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Versions: 00                                                            
Internet Engineering Task Force                                 H. Kario
Internet-Draft                                             Red Hat, Inc.
Updates: 4462 (if approved)                                 Sep 30, 2020
Intended status: Standards Track
Expires: April 3, 2021


             Quantum-Resistant GSS-API Key Exchange for SSH
                       draft-kario-gss-qr-kex-00

Abstract

   This document specifies additions and amendments to RFC4462.  It
   defines a new key exchange method that uses GSS-API in a way to
   provide key exchange method that is resistant to attacks by quantum
   computers.  The purpose of this specification is to provide an easy-
   to-implement upgrade to environments that require resistance against
   quantum computers before widely accepted post-quantum cryptography
   algorithms are established.

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 https://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 April 3, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://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



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   include Simplified BSD License text as described in Section 4.e of
   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 Quantum Resistant Key Exchange Methods  . . . . . . . . .   3
     4.1.  Generic Quantum Resistant GSS-API key Exchange  . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     6.1.  Symmetric cipher security . . . . . . . . . . . . . . . .   7
     6.2.  User authentication . . . . . . . . . . . . . . . . . . .   8
     6.3.  Used GSSAPI Mechanisms  . . . . . . . . . . . . . . . . .   8
     6.4.  GSSAPI Delegation . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   SSH GSS-API Methods [RFC4462] allows the use of GSSAPI for
   authentication and key exchange in SSH.  Unfortunately for resistance
   against quantum computers all of the methods in RFC 4462 as well as
   all of the new methods introduced in SSH GSS-API SHA-2 Methods
   [RFC8732] derive their security from Finite-Field Diffie-Hellman or
   Elliptic Curve Diffie-Hellman key exchanges.  Both FFDH and ECDH are
   believed to be vulnerable to Shor's algorithm running on quantum
   computers.  This document updates RFC4462 with new methods intended
   for use in environments where use of quantum resistant algorithms is
   more important that the forward secrecy provided by FFDH and ECDH.

2.  Rationale

   Due to security concerns with FFDH and ECDH against attacks using
   quantum computers, we propose a new key exchange method that does not
   use FFDH or ECDH to agree on a shared secret to derive later
   encryption keys but rather uses GSS-API as a secure communication
   channel to exchange secrets that are then used to derive encryption
   keys.

   To provide resistance against quantum computer attacks the connection
   needs to also carefully select encryption ciphers, and host
   authentication methods.




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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 [RFC2119] RFC8174 [RFC8174] when, and only when, they
   appear in all capitals, as shown here.

4.  New Quantum Resistant Key Exchange Methods

   This document adopts the same naming convention as defined in
   [RFC4462] to define families of methods that cover any GSS-API
   mechanism used with a specific SHA-2 Hash.  It also reuses much of
   the the scheme defined in Section 2.1 of [RFC4462].

          The following new key exchange algorithms are defined:

       +--------------------------+--------------------------------+
       | Key Exchange Method Name | Implementation Recommendations |
       +--------------------------+--------------------------------+
       | gss-qr-sha256-*          | SHOULD/RECOMMENDED             |
       | gss-qr-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.

   Each method in any family of methods specifies GSS-API-authenticated
   exchanges as described in Section 2.1 of [RFC4462].  The method name
   for each method is the concatenation of the family name prefix 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 references

                  +--------------------+---------------+
                  | Family Name prefix | Hash Function |
                  +--------------------+---------------+
                  | gss-qr-sha256-     | SHA-256       |
                  | gss-qr-sha512-     | SHA-512       |
                  +--------------------+---------------+







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4.1.  Generic Quantum Resistant GSS-API key Exchange

   This section reuses much of the scheme defined in Section 2.1 of
   [RFC4462] though it does not transport FFDH key shares in the
   exchanged messages.

   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 parties generate nonces in the key exchange.  The generated
   nonces MUST be at least 256 bits long and come from a quantum safe
   CSPRNG.  The nonces MUST NOT be reused in other key exchanges.

   The client initiates negotiation by calling GSS_Init_sec_context()
   and the server responds to it by calling GSS_Accept_sec_context().
   For the negotiation, client MUST set the mutual_req_flag,
   conf_req_flag, and integ_req_flag 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".

   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.

   To verify the integrity of the handshake both peers use the Hash
   Function defined by the selected Key Exchange method to calculate the
   running hash of exchanged messages, H_S and H_C.

   H_S = hash(V_C || V_S || I_C || KC_S || ... || KC_C).

   H_C = hash(V_C || V_S || I_C || KC_S || ... || KC_C || KC).

   The GSS_wrap() call is used by the server and client to encrypt the
   calculated hash and the selected nonce.  The peers use the




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   GSS_unwrap() to decrypt the value used to check if the other peer has
   received the same messages and to get the nonce it selected.

   Peers MUST verify if the length of the selected nonce is not shorter
   than 32 octets.  If the received nonce is shorter, the key exchange
   MUST fail.

   The following is an overview of the key exchange process:

        Client                                                Server
        ------                                                ------
        Calls GSS_Init_sec_context().
        SSH_MSG_KEXGSS_INIT  --------------->

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

                                         Calls GSS_Accept_sec_context().
                                              Generates ephemeral nonce.
                                                      Computes hash H_S.
                                       Calls GSS_wrap( H_S || nonce_S ).
                                <------------ SSH_MSG_KEXGSS_COMPLETE

        Computes hash H_S.
        Calls GSS_unwrap().
        Verifies that computed H_S matches received value.
        Computes hash H_C.
        Generates ephemeral nonce.
        Calls GSS_wrap( H_C || nonce_C ).
        SSH_MSG_KEXGSS_COMPLETE ------------>
                                                      Computes hash H_C.
                                                     Calls GSS_unwrap().
                      Verifies that computed H_C matches received value.

   This is implemented with the following messages:

   The client sends:

       byte      SSH_MSG_KEXGSS_INIT
       string    output_token (from GSS_Init_sec_context())

   The server sends:

       byte      SSH_MSG_KEXGSS_CONTINUE
       string    output_token (from GSS_Accept_sec_context())



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   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 final server message is either:

       byte      SSH_MSG_KEXGSS_COMPLETE
       string    enc_nonce (GSS_wrap() of H_S and nonce_S)
       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    enc_nonce (GSS_wrap() of H_S and nonce_S)
       boolean   FALSE

   As the final message the client sends either:

       byte      SSH_MSG_KEXGSS_COMPLETE
       string    enc_nonce (GSS_wrap() of H_C and nonce_C)
       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    enc_nonce (GSS_wrap() of H_C and nonce_C)
       boolean   FALSE

   The hashes H_S and H_C are 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    KC_S, payload of the server's SSH_MSG_KEXGSS_CONTINUE
    string    KC_C, payload of the client's SSH_MSG_KEXGSS_CONTINUE
    string    KC_S, payload of the server's second SSH_MSG_KEXGSS_CONTINUE
    string    KC_C, payload of the client's second SSH_MSG_KEXGSS_CONTINUE
    ...
    string    KC, payload of the server's SSH_MSG_KEXGSS_COMPLETE




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   Those values are called exchange hashes, and they are used to
   authenticate the key exchange.  The exchange hashes SHOULD be kept
   secret.  If no SSH_MSG_KEXGSS_CONTINUE messages have been sent by the
   server or received by the client, then an empty string is used in
   place of KC_S and KC_C when computing the exchange hash.  When
   multiple SSH_MSG_KEXGSS_CONTINUE messages have been sent by either
   side, then they should all be included in the exchange hash, in order
   they have been processed by both sides of the connection.  For the
   H_S hash, the KC is an empty string.

   Once a party has both the server nonce (nonce_S) and the client nonce
   (nonce_C) it concatenates them, in this order, to compute the used
   shared secret K:

   K = nonce_S || nonce_C

   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.

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  |
                 +--------------------------+------------+
                 | gss-qr-sha256-*          | This draft |
                 | gss-qr-sha512-*          | This draft |
                 +--------------------------+------------+

6.  Security Considerations

6.1.  Symmetric cipher security

   Current understanding of quantum computer capabilities suggest that
   symmetric ciphers with keys smaller than 256 bits will require less
   than the current recommended minimal work factor of 2^128 operations.



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   As such, connections that use this key exchange methods MUST use
   ciphers with at least 256 bit keys to retain quantum resistance.

6.2.  User authentication

   For the connection to remain resistant against quantum computers, the
   user authentication needs to also use quantum resistant algorithms.
   In particular, it's RECOMMENDED that connections use gssapi-keyex for
   client authentication.  The publickey mechanism MUST NOT be used
   unless the asymmetric keys used for it use post-quantum algorithms.
   DSA, ECDSA, and RSA keys MUST NOT be used.

6.3.  Used GSSAPI Mechanisms

   The security of the key exchange depends on the security of the used
   GSSAPI mechanism.  The described key exchange will be quantum
   resistant only in case the used GSSAPI mechanism is quantum
   resistant.

   For example, the Kerberos 5 mechanism is quantum resistant only when
   it's used together with algorithms and key sizes that are quantum
   resistant.  Quantum safe algorithm SHOULD be used throught the
   kerberos infrastructure, both for authentication and encryption.
   Currently aes256-cts-hmac-sha384-192 mechanism defined in [RFC8009]
   for encryption is an example of such an algorithm.

6.4.  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
   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

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






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   [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>.

   [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>.

   [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>.

   [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>.

   [RFC8732]  Sorce, S. and H. Kario, "Generic Security Service
              Application Program Interface (GSS-API) Key Exchange with
              SHA-2", RFC 8732, DOI 10.17487/RFC8732, February 2020,
              <https://www.rfc-editor.org/info/rfc8732>.

7.2.  Informative References

   [IANA-KEX-NAMES]
              Internet Assigned Numbers Authority, "Secure Shell (SSH)
              Protocol Parameters: Key Exchange Method Names", 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, November 2015,
              <http://standards.iso.org/ittf/PubliclyAvailableStandards/
              c068345_ISO_IEC_8825-1_2015.zip>.






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   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc6194>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC8009]  Jenkins, M., Peck, M., and K. Burgin, "AES Encryption with
              HMAC-SHA2 for Kerberos 5", RFC 8009, DOI 10.17487/RFC8009,
              October 2016, <https://www.rfc-editor.org/info/rfc8009>.

Author's Address

   Hubert Kario
   Red Hat, Inc.
   Purkynova 115
   Brno  612 00
   Czech Republic

   Email: hkario@redhat.com




















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