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RFC 5280

 
 
 

Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile

Part 2 of 7, p. 16 to 43
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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

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        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.

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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

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   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

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   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

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   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

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   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

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   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),

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   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.

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      (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

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   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

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   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

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   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 STRING

4.2.1.2.  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).

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      (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.

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   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.

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      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.

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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

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   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.

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   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)) }

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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

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   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

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   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

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   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

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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 Attribute

4.2.1.9.  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

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   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.

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   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

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   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

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      GeneralSubtree ::= SEQUENCE {
           base                    GeneralName,
           minimum         [0]     BaseDistance DEFAULT 0,
           maximum         [1]     BaseDistance OPTIONAL }

      BaseDistance ::= INTEGER (0..MAX)



(page 43 continued on part 3)

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