.. _mozilla_projects_nss_nss_tech_notes_nss_tech_note3:

nss tech note3
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.. _all_about_certificate_extensions:

`All About Certificate Extensions <#all_about_certificate_extensions>`__
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.. _nss_technical_note_3:

`NSS Technical Note: 3 <#nss_technical_note_3>`__
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      09 May 2002
      Nelson B. Bolyard

      This week at least 5 different people came to me with variants of the
      same question:

        What certificate extensions do I have to put into my cert for NSS to
        allow it to be used for purpose <x>??

      This message attempts to answer that question, and to document NSS's
      approach to validating certificates for certain purposes.

      When NSS is asked to verify the validity of a certificate chain, it
      verifies the validity of that cert chain for a particular purpose,
      known as a SECCertUsage, as of a specific date and time.

      The list of known SECCertUsages is short:

      certUsageSSLClient ...........  An SSL client authentication cert
      certUsageSSLServer ...........  An ordinary SSL server cert
      certUsageSSLServerWithStepUp..  An SSL server cert that allows export
                      clients to use strong crypto.
      certUsageSSLCA ...............  An intermediate or root CA cert allowed
                      to issue SSL client or SSL server certs
                      or other intermediate SSL CA certs.
      certUsageEmailSigner .........  Used to verify S/MIME email signatures
      certUsageEmailRecipient ......  Used to encrypt S/MIME emails.
      certUsageObjectSigner ........  Used to verify signatures on files of
                      executable code, e.g. jar files.
      certUsageStatusResponder .....  Used by an OCSP responder
      certUsageVerifyCA ............  A CA of any kind.

      Each cert has a "type" and a "key usage", each of which may contain one
      or more valid values.

      Each of the above SECCertUsages translates into a required set of
      cert type and key usage for the certificate itself, and into another
      set of required cert type and key usage for all the CA certs in the
      cert chain.

      To determine if a cert is valid for a given cert usage, it must have the
      the cert type and key usage required for that cert usage, and all the
      CA certs in the cert chain must have the cert type and key usage required
      for CA certs for that cert usage.

      There are 8 Key Usages:
          CERT_SIGN
          CRL_SIGN
          DATA_ENCIPHERMENT
          DIGITAL_SIGNATURE
          GOVT_APPROVED
          KEY_AGREEMENT
          KEY_ENCIPHERMENT
          NON_REPUDIATION

      There are 9 Cert types:
          EMAIL
          EMAIL_CA
          OBJECT_SIGNING
          OBJECT_SIGNING_CA
          SSL_CA
          SSL_CLIENT
          SSL_SERVER
          STATUS_RESPONDER
          TIME_STAMP


      For the cert being checked, the requirements are:

      Cert Usage      Requried Key Usage      Required Cert Type
      --------------------    --------------------        -----------------------
      SSLClient:      DIGITAL_SIGNATURE;      SSL_CLIENT;

      SSLServer:      KEY_AGREEMENT OR
                  KEY_ENCIPHERMENT;       SSL_SERVER;

      SSLServerWithStepUp:    GOVT_APPROVED  AND      SSL_SERVER
                  KEY_AGREEMENT  or
                  KEY_ENCIPHERMENT

      SSLCA:          CERT_SIGN;          SSL_CA;

      EmailSigner:        DIGITAL_SIGNATURE;      EMAIL;

      EmailRecipient:     KEY_AGREEMENT OR
                  KEY_ENCIPHERMENT;       EMAIL;

      ObjectSigner:       DIGITAL_SIGNATURE;      OBJECT_SIGNING;

      StatusResponder:    DIGITAL_SIGNATURE;      STATUS_RESPONDER;

      VerifyCA        CERT_SIGN           SSL_CA OR
                                  EMAIL_CA OR
                                  OBJECT_SIGNING_CA OR
                                  STATUS_RESPONDER

      For CA certs in the cert chain, the requirements are:

      Cert Usage      Requried Key Usage  Required Cert Type
      --------------------    --------------------    -----------------------
      SSLServerWithStepUp:    GOVT_APPROVED AND
                  CERT_SIGN;      SSL_CA;

      SSLClient:      CERT_SIGN;      SSL_CA;

      SSLServer:      CERT_SIGN;      SSL_CA;

      SSLCA:          CERT_SIGN;      SSL_CA;

      EmailSigner:        CERT_SIGN;      EMAIL_CA or SSL_CA

      EmailRecipient:     CERT_SIGN;      EMAIL_CA or SSL_CA

      ObjectSigner:       CERT_SIGN;      OBJECT_SIGNING_CA;

      UsageAnyCA:     CERT_SIGN;      OBJECT_SIGNING_CA OR
                                  EMAIL_CA OR
                                  SSL_CA;

      StatusResponder:    CERT_SIGN;      OBJECT_SIGNING_CA OR
                                  EMAIL_CA OR
                                  SSL_CA;

      Note:  When the required key usage is KEY_AGREEMENT OR KEY_ENCIPHERMENT,
      the actual key usage required depends on the key's algorithm.  For
      RSA keys, the required usage is KEY_ENCIPHERMENT. For other types of
      keys, it is KEY_AGREEMENT.


      Cert Extensions:

      One vital Certificate extension is the "Basic Constraints" extension.
      It tells NSS whether the cert is a CA cert, or not, and affects every
      other aspect of how the cert is interpreted by NSS.  The OID for this
      extension is { 2 5 29 19 }, encoded in hex as 0x55, 0x1d, 0x13.
      If the extension is present and has the value TRUE, then this cert is
      taken to be a CA cert.  Otherwise it is not (except that trust flags
      may override this, see discussion of trust flags farther below).

      Netscape has its own openly defined Cert Type extension, which can be used
      to explicitly set the Cert Type in any Cert.  The Cert Type extension has
      bits in it that correspond directly to the cert types named above.
      The OID for this extension is { 2 16 840 1 113730 1 1 }
      encoded in hex as  0x60, 0x86, 0x48, 0x01, 0x86, 0xf8, 0x42, 0x01, 0x01

      In addition to Netscape's own Cert Type extension, NSS recognizes various
      X.509 extensions.

      The X.509 key usage extension has OID { 2 5 29 0F } encoded in hex as
      0x55, 0x1d, 0x0f.  If present, this extension directly determines the
      values of the 8 key usages defined above.  If absent, the cert is
      assumed to be valid for all key usages.

      The X.509v3 extended Key usage extension as OID { 2 5 29 37 } encoded in
      hex as 0x55, 0x1d, 0x25.  That extension contains a sequence of OIDs, each
      of which signifies one or more Cert Types, depending on the presence or
      absence of of the True Basic Constraints extension; that is, the
      interpretation of the extended Key Usage extension is controlled by
      whether the cert is a CA cert, or not.

      The following table shows the OIDs recognized in the extended key usage
      extension, and how they map to cert types and key usages for CA and non-CA
      certs.

      extended key usage OID          non-CA cert CA cert
      ----------------------------------- --------------  ----------------
      SEC_OID_EXT_KEY_USAGE_EMAIL_PROTECT     EMAIL_CA    EMAIL_CA
      SEC_OID_EXT_KEY_USAGE_SERVER_AUTH   SSL_SERVER  SSL_CA
      SEC_OID_EXT_KEY_USAGE_CLIENT_AUTH   SSL_CLIENT  SSL_CA
      SEC_OID_EXT_KEY_USAGE_CODE_SIGN     OBJECT_SIGNING  OBJECT_SIGNING_CA
      SEC_OID_EXT_KEY_USAGE_TIME_STAMP    TIME_STAMP  TIME_STAMP
      SEC_OID_OCSP_RESPONDER          OCSP_RESPONDER  OCSP_RESPONDER

      SEC_OID_NS_KEY_USAGE_GOVT_APPROVED  GOVT_APPROVED   GOVT_APPROVED

      If the extended key usage extension is absent, the cert is assumed to have
      the cert types SSL_CLIENT, SSL_SERVER and EMAIL, and if the cert is a CA
      cert (as indicated by the presence of a true basic constraints extension),
      the cert is also assumed to have the cert types SSL_CA, EMAIL_CA and
      STATUS_RESPONDER.  If the basic constraints extension is missing, but the
      user has trusted the cert as a CA cert, the cert also gets the
      STATUS_RESPONDER cert type.  If the cert has a Fortezza type public key
      with the magic bits that signify that it is a CA, it is given cert types
      SSL_CA and EMAIL_CA.

      A cert with the extended key usage extension and the Netscape cert type
      extension that has the cert type SSL_CLIENT and also has an email address
      in the subject is also given the cert type EMAIL.  This allows all SSL
      client authentication certs with email addresses to also be used as email
      certs (provded they have adequate key usage).

      A cert with the extended key usage extension  and the Netscape cert type
      extension that as cert type SSL_CA is also always given cert type EMAIL_CA.
      This allows all SSL intermediate CAs to also be used as email intermediate CAs.

      /* X.509 v3 Key Usage Extension flags */
      #define KU_DIGITAL_SIGNATURE            (0x80)  /* bit 0 */
      #define KU_NON_REPUDIATION              (0x40)  /* bit 1 */
      #define KU_KEY_ENCIPHERMENT             (0x20)  /* bit 2 */
      #define KU_DATA_ENCIPHERMENT            (0x10)  /* bit 3 */
      #define KU_KEY_AGREEMENT                (0x08)  /* bit 4 */
      #define KU_KEY_CERT_SIGN                (0x04)  /* bit 5 */
      #define KU_CRL_SIGN                     (0x02)  /* bit 6 */

      #define NS_CERT_TYPE_SSL_CLIENT         (0x80)  /* bit 0 */
      #define NS_CERT_TYPE_SSL_SERVER         (0x40)  /* bit 1 */
      #define NS_CERT_TYPE_EMAIL              (0x20)  /* bit 2 */
      #define NS_CERT_TYPE_OBJECT_SIGNING     (0x10)  /* bit 3 */
      #define NS_CERT_TYPE_RESERVED           (0x08)  /* bit 4 */
      #define NS_CERT_TYPE_SSL_CA             (0x04)  /* bit 5 */
      #define NS_CERT_TYPE_EMAIL_CA           (0x02)  /* bit 6 */
      #define NS_CERT_TYPE_OBJECT_SIGNING_CA  (0x01)  /* bit 7 */
      </x>