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Versions: (draft-ssorce-gss-keyex-sha2) 00 01 02

Internet Engineering Task Force                                 S. Sorce
Internet-Draft                                                  H. Kario
Updates: 4462 (if approved)                                Red Hat, Inc.
Intended status: Standards Track                           June 15, 2017
Expires: December 17, 2017


                     GSS-API Key Exchange with SHA2
                  draft-ietf-curdle-gss-keyex-sha2-02

Abstract

   This document specifies additions and amendments to SSH GSS-API
   Methods [RFC4462].  It defines a new key exchange method that uses
   SHA-2 for integrity and deprecates weak DH groups.  The purpose of
   this specification is to modernize the cryptographic primitives used
   by GSS Key Exchanges.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 17, 2017.

Copyright Notice

   Copyright (c) 2017 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Document Conventions  . . . . . . . . . . . . . . . . . . . .   3
   4.  New Diffie-Hellman Key Exchange methods . . . . . . . . . . .   3
     4.1.  gss-group14-sha256-*  . . . . . . . . . . . . . . . . . .   3
     4.2.  gss-group15-sha512-*  . . . . . . . . . . . . . . . . . .   3
     4.3.  gss-group16-sha512-*  . . . . . . . . . . . . . . . . . .   4
     4.4.  gss-group17-sha512-*  . . . . . . . . . . . . . . . . . .   4
     4.5.  gss-group18-sha512-*  . . . . . . . . . . . . . . . . . .   4
   5.  New Elliptic Curve Diffie-Hellman Key Exchange methods  . . .   4
     5.1.  Generic GSS-API Key Exchange with ECDH  . . . . . . . . .   4
     5.2.  ECDH Key Exchange Methods . . . . . . . . . . . . . . . .  11
       5.2.1.  gss-nistp256-sha256-* . . . . . . . . . . . . . . . .  12
       5.2.2.  gss-nistp384-sha384-* . . . . . . . . . . . . . . . .  12
       5.2.3.  gss-nistp521-sha512-* . . . . . . . . . . . . . . . .  12
       5.2.4.  gss-curve25519-sha256-* . . . . . . . . . . . . . . .  12
       5.2.5.  gss-curve448-sha512-* . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     7.1.  New Finite Field DH mechanisms  . . . . . . . . . . . . .  13
     7.2.  New Elliptic Curve DH mechanisms  . . . . . . . . . . . .  13
     7.3.  GSSAPI Delegation . . . . . . . . . . . . . . . . . . . .  14
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   SSH GSS-API Methods [RFC4462] allows the use of GSSAPI for
   authentication and key exchange in SSH.  It defines three exchange
   methods all based on DH groups and SHA-1.  The new methods described
   in this document are intended to support environments that desire to
   use the SHA-2 cryptographic hash functions.

2.  Rationale

   Due to security concerns with SHA-1 [RFC6194] and with MODP groups
   with less than 2048 bits [NIST-SP-800-131Ar1] we propose the use of
   the SHA-2 based hashes with DH group14, group15, group16, group17 and
   group18 [RFC3526].  Additionally we add support for key exchange
   based on Elliptic Curve Diffie Hellman with NIST P-256, P-384 and
   P-521 as well as X25519 and X448 curves.  Following the rationale of
   [I-D.ietf-curdle-ssh-modp-dh-sha2] only SHA-256 and SHA-512 hashes




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   are used for DH groups.  For NIST curves the same curve-to-hashing
   algorithm pairing used in [RFC5656] is adopted for consistency.

3.  Document Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

4.  New Diffie-Hellman Key Exchange methods

   This document adopts the same naming convention defined in [RFC4462]
   to define families of methods that cover any GSS-API mechanism used
   with a specific Diffie-Hellman group and SHA-2 Hash combination.

          The following new key exchange algorithms are defined:

       +--------------------------+--------------------------------+
       | Key Exchange Method Name | Implementation Recommendations |
       +--------------------------+--------------------------------+
       | gss-group14-sha256-*     | SHOULD/RECOMMENDED             |
       | gss-group15-sha512-*     | MAY/OPTIONAL                   |
       | gss-group16-sha512-*     | SHOULD/RECOMMENDED             |
       | gss-group17-sha512-*     | MAY/OPTIONAL                   |
       | gss-group18-sha512-*     | MAY/OPTIONAL                   |
       +--------------------------+--------------------------------+

   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.

4.1.  gss-group14-sha256-*

   Each of these methods specifies GSS-API-authenticated Diffie-Hellman
   key exchange as described in Section 2.1 of [RFC4462] with SHA-256 as
   HASH, and the group defined in Section 8.2 of [RFC4253] The method
   name for each method is the concatenation of the string "gss-
   group14-sha256-" 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].

4.2.  gss-group15-sha512-*

   Each of these methods specifies GSS-API-authenticated Diffie-Hellman
   key exchange as described in Section 2.1 of [RFC4462] with SHA-512 as
   HASH, and the group defined in Section 4 of [RFC3526] The method name



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   for each method is the concatenation of the string "gss-
   group15-sha512-" with the Base64 encoding of the MD5 hash of the
   ASN.1 DER encoding of the underlying GSS-API mechanism's OID.

4.3.  gss-group16-sha512-*

   Each of these methods specifies GSS-API-authenticated Diffie-Hellman
   key exchange as described in Section 2.1 of [RFC4462] with SHA-512 as
   HASH, and the group defined in Section 5 of [RFC3526] The method name
   for each method is the concatenation of the string "gss-
   group16-sha512-" with the Base64 encoding of the MD5 hash of the
   ASN.1 DER encoding of the underlying GSS-API mechanism's OID.

4.4.  gss-group17-sha512-*

   Each of these methods specifies GSS-API-authenticated Diffie-Hellman
   key exchange as described in Section 2.1 of [RFC4462] with SHA-512 as
   HASH, and the group defined in Section 6 of [RFC3526] The method name
   for each method is the concatenation of the string "gss-
   group17-sha512-" with the Base64 encoding of the MD5 hash of the
   ASN.1 DER encoding of the underlying GSS-API mechanism's OID.

4.5.  gss-group18-sha512-*

   Each of these methods specifies GSS-API-authenticated Diffie-Hellman
   key exchange as described in Section 2.1 of [RFC4462] with SHA-512 as
   HASH, and the group defined in Section 7 of [RFC3526] The method name
   for each method is the concatenation of the string "gss-
   group18-sha512-" with the Base64 encoding of the MD5 hash of the
   ASN.1 DER encoding of the underlying GSS-API mechanism's OID.

5.  New Elliptic Curve Diffie-Hellman Key Exchange methods

   In [RFC5656] new SSH key exchange algorithms based on Elliptic Curve
   Cryptography are introduced.  We reuse much of section 4 to implement
   GSS-API-authenticated ECDH Key Exchanges.

   Additionally we utilize also the curves defined in
   [I-D.ietf-curdle-ssh-curves] to complement the 3 classic NIST defined
   curves required by [RFC5656].

5.1.  Generic GSS-API Key Exchange with ECDH

   This section reuses much of the scheme defined in Section 2.1 of
   [RFC4462] and combines it with the scheme defined in Section 4 of
   [RFC5656]; in particular, all checks and verification steps
   prescribed in Section 4 of [RFC5656] apply here as well.




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   The symbols used in this description conform to the symbols used in
   Section 2.1 of [RFC4462].  Additionally, the following symbols are
   defined:

   Q_C is the client ephemeral public key octet string

   Q_S is the server ephemeral public key octet string

   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.

   The Client:

   1.  C generates an ephemeral key pair with public key Q_C.  It does
   that by:

      For NIST curves:

         Selecting a value d_C uniformly at random from the interval [1,
         n-1] where n is the order of generator of the curve associated
         with the selected key exchange method.

         Performing point multiplication between the curve base point
         and selected integer d_C to get the public point q_C.

         Converts the point q_C to the Q_C octet string by concatenation
         of value 0x04 and big-endian representation of the x coordinate
         and then y coordinate.  The coordinate coversion MUST preserve
         leading zero octets.  Thus for nistp521 curve the encoded x
         coordinate will always have a length of 66 octets while the Q_C
         representation will be 133 octets long.  This is the
         uncompressed representation specified in Section 4.3.6 of
         [ANSI-X9-62-2005].

      For curve25519 and curve448:

         Selecting d_C as 32 uniformly distributed random octets for
         curve25519 and 56 octets for curve448.

         Preparing the generator g as the number 9 little-endian encoded
         in 32 octets for curve25519 and number 5 in 56 octets for
         curve448.  This is the same as an octet of value 0x09 followed
         by 31 zero octets for curve255519 and as an octect of value
         0x05 followed by 55 zero octets.





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         Calculating Q_C as the result of the call to X25519 or X448
         function, respectively for curve25519 and curve448 key
         exchange, with parameters d_C and g.

   2.  C calls GSS_Init_sec_context(), using the most recent reply token
   received from S during this exchange, if any.  For this call, the
   client MUST set mutual_req_flag to "true" to request that mutual
   authentication be performed.  It also MUST set integ_req_flag to
   "true" to request that per-message integrity protection be supported
   for this context.  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.  Since the
   key exchange process will involve the exchange of only a single token
   once the context has been established, it is not necessary that the
   GSS-API context support detection of replayed or out-of-sequence
   tokens.  Thus, replay_det_req_flag and sequence_req_flag need not be
   set for this process.  These flags SHOULD be set to "false".

      If the resulting major_status code is GSS_S_COMPLETE and the
      mutual_state flag is not true, then mutual authentication has not
      been established, and the key exchange MUST fail.

      If the resulting major_status code is GSS_S_COMPLETE and the
      integ_avail flag is not true, then per-message integrity
      protection is not available, and the key exchange MUST fail.

      If the resulting major_status code is GSS_S_COMPLETE and both the
      mutual_state and integ_avail flags are true, the resulting output
      token is sent to S.

      If the resulting major_status code is GSS_S_CONTINUE_NEEDED, the
      output_token is sent to S, which will reply with a new token to be
      provided to GSS_Init_sec_context().

      The client MUST also include Q_C with the first message it sends
      to the server during this process; if the server receives more
      than one Q_C or none at all, the key exchange MUST fail.

      It is an error if the call does not produce a token of non-zero
      length to be sent to the server.  In this case, the key exchange
      MUST fail.




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   3.  When a Q_C key is received, S verifies that the key is valid.  If
   the key is not valid the key exchange MUST fail.

      The server first checks if the length of the Q_C matches the
      selected key exchange: 65 octets for nistp256, 97 octets for
      nistp384, 133 octets for nistp521, 32 octets for curve25519 or 56
      octets for curve448.  If the value does not have matching length
      the key exchange MUST fail.

      In case of key exchanges that use NIST curves, the server MUST
      check if the first octet of the Q_C is equal to 0x04.  If the
      octet has different value the key exchange MUST fail.

      For NIST curves, the server converts the octet representation of
      the key to q_C point representation by interpreting the first half
      of remaining octets as the unsigned big-endian representation of
      the x coordinate of the point and the second half as the unsigned
      big-endian representation of the y coordinate.

      For NIST curves, the server verifies that the q_C is not a point
      at infinity, that both coordinates are in the interval [0, p - 1],
      where p is the prime associated with the curve of the selected key
      exchange and that the point lies on the curve (satisfies the curve
      equation).

      For curve25519, the server verifies that the the high-order bit of
      the last octet is not set - this prevents distinguishing attacks
      between implementations that use Montgomery ladder implementation
      of the curve and ones that use generic elliptic-curve libraries.
      If the bit is set, the key exchange SHOULD fail.  For curve448 any
      bit can be set.

      For curve25519 and curve448, the point is not decoded but used as
      is.  Q_C and q_C are considered equivalent.

   4.  S calls GSS_Accept_sec_context(), using the token received from
   C.

      If the resulting major_status code is GSS_S_COMPLETE and the
      mutual_state flag is not true, then mutual authentication has not
      been established, and the key exchange MUST fail.

      If the resulting major_status code is GSS_S_COMPLETE and the
      integ_avail flag is not true, then per-message integrity
      protection is not available, and the key exchange MUST fail.

      If the resulting major_status code is GSS_S_COMPLETE and both the
      mutual_state and integ_avail flags are true, then the security



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      context has been established, and processing continues with step
      5.

      If the resulting major_status code is GSS_S_CONTINUE_NEEDED, then
      the output token is sent to C, and processing continues with step
      2.

      If the resulting major_status code is GSS_S_COMPLETE, but a non-
      zero-length reply token is returned, then that token is sent to
      the client.

   5.  S generates an ephemeral key pair with public key Q_S calculated
   the same way it is done in step 1 and peforms the following
   computations:

      K a shared secret obtained using ECDH key exchange:



         Both client and server perform the same calculation where d_U
         is the secret value, d_C for client and d_S for server and q_V
         is the received public value, q_S for client and q_C for
         server.

         For NIST curves, the peers perform point multiplication using
         d_U and q_V to get point P.

         For NIST curves, peers verify that P is not a point at
         infinity.  If P is a point at infinity, the key exchange MUST
         fail.

         For NIST curves, the shared secret is the zero-padded big-
         endian representation of the x coordinate of P.

         For curve25519 and curve448, the peers apply the X25519 or X448
         function, respectively for curve25519 and curve448, on the d_U
         and q_V.  The result of the function is the shared secret.

         For curve25519 and curve448, if all the octets of the shared
         secret are zero octets, the key exchange MUST fail.

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

      MIC is the GSS-API message integrity code for H computed by
      calling GSS_GetMIC().

   6.  This step is performed only if the server's final call to
   GSS_Accept_sec_context() produced a non-zero-length final reply token



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   to be sent to the client and if no previous call by the client to
   GSS_Init_sec_context() has resulted in a major_status of
   GSS_S_COMPLETE.  Under these conditions, the client makes an
   additional call to GSS_Init_sec_context() to process the final reply
   token.  This call is made exactly as described above.  However, if
   the resulting major_status is anything other than GSS_S_COMPLETE, or
   a non-zero-length token is returned, it is an error and the key
   exchange MUST fail.

   7.  C verifies that the key Q_S is valid the same way it is done in
   step 3.  If the key is not valid the key exchange MUST fail.

   8.  C computes the shared secret K and H the same way it is done in
   step 5.  It then calls GSS_VerifyMIC() to check that the MIC sent by
   S verifies H's integrity.  If the MIC is not successfully verified,
   the key exchange MUST fail.

   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.

   This exchange 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 key octet string

   The server may responds with:

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

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

   Each time the server's call to GSS_Accept_sec_context() returns a
   major_status code of GSS_S_CONTINUE_NEEDED







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   The server replies:

       byte      SSH_MSG_KEXGSS_CONTINUE
       string    output_token (from GSS_Accept_sec_context())

   If the client receives this 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.

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

   The client sends:

       byte      SSH_MSG_KEXGSS_CONTINUE
       string    output_token (from GSS_Init_sec_context())

   The server and client continue to trade these two messages as long as
   the server's calls to GSS_Accept_sec_context() result in major_status
   codes of GSS_S_CONTINUE_NEEDED.  When a call results in a
   major_status code of GSS_S_COMPLETE, it sends one of two final
   messages.

   If the server's final call to GSS_Accept_sec_context() (resulting in
   a major_status code of GSS_S_COMPLETE) returns a non-zero-length
   token to be sent to the client, it sends the following:

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

   If the client receives this 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 server's final call to GSS_Accept_sec_context() (resulting in
   a major_status code of GSS_S_COMPLETE) returns a zero-length token or
   no token at all, it sends the following:

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




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   If the client receives this message when no call to
   GSS_Init_sec_context() has yet resulted in a major_status code of
   GSS_S_COMPLETE, a protocol error has occurred and the key exchange
   MUST fail.

   In case of errors the messages described in Section 2.1 of [RFC4462]
   are used as well as the recommendation about the messages' order.

   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 key octet string
       string    Q_S, server's ephemeral public 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.

   The GSS_GetMIC call MUST be applied over H, not the original data.

5.2.  ECDH Key Exchange Methods

            The following new key exchange methods are defined:

       +--------------------------+--------------------------------+
       | Key Exchange Method Name | Implementation Recommendations |
       +--------------------------+--------------------------------+
       | gss-nistp256-sha256-*    | SHOULD/RECOMMENDED             |
       | gss-nistp384-sha384-*    | MAY/OPTIONAL                   |
       | gss-nistp521-sha512-*    | MAY/OPTIONAL                   |
       | gss-curve25519-sha256-*  | SHOULD/RECOMMENDED             |
       | gss-curve448-sha512-*    | MAY/OPTIONAL                   |
       +--------------------------+--------------------------------+

   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.





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5.2.1.  gss-nistp256-sha256-*

   Each of these methods specifies GSS-API-authenticated Elliptic Curve
   Diffie-Hellman key exchange as described in Section 5.1 of this
   document with SHA-256 as HASH, and the curve and base point defined
   in section 2.4.2 of [SEC2v2] as secp256r1.  The method name for each
   method is the concatenation of the string "gss-nistp256-sha256-" 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].

5.2.2.  gss-nistp384-sha384-*

   Each of these methods specifies GSS-API-authenticated Elliptic Curve
   Diffie-Hellman key exchange as described in Section 5.1 of this
   document with SHA-384 as HASH, and the curve and base point defined
   in section 2.5.1 of [SEC2v2] as secp384r1.  The method name for each
   method is the concatenation of the string "gss-nistp384-sha384-" 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].

5.2.3.  gss-nistp521-sha512-*

   Each of these methods specifies GSS-API-authenticated Elliptic Curve
   Diffie-Hellman key exchange as described in Section 5.1 of this
   document with SHA-512 as HASH, and the curve and base point defined
   in section 2.6.1 of [SEC2v2] as secp521r1.  The method name for each
   method is the concatenation of the string "gss-nistp521-sha512-" 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].

5.2.4.  gss-curve25519-sha256-*

   Each of these methods specifies GSS-API-authenticated Elliptic Curve
   Diffie-Hellman key exchange as described in Section 5.1 of this
   document with SHA-256 as HASH, and the X25519 function defined in
   section 5 of [RFC7748].  The method name for each method is the
   concatenation of the string "gss-curve25519-sha256-" 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].








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5.2.5.  gss-curve448-sha512-*

   Each of these methods specifies GSS-API-authenticated Elliptic Curve
   Diffie-Hellman key exchange as described in Section 5.1 of this
   document with SHA-512 as HASH, and the X448 function defined in
   section 5 of [RFC7748].  The method name for each method is the
   concatenation of the string "gss-curve448-sha512-" 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].

6.  IANA Considerations

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

      IANA is requested to update the SSH algorithm registry with the
                            following entries:

    +--------------------------+------------+------------------------+
    | Key Exchange Method Name | Reference  | Implementation Support |
    +--------------------------+------------+------------------------+
    | gss-group14-sha256-*     | This draft | SHOULD                 |
    | gss-group15-sha512-*     | This draft | MAY                    |
    | gss-group16-sha512-*     | This draft | SHOULD                 |
    | gss-group17-sha512-*     | This draft | MAY                    |
    | gss-group18-sha512-*     | This draft | MAY                    |
    | gss-nistp256-sha256-*    | This draft | SHOULD                 |
    | gss-nistp384-sha384-*    | This draft | MAY                    |
    | gss-nistp521-sha512-*    | This draft | MAY                    |
    | gss-curve25519-sha256-*  | This draft | SHOULD                 |
    | gss-curve448-sha512-*    | This draft | MAY                    |
    +--------------------------+------------+------------------------+

7.  Security Considerations

7.1.  New Finite Field DH mechanisms

   Except for the use of a different secure hash function and larger DH
   groups, no significant changes has been made to the protocol
   described by [RFC4462]; therefore all the original Security
   Considerations apply.

7.2.  New Elliptic Curve DH mechanisms

   Although a new cryptographic primitive is used with these methods the
   actual key exchange closely follows the key exchange defined in




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   [RFC5656]; therefore all the original Security Considerations as well
   as those expressed in [RFC5656] apply.

7.3.  GSSAPI Delegation

   Some GSSAPI mechanisms can optionally delegate credentials to the
   target host by setting the deleg_ret_flag.  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 may results in the user
   silently delegating credentials to the attacker, who can then
   impersonate the user at will.

8.  Normative References

   [ANSI-X9-62-2005]
              American National Standards Institute, "Public Key
              Cryptography for the Financial Services Industry, The
              Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI
              Standard X9.62, 2005.

   [FIPS-180-4]
              National Institute of Standards and Technology, "FIPS PUB
              180-4: Secure Hash Standard (SHS)", FIPS PUB 180-4, August
              2015, <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

   [I-D.ietf-curdle-ssh-curves]
              Adamantiadis, A., Josefsson, S., and M. Baushke, "Secure
              Shell (SSH) Key Exchange Method using Curve25519 and
              Curve448", draft-ietf-curdle-ssh-curves-04 (work in
              progress), April 2017.

   [I-D.ietf-curdle-ssh-modp-dh-sha2]
              Baushke, M., "More Modular Exponential (MODP) Diffie-
              Hellman (DH) Key Exchange (KEX) Groups for Secure Shell
              (SSH)", draft-ietf-curdle-ssh-modp-dh-sha2-04 (work in
              progress), April 2017.











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   [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, November 2015,
              <http://standards.iso.org/ittf/PubliclyAvailableStandards/
              c068345_ISO_IEC_8825-1_2015.zip>.

   [NIST-SP-800-131Ar1]
              National Institute of Standards and Technology,
              "Transitions: Recommendation for Transitioning of the Use
              of Cryptographic Algorithms and Key Lengths", NIST Special
              Publication 800-131A Revision 1, November 2015,
              <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-131Ar1.pdf>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3526]  Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
              Diffie-Hellman groups for Internet Key Exchange (IKE)",
              RFC 3526, DOI 10.17487/RFC3526, May 2003,
              <http://www.rfc-editor.org/info/rfc3526>.

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <http://www.rfc-editor.org/info/rfc4253>.

   [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, <http://www.rfc-editor.org/info/rfc4462>.






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

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
              <http://www.rfc-editor.org/info/rfc6194>.

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

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <http://www.rfc-editor.org/info/rfc7748>.

   [SEC2v2]   Certicom Research, "SEC 2: Recommended Elliptic Curve
              Domain Parameters", Standards for Efficient
              Cryptography SEC 2, 2010.

Authors' Addresses

   Simo Sorce
   Red Hat, Inc.
   140 Broadway
   24th Floor
   New York, NY  10025
   USA

   Email: simo@redhat.com


   Hubert Kario
   Red Hat, Inc.
   Purkynova 99/71
   Brno  612 45
   Czech Republic

   Email: hkario@redhat.com










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