Digital Certificates and Digital Signatures

PKIv3 architectual model and datastructures conforming to ITU-T X.509

  Users of a public key shall be confident that the associated private
  key is owned by the correct remote subject (person or system) with
  which an encryption or digital signature mechanism will be used.
  This confidence is obtained through the use of public key
  certificates, which are data structures that bind public key values
  to subjects.  The binding is asserted by having a trusted CA
  digitally sign each certificate. The CA may base this assertion upon
  technical means (a.k.a., proof of posession through a challenge-
  response protocol), presentation of the private key, or on an
  assertion by the subject.  A certificate has a limited valid lifetime
  which is indicated in its signed contents.  Because a certificate's
  signature and timeliness can be independently checked by a
  certificate-using client, certificates can be distributed via
  untrusted communications and server systems, and can be cached in
  unsecured storage in certificate-using systems.

  ITU-T X.509 (formerly CCITT X.509) or ISO/IEC/ITU 9594-8, which was
  first published in 1988 as part of the X.500 Directory
  recommendations, defines a standard certificate format [X.509]. The
  certificate format in the 1988 standard is called the version 1 (v1)
  format.  When X.500 was revised in 1993, two more fields were added,
  resulting in the version 2 (v2) format. These two fields may be used
  to support directory access control.

X.509v3

  The v3 format extends the v2 format by
  adding provision for additional extension fields.  Particular
  extension field types may be specified in standards or may be defined
  and registered by any organization or community. In June 1996,
  standardization of the basic v3 format was completed [X.509].

  ISO/IEC/ITU and ANSI X9 have also developed standard extensions for
  use in the v3 extensions field [X.509][X9.55].  These extensions can
  convey such data as additional subject identification information,
  key attribute information, policy information, and certification path
  constraints.

Basic Certificate Fields

  The X.509 v3 certificate basic syntax is as follows.  For signature
  calculation, the certificate is encoded using the ASN.1 distinguished
  encoding rules (DER) [X.208].  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 shall be v2 or v3
       subjectUniqueID [2]  IMPLICIT UniqueIdentifier OPTIONAL,
                            -- If present, version shall be v2 or v3
       extensions      [3]  EXPLICIT Extensions OPTIONAL
                            -- If present, version shall 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  }

Revocation

  X.509 defines one method of certificate revocation.  This method
  involves each CA periodically issuing a signed data structure called
  a certificate revocation list (CRL).  A CRL is a time stamped list
  identifying revoked certificates which is signed by a CA and made
  freely available in a public repository.  Each revoked certificate is
  identified in a CRL by its certificate serial number.