but other signature algorithms MAY also be supported.4.1.2.4. Issuer
The issuer field identifies the entity that has signed and issued the certificate. The issuer field MUST contain a non-empty distinguished name (DN). The issuer field is defined as the X.501 type Name [X.501]. Name is defined by the following ASN.1 structures: Name ::= CHOICE { -- only one possibility for now -- rdnSequence RDNSequence } RDNSequence ::= SEQUENCE OF RelativeDistinguishedName RelativeDistinguishedName ::= SET SIZE (1..MAX) OF AttributeTypeAndValue AttributeTypeAndValue ::= SEQUENCE { type AttributeType, value AttributeValue } AttributeType ::= OBJECT IDENTIFIER AttributeValue ::= ANY -- DEFINED BY AttributeType DirectoryString ::= CHOICE { teletexString TeletexString (SIZE (1..MAX)), printableString PrintableString (SIZE (1..MAX)), universalString UniversalString (SIZE (1..MAX)), utf8String UTF8String (SIZE (1..MAX)), bmpString BMPString (SIZE (1..MAX)) } The Name describes a hierarchical name composed of attributes, such as country name, and corresponding values, such as US. The type of the component AttributeValue is determined by the AttributeType; in general it will be a DirectoryString. The DirectoryString type is defined as a choice of PrintableString, TeletexString, BMPString, UTF8String, and UniversalString. CAs conforming to this profile MUST use either the PrintableString or UTF8String encoding of DirectoryString, with two exceptions. When CAs have previously issued certificates with issuer fields with attributes encoded using TeletexString, BMPString, or UniversalString, then the CA MAY continue to use these encodings of the DirectoryString to preserve backward compatibility. Also, new
CAs that are added to a domain where existing CAs issue certificates with issuer fields with attributes encoded using TeletexString, BMPString, or UniversalString MAY encode attributes that they share with the existing CAs using the same encodings as the existing CAs use. As noted above, distinguished names are composed of attributes. This specification does not restrict the set of attribute types that may appear in names. However, conforming implementations MUST be prepared to receive certificates with issuer names containing the set of attribute types defined below. This specification RECOMMENDS support for additional attribute types. Standard sets of attributes have been defined in the X.500 series of specifications [X.520]. Implementations of this specification MUST be prepared to receive the following standard attribute types in issuer and subject (Section 4.1.2.6) names: * country, * organization, * organizational unit, * distinguished name qualifier, * state or province name, * common name (e.g., "Susan Housley"), and * serial number. In addition, implementations of this specification SHOULD be prepared to receive the following standard attribute types in issuer and subject names: * locality, * title, * surname, * given name, * initials, * pseudonym, and * generation qualifier (e.g., "Jr.", "3rd", or "IV"). The syntax and associated object identifiers (OIDs) for these attribute types are provided in the ASN.1 modules in Appendix A. In addition, implementations of this specification MUST be prepared to receive the domainComponent attribute, as defined in [RFC4519]. The Domain Name System (DNS) provides a hierarchical resource labeling system. This attribute provides a convenient mechanism for organizations that wish to use DNs that parallel their DNS names. This is not a replacement for the dNSName component of the alternative name extensions. Implementations are not required to
convert such names into DNS names. The syntax and associated OID for this attribute type are provided in the ASN.1 modules in Appendix A. Rules for encoding internationalized domain names for use with the domainComponent attribute type are specified in Section 7.3. Certificate users MUST be prepared to process the issuer distinguished name and subject distinguished name (Section 4.1.2.6) fields to perform name chaining for certification path validation (Section 6). Name chaining is performed by matching the issuer distinguished name in one certificate with the subject name in a CA certificate. Rules for comparing distinguished names are specified in Section 7.1. If the names in the issuer and subject field in a certificate match according to the rules specified in Section 7.1, then the certificate is self-issued.4.1.2.5. Validity
The certificate validity period is the time interval during which the CA warrants that it will maintain information about the status of the certificate. The field is represented as a SEQUENCE of two dates: the date on which the certificate validity period begins (notBefore) and the date on which the certificate validity period ends (notAfter). Both notBefore and notAfter may be encoded as UTCTime or GeneralizedTime. CAs conforming to this profile MUST always encode certificate validity dates through the year 2049 as UTCTime; certificate validity dates in 2050 or later MUST be encoded as GeneralizedTime. Conforming applications MUST be able to process validity dates that are encoded in either UTCTime or GeneralizedTime. The validity period for a certificate is the period of time from notBefore through notAfter, inclusive. In some situations, devices are given certificates for which no good expiration date can be assigned. For example, a device could be issued a certificate that binds its model and serial number to its public key; such a certificate is intended to be used for the entire lifetime of the device. To indicate that a certificate has no well-defined expiration date, the notAfter SHOULD be assigned the GeneralizedTime value of 99991231235959Z. When the issuer will not be able to maintain status information until the notAfter date (including when the notAfter date is 99991231235959Z), the issuer MUST ensure that no valid certification path exists for the certificate after maintenance of status
information is terminated. This may be accomplished by expiration or revocation of all CA certificates containing the public key used to verify the signature on the certificate and discontinuing use of the public key used to verify the signature on the certificate as a trust anchor.4.1.2.5.1. UTCTime
The universal time type, UTCTime, is a standard ASN.1 type intended for representation of dates and time. UTCTime specifies the year through the two low-order digits and time is specified to the precision of one minute or one second. UTCTime includes either Z (for Zulu, or Greenwich Mean Time) or a time differential. For the purposes of this profile, UTCTime values MUST be expressed in Greenwich Mean Time (Zulu) and MUST include seconds (i.e., times are YYMMDDHHMMSSZ), even where the number of seconds is zero. Conforming systems MUST interpret the year field (YY) as follows: Where YY is greater than or equal to 50, the year SHALL be interpreted as 19YY; and Where YY is less than 50, the year SHALL be interpreted as 20YY.4.1.2.5.2. GeneralizedTime
The generalized time type, GeneralizedTime, is a standard ASN.1 type for variable precision representation of time. Optionally, the GeneralizedTime field can include a representation of the time differential between local and Greenwich Mean Time. For the purposes of this profile, GeneralizedTime values MUST be expressed in Greenwich Mean Time (Zulu) and MUST include seconds (i.e., times are YYYYMMDDHHMMSSZ), even where the number of seconds is zero. GeneralizedTime values MUST NOT include fractional seconds.4.1.2.6. Subject
The subject field identifies the entity associated with the public key stored in the subject public key field. The subject name MAY be carried in the subject field and/or the subjectAltName extension. If the subject is a CA (e.g., the basic constraints extension, as discussed in Section 4.2.1.9, is present and the value of cA is TRUE), then the subject field MUST be populated with a non-empty distinguished name matching the contents of the issuer field (Section 4.1.2.4) in all certificates issued by the subject CA. If the subject is a CRL issuer (e.g., the key usage extension, as discussed in Section 4.2.1.3, is present and the value of cRLSign is TRUE),
then the subject field MUST be populated with a non-empty distinguished name matching the contents of the issuer field (Section 5.1.2.3) in all CRLs issued by the subject CRL issuer. If subject naming information is present only in the subjectAltName extension (e.g., a key bound only to an email address or URI), then the subject name MUST be an empty sequence and the subjectAltName extension MUST be critical. Where it is non-empty, the subject field MUST contain an X.500 distinguished name (DN). The DN MUST be unique for each subject entity certified by the one CA as defined by the issuer field. A CA MAY issue more than one certificate with the same DN to the same subject entity. The subject field is defined as the X.501 type Name. Implementation requirements for this field are those defined for the issuer field (Section 4.1.2.4). Implementations of this specification MUST be prepared to receive subject names containing the attribute types required for the issuer field. Implementations of this specification SHOULD be prepared to receive subject names containing the recommended attribute types for the issuer field. The syntax and associated object identifiers (OIDs) for these attribute types are provided in the ASN.1 modules in Appendix A. Implementations of this specification MAY use the comparison rules in Section 7.1 to process unfamiliar attribute types (i.e., for name chaining) whose attribute values use one of the encoding options from DirectoryString. Binary comparison should be used when unfamiliar attribute types include attribute values with encoding options other than those found in DirectoryString. This allows implementations to process certificates with unfamiliar attributes in the subject name. When encoding attribute values of type DirectoryString, conforming CAs MUST use PrintableString or UTF8String encoding, with the following exceptions: (a) When the subject of the certificate is a CA, the subject field MUST be encoded in the same way as it is encoded in the issuer field (Section 4.1.2.4) in all certificates issued by the subject CA. Thus, if the subject CA encodes attributes in the issuer fields of certificates that it issues using the TeletexString, BMPString, or UniversalString encodings, then the subject field of certificates issued to that CA MUST use the same encoding. (b) When the subject of the certificate is a CRL issuer, the subject field MUST be encoded in the same way as it is encoded in the issuer field (Section 5.1.2.3) in all CRLs issued by the subject CRL issuer.
(c) TeletexString, BMPString, and UniversalString are included
for backward compatibility, and SHOULD NOT be used for
certificates for new subjects. However, these types MAY be
used in certificates where the name was previously
established, including cases in which a new certificate is
being issued to an existing subject or a certificate is being
issued to a new subject where the attributes being encoded
have been previously established in certificates issued to
other subjects. Certificate users SHOULD be prepared to
receive certificates with these types.
Legacy implementations exist where an electronic mail address is
embedded in the subject distinguished name as an emailAddress
attribute [RFC2985]. The attribute value for emailAddress is of type
IA5String to permit inclusion of the character '@', which is not part
of the PrintableString character set. emailAddress attribute values
are not case-sensitive (e.g., "subscriber@example.com" is the same as
"SUBSCRIBER@EXAMPLE.COM").
Conforming implementations generating new certificates with
electronic mail addresses MUST use the rfc822Name in the subject
alternative name extension (Section 4.2.1.6) to describe such
identities. Simultaneous inclusion of the emailAddress attribute in
the subject distinguished name to support legacy implementations is
deprecated but permitted.
4.1.2.7. Subject Public Key Info
This field is used to carry the public key and identify the algorithm
with which the key is used (e.g., RSA, DSA, or Diffie-Hellman). The
algorithm is identified using the AlgorithmIdentifier structure
specified in Section 4.1.1.2. The object identifiers for the
supported algorithms and the methods for encoding the public key
materials (public key and parameters) are specified in [RFC3279],
[RFC4055], and [RFC4491].
4.1.2.8. Unique Identifiers
These fields MUST only appear if the version is 2 or 3 (Section
4.1.2.1). These fields MUST NOT appear if the version is 1. The
subject and issuer unique identifiers are present in the certificate
to handle the possibility of reuse of subject and/or issuer names
over time. This profile RECOMMENDS that names not be reused for
different entities and that Internet certificates not make use of
unique identifiers. CAs conforming to this profile MUST NOT generate
certificates with unique identifiers. Applications conforming to
this profile SHOULD be capable of parsing certificates that include unique identifiers, but there are no processing requirements associated with the unique identifiers.4.1.2.9. Extensions
This field MUST only appear if the version is 3 (Section 4.1.2.1). If present, this field is a SEQUENCE of one or more certificate extensions. The format and content of certificate extensions in the Internet PKI are defined in Section 4.2.4.2. Certificate Extensions
The extensions defined for X.509 v3 certificates provide methods for associating additional attributes with users or public keys and for managing relationships between CAs. The X.509 v3 certificate format also allows communities to define private extensions to carry information unique to those communities. Each extension in a certificate is designated as either critical or non-critical. A certificate-using system MUST reject the certificate if it encounters a critical extension it does not recognize or a critical extension that contains information that it cannot process. A non-critical extension MAY be ignored if it is not recognized, but MUST be processed if it is recognized. The following sections present recommended extensions used within Internet certificates and standard locations for information. Communities may elect to use additional extensions; however, caution ought to be exercised in adopting any critical extensions in certificates that might prevent use in a general context. Each extension includes an OID and an ASN.1 structure. When an extension appears in a certificate, the OID appears as the field extnID and the corresponding ASN.1 DER encoded structure is the value of the octet string extnValue. A certificate MUST NOT include more than one instance of a particular extension. For example, a certificate may contain only one authority key identifier extension (Section 4.2.1.1). An extension includes the boolean critical, with a default value of FALSE. The text for each extension specifies the acceptable values for the critical field for CAs conforming to this profile. Conforming CAs MUST support key identifiers (Sections 4.2.1.1 and 4.2.1.2), basic constraints (Section 4.2.1.9), key usage (Section 4.2.1.3), and certificate policies (Section 4.2.1.4) extensions. If the CA issues certificates with an empty sequence for the subject field, the CA MUST support the subject alternative name extension (Section 4.2.1.6). Support for the remaining extensions is OPTIONAL. Conforming CAs MAY support extensions that are not identified within
this specification; certificate issuers are cautioned that marking such extensions as critical may inhibit interoperability. At a minimum, applications conforming to this profile MUST recognize the following extensions: key usage (Section 4.2.1.3), certificate policies (Section 4.2.1.4), subject alternative name (Section 4.2.1.6), basic constraints (Section 4.2.1.9), name constraints (Section 4.2.1.10), policy constraints (Section 4.2.1.11), extended key usage (Section 4.2.1.12), and inhibit anyPolicy (Section 4.2.1.14). In addition, applications conforming to this profile SHOULD recognize the authority and subject key identifier (Sections 4.2.1.1 and 4.2.1.2) and policy mappings (Section 4.2.1.5) extensions.4.2.1. Standard Extensions
This section identifies standard certificate extensions defined in [X.509] for use in the Internet PKI. Each extension is associated with an OID defined in [X.509]. These OIDs are members of the id-ce arc, which is defined by the following: id-ce OBJECT IDENTIFIER ::= { joint-iso-ccitt(2) ds(5) 29 }4.2.1.1. Authority Key Identifier
The authority key identifier extension provides a means of identifying the public key corresponding to the private key used to sign a certificate. This extension is used where an issuer has multiple signing keys (either due to multiple concurrent key pairs or due to changeover). The identification MAY be based on either the key identifier (the subject key identifier in the issuer's certificate) or the issuer name and serial number. The keyIdentifier field of the authorityKeyIdentifier extension MUST be included in all certificates generated by conforming CAs to facilitate certification path construction. There is one exception; where a CA distributes its public key in the form of a "self-signed" certificate, the authority key identifier MAY be omitted. The signature on a self-signed certificate is generated with the private key associated with the certificate's subject public key. (This proves that the issuer possesses both the public and private keys.) In this case, the subject and authority key identifiers would be identical, but only the subject key identifier is needed for certification path building. The value of the keyIdentifier field SHOULD be derived from the public key used to verify the certificate's signature or a method
that generates unique values. Two common methods for generating key identifiers from the public key are described in Section 4.2.1.2. Where a key identifier has not been previously established, this specification RECOMMENDS use of one of these methods for generating keyIdentifiers or use of a similar method that uses a different hash algorithm. Where a key identifier has been previously established, the CA SHOULD use the previously established identifier. This profile RECOMMENDS support for the key identifier method by all certificate users. Conforming CAs MUST mark this extension as non-critical. id-ce-authorityKeyIdentifier OBJECT IDENTIFIER ::= { id-ce 35 } AuthorityKeyIdentifier ::= SEQUENCE { keyIdentifier [0] KeyIdentifier OPTIONAL, authorityCertIssuer [1] GeneralNames OPTIONAL, authorityCertSerialNumber [2] CertificateSerialNumber OPTIONAL } KeyIdentifier ::= OCTET STRING4.2.1.2. Subject Key Identifier
The subject key identifier extension provides a means of identifying certificates that contain a particular public key. To facilitate certification path construction, this extension MUST appear in all conforming CA certificates, that is, all certificates including the basic constraints extension (Section 4.2.1.9) where the value of cA is TRUE. In conforming CA certificates, the value of the subject key identifier MUST be the value placed in the key identifier field of the authority key identifier extension (Section 4.2.1.1) of certificates issued by the subject of this certificate. Applications are not required to verify that key identifiers match when performing certification path validation. For CA certificates, subject key identifiers SHOULD be derived from the public key or a method that generates unique values. Two common methods for generating key identifiers from the public key are: (1) The keyIdentifier is composed of the 160-bit SHA-1 hash of the value of the BIT STRING subjectPublicKey (excluding the tag, length, and number of unused bits).
(2) The keyIdentifier is composed of a four-bit type field with
the value 0100 followed by the least significant 60 bits of
the SHA-1 hash of the value of the BIT STRING
subjectPublicKey (excluding the tag, length, and number of
unused bits).
Other methods of generating unique numbers are also acceptable.
For end entity certificates, the subject key identifier extension
provides a means for identifying certificates containing the
particular public key used in an application. Where an end entity
has obtained multiple certificates, especially from multiple CAs, the
subject key identifier provides a means to quickly identify the set
of certificates containing a particular public key. To assist
applications in identifying the appropriate end entity certificate,
this extension SHOULD be included in all end entity certificates.
For end entity certificates, subject key identifiers SHOULD be
derived from the public key. Two common methods for generating key
identifiers from the public key are identified above.
Where a key identifier has not been previously established, this
specification RECOMMENDS use of one of these methods for generating
keyIdentifiers or use of a similar method that uses a different hash
algorithm. Where a key identifier has been previously established,
the CA SHOULD use the previously established identifier.
Conforming CAs MUST mark this extension as non-critical.
id-ce-subjectKeyIdentifier OBJECT IDENTIFIER ::= { id-ce 14 }
SubjectKeyIdentifier ::= KeyIdentifier
4.2.1.3. Key Usage
The key usage extension defines the purpose (e.g., encipherment,
signature, certificate signing) of the key contained in the
certificate. The usage restriction might be employed when a key that
could be used for more than one operation is to be restricted. For
example, when an RSA key should be used only to verify signatures on
objects other than public key certificates and CRLs, the
digitalSignature and/or nonRepudiation bits would be asserted.
Likewise, when an RSA key should be used only for key management, the
keyEncipherment bit would be asserted.
Conforming CAs MUST include this extension in certificates that
contain public keys that are used to validate digital signatures on
other public key certificates or CRLs. When present, conforming CAs
SHOULD mark this extension as critical.
id-ce-keyUsage OBJECT IDENTIFIER ::= { id-ce 15 }
KeyUsage ::= BIT STRING {
digitalSignature (0),
nonRepudiation (1), -- recent editions of X.509 have
-- renamed this bit to contentCommitment
keyEncipherment (2),
dataEncipherment (3),
keyAgreement (4),
keyCertSign (5),
cRLSign (6),
encipherOnly (7),
decipherOnly (8) }
Bits in the KeyUsage type are used as follows:
The digitalSignature bit is asserted when the subject public key
is used for verifying digital signatures, other than signatures on
certificates (bit 5) and CRLs (bit 6), such as those used in an
entity authentication service, a data origin authentication
service, and/or an integrity service.
The nonRepudiation bit is asserted when the subject public key is
used to verify digital signatures, other than signatures on
certificates (bit 5) and CRLs (bit 6), used to provide a non-
repudiation service that protects against the signing entity
falsely denying some action. In the case of later conflict, a
reliable third party may determine the authenticity of the signed
data. (Note that recent editions of X.509 have renamed the
nonRepudiation bit to contentCommitment.)
The keyEncipherment bit is asserted when the subject public key is
used for enciphering private or secret keys, i.e., for key
transport. For example, this bit shall be set when an RSA public
key is to be used for encrypting a symmetric content-decryption
key or an asymmetric private key.
The dataEncipherment bit is asserted when the subject public key
is used for directly enciphering raw user data without the use of
an intermediate symmetric cipher. Note that the use of this bit
is extremely uncommon; almost all applications use key transport
or key agreement to establish a symmetric key.
The keyAgreement bit is asserted when the subject public key is
used for key agreement. For example, when a Diffie-Hellman key is
to be used for key management, then this bit is set.
The keyCertSign bit is asserted when the subject public key is
used for verifying signatures on public key certificates. If the
keyCertSign bit is asserted, then the cA bit in the basic
constraints extension (Section 4.2.1.9) MUST also be asserted.
The cRLSign bit is asserted when the subject public key is used
for verifying signatures on certificate revocation lists (e.g.,
CRLs, delta CRLs, or ARLs).
The meaning of the encipherOnly bit is undefined in the absence of
the keyAgreement bit. When the encipherOnly bit is asserted and
the keyAgreement bit is also set, the subject public key may be
used only for enciphering data while performing key agreement.
The meaning of the decipherOnly bit is undefined in the absence of
the keyAgreement bit. When the decipherOnly bit is asserted and
the keyAgreement bit is also set, the subject public key may be
used only for deciphering data while performing key agreement.
If the keyUsage extension is present, then the subject public key
MUST NOT be used to verify signatures on certificates or CRLs unless
the corresponding keyCertSign or cRLSign bit is set. If the subject
public key is only to be used for verifying signatures on
certificates and/or CRLs, then the digitalSignature and
nonRepudiation bits SHOULD NOT be set. However, the digitalSignature
and/or nonRepudiation bits MAY be set in addition to the keyCertSign
and/or cRLSign bits if the subject public key is to be used to verify
signatures on certificates and/or CRLs as well as other objects.
Combining the nonRepudiation bit in the keyUsage certificate
extension with other keyUsage bits may have security implications
depending on the context in which the certificate is to be used.
Further distinctions between the digitalSignature and nonRepudiation
bits may be provided in specific certificate policies.
This profile does not restrict the combinations of bits that may be
set in an instantiation of the keyUsage extension. However,
appropriate values for keyUsage extensions for particular algorithms
are specified in [RFC3279], [RFC4055], and [RFC4491]. When the
keyUsage extension appears in a certificate, at least one of the bits
MUST be set to 1.
4.2.1.4. Certificate Policies
The certificate policies extension contains a sequence of one or more policy information terms, each of which consists of an object identifier (OID) and optional qualifiers. Optional qualifiers, which MAY be present, are not expected to change the definition of the policy. A certificate policy OID MUST NOT appear more than once in a certificate policies extension. In an end entity certificate, these policy information terms indicate the policy under which the certificate has been issued and the purposes for which the certificate may be used. In a CA certificate, these policy information terms limit the set of policies for certification paths that include this certificate. When a CA does not wish to limit the set of policies for certification paths that include this certificate, it MAY assert the special policy anyPolicy, with a value of { 2 5 29 32 0 }. Applications with specific policy requirements are expected to have a list of those policies that they will accept and to compare the policy OIDs in the certificate to that list. If this extension is critical, the path validation software MUST be able to interpret this extension (including the optional qualifier), or MUST reject the certificate. To promote interoperability, this profile RECOMMENDS that policy information terms consist of only an OID. Where an OID alone is insufficient, this profile strongly recommends that the use of qualifiers be limited to those identified in this section. When qualifiers are used with the special policy anyPolicy, they MUST be limited to the qualifiers identified in this section. Only those qualifiers returned as a result of path validation are considered. This specification defines two policy qualifier types for use by certificate policy writers and certificate issuers. The qualifier types are the CPS Pointer and User Notice qualifiers. The CPS Pointer qualifier contains a pointer to a Certification Practice Statement (CPS) published by the CA. The pointer is in the form of a URI. Processing requirements for this qualifier are a local matter. No action is mandated by this specification regardless of the criticality value asserted for the extension. User notice is intended for display to a relying party when a certificate is used. Only user notices returned as a result of path validation are intended for display to the user. If a notice is
duplicated, only one copy need be displayed. To prevent such
duplication, this qualifier SHOULD only be present in end entity
certificates and CA certificates issued to other organizations.
The user notice has two optional fields: the noticeRef field and the
explicitText field. Conforming CAs SHOULD NOT use the noticeRef
option.
The noticeRef field, if used, names an organization and
identifies, by number, a particular textual statement prepared by
that organization. For example, it might identify the
organization "CertsRUs" and notice number 1. In a typical
implementation, the application software will have a notice file
containing the current set of notices for CertsRUs; the
application will extract the notice text from the file and display
it. Messages MAY be multilingual, allowing the software to select
the particular language message for its own environment.
An explicitText field includes the textual statement directly in
the certificate. The explicitText field is a string with a
maximum size of 200 characters. Conforming CAs SHOULD use the
UTF8String encoding for explicitText, but MAY use IA5String.
Conforming CAs MUST NOT encode explicitText as VisibleString or
BMPString. The explicitText string SHOULD NOT include any control
characters (e.g., U+0000 to U+001F and U+007F to U+009F). When
the UTF8String encoding is used, all character sequences SHOULD be
normalized according to Unicode normalization form C (NFC) [NFC].
If both the noticeRef and explicitText options are included in the
one qualifier and if the application software can locate the notice
text indicated by the noticeRef option, then that text SHOULD be
displayed; otherwise, the explicitText string SHOULD be displayed.
Note: While the explicitText has a maximum size of 200 characters,
some non-conforming CAs exceed this limit. Therefore, certificate
users SHOULD gracefully handle explicitText with more than 200
characters.
id-ce-certificatePolicies OBJECT IDENTIFIER ::= { id-ce 32 }
anyPolicy OBJECT IDENTIFIER ::= { id-ce-certificatePolicies 0 }
certificatePolicies ::= SEQUENCE SIZE (1..MAX) OF PolicyInformation
PolicyInformation ::= SEQUENCE {
policyIdentifier CertPolicyId,
policyQualifiers SEQUENCE SIZE (1..MAX) OF
PolicyQualifierInfo OPTIONAL }
CertPolicyId ::= OBJECT IDENTIFIER
PolicyQualifierInfo ::= SEQUENCE {
policyQualifierId PolicyQualifierId,
qualifier ANY DEFINED BY policyQualifierId }
-- policyQualifierIds for Internet policy qualifiers
id-qt OBJECT IDENTIFIER ::= { id-pkix 2 }
id-qt-cps OBJECT IDENTIFIER ::= { id-qt 1 }
id-qt-unotice OBJECT IDENTIFIER ::= { id-qt 2 }
PolicyQualifierId ::= OBJECT IDENTIFIER ( id-qt-cps | id-qt-unotice )
Qualifier ::= CHOICE {
cPSuri CPSuri,
userNotice UserNotice }
CPSuri ::= IA5String
UserNotice ::= SEQUENCE {
noticeRef NoticeReference OPTIONAL,
explicitText DisplayText OPTIONAL }
NoticeReference ::= SEQUENCE {
organization DisplayText,
noticeNumbers SEQUENCE OF INTEGER }
DisplayText ::= CHOICE {
ia5String IA5String (SIZE (1..200)),
visibleString VisibleString (SIZE (1..200)),
bmpString BMPString (SIZE (1..200)),
utf8String UTF8String (SIZE (1..200)) }
4.2.1.5. Policy Mappings
This extension is used in CA certificates. It lists one or more pairs of OIDs; each pair includes an issuerDomainPolicy and a subjectDomainPolicy. The pairing indicates the issuing CA considers its issuerDomainPolicy equivalent to the subject CA's subjectDomainPolicy. The issuing CA's users might accept an issuerDomainPolicy for certain applications. The policy mapping defines the list of policies associated with the subject CA that may be accepted as comparable to the issuerDomainPolicy. Each issuerDomainPolicy named in the policy mappings extension SHOULD also be asserted in a certificate policies extension in the same certificate. Policies MUST NOT be mapped either to or from the special value anyPolicy (Section 4.2.1.4). In general, certificate policies that appear in the issuerDomainPolicy field of the policy mappings extension are not considered acceptable policies for inclusion in subsequent certificates in the certification path. In some circumstances, a CA may wish to map from one policy (p1) to another (p2), but still wants the issuerDomainPolicy (p1) to be considered acceptable for inclusion in subsequent certificates. This may occur, for example, if the CA is in the process of transitioning from the use of policy p1 to the use of policy p2 and has valid certificates that were issued under each of the policies. A CA may indicate this by including two policy mappings in the CA certificates that it issues. Each policy mapping would have an issuerDomainPolicy of p1; one policy mapping would have a subjectDomainPolicy of p1 and the other would have a subjectDomainPolicy of p2. This extension MAY be supported by CAs and/or applications. Conforming CAs SHOULD mark this extension as critical. id-ce-policyMappings OBJECT IDENTIFIER ::= { id-ce 33 } PolicyMappings ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE { issuerDomainPolicy CertPolicyId, subjectDomainPolicy CertPolicyId }4.2.1.6. Subject Alternative Name
The subject alternative name extension allows identities to be bound to the subject of the certificate. These identities may be included in addition to or in place of the identity in the subject field of the certificate. Defined options include an Internet electronic mail
address, a DNS name, an IP address, and a Uniform Resource Identifier (URI). Other options exist, including completely local definitions. Multiple name forms, and multiple instances of each name form, MAY be included. Whenever such identities are to be bound into a certificate, the subject alternative name (or issuer alternative name) extension MUST be used; however, a DNS name MAY also be represented in the subject field using the domainComponent attribute as described in Section 4.1.2.4. Note that where such names are represented in the subject field implementations are not required to convert them into DNS names. Because the subject alternative name is considered to be definitively bound to the public key, all parts of the subject alternative name MUST be verified by the CA. Further, if the only subject identity included in the certificate is an alternative name form (e.g., an electronic mail address), then the subject distinguished name MUST be empty (an empty sequence), and the subjectAltName extension MUST be present. If the subject field contains an empty sequence, then the issuing CA MUST include a subjectAltName extension that is marked as critical. When including the subjectAltName extension in a certificate that has a non-empty subject distinguished name, conforming CAs SHOULD mark the subjectAltName extension as non-critical. When the subjectAltName extension contains an Internet mail address, the address MUST be stored in the rfc822Name. The format of an rfc822Name is a "Mailbox" as defined in Section 4.1.2 of [RFC2821]. A Mailbox has the form "Local-part@Domain". Note that a Mailbox has no phrase (such as a common name) before it, has no comment (text surrounded in parentheses) after it, and is not surrounded by "<" and ">". Rules for encoding Internet mail addresses that include internationalized domain names are specified in Section 7.5. When the subjectAltName extension contains an iPAddress, the address MUST be stored in the octet string in "network byte order", as specified in [RFC791]. The least significant bit (LSB) of each octet is the LSB of the corresponding byte in the network address. For IP version 4, as specified in [RFC791], the octet string MUST contain exactly four octets. For IP version 6, as specified in [RFC2460], the octet string MUST contain exactly sixteen octets. When the subjectAltName extension contains a domain name system label, the domain name MUST be stored in the dNSName (an IA5String). The name MUST be in the "preferred name syntax", as specified by Section 3.5 of [RFC1034] and as modified by Section 2.1 of [RFC1123]. Note that while uppercase and lowercase letters are allowed in domain names, no significance is attached to the case. In
addition, while the string " " is a legal domain name, subjectAltName extensions with a dNSName of " " MUST NOT be used. Finally, the use of the DNS representation for Internet mail addresses (subscriber.example.com instead of subscriber@example.com) MUST NOT be used; such identities are to be encoded as rfc822Name. Rules for encoding internationalized domain names are specified in Section 7.2. When the subjectAltName extension contains a URI, the name MUST be stored in the uniformResourceIdentifier (an IA5String). The name MUST NOT be a relative URI, and it MUST follow the URI syntax and encoding rules specified in [RFC3986]. The name MUST include both a scheme (e.g., "http" or "ftp") and a scheme-specific-part. URIs that include an authority ([RFC3986], Section 3.2) MUST include a fully qualified domain name or IP address as the host. Rules for encoding Internationalized Resource Identifiers (IRIs) are specified in Section 7.4. As specified in [RFC3986], the scheme name is not case-sensitive (e.g., "http" is equivalent to "HTTP"). The host part, if present, is also not case-sensitive, but other components of the scheme- specific-part may be case-sensitive. Rules for comparing URIs are specified in Section 7.4. When the subjectAltName extension contains a DN in the directoryName, the encoding rules are the same as those specified for the issuer field in Section 4.1.2.4. The DN MUST be unique for each subject entity certified by the one CA as defined by the issuer field. A CA MAY issue more than one certificate with the same DN to the same subject entity. The subjectAltName MAY carry additional name types through the use of the otherName field. The format and semantics of the name are indicated through the OBJECT IDENTIFIER in the type-id field. The name itself is conveyed as value field in otherName. For example, Kerberos [RFC4120] format names can be encoded into the otherName, using a Kerberos 5 principal name OID and a SEQUENCE of the Realm and the PrincipalName. Subject alternative names MAY be constrained in the same manner as subject distinguished names using the name constraints extension as described in Section 4.2.1.10. If the subjectAltName extension is present, the sequence MUST contain at least one entry. Unlike the subject field, conforming CAs MUST NOT issue certificates with subjectAltNames containing empty GeneralName fields. For example, an rfc822Name is represented as an IA5String. While an empty string is a valid IA5String, such an rfc822Name is not permitted by this profile. The behavior of clients
that encounter such a certificate when processing a certification
path is not defined by this profile.
Finally, the semantics of subject alternative names that include
wildcard characters (e.g., as a placeholder for a set of names) are
not addressed by this specification. Applications with specific
requirements MAY use such names, but they must define the semantics.
id-ce-subjectAltName OBJECT IDENTIFIER ::= { id-ce 17 }
SubjectAltName ::= GeneralNames
GeneralNames ::= SEQUENCE SIZE (1..MAX) OF GeneralName
GeneralName ::= CHOICE {
otherName [0] OtherName,
rfc822Name [1] IA5String,
dNSName [2] IA5String,
x400Address [3] ORAddress,
directoryName [4] Name,
ediPartyName [5] EDIPartyName,
uniformResourceIdentifier [6] IA5String,
iPAddress [7] OCTET STRING,
registeredID [8] OBJECT IDENTIFIER }
OtherName ::= SEQUENCE {
type-id OBJECT IDENTIFIER,
value [0] EXPLICIT ANY DEFINED BY type-id }
EDIPartyName ::= SEQUENCE {
nameAssigner [0] DirectoryString OPTIONAL,
partyName [1] DirectoryString }
4.2.1.7. Issuer Alternative Name
As with Section 4.2.1.6, this extension is used to associate Internet
style identities with the certificate issuer. Issuer alternative
name MUST be encoded as in 4.2.1.6. Issuer alternative names are not
processed as part of the certification path validation algorithm in
Section 6. (That is, issuer alternative names are not used in name
chaining and name constraints are not enforced.)
Where present, conforming CAs SHOULD mark this extension as non-
critical.
id-ce-issuerAltName OBJECT IDENTIFIER ::= { id-ce 18 }
IssuerAltName ::= GeneralNames
4.2.1.8. Subject Directory Attributes
The subject directory attributes extension is used to convey identification attributes (e.g., nationality) of the subject. The extension is defined as a sequence of one or more attributes. Conforming CAs MUST mark this extension as non-critical. id-ce-subjectDirectoryAttributes OBJECT IDENTIFIER ::= { id-ce 9 } SubjectDirectoryAttributes ::= SEQUENCE SIZE (1..MAX) OF Attribute4.2.1.9. Basic Constraints
The basic constraints extension identifies whether the subject of the certificate is a CA and the maximum depth of valid certification paths that include this certificate. The cA boolean indicates whether the certified public key may be used to verify certificate signatures. If the cA boolean is not asserted, then the keyCertSign bit in the key usage extension MUST NOT be asserted. If the basic constraints extension is not present in a version 3 certificate, or the extension is present but the cA boolean is not asserted, then the certified public key MUST NOT be used to verify certificate signatures. The pathLenConstraint field is meaningful only if the cA boolean is asserted and the key usage extension, if present, asserts the keyCertSign bit (Section 4.2.1.3). In this case, it gives the maximum number of non-self-issued intermediate certificates that may follow this certificate in a valid certification path. (Note: The last certificate in the certification path is not an intermediate certificate, and is not included in this limit. Usually, the last certificate is an end entity certificate, but it can be a CA certificate.) A pathLenConstraint of zero indicates that no non- self-issued intermediate CA certificates may follow in a valid certification path. Where it appears, the pathLenConstraint field MUST be greater than or equal to zero. Where pathLenConstraint does not appear, no limit is imposed. Conforming CAs MUST include this extension in all CA certificates that contain public keys used to validate digital signatures on certificates and MUST mark the extension as critical in such certificates. This extension MAY appear as a critical or non- critical extension in CA certificates that contain public keys used exclusively for purposes other than validating digital signatures on certificates. Such CA certificates include ones that contain public keys used exclusively for validating digital signatures on CRLs and ones that contain key management public keys used with certificate
enrollment protocols. This extension MAY appear as a critical or
non-critical extension in end entity certificates.
CAs MUST NOT include the pathLenConstraint field unless the cA
boolean is asserted and the key usage extension asserts the
keyCertSign bit.
id-ce-basicConstraints OBJECT IDENTIFIER ::= { id-ce 19 }
BasicConstraints ::= SEQUENCE {
cA BOOLEAN DEFAULT FALSE,
pathLenConstraint INTEGER (0..MAX) OPTIONAL }
4.2.1.10. Name Constraints
The name constraints extension, which MUST be used only in a CA
certificate, indicates a name space within which all subject names in
subsequent certificates in a certification path MUST be located.
Restrictions apply to the subject distinguished name and apply to
subject alternative names. Restrictions apply only when the
specified name form is present. If no name of the type is in the
certificate, the certificate is acceptable.
Name constraints are not applied to self-issued certificates (unless
the certificate is the final certificate in the path). (This could
prevent CAs that use name constraints from employing self-issued
certificates to implement key rollover.)
Restrictions are defined in terms of permitted or excluded name
subtrees. Any name matching a restriction in the excludedSubtrees
field is invalid regardless of information appearing in the
permittedSubtrees. Conforming CAs MUST mark this extension as
critical and SHOULD NOT impose name constraints on the x400Address,
ediPartyName, or registeredID name forms. Conforming CAs MUST NOT
issue certificates where name constraints is an empty sequence. That
is, either the permittedSubtrees field or the excludedSubtrees MUST
be present.
Applications conforming to this profile MUST be able to process name
constraints that are imposed on the directoryName name form and
SHOULD be able to process name constraints that are imposed on the
rfc822Name, uniformResourceIdentifier, dNSName, and iPAddress name
forms. If a name constraints extension that is marked as critical
imposes constraints on a particular name form, and an instance of
that name form appears in the subject field or subjectAltName
extension of a subsequent certificate, then the application MUST
either process the constraint or reject the certificate.
Within this profile, the minimum and maximum fields are not used with any name forms, thus, the minimum MUST be zero, and maximum MUST be absent. However, if an application encounters a critical name constraints extension that specifies other values for minimum or maximum for a name form that appears in a subsequent certificate, the application MUST either process these fields or reject the certificate. For URIs, the constraint applies to the host part of the name. The constraint MUST be specified as a fully qualified domain name and MAY specify a host or a domain. Examples would be "host.example.com" and ".example.com". When the constraint begins with a period, it MAY be expanded with one or more labels. That is, the constraint ".example.com" is satisfied by both host.example.com and my.host.example.com. However, the constraint ".example.com" is not satisfied by "example.com". When the constraint does not begin with a period, it specifies a host. If a constraint is applied to the uniformResourceIdentifier name form and a subsequent certificate includes a subjectAltName extension with a uniformResourceIdentifier that does not include an authority component with a host name specified as a fully qualified domain name (e.g., if the URI either does not include an authority component or includes an authority component in which the host name is specified as an IP address), then the application MUST reject the certificate. A name constraint for Internet mail addresses MAY specify a particular mailbox, all addresses at a particular host, or all mailboxes in a domain. To indicate a particular mailbox, the constraint is the complete mail address. For example, "root@example.com" indicates the root mailbox on the host "example.com". To indicate all Internet mail addresses on a particular host, the constraint is specified as the host name. For example, the constraint "example.com" is satisfied by any mail address at the host "example.com". To specify any address within a domain, the constraint is specified with a leading period (as with URIs). For example, ".example.com" indicates all the Internet mail addresses in the domain "example.com", but not Internet mail addresses on the host "example.com". DNS name restrictions are expressed as host.example.com. Any DNS name that can be constructed by simply adding zero or more labels to the left-hand side of the name satisfies the name constraint. For example, www.host.example.com would satisfy the constraint but host1.example.com would not. Legacy implementations exist where an electronic mail address is embedded in the subject distinguished name in an attribute of type emailAddress (Section 4.1.2.6). When constraints are imposed on the
rfc822Name name form, but the certificate does not include a subject alternative name, the rfc822Name constraint MUST be applied to the attribute of type emailAddress in the subject distinguished name. The ASN.1 syntax for emailAddress and the corresponding OID are supplied in Appendix A. Restrictions of the form directoryName MUST be applied to the subject field in the certificate (when the certificate includes a non-empty subject field) and to any names of type directoryName in the subjectAltName extension. Restrictions of the form x400Address MUST be applied to any names of type x400Address in the subjectAltName extension. When applying restrictions of the form directoryName, an implementation MUST compare DN attributes. At a minimum, implementations MUST perform the DN comparison rules specified in Section 7.1. CAs issuing certificates with a restriction of the form directoryName SHOULD NOT rely on implementation of the full ISO DN name comparison algorithm. This implies name restrictions MUST be stated identically to the encoding used in the subject field or subjectAltName extension. The syntax of iPAddress MUST be as described in Section 4.2.1.6 with the following additions specifically for name constraints. For IPv4 addresses, the iPAddress field of GeneralName MUST contain eight (8) octets, encoded in the style of RFC 4632 (CIDR) to represent an address range [RFC4632]. For IPv6 addresses, the iPAddress field MUST contain 32 octets similarly encoded. For example, a name constraint for "class C" subnet 192.0.2.0 is represented as the octets C0 00 02 00 FF FF FF 00, representing the CIDR notation 192.0.2.0/24 (mask 255.255.255.0). Additional rules for encoding and processing name constraints are specified in Section 7. The syntax and semantics for name constraints for otherName, ediPartyName, and registeredID are not defined by this specification, however, syntax and semantics for name constraints for other name forms may be specified in other documents. id-ce-nameConstraints OBJECT IDENTIFIER ::= { id-ce 30 } NameConstraints ::= SEQUENCE { permittedSubtrees [0] GeneralSubtrees OPTIONAL, excludedSubtrees [1] GeneralSubtrees OPTIONAL } GeneralSubtrees ::= SEQUENCE SIZE (1..MAX) OF GeneralSubtree
GeneralSubtree ::= SEQUENCE {
base GeneralName,
minimum [0] BaseDistance DEFAULT 0,
maximum [1] BaseDistance OPTIONAL }
BaseDistance ::= INTEGER (0..MAX)
(page 43 continued on part 3)