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RFC 3280

 
 
 

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

Part 2 of 5, p. 14 to 48
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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.

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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

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   Extension  ::=  SEQUENCE  {
        extnID      OBJECT IDENTIFIER,
        critical    BOOLEAN DEFAULT FALSE,
        extnValue   OCTET STRING  }

   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.
   [PKIXALGS] lists 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

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   signature value is encoded as a BIT STRING and included in the
   signature field.  The details of this process are specified for each
   of algorithms listed in [PKIXALGS].

   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 who 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 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.

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   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.  Conformant 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.  [PKIXALGS] lists the
   supported signature algorithms, but other signature algorithms MAY
   also be supported.

4.1.2.4  Issuer

   The issuer field identifies the entity who 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 {
     RDNSequence }

   RDNSequence ::= SEQUENCE OF RelativeDistinguishedName

   RelativeDistinguishedName ::=
     SET OF AttributeTypeAndValue

   AttributeTypeAndValue ::= SEQUENCE {
     type     AttributeType,
     value    AttributeValue }

   AttributeType ::= OBJECT IDENTIFIER

   AttributeValue ::= ANY DEFINED BY AttributeType

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   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.  The
   UTF8String encoding [RFC 2279] is the preferred encoding, and all
   certificates issued after December 31, 2003 MUST use the UTF8String
   encoding of DirectoryString (except as noted below).  Until that
   date, conforming CAs MUST choose from the following options when
   creating a distinguished name, including their own:

      (a)  if the character set is sufficient, the string MAY be
      represented as a PrintableString;

      (b)  failing (a), if the BMPString character set is sufficient the
      string MAY be represented as a BMPString; and

      (c)  failing (a) and (b), the string MUST be represented as a
      UTF8String.  If (a) or (b) is satisfied, the CA MAY still choose
      to represent the string as a UTF8String.

   Exceptions to the December 31, 2003 UTF8 encoding requirements are as
   follows:

      (a)  CAs MAY issue "name rollover" certificates to support an
      orderly migration to UTF8String encoding.  Such certificates would
      include the CA's UTF8String encoded name as issuer and and the old
      name encoding as subject, or vice-versa.

      (b)  As stated in section 4.1.2.6, the subject field MUST be
      populated with a non-empty distinguished name matching the
      contents of the issuer field in all certificates issued by the
      subject CA regardless of encoding.

   The TeletexString 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.  Certificate users SHOULD be
   prepared to receive certificates with these types.

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   In addition, many legacy implementations support names encoded in the
   ISO 8859-1 character set (Latin1String) [ISO 8859-1] but tag them as
   TeletexString.  TeletexString encodes a larger character set than ISO
   8859-1, but it encodes some characters differently.  Implementations
   SHOULD be prepared to handle both encodings.

   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 [RFC 2247].
   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

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   alternative name field.  Implementations are not required to convert
   such names into DNS names.  The syntax and associated OID for this
   attribute type is provided in the ASN.1 modules in Appendix A.

   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.

   This specification requires only a subset of the name comparison
   functionality specified in the X.500 series of specifications.
   Conforming implementations are REQUIRED to implement the following
   name comparison rules:

      (a)  attribute values encoded in different types (e.g.,
      PrintableString and BMPString) MAY be assumed to represent
      different strings;

      (b) attribute values in types other than PrintableString are case
      sensitive (this permits matching of attribute values as binary
      objects);

      (c)  attribute values in PrintableString are not case sensitive
      (e.g., "Marianne Swanson" is the same as "MARIANNE SWANSON"); and

      (d)  attribute values in PrintableString are compared after
      removing leading and trailing white space and converting internal
      substrings of one or more consecutive white space characters to a
      single space.

   These name comparison rules permit a certificate user to validate
   certificates issued using languages or encodings unfamiliar to the
   certificate user.

   In addition, implementations of this specification MAY use these
   comparison rules to process unfamiliar attribute types for name
   chaining.  This allows implementations to process certificates with
   unfamiliar attributes in the issuer name.

   Note that the comparison rules defined in the X.500 series of
   specifications indicate that the character sets used to encode data
   in distinguished names are irrelevant.  The characters themselves are
   compared without regard to encoding.  Implementations of this profile
   are permitted to use the comparison algorithm defined in the X.500
   series.  Such an implementation will recognize a superset of name
   matches recognized by the algorithm specified above.

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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.

   The validity period for a certificate is the period of time from
   notBefore through notAfter, inclusive.

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
   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 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.

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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 4.2.1.10, 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 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 4.1.2.4) 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 name field.
   A CA MAY issue more than one certificate with the same DN to the same
   subject entity.

   The subject name 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).  When encoding attribute values of
   type DirectoryString, the encoding rules for the issuer field MUST be
   implemented.  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 these comparison rules to process unfamiliar
   attribute types (i.e., for name chaining).  This allows
   implementations to process certificates with unfamiliar attributes in
   the subject name.

   In addition, legacy implementations exist where an RFC 822 name is
   embedded in the subject distinguished name as an EmailAddress
   attribute.  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., "fanfeedback@redsox.com" is the same as
   "FANFEEDBACK@REDSOX.COM").

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   Conforming implementations generating new certificates with
   electronic mail addresses MUST use the rfc822Name in the subject
   alternative name field (section 4.2.1.7) 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 [PKIXALGS].

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 SHOULD NOT
   generate certificates with unique identifiers.  Applications
   conforming to this profile SHOULD be capable of parsing 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 is 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 a certification hierarchy.  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; however, a non-critical
   extension MAY be ignored if it is not recognized.  The following
   sections present recommended extensions used within Internet

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   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 which
   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 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.

   Conforming CAs MUST support key identifiers (sections 4.2.1.1 and
   4.2.1.2), basic constraints (section 4.2.1.10), key usage (section
   4.2.1.3), and certificate policies (section 4.2.1.5) extensions.  If
   the CA issues certificates with an empty sequence for the subject
   field, the CA MUST support the subject alternative name extension
   (section 4.2.1.7).  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 4.2.1.5), the subject alternative name (section
   4.2.1.7), basic constraints (section 4.2.1.10), name constraints
   (section 4.2.1.11), policy constraints (section 4.2.1.12), extended
   key usage (section 4.2.1.13), and inhibit any-policy (section
   4.2.1.15).

   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 mapping (section 4.2.1.6) 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 }

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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 on 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, and one common method for generating
   unique values, 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.
   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.

   This extension MUST NOT be marked 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

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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 4.2.1.10) where
   the value of cA is TRUE.  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.

   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 bit string bits).

   One common method for generating unique values is a monotonically
   increasing sequence of integers.

   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.  Where a key identifier has been previously
   established, the CA SHOULD use the previously established identifier.

   This extension MUST NOT be marked critical.

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   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.

   This extension MUST appear in certificates that contain public keys
   that are used to validate digital signatures on other public key
   certificates or CRLs.  When this extension appears, it SHOULD be
   marked critical.

      id-ce-keyUsage OBJECT IDENTIFIER ::=  { id-ce 15 }

      KeyUsage ::= BIT STRING {
           digitalSignature        (0),
           nonRepudiation          (1),
           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 with a digital signature mechanism to support security
      services other than certificate signing (bit 5), or CRL signing
      (bit 6).  Digital signature mechanisms are often used for entity
      authentication and data origin authentication with integrity.

      The nonRepudiation bit is asserted when the subject public key is
      used to verify digital signatures used to provide a non-
      repudiation service which protects against the signing entity
      falsely denying some action, excluding certificate or CRL signing.
      In the case of later conflict, a reliable third party may
      determine the authenticity of the signed data.

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      Further distinctions between the digitalSignature and
      nonRepudiation bits may be provided in specific certificate
      policies.

      The keyEncipherment bit is asserted when the subject public key is
      used for key transport.  For example, when an RSA key is to be
      used for key management, then this bit is set.

      The dataEncipherment bit is asserted when the subject public key
      is used for enciphering user data, other than cryptographic keys.

      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 a signature on public key certificates.  If the
      keyCertSign bit is asserted, then the cA bit in the basic
      constraints extension (section 4.2.1.10) MUST also be asserted.

      The cRLSign bit is asserted when the subject public key is used
      for verifying a signature on certificate revocation list (e.g., a
      CRL, delta CRL, or an ARL).  This bit MUST be asserted in
      certificates that are used to verify signatures on CRLs.

      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.

   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 [PKIXALGS].

4.2.1.4  Private Key Usage Period

   This extension SHOULD NOT be used within the Internet PKI.  CAs
   conforming to this profile MUST NOT generate certificates that
   include a critical private key usage period extension.

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   The private key usage period extension allows the certificate issuer
   to specify a different validity period for the private key than the
   certificate.  This extension is intended for use with digital
   signature keys.  This extension consists of two optional components,
   notBefore and notAfter.  The private key associated with the
   certificate SHOULD NOT be used to sign objects before or after the
   times specified by the two components, respectively.  CAs conforming
   to this profile MUST NOT generate certificates with private key usage
   period extensions unless at least one of the two components is
   present and the extension is non-critical.

   Where used, notBefore and notAfter are represented as GeneralizedTime
   and MUST be specified and interpreted as defined in section
   4.1.2.5.2.

   id-ce-privateKeyUsagePeriod OBJECT IDENTIFIER ::=  { id-ce 16 }

   PrivateKeyUsagePeriod ::= SEQUENCE {
        notBefore       [0]     GeneralizedTime OPTIONAL,
        notAfter        [1]     GeneralizedTime OPTIONAL }

4.2.1.5  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.

   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 which include this certificate.  When a CA does
   not wish to limit the set of policies for certification paths which
   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 which 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 use of qualifiers

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   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.

   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.  The application software SHOULD display all
   user notices in all certificates of the certification path used,
   except that 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.

      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.

   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-certificate-policies 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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4.2.1.6  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 mapping extension SHOULD
   also be asserted in a certificate policies extension in the same
   certificate.  Policies SHOULD NOT be mapped either to or from the
   special value anyPolicy (section 4.2.1.5).

   This extension MAY be supported by CAs and/or applications, and it
   MUST be non-critical.

   id-ce-policyMappings OBJECT IDENTIFIER ::=  { id-ce 33 }

   PolicyMappings ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
        issuerDomainPolicy      CertPolicyId,
        subjectDomainPolicy     CertPolicyId }

4.2.1.7  Subject Alternative Name

   The subject alternative names extension allows additional identities
   to be bound to the subject 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 be represented in the subject field using the
   domainComponent attribute as described in section 4.1.2.4.

   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

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   subjectAltName extension MUST be present.  If the subject field
   contains an empty sequence, the subjectAltName extension MUST be
   marked critical.

   When the subjectAltName extension contains an Internet mail address,
   the address MUST be included as an rfc822Name.  The format of an
   rfc822Name is an "addr-spec" as defined in RFC 822 [RFC 822].  An
   addr-spec has the form "local-part@domain".  Note that an addr-spec
   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
   ">".  Note that while upper and lower case letters are allowed in an
   RFC 822 addr-spec, no significance is attached to the case.

   When the subjectAltName extension contains a iPAddress, the address
   MUST be stored in the octet string in "network byte order," as
   specified in RFC 791 [RFC 791].  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 RFC 791, the octet string
   MUST contain exactly four octets.  For IP Version 6, as specified in
   RFC 1883, the octet string MUST contain exactly sixteen octets [RFC
   1883].

   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 RFC
   1034 [RFC 1034].  Note that while upper and lower case letters are
   allowed in domain names, no signifigance 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 (wpolk.nist.gov
   instead of wpolk@nist.gov) MUST NOT be used; such identities are to
   be encoded as rfc822Name.

   Note: work is currently underway to specify domain names in
   international character sets.  Such names will likely not be
   accommodated by IA5String.  Once this work is complete, this profile
   will be revisited and the appropriate functionality will be added.

   When the subjectAltName extension contains a URI, the name MUST be
   stored in the uniformResourceIdentifier (an IA5String).  The name
   MUST NOT be a relative URL, and it MUST follow the URL syntax and
   encoding rules specified in [RFC 1738].  The name MUST include both a
   scheme (e.g., "http" or "ftp") and a scheme-specific-part.  The
   scheme-specific-part MUST include a fully qualified domain name or IP
   address as the host.

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   As specified in [RFC 1738], the scheme name is not case-sensitive
   (e.g., "http" is equivalent to "HTTP").  The host part is also not
   case-sensitive, but other components of the scheme-specific-part may
   be case-sensitive.  When comparing URIs, conforming implementations
   MUST compare the scheme and host without regard to case, but assume
   the remainder of the scheme-specific-part is case sensitive.

   When the subjectAltName extension contains a DN in the directoryName,
   the DN MUST be unique for each subject entity certified by the one CA
   as defined by the issuer name 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 [RFC 1510] format names can be encoded into the otherName,
   using 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 4.2.1.11.

   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 certificication
   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

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   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 }

4.2.1.8  Issuer Alternative Names

   As with 4.2.1.7, this extension is used to associate Internet style
   identities with the certificate issuer.  Issuer alternative names
   MUST be encoded as in 4.2.1.7.

   Where present, this extension SHOULD NOT be marked critical.

   id-ce-issuerAltName OBJECT IDENTIFIER ::=  { id-ce 18 }

   IssuerAltName ::= GeneralNames

4.2.1.9  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.  This
   extension MUST be non-critical.

   id-ce-subjectDirectoryAttributes OBJECT IDENTIFIER ::=  { id-ce 9 }

   SubjectDirectoryAttributes ::= SEQUENCE SIZE (1..MAX) OF Attribute

4.2.1.10  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.

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   The cA boolean indicates whether the certified public key belongs to
   a CA.  If the cA boolean is not asserted, then the keyCertSign bit in
   the key usage extension MUST NOT be asserted.

   The pathLenConstraint field is meaningful only if the cA boolean is
   asserted and the key usage extension 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.  A certificate is self-
   issued if the DNs that appear in the subject and issuer fields are
   identical and are not empty.  (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 only one more certificate
   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.

   This extension MUST appear as a critical extension in all CA
   certificates that contain public keys used to validate digital
   signatures on 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 }

4.2.1.11  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

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   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 certificates whose issuer and
   subject are identical (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.  This extension MUST be critical.

   Within this profile, the minimum and maximum fields are not used with
   any name forms, thus minimum MUST be zero, and maximum MUST be
   absent.

   For URIs, the constraint applies to the host part of the name.  The
   constraint MAY specify a host or a domain.  Examples would be
   "foo.bar.com";  and ".xyz.com".  When the the constraint begins with
   a period, it MAY be expanded with one or more subdomains.  That is,
   the constraint ".xyz.com" is satisfied by both abc.xyz.com and
   abc.def.xyz.com.  However, the constraint ".xyz.com" is not satisfied
   by "xyz.com".  When the constraint does not begin with a period, it
   specifies a host.

   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@xyz.com"
   indicates the root mailbox on the host "xyz.com".  To indicate all
   Internet mail addresses on a particular host, the constraint is
   specified as the host name.  For example, the constraint "xyz.com" is
   satisfied by any mail address at the host "xyz.com".  To specify any
   address within a domain, the constraint is specified with a leading
   period (as with URIs).  For example, ".xyz.com" indicates all the
   Internet mail addresses in the domain "xyz.com", but not Internet
   mail addresses on the host "xyz.com".

   DNS name restrictions are expressed as foo.bar.com.  Any DNS name
   that can be constructed by simply adding to the left hand side of the
   name satisfies the name constraint.  For example, www.foo.bar.com
   would satisfy the constraint but foo1.bar.com would not.

   Legacy implementations exist where an RFC 822 name is embedded in the
   subject distinguished name in an attribute of type EmailAddress
   (section 4.1.2.6).  When rfc822 names are constrained, but the

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   certificate does not include a subject alternative name, the rfc822
   name 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 and to the subjectAltName extensions of type
   directoryName.  Restrictions of the form x400Address MUST be applied
   to subjectAltName extensions of type x400Address.

   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 4.1.2.4.  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 4.2.1.7 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 1519 (CIDR) to represent an
   address range [RFC 1519].  For IPv6 addresses, the ipAddress field
   MUST contain 32 octets similarly encoded.  For example, a name
   constraint for "class C" subnet 10.9.8.0 is represented as the octets
   0A 09 08 00 FF FF FF 00, representing the CIDR notation
   10.9.8.0/255.255.255.0.

   The syntax and semantics for name constraints for otherName,
   ediPartyName, and registeredID are not defined by this specification.

      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)

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4.2.1.12  Policy Constraints

   The policy constraints extension can be used in certificates issued
   to CAs.  The policy constraints extension constrains path validation
   in two ways.  It can be used to prohibit policy mapping or require
   that each certificate in a path contain an acceptable policy
   identifier.

   If the inhibitPolicyMapping field is present, the value indicates the
   number of additional certificates that may appear in the path before
   policy mapping is no longer permitted.  For example, a value of one
   indicates that policy mapping may be processed in certificates issued
   by the subject of this certificate, but not in additional
   certificates in the path.

   If the requireExplicitPolicy field is present, the value of
   requireExplicitPolicy indicates the number of additional certificates
   that may appear in the path before an explicit policy is required for
   the entire path.  When an explicit policy is required, it is
   necessary for all certificates in the path to contain an acceptable
   policy identifier in the certificate policies extension.  An
   acceptable policy identifier is the identifier of a policy required
   by the user of the certification path or the identifier of a policy
   which has been declared equivalent through policy mapping.

   Conforming CAs MUST NOT issue certificates where policy constraints
   is a empty sequence.  That is, at least one of the
   inhibitPolicyMapping field or the requireExplicitPolicy field MUST be
   present.  The behavior of clients that encounter a empty policy
   constraints field is not addressed in this profile.

   This extension MAY be critical or non-critical.

   id-ce-policyConstraints OBJECT IDENTIFIER ::=  { id-ce 36 }

   PolicyConstraints ::= SEQUENCE {
        requireExplicitPolicy           [0] SkipCerts OPTIONAL,
        inhibitPolicyMapping            [1] SkipCerts OPTIONAL }

   SkipCerts ::= INTEGER (0..MAX)

4.2.1.13  Extended Key Usage

   This extension indicates one or more purposes for which the certified
   public key may be used, in addition to or in place of the basic
   purposes indicated in the key usage extension.  In general, this
   extension will appear only in end entity certificates.  This
   extension is defined as follows:

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   id-ce-extKeyUsage OBJECT IDENTIFIER ::= { id-ce 37 }

   ExtKeyUsageSyntax ::= SEQUENCE SIZE (1..MAX) OF KeyPurposeId

   KeyPurposeId ::= OBJECT IDENTIFIER

   Key purposes may be defined by any organization with a need.  Object
   identifiers used to identify key purposes MUST be assigned in
   accordance with IANA or ITU-T Recommendation X.660 [X.660].

   This extension MAY, at the option of the certificate issuer, be
   either critical or non-critical.

   If the extension is present, then the certificate MUST only be used
   for one of the purposes indicated.  If multiple purposes are
   indicated the application need not recognize all purposes indicated,
   as long as the intended purpose is present.  Certificate using
   applications MAY require that a particular purpose be indicated in
   order for the certificate to be acceptable to that application.

   If a CA includes extended key usages to satisfy such applications,
   but does not wish to restrict usages of the key, the CA can include
   the special keyPurposeID anyExtendedKeyUsage.  If the
   anyExtendedKeyUsage keyPurposeID is present, the extension SHOULD NOT
   be critical.

   If a certificate contains both a key usage extension and an extended
   key usage extension, then both extensions MUST be processed
   independently and the certificate MUST only be used for a purpose
   consistent with both extensions.  If there is no purpose consistent
   with both extensions, then the certificate MUST NOT be used for any
   purpose.

   The following key usage purposes are defined:

   anyExtendedKeyUsage OBJECT IDENTIFIER ::= { id-ce-extKeyUsage 0 }

   id-kp OBJECT IDENTIFIER ::= { id-pkix 3 }

   id-kp-serverAuth             OBJECT IDENTIFIER ::= { id-kp 1 }
   -- TLS WWW server authentication
   -- Key usage bits that may be consistent: digitalSignature,
   -- keyEncipherment or keyAgreement

   id-kp-clientAuth             OBJECT IDENTIFIER ::= { id-kp 2 }
   -- TLS WWW client authentication
   -- Key usage bits that may be consistent: digitalSignature
   -- and/or keyAgreement

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   id-kp-codeSigning             OBJECT IDENTIFIER ::= { id-kp 3 }
   -- Signing of downloadable executable code
   -- Key usage bits that may be consistent: digitalSignature

   id-kp-emailProtection         OBJECT IDENTIFIER ::= { id-kp 4 }
   -- E-mail protection
   -- Key usage bits that may be consistent: digitalSignature,
   -- nonRepudiation, and/or (keyEncipherment or keyAgreement)

   id-kp-timeStamping            OBJECT IDENTIFIER ::= { id-kp 8 }
   -- Binding the hash of an object to a time
   -- Key usage bits that may be consistent: digitalSignature
   -- and/or nonRepudiation

   id-kp-OCSPSigning            OBJECT IDENTIFIER ::= { id-kp 9 }
   -- Signing OCSP responses
   -- Key usage bits that may be consistent: digitalSignature
   -- and/or nonRepudiation

4.2.1.14  CRL Distribution Points

   The CRL distribution points extension identifies how CRL information
   is obtained.  The extension SHOULD be non-critical, but this profile
   RECOMMENDS support for this extension by CAs and applications.
   Further discussion of CRL management is contained in section 5.

   The cRLDistributionPoints extension is a SEQUENCE of
   DistributionPoint.  A DistributionPoint consists of three fields,
   each of which is optional: distributionPoint, reasons, and cRLIssuer.
   While each of these fields is optional, a DistributionPoint MUST NOT
   consist of only the reasons field; either distributionPoint or
   cRLIssuer MUST be present.  If the certificate issuer is not the CRL
   issuer, then the cRLIssuer field MUST be present and contain the Name
   of the CRL issuer.  If the certificate issuer is also the CRL issuer,
   then the cRLIssuer field MUST be omitted and the distributionPoint
   field MUST be present.  If the distributionPoint field is omitted,
   cRLIssuer MUST be present and include a Name corresponding to an
   X.500 or LDAP directory entry where the CRL is located.

   When the distributionPoint field is present, it contains either a
   SEQUENCE of general names or a single value, nameRelativeToCRLIssuer.
   If the cRLDistributionPoints extension contains a general name of
   type URI, the following semantics MUST be assumed: the URI is a
   pointer to the current CRL for the associated reasons and will be
   issued by the associated cRLIssuer.  The expected values for the URI
   are those defined in 4.2.1.7.  Processing rules for other values are
   not defined by this specification.

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   If the DistributionPointName contains multiple values, each name
   describes a different mechanism to obtain the same CRL.  For example,
   the same CRL could be available for retrieval through both LDAP and
   HTTP.

   If the DistributionPointName contains the single value
   nameRelativeToCRLIssuer, the value provides a distinguished name
   fragment.  The fragment is appended to the X.500 distinguished name
   of the CRL issuer to obtain the distribution point name.  If the
   cRLIssuer field in the DistributionPoint is present, then the name
   fragment is appended to the distinguished name that it contains;
   otherwise, the name fragment is appended to the certificate issuer
   distinguished name.  The DistributionPointName MUST NOT use the
   nameRealtiveToCRLIssuer alternative when cRLIssuer contains more than
   one distinguished name.

   If the DistributionPoint omits the reasons field, the CRL MUST
   include revocation information for all reasons.

   The cRLIssuer identifies the entity who signs and issues the CRL.  If
   present, the cRLIssuer MUST contain at least one an X.500
   distinguished name (DN), and MAY also contain other name forms.
   Since the cRLIssuer is compared to the CRL issuer name, the X.501
   type Name MUST follow the encoding rules for the issuer name field in
   the certificate (section 4.1.2.4).

   id-ce-cRLDistributionPoints OBJECT IDENTIFIER ::=  { id-ce 31 }

   CRLDistributionPoints ::= SEQUENCE SIZE (1..MAX) OF DistributionPoint

   DistributionPoint ::= SEQUENCE {
        distributionPoint       [0]     DistributionPointName OPTIONAL,
        reasons                 [1]     ReasonFlags OPTIONAL,
        cRLIssuer               [2]     GeneralNames OPTIONAL }

   DistributionPointName ::= CHOICE {
        fullName                [0]     GeneralNames,
        nameRelativeToCRLIssuer [1]     RelativeDistinguishedName }

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   ReasonFlags ::= BIT STRING {
        unused                  (0),
        keyCompromise           (1),
        cACompromise            (2),
        affiliationChanged      (3),
        superseded              (4),
        cessationOfOperation    (5),
        certificateHold         (6),
        privilegeWithdrawn      (7),
        aACompromise            (8) }

4.2.1.15  Inhibit Any-Policy

   The inhibit any-policy extension can be used in certificates issued
   to CAs.  The inhibit any-policy indicates that the special anyPolicy
   OID, with the value { 2 5 29 32 0 }, is not considered an explicit
   match for other certificate policies.  The value indicates the number
   of additional certificates that may appear in the path before
   anyPolicy is no longer permitted.  For example, a value of one
   indicates that anyPolicy may be processed in certificates issued by
   the subject of this certificate, but not in additional certificates
   in the path.

   This extension MUST be critical.

   id-ce-inhibitAnyPolicy OBJECT IDENTIFIER ::=  { id-ce 54 }

   InhibitAnyPolicy ::= SkipCerts

   SkipCerts ::= INTEGER (0..MAX)

4.2.1.16  Freshest CRL (a.k.a. Delta CRL Distribution Point)

   The freshest CRL extension identifies how delta CRL information is
   obtained.  The extension MUST be non-critical.  Further discussion of
   CRL management is contained in section 5.

   The same syntax is used for this extension and the
   cRLDistributionPoints extension, and is described in section
   4.2.1.14.  The same conventions apply to both extensions.

   id-ce-freshestCRL OBJECT IDENTIFIER ::=  { id-ce 46 }

   FreshestCRL ::= CRLDistributionPoints

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4.2.2  Private Internet Extensions

   This section defines two extensions for use in the Internet Public
   Key Infrastructure.  These extensions may be used to direct
   applications to on-line information about the issuing CA or the
   subject.  As the information may be available in multiple forms, each
   extension is a sequence of IA5String values, each of which represents
   a URI.  The URI implicitly specifies the location and format of the
   information and the method for obtaining the information.

   An object identifier is defined for the private extension.  The
   object identifier associated with the private extension is defined
   under the arc id-pe within the arc id-pkix.  Any future extensions
   defined for the Internet PKI are also expected to be defined under
   the arc id-pe.

      id-pkix  OBJECT IDENTIFIER  ::=
               { iso(1) identified-organization(3) dod(6) internet(1)
                       security(5) mechanisms(5) pkix(7) }

      id-pe  OBJECT IDENTIFIER  ::=  { id-pkix 1 }

4.2.2.1  Authority Information Access

   The authority information access extension indicates how to access CA
   information and services for the issuer of the certificate in which
   the extension appears.  Information and services may include on-line
   validation services and CA policy data.  (The location of CRLs is not
   specified in this extension; that information is provided by the
   cRLDistributionPoints extension.)  This extension may be included in
   end entity or CA certificates, and it MUST be non-critical.

   id-pe-authorityInfoAccess OBJECT IDENTIFIER ::= { id-pe 1 }

   AuthorityInfoAccessSyntax  ::=
           SEQUENCE SIZE (1..MAX) OF AccessDescription

   AccessDescription  ::=  SEQUENCE {
           accessMethod          OBJECT IDENTIFIER,
           accessLocation        GeneralName  }

   id-ad OBJECT IDENTIFIER ::= { id-pkix 48 }

   id-ad-caIssuers OBJECT IDENTIFIER ::= { id-ad 2 }

   id-ad-ocsp OBJECT IDENTIFIER ::= { id-ad 1 }

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   Each entry in the sequence AuthorityInfoAccessSyntax describes the
   format and location of additional information provided by the CA that
   issued the certificate in which this extension appears.  The type and
   format of the information is specified by the accessMethod field; the
   accessLocation field specifies the location of the information.  The
   retrieval mechanism may be implied by the accessMethod or specified
   by accessLocation.

   This profile defines two accessMethod OIDs: id-ad-caIssuers and
   id-ad-ocsp.

   The id-ad-caIssuers OID is used when the additional information lists
   CAs that have issued certificates superior to the CA that issued the
   certificate containing this extension.  The referenced CA issuers
   description is intended to aid certificate users in the selection of
   a certification path that terminates at a point trusted by the
   certificate user.

   When id-ad-caIssuers appears as accessMethod, the accessLocation
   field describes the referenced description server and the access
   protocol to obtain the referenced description.  The accessLocation
   field is defined as a GeneralName, which can take several forms.
   Where the information is available via http, ftp, or ldap,
   accessLocation MUST be a uniformResourceIdentifier.  Where the
   information is available via the Directory Access Protocol (DAP),
   accessLocation MUST be a directoryName.  The entry for that
   directoryName contains CA certificates in the crossCertificatePair
   attribute.  When the information is available via electronic mail,
   accessLocation MUST be an rfc822Name.  The semantics of other
   id-ad-caIssuers accessLocation name forms are not defined.

   The id-ad-ocsp OID is used when revocation information for the
   certificate containing this extension is available using the Online
   Certificate Status Protocol (OCSP) [RFC 2560].

   When id-ad-ocsp appears as accessMethod, the accessLocation field is
   the location of the OCSP responder, using the conventions defined in
   [RFC 2560].

   Additional access descriptors may be defined in other PKIX
   specifications.

4.2.2.2  Subject Information Access

   The subject information access extension indicates how to access
   information and services for the subject of the certificate in which
   the extension appears.  When the subject is a CA, information and
   services may include certificate validation services and CA policy

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   data.  When the subject is an end entity, the information describes
   the type of services offered and how to access them.  In this case,
   the contents of this extension are defined in the protocol
   specifications for the suported services.  This extension may be
   included in subject or CA certificates, and it MUST be non-critical.

   id-pe-subjectInfoAccess OBJECT IDENTIFIER ::= { id-pe 11 }

   SubjectInfoAccessSyntax  ::=
           SEQUENCE SIZE (1..MAX) OF AccessDescription

   AccessDescription  ::=  SEQUENCE {
           accessMethod          OBJECT IDENTIFIER,
           accessLocation        GeneralName  }

   Each entry in the sequence SubjectInfoAccessSyntax describes the
   format and location of additional information provided by the subject
   of the certificate in which this extension appears.  The type and
   format of the information is specified by the accessMethod field; the
   accessLocation field specifies the location of the information.  The
   retrieval mechanism may be implied by the accessMethod or specified
   by accessLocation.

   This profile defines one access method to be used when the subject is
   a CA, and one access method to be used when the subject is an end
   entity.  Additional access methods may be defined in the future in
   the protocol specifications for other services.

   The id-ad-caRepository OID is used when the subject is a CA, and
   publishes its certificates and CRLs (if issued) in a repository.  The
   accessLocation field is defined as a GeneralName, which can take
   several forms.  Where the information is available via http, ftp, or
   ldap, accessLocation MUST be a uniformResourceIdentifier.  Where the
   information is available via the directory access protocol (dap),
   accessLocation MUST be a directoryName.  When the information is
   available via electronic mail, accessLocation MUST be an rfc822Name.
   The semantics of other name forms of of accessLocation (when
   accessMethod is id-ad-caRepository) are not defined by this
   specification.

   The id-ad-timeStamping OID is used when the subject offers
   timestamping services using the Time Stamp Protocol defined in
   [PKIXTSA].  Where the timestamping services are available via http or
   ftp, accessLocation MUST be a uniformResourceIdentifier.  Where the
   timestamping services are available via electronic mail,
   accessLocation MUST be an rfc822Name.  Where timestamping services

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   are available using TCP/IP, the dNSName or ipAddress name forms may
   be used.  The semantics of other name forms of accessLocation (when
   accessMethod is id-ad-timeStamping) are not defined by this
   specification.

   Additional access descriptors may be defined in other PKIX
   specifications.

   id-ad OBJECT IDENTIFIER ::= { id-pkix 48 }

   id-ad-caRepository OBJECT IDENTIFIER ::= { id-ad 5 }

   id-ad-timeStamping OBJECT IDENTIFIER ::= { id-ad 3 }



(page 48 continued on part 3)

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