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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12         Standards Track
Internet Engineering Task Force                          A. Adamantiadis
Internet-Draft                                                    libssh
Intended status: Standards Track                            S. Josefsson
Expires: February 3, 2020                                         SJD AB
                                                              M. Baushke
                                                  Juniper Networks, Inc.
                                                          August 2, 2019

  Secure Shell (SSH) Key Exchange Method using Curve25519 and Curve448


   This document describes the conventions for using Curve25519 and
   Curve448 key exchange methods in the Secure Shell (SSH) protocol.

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 February 3, 2020.

Copyright Notice

   Copyright (c) 2019 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Key Exchange Methods  . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Shared Secret Encoding  . . . . . . . . . . . . . . . . .   3
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Copying conditions . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Secure Shell (SSH) [RFC4251] is a secure remote login protocol.  The
   key exchange protocol described in [RFC4253] supports an extensible
   set of methods.  [RFC5656] describes how elliptic curves are
   integrated in SSH, and this document reuses those protocol messages.

   This document describes how to implement key exchange based on
   Curve25519 and Curve448 [RFC7748] in SSH.  For Curve25519 with
   SHA-256 [RFC6234], the algorithm we describe is equivalent to the
   privately defined algorithm "curve25519-sha256@libssh.org", which is
   currently implemented and widely deployed in libssh and OpenSSH.  The
   Curve448 key exchange method is novel but similar in spirit, and we
   chose to couple it with SHA-512 [RFC6234] to further separate it from
   the Curve25519 alternative.

   This document provide Curve25519 as the preferred choice, but
   suggests that the fall back option Curve448 is implemented to provide
   an hedge against unforeseen analytical advances against Curve25519
   and SHA-256.  Due to different implementation status of these two
   curves (high-quality free implementations of Curve25519 has been in
   deployed use for several years, while Curve448 implementations are
   slowly appearing), it is accepted that adoption of Curve448 will be

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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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3.  Key Exchange Methods

   The key exchange procedure is similar to the ECDH method described in
   chapter 4 of [RFC5656], though with a different wire encoding used
   for public values and the final shared secret.  Public ephemeral keys
   are encoded for transmission as standard SSH strings.

   The protocol flow, the SSH_MSG_KEX_ECDH_INIT and
   SSH_MSG_KEX_ECDH_REPLY messages, and the structure of the exchange
   hash are identical to chapter 4 of [RFC5656].

   The method names registered by this document are "curve25519-sha256"
   and "curve448-sha512".

   The methods are based on Curve25519 and Curve448 scalar
   multiplication, as described in [RFC7748].  Private and public keys
   are generated as described therein.  Public keys are defined as
   strings of 32 bytes for Curve25519 and 56 bytes for Curve448.

   Key-agreement schemes Curve25519 and Curve448 perform the Diffie-
   Helman protocol using the functions X25519 and X448, respectively.
   Implementations SHOULD compute these functions using the algorithms
   described in [RFC7748].  When they do so, implementations MUST check
   whether the computed Diffie-Hellman shared secret is the all-zero
   value and abort if so, as described in Section 6 of [RFC7748].
   Alternative implementations of these functions SHOULD abort when
   either input forces the shared secret to one of a small set of
   values, as discussed in Section 7 of [RFC7748].  Clients and servers
   MUST fail the key exchange if the length of the received public keys
   are not the expected lengths.  No further validation is required
   beyond what is discussed in [RFC7748].  The derived shared secret is
   32 bytes when Curve25519 is used and 56 bytes when Curve448 is used.
   The encodings of all values are defined in [RFC7748].  The hash used
   is SHA-256 for Curve25519 and SHA-512 for Curve448.

3.1.  Shared Secret Encoding

   The following step differs from [RFC5656], which uses a different
   conversion.  This is not intended to modify that text generally, but
   only to be applicable to the scope of the mechanism described in this

   The shared secret, K, is defined in [RFC4253] and [RFC5656] as an
   integer encoded as a multiple precision integer (mpint).
   Curve25519/448 outputs a binary string X, which is the 32 or 56 byte
   point obtained by scalar multiplication of the other side's public
   key and the local private key scalar.  The 32 or 56 bytes of X are

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   converted into K by interpreting the octets as an unsigned fixed-
   length integer encoded in network byte order.

   The integer K is then encoded as an mpint using the process described
   in section 5 of [RFC4251] and the resulting bytes are fed as
   described in [RFC4253] to the key exchange method's hash function to
   generate encryption keys.

   When performing the X25519 or X448 operations, the integer values
   there will be encoded into byte strings by doing a fixed-length
   unsigned little-endian conversion, per [RFC7748].  It is only later
   when these byte strings are then passed to the ECDH code in SSH that
   the bytes are re-interpreted as a fixed-length unsigned big-endian
   integer value K, and then later that K value is encoded as a
   variable-length signed "mpint" before being fed to the hash algorithm
   used for key generation.  The mpint K is then fed along with other
   data to the key exchange method's hash function to generate
   encryption keys.

4.  Acknowledgements

   The "curve25519-sha256" key exchange method is identical to the
   "curve25519-sha256@libssh.org" key exchange method created by Aris
   Adamantiadis and implemented in libssh and OpenSSH.

   Thanks to the following people for review and comments: Denis Bider,
   Damien Miller, Niels Moeller, Matt Johnston, Eric Rescorla, Ron
   Frederick, Stefan Buehler.

5.  Security Considerations

   The security considerations of [RFC4251], [RFC5656], and [RFC7748]
   are inherited.

   Curve25519 provide strong security and is efficient on a wide range
   of architectures, and has properties that allows better
   implementation properties compared to traditional elliptic curves.
   Curve448 with SHA-512 is similar, but has not received the same
   cryptographic review as Curve25519, and is slower, but it is provided
   as an hedge to combat unforeseen analytical advances against
   Curve25519 and SHA-256.

   The way the derived binary secret string is encoded into a mpint
   before it is hashed (i.e., adding or removing zero-bytes for
   encoding) raises the potential for a side-channel attack which could
   determine the length of what is hashed.  This would leak the most
   significant bit of the derived secret, and/or allow detection of when

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   the most significant bytes are zero.  For backwards compatibility
   reasons it was decided not to address this potential problem.

6.  IANA Considerations

   IANA is requested to add "curve25519-sha256" and "curve448-sha512" to
   the "Key Exchange Method Names" registry for SSH [IANA-KEX] that was
   created in RFC 4250 section 4.10 [RFC4250].

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250,
              DOI 10.17487/RFC4250, January 2006,

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
              January 2006, <https://www.rfc-editor.org/info/rfc4251>.

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

   [RFC5656]  Stebila, D. and J. Green, "Elliptic Curve Algorithm
              Integration in the Secure Shell Transport Layer",
              RFC 5656, DOI 10.17487/RFC5656, December 2009,

   [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

              Internet Assigned Numbers Authority (IANA), "Secure Shell
              (SSH) Protocol Parameters: Key Exchange Method Names",
              June 2018, <http://www.iana.org/assignments/ssh-

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   [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,

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

Appendix A.  Copying conditions

   Regarding this entire document or any portion of it, the authors make
   no guarantees and are not responsible for any damage resulting from
   its use.  The authors grant irrevocable permission to anyone to use,
   modify, and distribute it in any way that does not diminish the
   rights of anyone else to use, modify, and distribute it, provided
   that redistributed derivative works do not contain misleading author
   or version information.  Derivative works need not be licensed under
   similar terms.

Authors' Addresses

   Aris Adamantiadis

   Email: aris@badcode.be

   Simon Josefsson

   Email: simon@josefsson.org

   Mark D. Baushke
   Juniper Networks, Inc.

   Email: mdb@juniper.net

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