<pre>Network Working Group                                           C. Adams
Request for Comments: 2025                        Bell-Northern Research
Category: Standards Track                                   October 1996


             <span class="h1">The Simple Public-Key GSS-API Mechanism (SPKM)</span>

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This specification defines protocols, procedures, and conventions to
   be employed by peers implementing the Generic Security Service
   Application Program Interface (as specified in RFCs 1508 and 1509)
   when using the Simple Public-Key Mechanism.

Background

   Although the Kerberos Version 5 GSS-API mechanism [KRB5] is becoming
   well-established in many environments, it is important in some
   applications to have a GSS-API mechanism which is based on a public-
   key, rather than a symmetric-key, infrastructure.  The mechanism
   described in this document has been proposed to meet this need and to
   provide the following features.

     1)  The SPKM allows both unilateral and mutual authentication
         to be accomplished without the use of secure timestamps.  This
         enables environments which do not have access to secure time
         to nevertheless have access to secure authentication.

     2)  The SPKM uses Algorithm Identifiers to specify various
         algorithms to be used by the communicating peers.  This allows
         maximum flexibility for a variety of environments, for future
         enhancements, and for alternative algorithms.

     3)  The SPKM allows the option of a true, asymmetric algorithm-
         based, digital signature in the gss_sign() and gss_seal()
         operations (now called gss_getMIC() and gss_wrap() in
         [GSSv2]), rather than an integrity checksum based on a MAC
         computed with a symmetric algorithm (e.g., DES).  For some
         environments, the availability of true digital signatures
         supporting non-repudiation is a necessity.



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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


     4)  SPKM data formats and procedures are designed to be as similar
         to those of the Kerberos mechanism as is practical.  This is
         done for ease of implementation in those environments where
         Kerberos has already been implemented.

   For the above reasons, it is felt that the SPKM will offer
   flexibility and functionality, without undue complexity or overhead.

Key Management

   The key management employed in SPKM is intended to be as compatible
   as possible with both X.509 [X.509] and PEM [<a href="./rfc1422">RFC-1422</a>], since these
   represent large communities of interest and show relative maturity in
   standards.

Acknowledgments

   Much of the material in this document is based on the Kerberos
   Version 5 GSS-API mechanism [KRB5], and is intended to be as
   compatible with it as possible.  This document also owes a great debt
   to Warwick Ford and Paul Van Oorschot of Bell-Northern Research for
   many fruitful discussions, to Kelvin Desplanque for implementation-
   related clarifications, to John Linn of OpenVision Technologies for
   helpful comments, and to Bancroft Scott of OSS for ASN.1 assistance.

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Overview</span>

   The goal of the Generic Security Service Application Program
   Interface (GSS-API) is stated in the abstract of [<a href="./rfc1508">RFC-1508</a>] as
   follows:

     "This Generic Security Service Application Program Interface (GSS-
     API) definition provides security services to callers in a generic
     fashion, supportable with a range of underlying mechanisms and
     technologies and hence allowing source-level portability of
     applications to different environments. This specification defines
     GSS-API services and primitives at a level independent of
     underlying mechanism and programming language environment, and is
     to be complemented by other, related specifications:

       - documents defining specific parameter bindings for particular
         language environments;

       - documents defining token formats, protocols, and procedures to
         be implemented in order to realize GSS-API services atop
         particular security mechanisms."





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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   The SPKM is an instance of the latter type of document and is
   therefore termed a "GSS-API Mechanism".  This mechanism provides
   authentication, key establishment, data integrity, and data
   confidentiality in an on-line distributed application environment
   using a public-key infrastructure.  Because it conforms to the
   interface defined by [<a href="./rfc1508">RFC-1508</a>], SPKM can be used as a drop-in
   replacement by any application which makes use of security services
   through GSS-API calls (for example, any application which already
   uses the Kerberos GSS-API for security).  The use of a public-key
   infrastructure allows digital signatures supporting non-repudiation
   to be employed for message exchanges, and provides other benefits
   such as scalability to large user populations.

   The tokens defined in SPKM are intended to be used by application
   programs according to the GSS API "operational paradigm" (see [RFC-
   1508] for further details):

     The operational paradigm in which GSS-API operates is as follows.
     A typical GSS-API caller is itself a communications protocol [or is
     an application program which uses a communications protocol],
     calling on GSS-API in order to protect its communications with
     authentication, integrity, and/or confidentiality security
     services.  A GSS-API caller accepts tokens provided to it by its
     local GSS-API implementation [i.e., its GSS-API mechanism] and
     transfers the tokens to a peer on a remote system; that peer passes
     the received tokens to its local GSS-API implementation for
     processing.

     This document defines two separate GSS-API mechanisms, SPKM-1 and
     SPKM-2, whose primary difference is that SPKM-2 requires the
     presence of secure timestamps for the purpose of replay detection
     during context establishment and SPKM-1 does not.  This allows
     greater flexibility for applications since secure timestamps cannot
     always be guaranteed to be available in a given environment.

















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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Algorithms</span>

   A number of algorithm types are employed in SPKM.  Each type, along
   with its purpose and a set of specific examples, is described in this
   section.  In order to ensure at least a minimum level of
   interoperability among various implementations of SPKM, one of the
   integrity algorithms is specified as MANDATORY; all remaining
   examples (and any other algorithms) may optionally be supported by a
   given SPKM implementation (note that a GSS-conformant mechanism need
   not support confidentiality).  Making a confidentiality algorithm
   mandatory may preclude exportability of the mechanism implementation;
   this document therefore specifies certain algorithms as RECOMMENDED
   (that is, interoperability will be enhanced if these algorithms are
   included in all SPKM implementations for which exportability is not a
   concern).

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a> Integrity Algorithm (I-ALG):</span>

         Purpose:

         This algorithm is used to ensure that a message has not been
         altered in any way after being constructed by the legitimate
         sender.  Depending on the algorithm used, the application of
         this algorithm may also provide authenticity and support non-
         repudiation for the message.

       Examples:

         md5WithRSAEncryption OBJECT IDENTIFIER ::= {
           iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
           pkcs-1(1) 4        -- imported from [PKCS1]
         }

            This algorithm (MANDATORY) provides data integrity and
            authenticity and supports non-repudiation by computing an
            RSA signature on the MD5 hash of that data.  This is
            essentially equivalent to md5WithRSA {1 3 14 3 2 3},
            which is defined by OIW (the Open Systems Environment
            Implementors' Workshop).

            Note that since this is the only integrity/authenticity
            algorithm specified to be mandatory at this time, for
            interoperability reasons it is also stipulated that
            md5WithRSA be the algorithm used to sign all context
            establishment tokens which are signed rather than MACed --
            see <a href="#section-3.1.1">Section 3.1.1</a> for details.  In future versions of this
            document, alternate or additional algorithms may be
            specified to be mandatory and so this stipulation on the



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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


            context establishment tokens may be removed.

         DES-MAC OBJECT IDENTIFIER ::= {
            iso(1) identified-organization(3) oiw(14) secsig(3)
            algorithm(2) 10  -- carries length in bits of the MAC as
         }                   -- an INTEGER parameter, constrained to
                             -- multiples of eight from 16 to 64

            This algorithm (RECOMMENDED) provides integrity by computing
            a DES MAC (as specified by [FIPS-113]) on that data.


         md5-DES-CBC OBJECT IDENTIFIER ::= {
            iso(1) identified-organization(3) dod(6) internet(1)
            security(5) integrity(3) md5-DES-CBC(1)
         }

            This algorithm provides data integrity by encrypting, using
            DES CBC, the "confounded" MD5 hash of that data (see <a href="#section-3.2.2.1">Section</a>
            <a href="#section-3.2.2.1">3.2.2.1</a> for the definition and purpose of confounding).
            This will typically be faster in practice than computing a
            DES MAC unless the input data is extremely short (e.g., a
            few bytes).  Note that without the confounder the strength
            of this integrity mechanism is (at most) equal to the
            strength of DES under a known-plaintext attack.


         sum64-DES-CBC OBJECT IDENTIFIER ::= {
            iso(1) identified-organization(3) dod(6) internet(1)
            security(5) integrity(3) sum64-DES-CBC(2)
         }

            This algorithm provides data integrity by encrypting, using
            DES CBC, the concatenation of the confounded data and the
            sum of all the input data blocks (the sum computed using
            addition modulo 2**64 - 1).  Thus, in this algorithm,
            encryption is a requirement for the integrity to be secure.

            For comments regarding the security of this integrity
            algorithm, see [Juen84, Davi89].











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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a> Confidentiality Algorithm (C-ALG):</span>

       Purpose:

         This symmetric algorithm is used to generate the encrypted
         data for gss_seal() / gss_wrap().

       Example:

         DES-CBC OBJECT IDENTIFIER ::= {
            iso(1) identified-organization(3) oiw(14) secsig(3)
            algorithm(2) 7 -- carries IV (OCTET STRING) as a parameter;
         }                 -- this (optional) parameter is unused in
                           -- SPKM due to the use of confounding

            This algorithm is RECOMMENDED.

<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a> Key Establishment Algorithm (K-ALG):</span>

       Purpose:

         This algorithm is used to establish a symmetric key for use
         by both the initiator and the target over the established
         context.  The keys used for C-ALG and any keyed I-ALGs (for
         example, DES-MAC) are derived from this context key.  As will
         be seen in <a href="#section-3.1">Section 3.1</a>, key establishment is done within the
         X.509 authentication exchange and so the resulting shared
         symmetric key is authenticated.

       Examples:

         RSAEncryption OBJECT IDENTIFIER ::= {
           iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
           pkcs-1(1) 1        -- imported from [PKCS1] and [<a href="./rfc1423">RFC-1423</a>]
         }

            In this algorithm (MANDATORY), the context key is generated
            by the initiator, encrypted with the RSA public key of the
            target, and sent to the target.  The target need not respond
            to the initiator for the key to be established.

         id-rsa-key-transport OBJECT IDENTIFIER ::= {
            iso(1) identified-organization(3) oiw(14) secsig(3)
            algorithm(2) 22   -- imported from [X9.44]
         }

            Similar to RSAEncryption, but source authenticating info.
            is also encrypted with the target's RSA public key.



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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


        dhKeyAgreement OBJECT IDENTIFIER ::= {
           iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
           pkcs-3(3) 1
        }

            In this algorithm, the context key is generated jointly by
            the initiator and the target using the Diffie-Hellman key
            establishment algorithm.  The target must therefore respond
            to the initiator for the key to be established (so this
            K-ALG cannot be used with unilateral authentication in
            SPKM-2 (see <a href="#section-3.1">Section 3.1</a>)).

<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a> One-Way Function (O-ALG) for Subkey Derivation Algorithm:</span>

       Purpose:

         Having established a context key using the negotiated K-ALG,
         both initiator and target must be able to derive a set of
         subkeys for the various C-ALGs and keyed I-ALGs supported over
         the context.  Let the (ordered) list of agreed C-ALGs be
         numbered consecutively, so that the first algorithm (the
         "default") is numbered "0", the next is numbered "1", and so
         on.  Let the numbering for the (ordered) list of agreed I-ALGs
         be identical.  Finally, let the context key be a binary string
         of arbitrary length "M", subject to the following constraint:
         L &lt;= M &lt;= U  (where the lower limit "L" is the bit length of
         the longest key needed by any agreed C-ALG or keyed I-ALG, and
         the upper limit "U" is the largest bit size which will fit
         within the K-ALG parameters).

         For example, if DES and two-key-triple-DES are the negotiated
         confidentiality algorithms and DES-MAC is the negotiated keyed
         integrity algorithm (note that digital signatures do not use a
         context key), then the context key must be at least 112 bits
         long.  If 512-bit RSAEncryption is the K-ALG in use then the
         originator can randomly generate a context key of any greater
         length up to 424 bits (the longest allowable RSA input
         specified in [PKCS-1]) -- the target can determine the length
         which was chosen by removing the padding bytes during the RSA
         decryption operation.  On the other hand, if dhKeyAgreement is
         the K-ALG in use then the context key is the result of the
         Diffie-Hellman computation (with the exception of the high-
         order byte, which is discarded for security reasons), so that
         its length is that of the Diffie-Hellman modulus, p, minus 8
         bits.






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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


         The derivation algorithm for a k-bit subkey is specified as
         follows:

      rightmost_k_bits (OWF(context_key || x || n || s || context_key))

         where

          - "x" is the ASCII character "C" (0x43) if the subkey is
            for a confidentiality algorithm or the ASCII character "I"
            (0x49) if the subkey is for a keyed integrity algorithm;
          - "n" is the number of the algorithm in the appropriate agreed
            list for the context (the ASCII character "0" (0x30), "1"
            (0x31), and so on);
          - "s" is the "stage" of processing -- always the ASCII
            character "0" (0x30), unless "k" is greater than the output
            size of OWF, in which case the OWF is computed repeatedly
            with increasing ASCII values of "stage" (each OWF output
            being concatenated to the end of previous OWF outputs),
            until "k" bits have been generated;
          - "||" is the concatenation operation; and
          - "OWF" is any appropriate One-Way Function.

       Examples:

         MD5 OBJECT IDENTIFIER ::= {
           iso(1) member-body(2) US(840) rsadsi(113549)
           digestAlgorithm(2) 5
         }

           This algorithm is MANDATORY.

         SHA OBJECT IDENTIFIER ::= {
            iso(1) identified-organization(3) oiw(14) secsig(3)
            algorithm(2) 18
         }

         It is recognized that existing hash functions may not satisfy
         all required properties of OWFs.  This is the reason for
         allowing negotiation of the O-ALG OWF during the context
         establishment process (see <a href="#section-2.5">Section 2.5</a>), since in this way
         future improvements in OWF design can easily be accommodated.
         For example, in some environments a preferred OWF technique
         might be an encryption algorithm which encrypts the input
         specified above using the context_key as the encryption key.







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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h3"><a class="selflink" id="section-2.5" href="#section-2.5">2.5</a> Negotiation:</span>

   During context establishment in SPKM, the initiator offers a set of
   possible confidentiality algorithms and a set of possible integrity
   algorithms to the target (note that the term "integrity algorithms"
   includes digital signature algorithms).  The confidentiality
   algorithms selected by the target become ones that may be used for
   C-ALG over the established context, and the integrity algorithms
   selected by the target become ones that may be used for I-ALG over
   the established context (the target "selects" algorithms by
   returning, in the same relative order, the subset of each offered
   list that it supports).  Note that any C-ALG and I-ALG may be used
   for any message over the context and that the first confidentiality
   algorithm and the first integrity algorithm in the agreed sets become
   the default algorithms for that context.

   The agreed confidentiality and integrity algorithms for a specific
   context define the valid values of the Quality of Protection (QOP)
   parameter used in the gss_getMIC() and gss_wrap() calls -- see
   <a href="#section-5.2">Section 5.2</a> for further details.  If no response is expected from the
   target (unilateral authentication in SPKM-2) then the algorithms
   offered by the initiator are the ones that may be used over the
   context (if this is unacceptable to the target then a delete token
   must be sent to the initiator so that the context is never
   established).

   Furthermore, in the first context establishment token the initiator
   offers a set of possible K-ALGs, along with the key (or key half)
   corresponding to the first algorithm in the set (its preferred
   algorithm).  If this K-ALG is unacceptable to the target then the
   target must choose one of the other K-ALGs in the set and send this
   choice along with the key (or key half) corresponding to this choice
   in its response (otherwise a delete token must be sent so that the
   context is never established).  If necessary (that is, if the target
   chooses a 2-pass K-ALG such as dhKeyAgreement), the initiator will
   send its key half in a response to the target.

   Finally, in the first context establishment token the initiator
   offers a set of possible O-ALGs (only a single O-ALG if no response
   is expected).  The (single) O-ALG chosen by the target becomes the
   subkey derivation algorithm OWF to be used over the context.

   In future versions of SPKM, other algorithms may be specified for any
   or all of I-ALG, C-ALG, K-ALG, and O-ALG.







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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Token Formats</span>

   This section discusses protocol-visible characteristics of the SPKM;
   it defines elements of protocol for interoperability and is
   independent of language bindings per [<a href="./rfc1509">RFC-1509</a>].

   The SPKM GSS-API mechanism will be identified by an Object Identifier
   representing "SPKM-1" or "SPKM-2", having the value {spkm spkm-1(1)}
   or {spkm spkm-2(2)}, where spkm has the value {iso(1) identified-
   organization(3) dod(6) internet(1) security(5) mechanisms(5)
   spkm(1)}.  SPKM-1 uses random numbers for replay detection during
   context establishment and SPKM-2 uses timestamps (note that for both
   mechanisms, sequence numbers are used to provide replay and out-of-
   sequence detection during the context, if this has been requested by
   the application).

   Tokens transferred between GSS-API peers (for security context
   management and per-message protection purposes) are defined.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. Context Establishment Tokens</span>

   Three classes of tokens are defined in this section:  "Initiator"
   tokens, emitted by calls to gss_init_sec_context() and consumed by
   calls to gss_accept_sec_context(); "Target" tokens, emitted by calls
   to gss_accept_sec_context() and consumed by calls to
   gss_init_sec_context(); and "Error" tokens, potentially emitted by
   calls to gss_init_sec_context() or gss_accept_sec_context(), and
   potentially consumed by calls to gss_init_sec_context() or
   gss_accept_sec_context().

   Per <a href="./rfc1508#appendix-B">RFC-1508, Appendix&nbsp;B</a>, the initial context establishment token
   will be enclosed within framing as follows:

   InitialContextToken ::= [APPLICATION 0] IMPLICIT SEQUENCE {
           thisMech           MechType,
                   -- MechType is OBJECT IDENTIFIER
                   -- representing "SPKM-1" or "SPKM-2"
           innerContextToken  ANY DEFINED BY thisMech
   }               -- contents mechanism-specific












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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   When thisMech is SPKM-1 or SPKM-2, innerContextToken is defined as
   follows:

      SPKMInnerContextToken ::= CHOICE {
         req    [0] SPKM-REQ,
         rep-ti [1] SPKM-REP-TI,
         rep-it [2] SPKM-REP-IT,
         error  [3] SPKM-ERROR,
         mic    [4] SPKM-MIC,
         wrap   [5] SPKM-WRAP,
         del    [6] SPKM-DEL
      }

   The above GSS-API framing shall be applied to all tokens emitted by
   the SPKM GSS-API mechanism, including SPKM-REP-TI (the response from
   the Target to the Initiator), SPKM-REP-IT (the response from the
   Initiator to the Target), SPKM-ERROR, context-deletion, and per-
   message tokens, not just to the initial token in a context
   establishment exchange.  While not required by <a href="./rfc1508">RFC-1508</a>, this enables
   implementations to perform enhanced error-checking.  The tag values
   provided in SPKMInnerContextToken ("[0]" through "[6]") specify a
   token-id for each token; similar information is contained in each
   token's tok-id field.  While seemingly redundant, the tag value and
   tok-id actually perform different tasks:  the tag ensures that
   InitialContextToken can be properly decoded; tok-id ensures, among
   other things, that data associated with the per-message tokens is
   cryptographically linked to the intended token type.  Every
   innerContextToken also includes a context-id field; see <a href="#section-6">Section 6</a> for
   a discussion of both token-id and context-id information and their
   use in an SPKM support function).

   The innerContextToken field of context establishment tokens for the
   SPKM GSS-API mechanism will contain one of the following messages:
   SPKM-REQ; SPKM-REP-TI; SPKM-REP-IT; and SPKM-ERROR.  Furthermore, all
   innerContextTokens are encoded using ASN.1 BER (constrained, in the
   interests of parsing simplicity, to the DER subset defined in
   [X.509], clause 8.7).

   The SPKM context establishment tokens are defined according to
   [X.509] <a href="#section-10">Section 10</a> and are compatible with [9798].  SPKM-1 (random
   numbers) uses <a href="#section-10.3">Section 10.3</a>, "Two-way Authentication", when performing
   unilateral authentication of the target to the initiator and uses
   <a href="#section-10.4">Section 10.4</a>, "Three-way Authentication", when mutual authentication
   is requested by the initiator.  SPKM-2 (timestamps) uses <a href="#section-10.2">Section</a>
   <a href="#section-10.2">10.2</a>, "One-way Authentication", when performing unilateral
   authentication of the initiator to the target and uses <a href="#section-10.3">Section 10.3</a>,
   "Two-way Authentication", when mutual authentication is requested by
   the initiator.



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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   The implication of the previous paragraph is that for SPKM-2
   unilateral authentication no negotiation of K-ALG can be done (the
   target either accepts the K-ALG and context key given by the
   initiator or disallows the context).  For SPKM-2 mutual or SPKM-1
   unilateral authentication some negotiation is possible, but the
   target can only choose among the one-pass K-ALGs offered by the
   initiator (or disallow the context).  Alternatively, the initiator
   can request that the target generate and transmit the context key.
   For SPKM-1 mutual authentication the target can choose any one- or
   two-pass K-ALG offered by the initiator and, again, can be requested
   to generate and transmit the context key.

   It is envisioned that typical use of SPKM-1 or SPKM-2 will involve
   mutual authentication.  Although unilateral authentication is
   available for both mechanisms, its use is not generally recommended.

<span class="h4"><a class="selflink" id="section-3.1.1" href="#section-3.1.1">3.1.1</a>. Context Establishment Tokens - Initiator (first token)</span>

   In order to accomplish context establishment, it may be necessary
   that both the initiator and the target have access to the other
   partys public-key certificate(s).  In some environments the initiator
   may choose to acquire all certificates and send the relevant ones to
   the target in the first token.  In other environments the initiator
   may request that the target send certificate data in its response
   token, or each side may individually obtain the certificate data it
   needs.  In any case, however, the SPKM implementation must have the
   ability to obtain certificates which correspond to a supplied Name.
   The actual mechanism to be used to achieve this is a local
   implementation matter and is therefore outside the scope of this
   specification.


   Relevant SPKM-REQ syntax is as follows (note that imports from other
   documents are given in <a href="#appendix-A">Appendix A</a>):

   SPKM-REQ ::= SEQUENCE {
           requestToken      REQ-TOKEN,
           certif-data [0]   CertificationData OPTIONAL,
           auth-data [1]     AuthorizationData OPTIONAL
              -- see [<a href="./rfc1510">RFC-1510</a>] for a discussion of auth-data
   }

   CertificationData ::= SEQUENCE {
           certificationPath [0]          CertificationPath OPTIONAL,
           certificateRevocationList [1]  CertificateList OPTIONAL
   }  -- at least one of the above shall be present





<span class="grey">Adams                       Standards Track                    [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   CertificationPath ::= SEQUENCE {
           userKeyId [0]         OCTET STRING OPTIONAL,
              -- identifier for user's public key
           userCertif [1]        Certificate OPTIONAL,
              -- certificate containing user's public key
           verifKeyId [2]        OCTET STRING OPTIONAL,
              -- identifier for user's public verification key
           userVerifCertif [3]   Certificate OPTIONAL,
              -- certificate containing user's public verification key
           theCACertificates [4] SEQUENCE OF CertificatePair OPTIONAL
   }          -- certification path from target to source

   Having separate verification fields allows different key pairs
   (possibly corresponding to different algorithms) to be used for
   encryption/decryption and signing/verification.  Presence of [0] or
   [1] and absence of [2] and [3] implies that the same key pair is to
   be used for enc/dec and verif/signing (note that this practice is not
   typically recommended).  Presence of [2] or [3] implies that a
   separate key pair is to be used for verif/signing, and so [0] or [1]
   must also be present.  Presence of [4] implies that at least one of
   [0], [1], [2], and [3] must also be present.

      REQ-TOKEN ::= SEQUENCE {
              req-contents     Req-contents,
              algId            AlgorithmIdentifier,
              req-integrity    Integrity  -- "token" is Req-contents
      }

      Integrity ::= BIT STRING
        -- If corresponding algId specifies a signing algorithm,
        -- "Integrity" holds the result of applying the signing procedure
        -- specified in algId to the BER-encoded octet string which results
        -- from applying the hashing procedure (also specified in algId) to
        -- the DER-encoded octets of "token".
        -- Alternatively, if corresponding algId specifies a MACing
        -- algorithm, "Integrity" holds the result of applying the MACing
        -- procedure specified in algId to the DER-encoded octets of
        -- "token" (note that for MAC, algId must be one of the integrity
        -- algorithms offered by the initiator with the appropriate subkey
        -- derived from the context key (see <a href="#section-2.4">Section 2.4</a>) used as the key
        -- input)

   It is envisioned that typical use of the Integrity field for each of
   REQ-TOKEN, REP-TI-TOKEN, and REP-IT-TOKEN will be a true digital
   signature, providing unilateral or mutual authentication along with
   replay protection, as required.  However, there are situations in
   which the MAC choice will be appropriate.  One example is the case in
   which the initiator wishes to remain anonymous (so that the first, or



<span class="grey">Adams                       Standards Track                    [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   first and third, token(s) will be MACed and the second token will be
   signed).  Another example is the case in which a previously
   authenticated, established, and cached context is being re-
   established at some later time (here all exchanged tokens will be
   MACed).

   The primary advantage of the MAC choice is that it reduces processing
   overhead for cases in which either authentication is not required
   (e.g., anonymity) or authentication is established by some other
   means (e.g., ability to form the correct MAC on a "fresh" token in
   context re-establishment).

   Req-contents ::= SEQUENCE {
           tok-id           INTEGER (256),    -- shall contain 0100(hex)
           context-id       Random-Integer,   -- see <a href="#section-6.3">Section 6.3</a>
           pvno             BIT STRING,       -- protocol version number
           timestamp        UTCTime OPTIONAL, -- mandatory for SPKM-2
           randSrc          Random-Integer,
           targ-name        Name,
           src-name [0]     Name OPTIONAL,
              -- must be supplied unless originator is "anonymous"
           req-data         Context-Data,
           validity [1]     Validity OPTIONAL,
              -- validity interval for key (may be used in the
              -- computation of security context lifetime)
           key-estb-set     Key-Estb-Algs,
              -- specifies set of key establishment algorithms
           key-estb-req      BIT STRING OPTIONAL,
              -- key estb. parameter corresponding to first K-ALG in set
              -- (not used if initiator is unable or unwilling to
              -- generate and securely transmit key material to target).
              -- Established key must satisfy the key length constraints
              -- specified in <a href="#section-2.4">Section 2.4</a>.
           key-src-bind      OCTET STRING OPTIONAL
              -- Used to bind the source name to the symmetric key.
              -- This field must be present for the case of SPKM-2
              -- unilateral authen. if the K-ALG in use does not provide
              -- such a binding (but is optional for all other cases).
              -- The octet string holds the result of applying the
              -- mandatory hashing procedure MD5 (in MANDATORY I-ALG;
              -- see <a href="#section-2.1">Section 2.1</a>) as follows:  MD5(src || context_key),
              -- where "src" is the DER-encoded octets of src-name,
              -- "context-key" is the symmetric key (i.e., the
              -- unprotected version of what is transmitted in
              -- key-estb-req), and "||" is the concatenation operation.
           }





<span class="grey">Adams                       Standards Track                    [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   -- The protocol version number (pvno) parameter is a BIT STRING which
   -- uses as many bits as necessary to specify all the SPKM protocol
   -- versions supported by the initiator (one bit per protocol
   -- version).  The protocol specified by this document is version 0.
   -- Bit 0 of pvno is therefore set if this version is supported;
   -- similarly, bit 1 is set if version 1 (if defined in the future) is
   -- supported, and so on.  Note that for unilateral authentication
   -- using SPKM-2, no response token is expected during context
   -- establishment, so no protocol negotiation can take place; in this
   -- case, the initiator must set exactly one bit of pvno.  The version
   -- of REQ-TOKEN must correspond to the highest bit set in pvno.
   -- The "validity" parameter above is the only way within SPKM for
   -- the initiator to transmit desired context lifetime to the target.
   -- Since it cannot be guaranteed that the initiator and target have
   -- synchronized time, the span of time specified by "validity" is to
   -- be taken as definitive (rather than the actual times given in this
   -- parameter).

   Random-Integer ::= BIT STRING

   -- Each SPKM implementation is responsible for generating a "fresh"
   -- random number for the purpose of context establishment; that is,
   -- one which (with high probability) has not been used previously.
   -- There are no cryptographic requirements on this random number
   -- (i.e., it need not be unpredictable, it simply needs to be fresh).

   Context-Data ::= SEQUENCE {
           channelId       ChannelId OPTIONAL, -- channel bindings
           seq-number      INTEGER OPTIONAL,   -- sequence number
           options         Options,
           conf-alg        Conf-Algs,          -- confidentiality. algs.
           intg-alg        Intg-Algs,          -- integrity algorithm
           owf-alg         OWF-Algs            -- for subkey derivation
   }

   ChannelId ::= OCTET STRING

   Options ::= BIT STRING {
           delegation-state (0),
           mutual-state (1),
           replay-det-state (2), -- used for replay det. during context
           sequence-state (3),   -- used for sequencing during context
           conf-avail (4),
           integ-avail (5),
           target-certif-data-required (6)
                                 -- used to request targ's certif. data
   }




<span class="grey">Adams                       Standards Track                    [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   Conf-Algs ::= CHOICE {
           algs [0]        SEQUENCE OF AlgorithmIdentifier,
           null [1]        NULL
            -- used when conf. is not available over context
   } -- for C-ALG (see <a href="#section-5.2">Section 5.2</a> for discussion of QOP)

   Intg-Algs ::= SEQUENCE OF AlgorithmIdentifier
       -- for I-ALG (see <a href="#section-5.2">Section 5.2</a> for discussion of QOP)

   OWF-Algs ::= SEQUENCE OF AlgorithmIdentifier
       -- Contains exactly one algorithm in REQ-TOKEN for SPKM-2
       -- unilateral, and contains at least one algorithm otherwise.
       -- Always contains exactly one algorithm in REP-TOKEN.

   Key-Estb-Algs ::= SEQUENCE OF AlgorithmIdentifier
       -- to allow negotiation of K-ALG

   A context establishment sequence based on the SPKM will perform
   unilateral authentication if the mutual-req bit is not set in the
   application's call to gss_init_sec_context().  SPKM-2 accomplishes
   this using only SPKM-REQ (thereby authenticating the initiator to the
   target), while SPKM-1 accomplishes this using both SPKM-REQ and
   SPKM-REP-TI (thereby authenticating the target to the initiator).

   Applications requiring authentication of both peers (initiator as
   well as target) must request mutual authentication, resulting in
   "mutual-state" being set within SPKM-REQ Options.  In response to
   such a request, the context target will reply to the initiator with
   an SPKM-REP-TI token.  If mechanism SPKM-2 has been chosen, this
   completes the (timestamp-based) mutual authentication context
   establishment exchange.  If mechanism SPKM-1 has been chosen and
   SPKM-REP-TI is sent, the initiator will then reply to the target with
   an SPKM-REP-IT token, completing the (random-number-based) mutual
   authentication context establishment exchange.

   Other bits in the Options field of Context-Data are explained in
   <a href="./rfc1508">RFC-1508</a>, with the exception of target-certif-data-required, which
   the initiator sets to TRUE to request that the target return its
   certification data in the SPKM-REP-TI token.  For unilateral
   authentication in SPKM-2 (in which no SPKM-REP-TI token is
   constructed), this option bit is ignored by both initiator and
   target.









<span class="grey">Adams                       Standards Track                    [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h4"><a class="selflink" id="section-3.1.2" href="#section-3.1.2">3.1.2</a>. Context Establishment Tokens - Target</span>

   SPKM-REP-TI ::= SEQUENCE {
           responseToken    REP-TI-TOKEN,
           certif-data      CertificationData OPTIONAL
             -- included if target-certif-data-required option was
             -- set to TRUE in SPKM-REQ
   }


   REP-TI-TOKEN ::= SEQUENCE {
           rep-ti-contents Rep-ti-contents,
           algId           AlgorithmIdentifier,
           rep-ti-integ    Integrity  -- "token" is Rep-ti-contents
   }

   Rep-ti-contents ::= SEQUENCE {
           tok-id           INTEGER (512),   -- shall contain 0200 (hex)
           context-id       Random-Integer,  -- see <a href="#section-6.3">Section 6.3</a>
           pvno [0]         BIT STRING OPTIONAL, -- prot. version number
           timestamp        UTCTime OPTIONAL, -- mandatory for SPKM-2
           randTarg         Random-Integer,
           src-name [1]     Name OPTIONAL,
             -- must contain whatever value was supplied in REQ-TOKEN
           targ-name        Name,
           randSrc          Random-Integer,
           rep-data         Context-Data,
           validity [2]     Validity  OPTIONAL,
             -- validity interval for key (used if the target can only
             -- support a shorter context lifetime than was offered in
             -- REQ-TOKEN)
           key-estb-id      AlgorithmIdentifier OPTIONAL,
             -- used if target is changing key estb. algorithm (must be
             -- a member of initiators key-estb-set)
           key-estb-str      BIT STRING OPTIONAL
             -- contains (1) the response to the initiators
             -- key-estb-req (if init. used a 2-pass K-ALG), or (2) the
             -- key-estb-req corresponding to the K-ALG supplied in
             -- above key-estb-id, or (3) the key-estb-req corresponding
             -- to the first K-ALG supplied in initiator's key-estb-id,
             -- if initiator's (OPTIONAL) key-estb-req was not used
             -- (target's key-estb-str must be present in this case).
             -- Established key must satisfy the key length constraints
             -- specified in <a href="#section-2.4">Section 2.4</a>.
           }






<span class="grey">Adams                       Standards Track                    [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   The protocol version number (pvno) parameter is a BIT STRING which
   uses as many bits as necessary to specify a single SPKM protocol
   version offered by the initiator which is supported by the target
   (one bit per protocol version); that is, the target sets exactly one
   bit of pvno.  If none of the versions offered by the initiator are
   supported by the target, a delete token must be returned so that the
   context is never established.  If the initiator's pvno has only one
   bit set and the target happens to support this protocol version, then
   this version is used over the context and the pvno parameter of REP-
   TOKEN can be omitted.  Finally, if the initiator and target do have
   one or more versions in common but the version of the REQ-TOKEN
   received is not supported by the target, a REP-TOKEN must be sent
   with a desired version bit set in pvno (and dummy values used for all
   subsequent token fields).  The initiator can then respond with a new
   REQ-TOKEN of the proper version (essentially starting context
   establishment anew).

<span class="h4"><a class="selflink" id="section-3.1.3" href="#section-3.1.3">3.1.3</a>. Context Establishment Tokens - Initiator (second token)</span>

   Relevant SPKM-REP-IT syntax is as follows:

   SPKM-REP-IT ::= SEQUENCE {
           responseToken    REP-IT-TOKEN,
           algId            AlgorithmIdentifier,
           rep-it-integ     Integrity  -- "token" is REP-IT-TOKEN
   }

   REP-IT-TOKEN ::= SEQUENCE {
           tok-id           INTEGER (768), -- shall contain 0300 (hex)
           context-id       Random-Integer,
           randSrc          Random-Integer,
           randTarg         Random-Integer,
           targ-name        Name,  -- the targ-name specified in REP-TI
           src-name         Name OPTIONAL,
             -- must contain whatever value was supplied in REQ-TOKEN
           key-estb-rep     BIT STRING OPTIONAL
                 -- contains the response to targets key-estb-str
                 -- (if target selected a 2-pass K-ALG)
           }

<span class="h4"><a class="selflink" id="section-3.1.4" href="#section-3.1.4">3.1.4</a>. Error Token</span>

   The syntax of SPKM-ERROR is as follows:

   SPKM-ERROR ::= SEQUENCE {
           error-token      ERROR-TOKEN,
           algId            AlgorithmIdentifier,
           integrity        Integrity  -- "token" is ERROR-TOKEN



<span class="grey">Adams                       Standards Track                    [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   }

   ERROR-TOKRN ::=   SEQUENCE {
           tok-id           INTEGER (1024), -- shall contain 0400 (hex)
           context-id       Random-Integer
           }

   The SPKM-ERROR token is used only during the context establishment
   process.  If an SPKM-REQ or SPKM-REP-TI token is received in error,
   the receiving function (either gss_init_sec_context() or
   gss_accept_sec_context()) will generate an SPKM-ERROR token to be
   sent to the peer (if the peer is still in the context establishment
   process) and will return GSS_S_CONTINUE_NEEDED.  If, on the other
   hand, no context establishment response is expected from the peer
   (i.e., the peer has completed context establishment), the function
   will return the appropriate major status code (e.g., GSS_S_BAD_SIG)
   along with a minor status of GSS_SPKM_S_SG_CONTEXT_ESTB_ABORT and all
   context-relevant information will be deleted.  The output token will
   not be an SPKM-ERROR token but will instead be an SPKM-DEL token
   which will be processed by the peer's gss_process_context_token().

   If gss_init_sec_context() receives an error token (whether valid or
   invalid), it will regenerate SPKM-REQ as its output token and return
   a major status code of GSS_S_CONTINUE_NEEDED.  (Note that if the
   peer's gss_accept_sec_context() receives SPKM-REQ token when it is
   expecting a SPKM-REP-IT token, it will ignore SPKM-REQ and return a
   zero-length output token with a major status of
   GSS_S_CONTINUE_NEEDED.)

   Similarly, if gss_accept_sec_context() receives an error token
   (whether valid or invalid), it will regenerate SPKM-REP-TI as its
   output token and return a major status code of GSS_S_CONTINUE_NEEDED.

   md5WithRsa is currently stipulated for the signing of context
   establishment tokens.  Discrepancies involving modulus bitlength can
   be resolved through judicious use of the SPKM-ERROR token.  The
   context initiator signs REQ-TOKEN using the strongest RSA it supports
   (e.g., 1024 bits).  If the target is unable to verify signatures of
   this length, it sends SPKM-ERROR signed with the strongest RSA that
   it supports (e.g. 512).

   At the completion of this exchange, both sides know what RSA
   bitlength the other supports, since the size of the signature is
   equal to the size of the modulus.  Further exchanges can be made
   (using successively smaller supported bitlengths) until either an
   agreement is reached or context establishment is aborted because no
   agreement is possible.




<span class="grey">Adams                       Standards Track                    [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. Per-Message and Context Deletion Tokens</span>

   Three classes of tokens are defined in this section: "MIC" tokens,
   emitted by calls to gss_getMIC() and consumed by calls to
   gss_verifyMIC(); "Wrap" tokens, emitted by calls to gss_wrap() and
   consumed by calls to gss_unwrap(); and context deletion tokens,
   emitted by calls to gss_init_sec_context(), gss_accept_sec_context(),
   or gss_delete_sec_context() and consumed by calls to
   gss_process_context_token().

<span class="h4"><a class="selflink" id="section-3.2.1" href="#section-3.2.1">3.2.1</a>. Per-message Tokens - Sign / MIC</span>

   Use of the gss_sign() / gss_getMIC() call yields a token, separate
   from the user data being protected, which can be used to verify the
   integrity of that data as received.  The token and the data may be
   sent separately by the sending application and it is the receiving
   application's responsibility to associate the received data with the
   received token.

   The SPKM-MIC token has the following format:

   SPKM-MIC ::= SEQUENCE {
           mic-header       Mic-Header,
           int-cksum        BIT STRING
                                -- Checksum over header and data,
                                -- calculated according to algorithm
                                -- specified in int-alg field.
   }

   Mic-Header ::= SEQUENCE {
           tok-id           INTEGER (257),
                                -- shall contain 0101 (hex)
           context-id       Random-Integer,
           int-alg [0]      AlgorithmIdentifier OPTIONAL,
                                -- Integrity algorithm indicator (must
                                -- be one of the agreed integrity
                                -- algorithms for this context).
                                -- field not present = default id.
           snd-seq [1]      SeqNum OPTIONAL  -- sequence number field.
   }

   SeqNum ::= SEQUENCE {
           num      INTEGER, -- the sequence number itself
           dir-ind  BOOLEAN  -- a direction indicator
   }






<span class="grey">Adams                       Standards Track                    [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h5"><a class="selflink" id="section-3.2.1.1" href="#section-3.2.1.1">3.2.1.1</a>. Checksum</span>

   Checksum calculation procedure (common to all algorithms -- note that
   for SPKM the term "checksum" includes digital signatures as well as
   hashes and MACs): Checksums are calculated over the data field,
   logically prepended by the bytes of the plaintext token header (mic-
   header).  The result binds the data to the entire plaintext header,
   so as to minimize the possibility of malicious splicing.

   For example, if the int-alg specifies the md5WithRSA algorithm, then
   the checksum is formed by computing an MD5 [<a href="./rfc1321">RFC-1321</a>] hash over the
   plaintext data (prepended by the header), and then computing an RSA
   signature [PKCS1] on the 16-byte MD5 result.  The signature is
   computed using the RSA private key retrieved from the credentials
   structure and the result (whose length is implied by the "modulus"
   parameter in the private key) is stored in the int-cksum field.

   If the int-alg specifies a keyed hashing algorithm (for example,
   DES-MAC or md5-DES-CBC), then the key to be used is the appropriate
   subkey derived from the context key (see <a href="#section-2.4">Section 2.4</a>).  Again, the
   result (whose length is implied by int-alg) is stored in the int-
   cksum field.

<span class="h5"><a class="selflink" id="section-3.2.1.2" href="#section-3.2.1.2">3.2.1.2</a>. Sequence Number</span>

   It is assumed that the underlying transport layers (of whatever
   protocol stack is being used by the application) will provide
   adequate communications reliability (that is, non-malicious loss,
   re-ordering, etc., of data packets will be handled correctly).
   Therefore, sequence numbers are used in SPKM purely for security, as
   opposed to reliability, reasons (that is, to avoid malicious loss,
   replay, or re-ordering of SPKM tokens) -- it is therefore recommended
   that applications request sequencing and replay detection over all
   contexts.  Note that sequence numbers are used so that there is no
   requirement for secure timestamps in the message tokens.  The
   initiator's initial sequence number for the current context may be
   explicitly given in the Context-Data field of SPKM-REQ and the
   target's initial sequence number may be explicitly given in the
   Context-Data field of SPKM-REP-TI; if either of these is not given
   then the default value of 00 is to be used.

   Sequence number field: The sequence number field is formed from the
   sender's four-byte sequence number and a Boolean direction-indicator
   (FALSE - sender is the context initiator, TRUE - sender is the
   context acceptor).  After constructing a gss_sign/getMIC() or
   gss_seal/wrap() token, the sender's seq. number is incremented by 1.





<span class="grey">Adams                       Standards Track                    [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h5"><a class="selflink" id="section-3.2.1.3" href="#section-3.2.1.3">3.2.1.3</a>. Sequence Number Processing</span>

   The receiver of the token will verify the sequence number field by
   comparing the sequence number with the expected sequence number and
   the direction indicator with the expected direction indicator.  If
   the sequence number in the token is higher than the expected number,
   then the expected sequence number is adjusted and GSS_S_GAP_TOKEN is
   returned.  If the token sequence number is lower than the expected
   number, then the expected sequence number is not adjusted and
   GSS_S_DUPLICATE_TOKEN, GSS_S_UNSEQ_TOKEN, or GSS_S_OLD_TOKEN is
   returned, whichever is appropriate.  If the direction indicator is
   wrong, then the expected sequence number is not adjusted and
   GSS_S_UNSEQ_TOKEN is returned.

   Since the sequence number is used as part of the input to the
   integrity checksum, sequence numbers need not be encrypted, and
   attempts to splice a checksum and sequence number from different
   messages will be detected.  The direction indicator will detect
   tokens which have been maliciously reflected.

<span class="h4"><a class="selflink" id="section-3.2.2" href="#section-3.2.2">3.2.2</a>. Per-message Tokens - Seal / Wrap</span>

   Use of the gss_seal() / gss_wrap() call yields a token which
   encapsulates the input user data (optionally encrypted) along with
   associated integrity check quantities. The token emitted by
   gss_seal() / gss_wrap() consists of an integrity header followed by a
   body portion that contains either the plaintext data (if conf-alg =
   NULL) or encrypted data (using the appropriate subkey specified in
   <a href="#section-2.4">Section 2.4</a> for one of the agreed C-ALGs for this context).

   The SPKM-WRAP token has the following format:

   SPKM-WRAP ::= SEQUENCE {
           wrap-header       Wrap-Header,
           wrap-body         Wrap-Body
   }

   Wrap-Header ::= SEQUENCE {
           tok-id           INTEGER (513),
                                -- shall contain 0201 (hex)
           context-id       Random-Integer,
           int-alg [0]      AlgorithmIdentifier OPTIONAL,
                                -- Integrity algorithm indicator (must
                                -- be one of the agreed integrity
                                -- algorithms for this context).
                                -- field not present = default id.





<span class="grey">Adams                       Standards Track                    [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


           conf-alg [1]     Conf-Alg OPTIONAL,
                                -- Confidentiality algorithm indicator
                                -- (must be NULL or one of the agreed
                                -- confidentiality algorithms for this
                                -- context).
                                -- field not present = default id.
                                -- NULL = none (no conf. applied).
           snd-seq [2]      SeqNum OPTIONAL
                                -- sequence number field.
   }



   Wrap-Body ::= SEQUENCE {
           int-cksum        BIT STRING,
                                -- Checksum of header and data,
                                -- calculated according to algorithm
                                -- specified in int-alg field.
           data             BIT STRING
                                -- encrypted or plaintext data.
   }

   Conf-Alg ::= CHOICE {
           algId [0]        AlgorithmIdentifier,
           null [1]         NULL
   }


3.2.2.1: Confounding

   As in [KRB5], an 8-byte random confounder is prepended to the data to
   compensate for the fact that an IV of zero is used for encryption.
   The result is referred to as the "confounded" data field.

<span class="h5"><a class="selflink" id="section-3.2.2.2" href="#section-3.2.2.2">3.2.2.2</a>. Checksum</span>

   Checksum calculation procedure (common to all algorithms): Checksums
   are calculated over the plaintext data field, logically prepended by
   the bytes of the plaintext token header (wrap-header).  As with
   gss_sign() / gss_getMIC(), the result binds the data to the entire
   plaintext header, so as to minimize the possibility of malicious
   splicing.

   The examples for md5WithRSA and DES-MAC are exactly as specified in
   3.2.1.1.

   If int-alg specifies md5-DES-CBC and conf-alg specifies anything
   other than DES-CBC, then the checksum is computed according to



<span class="grey">Adams                       Standards Track                    [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   3.2.1.1 and the result is stored in int-cksum.  However, if conf-alg
   specifies DES-CBC then the encryption and the integrity are done as
   follows.  An MD5 [<a href="./rfc1321">RFC-1321</a>] hash is computed over the plaintext data
   (prepended by the header).  This 16-byte value is appended to the
   concatenation of the "confounded" data and 1-8 padding bytes (the
   padding is as specified in [KRB5] for DES-CBC).  The result is then
   CBC encrypted using the DES-CBC subkey (see <a href="#section-2.4">Section 2.4</a>) and placed
   in the "data" field of Wrap-Body.  The final two blocks of ciphertext
   (i.e., the encrypted MD5 hash) are also placed in the int-cksum field
   of Wrap-Body as the integrity checksum.

   If int-alg specifies sum64-DES-CBC then conf-alg must specify DES-CBC
   (i.e., confidentiality must be requested by the calling application
   or SPKM will return an error).  Encryption and integrity are done in
   a single pass using the DES-CBC subkey as follows.  The sum (modulo
   2**64 - 1) of all plaintext data blocks (prepended by the header) is
   computed.  This 8-byte value is appended to the concatenation of the
   "confounded" data and 1-8 padding bytes (the padding is as specified
   in [KRB5] for DES-CBC).  As above, the result is then CBC encrypted
   and placed in the "data" field of Wrap-Body. The final block of
   ciphertext (i.e., the encrypted sum) is also placed in the int-cksum
   field of Wrap-Body as the integrity checksum.

<span class="h5"><a class="selflink" id="section-3.2.2.3" href="#section-3.2.2.3">3.2.2.3</a> Sequence Number</span>

   Sequence numbers are computed and processed for gss_wrap() exactly as
   specified in 3.2.1.2 and 3.2.1.3.

3.2.2.4: Data Encryption

   The following procedure is followed unless (a) conf-alg is NULL (no
   encryption), or (b) conf-alg is DES-CBC and int-alg is md5-DES-CBC
   (encryption as specified in 3.2.2.2), or (c) int-alg is sum64-DES-CBC
   (encryption as specified in 3.2.2.2):

   The "confounded" data is padded and encrypted according to the
   algorithm specified in the conf-alg field.  The data is encrypted
   using CBC with an IV of zero.  The key used is the appropriate subkey
   derived from the established context key using the subkey derivation
   algorithm described in <a href="#section-2.4">Section 2.4</a> (this ensures that the subkey used
   for encryption and the subkey used for a separate, keyed integrity
   algorithm -- for example DES-MAC, but not sum64-DES-CBC -- are
   different).

<span class="h4"><a class="selflink" id="section-3.2.3" href="#section-3.2.3">3.2.3</a>. Context deletion token</span>

   The token emitted by gss_delete_sec_context() is based on the format
   for tokens emitted by gss_sign() / gss_getMIC().



<span class="grey">Adams                       Standards Track                    [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   The SPKM-DEL token has the following format:

   SPKM-DEL ::= SEQUENCE {
           del-header       Del-Header,
           int-cksum        BIT STRING
                                -- Checksum of header, calculated
                                -- according to algorithm specified
                                -- in int-alg field.
   }

   Del-Header ::= SEQUENCE {
           tok-id           INTEGER (769),
                                -- shall contain 0301 (hex)
           context-id       Random-Integer,
           int-alg [0]      AlgorithmIdentifier OPTIONAL,
                                -- Integrity algorithm indicator (must
                                -- be one of the agreed integrity
                                -- algorithms for this context).
                                -- field not present = default id.
           snd-seq [1]      SeqNum OPTIONAL
                                -- sequence number field.
   }

   The field snd-seq will be calculated as for tokens emitted by
   gss_sign() / gss_getMIC().  The field int-cksum will be calculated as
   for tokens emitted by gss_sign() / gss_getMIC(), except that the
   user-data component of the checksum data will be a zero-length
   string.

   If a valid delete token is received, then the SPKM implementation
   will delete the context and gss_process_context_token() will return a
   major status of GSS_S_COMPLETE and a minor status of
   GSS_SPKM_S_SG_CONTEXT_DELETED.  If, on the other hand, the delete
   token is invalid, the context will not be deleted and
   gss_process_context_token() will return the appropriate major status
   (GSS_S_BAD_SIG, for example) and a minor status of
   GSS_SPKM_S_SG_BAD_DELETE_TOKEN_RECD.  The application may wish to
   take some action at this point to check the context status (such as
   sending a sealed/wrapped test message to its peer and waiting for a
   sealed/wrapped response).

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Name Types and Object Identifiers</span>

   No mandatory name forms have yet been defined for SPKM.  This section
   is for further study.






<span class="grey">Adams                       Standards Track                    [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Optional Name Forms</span>

   This section discusses name forms which may optionally be supported
   by implementations of the SPKM GSS-API mechanism.  It is recognized
   that OS-specific functions outside GSS-API are likely to exist in
   order to perform translations among these forms, and that GSS-API
   implementations supporting these forms may themselves be layered atop
   such OS-specific functions.  Inclusion of this support within GSS-API
   implementations is intended as a convenience to applications.

<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>. User Name Form</span>

   This name form shall be represented by the Object Identifier {iso(1)
   member-body(2) United States(840) mit(113554) infosys(1) gssapi(2)
   generic(1) user_name(1)}.  The recommended symbolic name for this
   type is "GSS_SPKM_NT_USER_NAME".

   This name type is used to indicate a named user on a local system.
   Its interpretation is OS-specific.  This name form is constructed as:

      username

<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>. Machine UID Form</span>

   This name form shall be represented by the Object Identifier {iso(1)
   member-body(2) United States(840) mit(113554) infosys(1) gssapi(2)
   generic(1) machine_uid_name(2)}.  The recommended symbolic name for
   this type is "GSS_SPKM_NT_MACHINE_UID_NAME".

   This name type is used to indicate a numeric user identifier
   corresponding to a user on a local system.  Its interpretation is
   OS-specific.  The gss_buffer_desc representing a name of this type
   should contain a locally-significant uid_t, represented in host byte
   order.  The gss_import_name() operation resolves this uid into a
   username, which is then treated as the User Name Form.

<span class="h4"><a class="selflink" id="section-4.1.3" href="#section-4.1.3">4.1.3</a>. String UID Form</span>

   This name form shall be represented by the Object Identifier {iso(1)
   member-body(2) United States(840) mit(113554) infosys(1) gssapi(2)
   generic(1) string_uid_name(3)}.  The recommended symbolic name for
   this type is "GSS_SPKM_NT_STRING_UID_NAME".

   This name type is used to indicate a string of digits representing
   the numeric user identifier of a user on a local system.  Its
   interpretation is OS-specific. This name type is similar to the
   Machine UID Form, except that the buffer contains a string
   representing the uid_t.



<span class="grey">Adams                       Standards Track                    [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Parameter Definitions</span>

   This section defines parameter values used by the SPKM GSS-API
   mechanism.  It defines interface elements in support of portability.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Minor Status Codes</span>

   This section recommends common symbolic names for minor_status values
   to be returned by the SPKM GSS-API mechanism.  Use of these
   definitions will enable independent implementors to enhance
   application portability across different implementations of the
   mechanism defined in this specification.  (In all cases,
   implementations of gss_display_status() will enable callers to
   convert minor_status indicators to text representations.) Each
   implementation must make available, through include files or other
   means, a facility to translate these symbolic names into the concrete
   values which a particular GSS-API implementation uses to represent
   the minor_status values specified in this section.  It is recognized
   that this list may grow over time, and that the need for additional
   minor_status codes specific to particular implementations may arise.

<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>. Non-SPKM-specific codes (Minor Status Code MSB, bit 31, SET)</span>

<span class="h5"><a class="selflink" id="section-5.1.1.1" href="#section-5.1.1.1">5.1.1.1</a>. GSS-Related codes (Minor Status Code bit 30 SET)</span>

   GSS_S_G_VALIDATE_FAILED
       /* "Validation error" */
   GSS_S_G_BUFFER_ALLOC
       /* "Couldn't allocate gss_buffer_t data" */
   GSS_S_G_BAD_MSG_CTX
       /* "Message context invalid" */
   GSS_S_G_WRONG_SIZE
       /* "Buffer is the wrong size" */
   GSS_S_G_BAD_USAGE
       /* "Credential usage type is unknown" */
   GSS_S_G_UNAVAIL_QOP
       /* "Unavailable quality of protection specified" */

<span class="h5"><a class="selflink" id="section-5.1.1.2" href="#section-5.1.1.2">5.1.1.2</a>. Implementation-Related codes (Minor Status Code bit 30 OFF)</span>

   GSS_S_G_MEMORY_ALLOC
       /* "Couldn't perform requested memory allocation" */

<span class="h4"><a class="selflink" id="section-5.1.2" href="#section-5.1.2">5.1.2</a>. SPKM-specific-codes (Minor Status Code MSB, bit 31, OFF)</span>

   GSS_SPKM_S_SG_CONTEXT_ESTABLISHED
       /* "Context is already fully established" */
   GSS_SPKM_S_SG_BAD_INT_ALG_TYPE



<span class="grey">Adams                       Standards Track                    [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


       /* "Unknown integrity algorithm type in token" */
   GSS_SPKM_S_SG_BAD_CONF_ALG_TYPE
       /* "Unknown confidentiality algorithm type in token" */
   GSS_SPKM_S_SG_BAD_KEY_ESTB_ALG_TYPE
       /* "Unknown key establishment algorithm type in token" */
   GSS_SPKM_S_SG_CTX_INCOMPLETE
       /* "Attempt to use incomplete security context" */
   GSS_SPKM_S_SG_BAD_INT_ALG_SET
       /* "No integrity algorithm in common from offered set" */
   GSS_SPKM_S_SG_BAD_CONF_ALG_SET
       /* "No confidentiality algorithm in common from offered set" */
   GSS_SPKM_S_SG_BAD_KEY_ESTB_ALG_SET
       /* "No key establishment algorithm in common from offered set" */
   GSS_SPKM_S_SG_NO_PVNO_IN_COMMON
       /* "No protocol version number in common from offered set" */
   GSS_SPKM_S_SG_INVALID_TOKEN_DATA
       /* "Data is improperly formatted:  cannot encode into token" */
   GSS_SPKM_S_SG_INVALID_TOKEN_FORMAT
       /* "Received token is improperly formatted:  cannot decode" */
   GSS_SPKM_S_SG_CONTEXT_DELETED
       /* "Context deleted at peer's request" */
   GSS_SPKM_S_SG_BAD_DELETE_TOKEN_RECD
       /* "Invalid delete token received -- context not deleted" */
   GSS_SPKM_S_SG_CONTEXT_ESTB_ABORT
      /* "Unrecoverable context establishment error. Context deleted" */

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Quality of Protection Values</span>

   The Quality of Protection (QOP) parameter is used in the SPKM GSS-API
   mechanism as input to gss_sign() and gss_seal() (gss_getMIC() and
   gss_wrap()) to select among alternate confidentiality and integrity-
   checking algorithms.  Once these sets of algorithms have been agreed
   upon by the context initiator and target, the QOP parameter simply
   selects from these ordered sets.

   More specifically, the SPKM-REQ token sends an ordered sequence of
   Alg. IDs specifying integrity-checking algorithms supported by the
   initiator and an ordered sequence of Alg. IDs specifying
   confidentiality algorithms supported by the initiator.  The target
   returns the subset of the offered integrity-checking Alg. IDs which
   it supports and the subset of the offered confidentiality Alg. IDs
   which it supports in the SPKM-REP-TI token (in the same relative
   orders as those given by the initiator).  Thus, the initiator and
   target each know the algorithms which they themselves support and the
   algorithms which both sides support (the latter are defined to be
   those supported over the established context).  The QOP parameter has
   meaning and validity with reference to this knowledge.  For example,
   an application may request integrity algorithm number 3 as defined by



<span class="grey">Adams                       Standards Track                    [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   the mechanism specification.  If this algorithm is supported over
   this context then it is used; otherwise, GSS_S_FAILURE and an
   appropriate minor status code are returned.

   If the SPKM-REP-TI token is not used (unilateral authentication using
   SPKM-2), then the "agreed" sets of Alg. IDs are simply taken to be
   the initiator's sets (if this is unacceptable to the target then it
   must return an error token so that the context is never established).
   Note that, in the interest of interoperability, the initiator is not
   required to offer every algorithm it supports; rather, it may offer
   only the mandated/recommended SPKM algorithms since these are likely
   to be supported by the target.

   The QOP parameter for SPKM is defined to be a 32-bit unsigned integer
   (an OM_uint32) with the following bit-field assignments:

 Confidentiality                     Integrity
 31 (MSB)                         16 15                         (LSB) 0
------------------------------------|-----------------------------------
|  TS (5)  | U(3) | IA (4) | MA (4) |  TS (5)  | U(3) | IA (4) | MA(4) |
------------------------------------|-----------------------------------

   where

      TS is a 5-bit Type Specifier (a semantic qualifier whose value
      specifies the type of algorithm which may be used to protect the
      corresponding token -- see below for details);

      U is a 3-bit Unspecified field (available for future
      use/expansion);

      IA is a 4-bit field enumerating Implementation-specific
      Algorithms; and

      MA is a 4-bit field enumerating Mechanism-defined Algorithms.

   The interpretation of the QOP parameter is as follows (note that the
   same procedure is used for both the confidentiality and the integrity
   halves of the parameter).  The MA field is examined first.  If it is
   non-zero then the algorithm used to protect the token is the
   mechanism-specified algorithm corresponding to that integer value.

   If MA is zero then IA is examined.  If this field value is non-zero
   then the algorithm used to protect the token is the implementation-
   specified algorithm corresponding to that integer value (if this
   algorithm is available over the established context).  Note that use
   of this field may hinder portability since a particular value may
   specify one algorithm in one implementation of the mechanism and may



<span class="grey">Adams                       Standards Track                    [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   not be supported or may specify a completely different algorithm in
   another implementation of the mechanism.

   Finally, if both MA and IA are zero then TS is examined.  A value of
   zero for TS specifies the default algorithm for the established
   context, which is defined to be the first algorithm on the
   initiator's list of offered algorithms (confidentiality or integrity,
   depending on which half of QOP is being examined) which is supported
   over the context.  A non-zero value for TS corresponds to a
   particular algorithm qualifier and selects the first algorithm
   supported over the context which satisfies that qualifier.

   The following TS values (i.e., algorithm qualifiers) are specified;
   other values may be added in the future.

      For the Confidentiality TS field:

         00001 (1) = SPKM_SYM_ALG_STRENGTH_STRONG
         00010 (2) = SPKM_SYM_ALG_STRENGTH_MEDIUM
         00011 (3) = SPKM_SYM_ALG_STRENGTH_WEAK

      For the Integrity TS field:

         00001 (1) = SPKM_INT_ALG_NON_REP_SUPPORT
         00010 (2) = SPKM_INT_ALG_REPUDIABLE

   Clearly, qualifiers such as strong, medium, and weak are debatable
   and likely to change with time, but for the purposes of this version
   of the specification we define these terms as follows.  A
   confidentiality algorithm is "weak" if the effective key length of
   the cipher is 40 bits or less; it is "medium-strength" if the
   effective key length is strictly between 40 and 80 bits; and it is
   "strong" if the effective key length is 80 bits or greater.  (Note
   that "effective key length" describes the computational effort
   required to break a cipher using the best-known cryptanalytic attack
   against that cipher.)

   A five-bit TS field allows up to 31 qualifiers for each of
   confidentiality and integrity (since "0" is reserved for "default").
   This document specifies three for confidentiality and two for
   integrity, leaving a lot of room for future specification.
   Suggestions of qualifiers such as "fast", "medium-speed", and "slow"
   have been made, but such terms are difficult to quantify (and in any
   case are platform- and processor-dependent), and so have been left
   out of this initial specification.  The intention is that the TS
   terms be quantitative, environment-independent qualifiers of
   algorithms, as much as this is possible.




<span class="grey">Adams                       Standards Track                    [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   Use of the QOP structure as defined above is ultimately meant to be
   as follows.

    - TS values are specified at the GSS-API level and are therefore
      portable across mechanisms.  Applications which know nothing about
      algorithms are still able to choose "quality" of protection for
      their message tokens.

    - MA values are specified at the mechanism level and are therefore
      portable across implementations of a mechanism.  For example, all
      implementations of the Kerberos V5 GSS mechanism must support

         GSS_KRB5_INTEG_C_QOP_MD5     (value: 1)
         GSS_KRB5_INTEG_C_QOP_DES_MD5 (value: 2)
         GSS_KRB5_INTEG_C_QOP_DES_MAC (value: 3).

      (Note that these Kerberos-specified integrity QOP values do not
      conflict with the QOP structure defined above.)

    - IA values are specified at the implementation level (in user
      documentation, for example) and are therefore typically non-
      portable.  An application which is aware of its own mechanism
      implementation and the mechanism implementation of its peer,
      however, is free to use these values since they will be perfectly
      valid and meaningful over that context and between those peers.

   The receiver of a token must pass back to its calling application a
   QOP parameter with all relevant fields set.  For example, if triple-
   DES has been specified by a mechanism as algorithm 8, then a receiver
   of a triple-DES-protected token must pass to its application (QOP
   Confidentiality TS=1, IA=0, MA=8).  In this way, the application is
   free to read whatever part of the QOP it understands (TS or IA/MA).

   To aid in implementation and interoperability, the following
   stipulation is made.  The set of integrity Alg. IDs sent by the
   initiator must contain at least one specifying an algorithm which
   computes a digital signature supporting non-repudiation, and must
   contain at least one specifying any other (repudiable) integrity
   algorithm.  The subset of integrity Alg. IDs returned by the target
   must also contain at least one specifying an algorithm which computes
   a digital signature supporting non-repudiation, and at least one
   specifying a repudiable integrity algorithm.

   The reason for this stipulation is to ensure that every SPKM
   implementation will provide an integrity service which supports non-
   repudiation and one which does not support non-repudiation.  An
   application with no knowledge of underlying algorithms can choose one
   or the other by passing (QOP Integrity TS=1, IA=MA=0) or (QOP



<span class="grey">Adams                       Standards Track                    [Page 31]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   Integrity TS=2, IA=MA=0).  Although an initiator who wishes to remain
   anonymous will never actually use the non-repudiable digital
   signature, this integrity service must be available over the context
   so that the target can use it if desired.

   Finally, in accordance with the MANDATORY and RECOMMENDED algorithms
   given in <a href="#section-2">Section 2</a>, the following QOP values are specified for SPKM.

   For the Confidentiality MA field:

      0001 (1) = DES-CBC

   For the Integrity MA field:

      0001 (1) = md5WithRSA
      0010 (2) = DES-MAC

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Support Functions</span>

   This section describes a mandatory support function for SPKM-
   conformant implementations which may, in fact, be of value in all
   GSS-API mechanisms.  It makes use of the token-id and context-id
   information which is included in SPKM context-establishment, error,
   context-deletion, and per-message tokens.  The function is defined in
   the following section.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. SPKM_Parse_token call</span>

   Inputs:

   o  input_token OCTET STRING

   Outputs:

   o  major_status INTEGER,

   o  minor_status INTEGER,

   o  mech_type OBJECT IDENTIFIER,

   o  token_type INTEGER,

   o  context_handle CONTEXT HANDLE,








<span class="grey">Adams                       Standards Track                    [Page 32]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-33" ></span>
<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   Return major_status codes:

   o  GSS_S_COMPLETE indicates that the input_token could be parsed for
      all relevant fields.  The resulting values are stored in
      mech_type, token_type and context_handle, respectively (with NULLs
      in any parameters which are not relevant).

   o  GSS_S_DEFECTIVE_TOKEN indicates that either the token-id or the
      context-id (if it was expected) information could not be parsed.
      A non-NULL return value in token_type indicates that the latter
      situation occurred.

   o  GSS_S_NO_TYPE indicates that the token-id information could be
      parsed, but it did not correspond to any valid token_type.

      (Note that this major status code has not been defined for GSS in
      <a href="./rfc1508">RFC-1508</a>.  Until such a definition is made (if ever), SPKM
      implementations should instead return GSS_S_DEFECTIVE_TOKEN with
      both token_type and context_handle set to NULL.  This essentially
      implies that unrecognized token-id information is considered to be
      equivalent to token-id information which could not be parsed.)

   o  GSS_S_NO_CONTEXT indicates that the context-id could be parsed,
      but it did not correspond to any valid context_handle.

   o  GSS_S_FAILURE indicates that the mechanism type could not be
      parsed (for example, the token may be corrupted).

   SPKM_Parse_token() is used to return to an application the mechanism
   type, token type, and context handle which correspond to a given
   input token.  Since GSS-API tokens are meant to be opaque to the
   calling application, this function allows the application to
   determine information about the token without having to violate the
   opaqueness intention of GSS.  Of primary importance is the token
   type, which the application can then use to decide which GSS function
   to call in order to have the token processed.

   If all tokens are framed as suggested in <a href="./rfc1508#appendix-B">RFC-1508, Appendix&nbsp;B</a>
   (specified both in the Kerberos V5 GSS mechanism [KRB5] and in this
   document), then any mechanism implementation should be able to return
   at least the mech_type parameter (the other parameters being NULL)
   for any uncorrupted input token.  If the mechanism implementation
   whose SPKM_Parse_token() function is being called does recognize the
   token, it can return token_type so that the application can
   subsequently call the correct GSS function.  Finally, if the
   mechanism provides a context-id field in its tokens (as SPKM does),
   then an implementation can map the context-id to a context_handle and
   return this to the application.  This is necessary for the situation



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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   where an application has multiple contexts open simultaneously, all
   using the same mechanism.  When an incoming token arrives, the
   application can use this function to determine not only which GSS
   function to call, but also which context_handle to use for the call.
   Note that this function does no cryptographic processing to determine
   the validity of tokens; it simply attempts to parse the mech_type,
   token_type, and context-id fields of any token it is given.  Thus, it
   is conceivable, for example, that an arbitrary buffer of data might
   start with random values which look like a valid mech_type and that
   SPKM_Parse_token() would return incorrect information if given this
   buffer.  While conceivable, however, such a situation is unlikely.

   The SPKM_Parse_token() function is mandatory for SPKM-conformant
   implementations, but it is optional for applications.  That is, if an
   application has only one context open and can guess which GSS
   function to call (or is willing to put up with some error codes),
   then it need never call SPKM_Parse_token().  Furthermore, if this
   function ever migrates up to the GSS-API level, then
   SPKM_Parse_token() will be deprecated at that time in favour of
   GSS_Parse_token(), or whatever the new name and function
   specification might be.  Note finally that no minor status return
   codes have been defined for this function at this time.

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. The token_type Output Parameter</span>

   The following token types are defined:

      GSS_INIT_TOKEN   = 1
      GSS_ACCEPT_TOKEN = 2
      GSS_ERROR_TOKEN  = 3
      GSS_SIGN_TOKEN   = GSS_GETMIC_TOKEN = 4
      GSS_SEAL_TOKEN   = GSS_WRAP_TOKEN   = 5
      GSS_DELETE_TOKEN = 6

   All SPKM mechanisms shall be able to perform the mapping from the
   token-id information which is included in every token (through the
   tag values in SPKMInnerContextToken or through the tok-id field) to
   one of the above token types.  Applications should be able to decide,
   on the basis of token_type, which GSS function to call (for example,
   if the token is a GSS_INIT_TOKEN then the application will call
   gss_accept_sec_context(), and if the token is a GSS_WRAP_TOKEN then
   the application will call gss_unwrap()).

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>. The context_handle Output Parameter</span>

   The SPKM mechanism implementation is responsible for maintaining a
   mapping between the context-id value which is included in every token
   and a context_handle, thus associating an individual token with its



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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   proper context.  Clearly the value of context_handle may be locally
   determined and may, in fact, be associated with memory containing
   sensitive data on the local system, and so having the context-id
   actually be set equal to a computed context_handle will not work in
   general.  Conversely, having the context_handle actually be set equal
   to a computed context-id will not work in general either, because
   context_handle must be returned to the application by the first call
   to gss_init_sec_context() or gss_accept_sec_context(), whereas
   uniqueness of the context-id (over all contexts at both ends) may
   require that both initiator and target be involved in the
   computation.  Consequently, context_handle and context-id must be
   computed separately and the mechanism implementation must be able to
   map from one to the other by the completion of context establishment
   at the latest.

   Computation of context-id during context establishment is
   accomplished as follows.  Each SPKM implementation is responsible for
   generating a "fresh" random number; that is, one which (with high
   probability) has not been used previously.  Note that there are no
   cryptographic requirements on this random number (i.e., it need not
   be unpredictable, it simply needs to be fresh).  The initiator passes
   its random number to the target in the context-id field of the SPKM-
   REQ token.  If no further context establishment tokens are expected
   (as for unilateral authentication in SPKM-2), then this value is
   taken to be the context-id (if this is unacceptable to the target
   then an error token must be generated).  Otherwise, the target
   generates its random number and concatenates it to the end of the
   initiator's random number.  This concatenated value is then taken to
   be the context-id and is used in SPKM-REP-TI and in all subsequent
   tokens over that context.

   Having both peers contribute to the context-id assures each peer of
   freshness and therefore precludes replay attacks between contexts
   (where a token from an old context between two peers is maliciously
   injected into a new context between the same or different peers).
   Such assurance is not available to the target in the case of
   unilateral authentication using SPKM-2, simply because it has not
   contributed to the freshness of the computed context-id (instead, it
   must trust the freshness of the initiator's random number, or reject
   the context).  The key-src-bind field in SPKM-REQ is required to be
   present for the case of SPKM-2 unilateral authentication precisely to
   assist the target in trusting the freshness of this token (and its
   proposed context key).

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Security Considerations</span>

   Security issues are discussed throughout this memo.




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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. References</span>

   [Davi89]:    D. W. Davies and W. L. Price, "Security for Computer
   Networks", Second Edition, John Wiley and Sons, New York, 1989.

   [FIPS-113]:  National Bureau of Standards, Federal Information
   Processing Standard 113, "Computer Data Authentication", May 1985.

   [GSSv2]:     Linn, J., "Generic Security Service Application Program
   Interface Version 2", Work in Progress.

   [Juen84]:    R. R. Jueneman, C. H. Meyer and S. M. Matyas, Message
   Authentication with Manipulation Detection Codes, in Proceedings of
   the 1983 IEEE Symposium on Security and Privacy, IEEE Computer
   Society Press, 1984, pp.33-54.

   [KRB5]:      Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
   <a href="./rfc1964">RFC 1964</a>, June 1996.

   [PKCS1]:     RSA Encryption Standard, Version 1.5, RSA Data Security,
   Inc., Nov. 1993.

   [PKCS3]:     Diffie-Hellman Key-Agreement Standard, Version 1.4, RSA
   Data Security, Inc., Nov. 1993.

   [<a href="./rfc1321">RFC-1321</a>]:  Rivest, R., "The MD5 Message-Digest Algorithm", <a href="./rfc1321">RFC 1321</a>.

   [<a href="./rfc1422">RFC-1422</a>]:  Kent, S., "Privacy Enhancement for Internet Electronic
   Mail:  Part II: Certificate-Based Key Management", <a href="./rfc1422">RFC 1422</a>.

   [<a href="./rfc1423">RFC-1423</a>]:  Balenson, D., "Privacy Enhancement for Internet
   Elecronic Mail: Part III: Algorithms, Modes, and Identifiers",
   <a href="./rfc1423">RFC 1423</a>.

   [<a href="./rfc1508">RFC-1508</a>]:  Linn, J., "Generic Security Service Application Program
   Interface", <a href="./rfc1508">RFC 1508</a>.

   [<a href="./rfc1509">RFC-1509</a>]:  Wray, J., "Generic Security Service Application Program
   Interface: C-bindings", <a href="./rfc1509">RFC 1509</a>.

   [<a href="./rfc1510">RFC-1510</a>]:  Kohl J., and C. Neuman, "The Kerberos Network
   Authentication Service (V5)", <a href="./rfc1510">RFC 1510</a>.

   [9798]:      ISO/IEC 9798-3, "Information technology - Security
   Techniques - Entity authentication mechanisms - Part 3:  Entitiy
   authentication using a public key algorithm", ISO/IEC, 1993.





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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   [X.501]:     ISO/IEC 9594-2, "Information Technology - Open Systems
   Interconnection - The Directory:  Models", CCITT/ITU Recommendation
   X.501, 1993.

   [X.509]:     ISO/IEC 9594-8, "Information Technology - Open Systems
   Interconnection - The Directory:  Authentication Framework",
   CCITT/ITU Recommendation X.509, 1993.

   [X9.44]:     ANSI, "Public Key Cryptography Using Reversible
    Algorithms for the Financial Services Industry:  Transport of
   Symmetric Algorithm Keys Using RSA", X9.44-1993.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. Author's Address</span>

   Carlisle Adams
   Bell-Northern Research
   P.O.Box 3511, Station C
   Ottawa, Ontario, CANADA  K1Y 4H7

   Phone: +1 613.763.9008
   EMail: cadams@bnr.ca






























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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


Appendix A:  ASN.1 Module Definition

SpkmGssTokens {iso(1) identified-organization(3) dod(6) internet(1)
               security(5) mechanisms(5) spkm(1) spkmGssTokens(10)}


DEFINITIONS IMPLICIT TAGS ::=
BEGIN


-- EXPORTS ALL --


IMPORTS

   Name
      FROM InformationFramework {joint-iso-ccitt(2) ds(5) module(1)
                                informationFramework(1) 2}

   Certificate, CertificateList, CertificatePair, AlgorithmIdentifier,
   Validity
      FROM AuthenticationFramework {joint-iso-ccitt(2) ds(5) module(1)
                                   authenticationFramework(7) 2}  ;



-- types --

   SPKM-REQ ::= SEQUENCE {
           requestToken      REQ-TOKEN,
           certif-data [0]   CertificationData OPTIONAL,
           auth-data [1]     AuthorizationData OPTIONAL
   }


   CertificationData ::= SEQUENCE {
           certificationPath [0]          CertificationPath OPTIONAL,
           certificateRevocationList [1]  CertificateList OPTIONAL
   } -- at least one of the above shall be present


   CertificationPath ::= SEQUENCE {
           userKeyId [0]         OCTET STRING OPTIONAL,
           userCertif [1]        Certificate OPTIONAL,
           verifKeyId [2]        OCTET STRING OPTIONAL,
           userVerifCertif [3]   Certificate OPTIONAL,
           theCACertificates [4] SEQUENCE OF CertificatePair OPTIONAL
   } -- Presence of [2] or [3] implies that [0] or [1] must also be



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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


     -- present.  Presence of [4] implies that at least one of [0], [1],
     -- [2], and [3] must also be present.

   REQ-TOKEN ::= SEQUENCE {
           req-contents     Req-contents,
           algId            AlgorithmIdentifier,
           req-integrity    Integrity  -- "token" is Req-contents
   }

  Integrity ::= BIT STRING
     -- If corresponding algId specifies a signing algorithm,
     -- "Integrity" holds the result of applying the signing procedure
     -- specified in algId to the BER-encoded octet string which results
     -- from applying the hashing procedure (also specified in algId) to
     -- the DER-encoded octets of "token".
     -- Alternatively, if corresponding algId specifies a MACing
     -- algorithm, "Integrity" holds the result of applying the MACing
     -- procedure specified in algId to the DER-encoded octets of
     -- "token"

   Req-contents ::= SEQUENCE {
           tok-id           INTEGER (256),  -- shall contain 0100 (hex)
           context-id       Random-Integer,
           pvno             BIT STRING,
           timestamp        UTCTime OPTIONAL, -- mandatory for SPKM-2
           randSrc          Random-Integer,
           targ-name        Name,
           src-name [0]     Name OPTIONAL,
           req-data         Context-Data,
           validity [1]     Validity OPTIONAL,
           key-estb-set     Key-Estb-Algs,
           key-estb-req     BIT STRING OPTIONAL,
           key-src-bind     OCTET STRING OPTIONAL
              -- This field must be present for the case of SPKM-2
              -- unilateral authen. if the K-ALG in use does not provide
              -- such a binding (but is optional for all other cases).
              -- The octet string holds the result of applying the
              -- mandatory hashing procedure (in MANDATORY I-ALG;
              -- see <a href="#section-2.1">Section 2.1</a>) as follows:  MD5(src || context_key),
              -- where "src" is the DER-encoded octets of src-name,
              -- "context-key" is the symmetric key (i.e., the
              -- unprotected version of what is transmitted in
              -- key-estb-req), and "||" is the concatenation operation.
   }

   Random-Integer ::= BIT STRING





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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   Context-Data ::= SEQUENCE {
           channelId       ChannelId OPTIONAL,
           seq-number      INTEGER OPTIONAL,
           options         Options,
           conf-alg        Conf-Algs,
           intg-alg        Intg-Algs,
           owf-alg         OWF-Algs
   }

   ChannelId ::= OCTET STRING

   Options ::= BIT STRING {
           delegation-state (0),
           mutual-state (1),
           replay-det-state (2),
           sequence-state (3),
           conf-avail (4),
           integ-avail (5),
           target-certif-data-required (6)
   }

   Conf-Algs ::= CHOICE {
           algs [0]         SEQUENCE OF AlgorithmIdentifier,
           null [1]         NULL
   }

   Intg-Algs ::= SEQUENCE OF AlgorithmIdentifier

   OWF-Algs ::= SEQUENCE OF AlgorithmIdentifier

   Key-Estb-Algs ::= SEQUENCE OF AlgorithmIdentifier


   SPKM-REP-TI ::= SEQUENCE {
           responseToken    REP-TI-TOKEN,
           certif-data      CertificationData OPTIONAL
             -- present if target-certif-data-required option was
   }         -- set to TRUE in SPKM-REQ

   REP-TI-TOKEN ::= SEQUENCE {
           rep-ti-contents  Rep-ti-contents,
           algId            AlgorithmIdentifier,
           rep-ti-integ     Integrity  -- "token" is Rep-ti-contents
   }

   Rep-ti-contents ::= SEQUENCE {
           tok-id           INTEGER (512),   -- shall contain 0200 (hex)
           context-id       Random-Integer,



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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


           pvno [0]         BIT STRING OPTIONAL,
           timestamp        UTCTime OPTIONAL, -- mandatory for SPKM-2
           randTarg         Random-Integer,
           src-name [1]     Name OPTIONAL,
           targ-name        Name,
           randSrc          Random-Integer,
           rep-data         Context-Data,
           validity [2]     Validity  OPTIONAL,
           key-estb-id      AlgorithmIdentifier OPTIONAL,
           key-estb-str     BIT STRING OPTIONAL
   }


   SPKM-REP-IT ::= SEQUENCE {
           responseToken    REP-IT-TOKEN,
           algId            AlgorithmIdentifier,
           rep-it-integ     Integrity  -- "token" is REP-IT-TOKEN
   }

   REP-IT-TOKEN ::= SEQUENCE {
           tok-id           INTEGER (768),  -- shall contain 0300 (hex)
           context-id       Random-Integer,
           randSrc          Random-Integer,
           randTarg         Random-Integer,
           targ-name        Name,
           src-name         Name OPTIONAL,
           key-estb-rep     BIT STRING OPTIONAL
   }

   SPKM-ERROR ::= SEQUENCE {
           errorToken       ERROR-TOKEN,
           algId            AlgorithmIdentifier,
           integrity        Integrity  -- "token" is ERROR-TOKEN
   }

   ERROR-TOKEN ::=   SEQUENCE {
           tok-id           INTEGER (1024), -- shall contain 0400 (hex)
           context-id       Random-Integer
   }

   SPKM-MIC ::= SEQUENCE {
           mic-header       Mic-Header,
           int-cksum        BIT STRING
   }

   Mic-Header ::= SEQUENCE {
           tok-id           INTEGER (257), -- shall contain 0101 (hex)
           context-id       Random-Integer,



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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


           int-alg [0]      AlgorithmIdentifier OPTIONAL,
           snd-seq [1]      SeqNum OPTIONAL
   }

   SeqNum ::= SEQUENCE {
           num              INTEGER,
           dir-ind          BOOLEAN
   }

   SPKM-WRAP ::= SEQUENCE {
           wrap-header       Wrap-Header,
           wrap-body         Wrap-Body
   }

   Wrap-Header ::= SEQUENCE {
           tok-id           INTEGER (513), -- shall contain 0201 (hex)
           context-id       Random-Integer,
           int-alg [0]      AlgorithmIdentifier OPTIONAL,
           conf-alg [1]     Conf-Alg OPTIONAL,
           snd-seq [2]      SeqNum OPTIONAL
   }

   Wrap-Body ::= SEQUENCE {
           int-cksum        BIT STRING,
           data             BIT STRING
   }

   Conf-Alg ::= CHOICE {
           algId [0]        AlgorithmIdentifier,
           null [1]         NULL
   }


   SPKM-DEL ::= SEQUENCE {
           del-header       Del-Header,
           int-cksum        BIT STRING
   }

   Del-Header ::= SEQUENCE {
           tok-id           INTEGER (769), -- shall contain 0301 (hex)
           context-id       Random-Integer,
           int-alg [0]      AlgorithmIdentifier OPTIONAL,
           snd-seq [1]      SeqNum OPTIONAL
   }


-- other types --




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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   -- from [<a href="./rfc1508">RFC-1508</a>] --

   MechType ::= OBJECT IDENTIFIER

   InitialContextToken ::= [APPLICATION 0] IMPLICIT SEQUENCE {
      thisMech              MechType,
      innerContextToken     SPKMInnerContextToken
   }     -- when thisMech is SPKM-1 or SPKM-2

   SPKMInnerContextToken ::= CHOICE {
      req    [0] SPKM-REQ,
      rep-ti [1] SPKM-REP-TI,
      rep-it [2] SPKM-REP-IT,
      error  [3] SPKM-ERROR,
      mic    [4] SPKM-MIC,
      wrap   [5] SPKM-WRAP,
      del    [6] SPKM-DEL
   }


   -- from [<a href="./rfc1510">RFC-1510</a>] --

   AuthorizationData ::= SEQUENCE OF SEQUENCE {
     ad-type  INTEGER,
     ad-data  OCTET STRING
   }


-- object identifier assignments --

   md5-DES-CBC OBJECT IDENTIFIER ::=
      {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       integrity(3) md5-DES-CBC(1)}

   sum64-DES-CBC OBJECT IDENTIFIER ::=
      {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       integrity(3) sum64-DES-CBC(2)}

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

   spkm-2 OBJECT IDENTIFIER ::=
      {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) spkm(1) spkm-2(2)}


END



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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


Appendix B:  Imported Types

   This appendix contains, for completeness, the relevant ASN.1 types
   imported from InformationFramework (1993), AuthenticationFramework
   (1993), and [PKCS3].

   AttributeType ::= OBJECT IDENTIFIER
   AttributeValue ::= ANY
   AttributeValueAssertion ::= SEQUENCE {AttributeType,AttributeValue}
   RelativeDistinguishedName ::= SET OF AttributeValueAssertion
      -- note that the 1993 InformationFramework module uses
      -- different syntax for the above constructs
   RDNSequence ::= SEQUENCE OF RelativeDistinguishedName
   DistinguishedName ::= RDNSequence
   Name ::= CHOICE {  -- only one for now
           rdnSequence       RDNSequence
   }

   Certificate ::= SEQUENCE {
           certContents      CertContents,
           algID             AlgorithmIdentifier,
           sig               BIT STRING
   }  -- sig holds the result of applying the signing procedure
      -- specified in algId to the BER-encoded octet string which
      -- results from applying the hashing procedure (also specified in
      -- algId) to the DER-encoded octets of CertContents

   CertContents ::= SEQUENCE {
           version [0]        Version DEFAULT v1,
           serialNumber       CertificateSerialNumber,
           signature          AlgorithmIdentifier,
           issuer             Name,
           validity           Validity,
           subject            Name,
           subjectPublicKeyInfo     SubjectPublicKeyInfo,
           issuerUID [1]      IMPLICIT UID OPTIONAL,  -- used in v2 only
           subjectUID [2]     IMPLICIT UID OPTIONAL   -- used in v2 only
   }

   Version ::= INTEGER {v1(0), v2(1)}
   CertificateSerialNumber ::= INTEGER
   UID ::= BIT STRING

   Validity ::= SEQUENCE {
           notBefore         UTCTime,
           notAfter          UTCTime
   }




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<span class="grey"><a href="./rfc2025">RFC 2025</a>                          SPKM                      October 1996</span>


   SubjectPublicKeyInfo ::= SEQUENCE {
           algorithm         AlgorithmIdentifier,
           subjectPublicKey  BIT STRING
   }

   CertificatePair ::= SEQUENCE {
           forward [0]      Certificate OPTIONAL,
           reverse [1]      Certificate OPTIONAL
   }         -- at least one of the pair shall be present

   CertificateList ::= SEQUENCE {
           certListContents        CertListContents,
           algId                   AlgorithmIdentifier,
           sig                     BIT STRING
   }  -- sig holds the result of applying the signing procedure
      -- specified in algId to the BER-encoded octet string which
      -- results from applying the hashing procedure (also specified in
      -- algId) to the DER-encoded octets of CertListContents

   CertListContents ::= SEQUENCE {
           signature               AlgorithmIdentifier,
           issuer                  Name,
           thisUpdate              UTCTime,
           nextUpdate              UTCTime OPTIONAL,
           revokedCertificates     SEQUENCE OF SEQUENCE {
                userCertificate       CertificateSerialNumber,
                revocationDate        UTCTime           } OPTIONAL
   }

   AlgorithmIdentifier ::= SEQUENCE {
           algorithm         OBJECT IDENTIFIER,
           parameter         ANY DEFINED BY algorithm OPTIONAL
   }  -- note that the 1993 AuthenticationFramework module uses
      -- different syntax for this construct



   --from [PKCS3] (the parameter to be used with dhKeyAgreement) --

   DHParameter ::= SEQUENCE {
     prime              INTEGER,  -- p
     base               INTEGER,  -- g
     privateValueLength INTEGER OPTIONAL
   }







Adams                       Standards Track                    [Page 45]
</pre>