



Internet Engineering Task Force                                 A. Kario
Internet-Draft                                             Red Hat, Inc.
Intended status: Standards Track                           22 April 2026
Expires: 24 October 2026


                GSS-API Key Exchange with hybrid ML-KEM
                      draft-kario-gss-keyex-pqc-00

Abstract

   This document specifies additions to RFC4462.  It defines a new key
   exchange methods that use hybrid Post-Quantum Traditional (PQ/T) key
   exchange.  The purpose of this specification is to modernize the
   cryptographic primitives used by Generic Security Service (GSS) key
   exchanges.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 24 October 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   provided without warranty as described in the Revised BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Document Conventions  . . . . . . . . . . . . . . . . . . . .   2
   4.  New PQ/T Hybrid Key Exchange methods  . . . . . . . . . . . .   3
     4.1.  Generic GSS-API Key Exchange with PQ/T Hybrid . . . . . .   3
     4.2.  PQ/T Key Exchange Methods . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     6.1.  New PQ/T key exchange mechanisms  . . . . . . . . . . . .   8
     6.2.  GSSAPI Delegation . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Secure Shell (SSH) Generic Security Service Application Program
   Interface (GSS-API) methods [RFC4462] allow the use of GSS-API
   [RFC2743] for authentication and key exchange in SSH.  This document
   updates [RFC4462] with new methods based on
   [I-D.ietf-sshm-mlkem-hybrid-kex] intended to support environments
   that desire to use key exchanges resistant to attacks by CRQC
   (Cryptographically Relevant Quantum Computers).

2.  Rationale

   As documented in [I-D.ietf-sshm-mlkem-hybrid-kex] traditional
   cryptography (Finite Field Diffie-Hellman and Elliptic Curve Diffie-
   Hellman) will not be secure agains CRQCs, to address that we propose
   use of hybrid Post-Quantum Traditional (PQ/T) cryptography together
   with GSS-API methods.

3.  Document Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.









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4.  New PQ/T Hybrid Key Exchange methods

   In [I-D.ietf-sshm-mlkem-hybrid-kex] new SSH key exchange algorithms
   based on PQ/T Hybrid Key Exchange Methods are introduced.  We reuse
   much of section 2 of [I-D.ietf-sshm-mlkem-hybrid-kex] to define GSS-
   API-authenticated PQ/T Hybrid Key Exchanges.

4.1.  Generic GSS-API Key Exchange with PQ/T Hybrid

   This section reuses much of the scheme defined in Section 2.1 of
   [RFC4462] and combines it with the scheme defined in Section 2 of
   [I-D.ietf-sshm-mlkem-hybrid-kex]; in particular, all checks and
   verification steps prescribed in Section 2.1 of
   [I-D.ietf-sshm-mlkem-hybrid-kex] apply here as well.

   This section defers to [RFC7546] as the source of information on GSS-
   API context establishment operations, Section 3 being the most
   relevant.  All Security Considerations described in [RFC7546] apply
   here too.

   A GSS Context is established according to Section 4 of [RFC5656]; The
   client initiates the establishment using GSS_Init_sec_context() and
   the server responds to it using GSS_Accept_sec_context().  For the
   negotiation, the client MUST set mutual_req_flag and integ_req_flag
   to "true".  In addition, deleg_req_flag MAY be set to "true" to
   request access delegation, if requested by the user.  Since the key
   exchange process authenticates only the host, the setting of
   anon_req_flag is immaterial to this process.  If the client does not
   support the "gssapi-keyex" user authentication method described in
   Section 4 of [RFC4462], or does not intend to use that method in
   conjunction with the GSS-API context established during key exchange,
   then anon_req_flag SHOULD be set to "true".  Otherwise, this flag MAY
   be set to true if the client wishes to hide its identity.  This key
   exchange process will exchange only a single message token once the
   context has been established, therefore the replay_det_req_flag and
   sequence_req_flag SHOULD be set to "false".

   The client MUST include its Traditional public key and Post-Quantum
   encapsulation key with the first message it sends to the server
   during this process; if the server receives more than one key or none
   at all, the key exchange MUST fail.  That is, the Q_C field of
   SSH_MSG_KEXGSS_INIT must contain the concatenation of C_PK2 and C_PK1
   from section 2.1 of [I-D.ietf-sshm-mlkem-hybrid-kex].

   During GSS Context establishment multiple tokens may be exchanged by
   the client and the server.  When the GSS Context is established
   (major_status is GSS_S_COMPLETE) the parties check that mutual_state
   and integ_avail are both "true".  If not the key exchange MUST fail.



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   When the GSS Context is established, the Q_S field in server's
   SSH_MSG_KEXGSS_COMPLETE message needs to contain the concatenation of
   of S_CT2 and S_PK1 from section 2.1 of
   [I-D.ietf-sshm-mlkem-hybrid-kex].

   Once a party receives the peer's public key it proceeds to compute a
   shared secret K.  This is done as specified in section 2.4 of
   [I-D.ietf-sshm-mlkem-hybrid-kex].

   To verify the integrity of the handshake, peers use the Hash Function
   defined by the selected Key Exchange method to calculate H:

   H = hash(V_C || V_S || I_C || I_S || K_S || Q_C || Q_S || K).

   The GSS_GetMIC() call is used by the server with H as the payload and
   generates a MIC.  The GSS_VerifyMIC() call is used by the client to
   verify the MIC.

   If any GSS_Init_sec_context() or GSS_Accept_sec_context() returns a
   major_status other than GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED, or
   any other GSS-API call returns a major_status other than
   GSS_S_COMPLETE, the key exchange MUST fail.  The same recommendations
   expressed in Section 2.1 of [RFC4462] are followed with regards to
   error reporting.

   The following is an overview of the key exchange process:

























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         Client                                                Server
         ------                                                ------
         Generate ephemeral key pairs.
         Calls GSS_Init_sec_context().
         SSH_MSG_KEXGSS_INIT  --------------->

                                         Verify received keys are valid.
     (Optional)                  <------------- SSH_MSG_KEXGSS_HOSTKEY

     (Loop)
     |                                 Calls GSS_Accept_sec_context().
     |                           <------------ SSH_MSG_KEXGSS_CONTINUE
     |   Calls GSS_Init_sec_context().
     |   SSH_MSG_KEXGSS_CONTINUE ------------>

                                       Calls GSS_Accept_sec_context().
                      Generate ephemeral key pair and encapsulate key.
                                                Compute shared secret.
                                                      Computes hash H.
                                          Calls GSS_GetMIC( H ) = MIC.
                              <------------ SSH_MSG_KEXGSS_COMPLETE

         Verify received key is valid, decapsulate key.
         Compute shared secret.
         Compute hash = H
         Calls GSS_VerifyMIC( MIC, H )

   This is implemented with the following messages:

   The client sends:

       byte      SSH_MSG_KEXGSS_INIT
       string    output_token (from GSS_Init_sec_context())
       string    Q_C, client's ephemeral public keys octet string

   The server may responds with:

       byte      SSH_MSG_KEXGSS_HOSTKEY
       string    server public host key and certificates (K_S)

   The server sends:

       byte      SSH_MSG_KEXGSS_CONTINUE
       string    output_token (from GSS_Accept_sec_context())

   Each time the client receives the message described above, it makes
   another call to GSS_Init_sec_context().




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   The client sends:

       byte      SSH_MSG_KEXGSS_CONTINUE
       string    output_token (from GSS_Init_sec_context())

   As the final message the server sends either:

       byte      SSH_MSG_KEXGSS_COMPLETE
       string    Q_S, server's ephemeral public key and encapsulated key
                 octet string
       string    mic_token (MIC of H)
       boolean   TRUE
       string    output_token (from GSS_Accept_sec_context())

   Or the following if no output_token is available:

       byte      SSH_MSG_KEXGSS_COMPLETE
       string    Q_S, server's ephemeral public key and encapsulated key
                 octet string
       string    mic_token (MIC of H)
       boolean   FALSE

   The hash H is computed as the HASH hash of the concatenation of the
   following:

       string    V_C, the client's version string (CR, NL excluded)
       string    V_S, server's version string (CR, NL excluded)
       string    I_C, payload of the client's SSH_MSG_KEXINIT
       string    I_S, payload of the server's SSH_MSG_KEXINIT
       string    K_S, server's public host key
       string    Q_C, client's ephemeral public keys octet string
       string    Q_S, server's ephemeral public key and encapsulated key
                 octet string
       mpint     K,   shared secret

   This value is called the exchange hash, and it is used to
   authenticate the key exchange.  The exchange hash SHOULD be kept
   secret.  If no SSH_MSG_KEXGSS_HOSTKEY message has been sent by the
   server or received by the client, then the empty string is used in
   place of K_S when computing the exchange hash.

   Since this key exchange method does not require the host key to be
   used for any encryption operations, the SSH_MSG_KEXGSS_HOSTKEY
   message is OPTIONAL.  If the "null" host key algorithm described in
   Section 5 of [RFC4462] is used, this message MUST NOT be sent.






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   If the client receives a SSH_MSG_KEXGSS_CONTINUE message after a call
   to GSS_Init_sec_context() has returned a major_status code of
   GSS_S_COMPLETE, a protocol error has occurred and the key exchange
   MUST fail.

   If the client receives a SSH_MSG_KEXGSS_COMPLETE message and a call
   to GSS_Init_sec_context() does not result in a major_status code of
   GSS_S_COMPLETE, a protocol error has occurred and the key exchange
   MUST fail.

4.2.  PQ/T Key Exchange Methods

   The following new key exchange methods are defined:

    +================================+================================+
    | Key Exchange Method Name       | Implementation Recommendations |
    +================================+================================+
    | gss-mlkem768nistp256-sha256-*  | SHOULD/RECOMMENDED             |
    +--------------------------------+--------------------------------+
    | gss-mlkem1024nistp384-sha384-* | MAY/OPTIONAL                   |
    +--------------------------------+--------------------------------+
    | gss-mlkem768x25519-sha256-*    | SHOULD/RECOMMENDED             |
    +--------------------------------+--------------------------------+

                                  Table 1

   Each key exchange method is implicitly registered by this document.
   The IESG is considered to be the owner of all these key exchange
   methods; this does NOT imply that the IESG is considered to be the
   owner of the underlying GSS-API mechanism.

   Each method in any family of methods specifies GSS-API-authenticated
   Post-Quantum Traditional Hybrid key exchanges as described in
   Section 4.1.  The method name for each method is the concatenation of
   the family method name with the Base64 encoding of the MD5 hash
   [RFC1321] of the ASN.1 DER encoding [ISO-IEC-8825-1] of the
   underlying GSS-API mechanism's OID.  Base64 encoding is described in
   Section 6.8 of [RFC2045].

   Family method refences

   +=========================+========+=================+================================+
   |Family Name prefix       |Hash    |Parameters /     |Definition                      |
   |                         |Function|Function Name    |                                |
   +=========================+========+=================+================================+
   |gss-                     |SHA-256 |mlkem768nistp256 |Section 2.3.1 of                |
   |mlkem768nistp256-sha256- |        |                 |[I-D.ietf-sshm-mlkem-hybrid-kex]|
   +-------------------------+--------+-----------------+--------------------------------+



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   |gss-                     |SHA-384 |mlkem1024nistp384|Section 2.3.2 of                |
   |mlkem1024nistp384-sha384-|        |                 |[I-D.ietf-sshm-mlkem-hybrid-kex]|
   +-------------------------+--------+-----------------+--------------------------------+
   |gss-                     |SHA-256 |mlkem768x25519   |Section 2.3.3 of                |
   |mlkem768x25519-sha256-   |        |                 |[I-D.ietf-sshm-mlkem-hybrid-kex]|
   +-------------------------+--------+-----------------+--------------------------------+

                                  Table 2

5.  IANA Considerations

   This document augments the SSH Key Exchange Method Names in
   [RFC4462].

   IANA is requested to update the SSH Protocol Parameters
   [IANA-KEX-NAMES] registry with the following entries:

     +================================+============+=================+
     | Key Exchange Method Name       | Reference  | OK to Implement |
     +================================+============+=================+
     | gss-mlkem768nistp256-sha256-*  | This draft | SHOULD          |
     +--------------------------------+------------+-----------------+
     | gss-mlkem1024nistp384-sha384-* | This draft | SHOULD          |
     +--------------------------------+------------+-----------------+
     | gss-mlkem768x25519-sha256-*    | This draft | SHOULD          |
     +--------------------------------+------------+-----------------+

                                  Table 3

6.  Security Considerations

6.1.  New PQ/T key exchange mechanisms

   Although a new cryptographic primitive is used with these methods the
   actual key exchange closely follows the key exchange defined in
   [I-D.ietf-sshm-mlkem-hybrid-kex]; therefore all the original Security
   Considerations as well as those expressed in
   [I-D.ietf-sshm-mlkem-hybrid-kex] apply.

6.2.  GSSAPI Delegation

   Some GSSAPI mechanisms can act on a request to delegate credentials
   to the target host when the deleg_req_flag is set.  In this case,
   extra care must be taken to ensure that the acceptor being
   authenticated matches the target the user intended.  Some mechanisms
   implementations (like commonly used krb5 libraries) may use insecure
   DNS resolution to canonicalize the target name; in these cases
   spoofing a DNS response that points to an attacker-controlled machine



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   may results in the user silently delegating credentials to the
   attacker, who can then impersonate the user at will.

7.  References

7.1.  Normative References

   [I-D.ietf-sshm-mlkem-hybrid-kex]
              Kampanakis, P., Stebila, D., and T. Hansen, "PQ/T Hybrid
              Key Exchange with ML-KEM in SSH", Work in Progress,
              Internet-Draft, draft-ietf-sshm-mlkem-hybrid-kex-10, 26
              February 2026, <https://datatracker.ietf.org/doc/html/
              draft-ietf-sshm-mlkem-hybrid-kex-10>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743,
              DOI 10.17487/RFC2743, January 2000,
              <https://www.rfc-editor.org/info/rfc2743>.

   [RFC4462]  Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
              "Generic Security Service Application Program Interface
              (GSS-API) Authentication and Key Exchange for the Secure
              Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May
              2006, <https://www.rfc-editor.org/info/rfc4462>.

   [RFC5656]  Stebila, D. and J. Green, "Elliptic Curve Algorithm
              Integration in the Secure Shell Transport Layer",
              RFC 5656, DOI 10.17487/RFC5656, December 2009,
              <https://www.rfc-editor.org/info/rfc5656>.

   [RFC7546]  Kaduk, B., "Structure of the Generic Security Service
              (GSS) Negotiation Loop", RFC 7546, DOI 10.17487/RFC7546,
              May 2015, <https://www.rfc-editor.org/info/rfc7546>.




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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [IANA-KEX-NAMES]
              IANA, "Secure Shell (SSH) Protocol Parameters: Key
              Exchange Method Names", 2 June 2005,
              <https://www.iana.org/assignments/ssh-parameters/ssh-
              parameters.xhtml#ssh-parameters-16>.

   [ISO-IEC-8825-1]
              International Organization for Standardization /
              International Electrotechnical Commission, "ASN.1 encoding
              rules: Specification of Basic Encoding Rules (BER),
              Canonical Encoding Rules (CER) and Distinguished Encoding
              Rules (DER)", ISO/IEC 8825-1, 15 November 2015,
              <http://standards.iso.org/ittf/PubliclyAvailableStandards/
              c068345_ISO_IEC_8825-1_2015.zip>.

Author's Address

   Alicja Kario
   Red Hat, Inc.
   Purkynova 115
   612 00 Brno
   Czech Republic
   Email: hkario@redhat.com






















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