



Internet Engineering Task Force                          A. Adamantiadis
Internet-Draft                                                    libssh
Intended status: Standards Track                            S. Josefsson
Expires: March 7, 2020                                            SJD AB
                                                              M. Baushke
                                                  Juniper Networks, Inc.
                                                       September 4, 2019


  Secure Shell (SSH) Key Exchange Method using Curve25519 and Curve448
                    draft-ietf-curdle-ssh-curves-12

Abstract

   This document describes the specification 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
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   This Internet-Draft will expire on March 7, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Key Exchange Methods  . . . . . . . . . . . . . . . . . . . .   2
     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  . . . . . . . . . . . . . . . . .   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] defines how elliptic curves are integrated
   into this extensible SSH framework, and this document reuses the
   Elliptic Curve Diffie-Hellman (ECDH) key exchange protocol messages
   defined in section 7.1 "ECDH Message Numbers" [RFC5656].  Other parts
   of [RFC5656], such as Elliptic Curve Menezes-Qu-Vanstone (ECMQV) key
   agreement, and Elliptic Curve Digital Signature Algorithm (ECDSA) are
   not considered in this document.

   This document describes how to implement key exchange based on
   Curve25519 and Curve448 [RFC7748] in SSH.  For Curve25519 with
   SHA-256 [RFC6234] and [SHS], the algorithm described is equivalent to
   the privately defined algorithm "curve25519-sha256@libssh.org", which
   at the time of publication was implemented and widely deployed in
   libssh [libssh] and OpenSSH [OpenSSH].  The Curve448 key exchange
   method is similar but uses SHA-512 [RFC6234] and [SHS].

2.  Requirements Language

   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.

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.



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   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-sha256" and "curve448-sha512"
   perform the Diffie-Hellman 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 described in Section 7 of [RFC7748].
   Clients and servers MUST also abort if the length of the received
   public keys are not the expected lengths.  An abort for these
   purposes is defined as a disconnect (SSH_MSG_DISCONNECT) of the
   session and SHOULD use the SSH_DISCONNECT_KEY_EXCHANGE_FAILED reason
   for the message [IANA-REASON].  No further validation is required
   beyond what is described in [RFC7748].  The derived shared secret is
   32 bytes when "curve25519-sha256" is used and 56 bytes when
   "curve448-sha512" is used.  The encodings of all values are defined
   in [RFC7748].  The hash used is SHA-256 for "curve25519-sha256" and
   SHA-512 for "curve448-sha512".

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
   document.

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





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   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 function 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 with SHA-256 provides strong (~128 bits) 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 provides stronger (~224 bits)
   security with similar implementation properties, but has not received
   the same cryptographic review as Curve25519, and is slower (larger
   key material and larger secure hash algorithm), but it is provided as
   a hedge to combat unforeseen analytical advances against Curve25519
   and SHA-256 due to the larger number of security bits.

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



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   This document provides "curve25519-sha256" as the preferred choice,
   but suggests that the "curve448-sha512" is implemented to provide
   more than 128 bits of security strength should that become a
   requirement.

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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250,
              DOI 10.17487/RFC4250, January 2006,
              <https://www.rfc-editor.org/info/rfc4250>.

   [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,
              <https://www.rfc-editor.org/info/rfc5656>.

   [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>.

   [SHS]      Information Technology Laboratory National Institute of
              Standards and Technology, "Secure Hash Standard (SHS)",
              August 2015, <http://dx.doi.org/10.6028/NIST.FIPS.180-4>.






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7.2.  Informative References

   [IANA-KEX]
              Internet Assigned Numbers Authority (IANA), "Secure Shell
              (SSH) Protocol Parameters: Key Exchange Method Names",
              August 2019, <http://www.iana.org/assignments/ssh-
              parameters/ssh-parameters.xhtml#ssh-parameters-16>.

   [IANA-REASON]
              Internet Assigned Numbers Authority (IANA), "Secure Shell
              (SSH) Protocol Parameters: Disconnection Messages Reason
              Codes and Descriptions", August 2019,
              <http://www.iana.org/assignments/ssh-parameters/
              ssh-parameters.xhtml#ssh-parameters-3>.

   [libssh]   libssh, "The SSH Library", September 2019,
              <https://www.libssh.org/>.

   [OpenSSH]  OpenSSH group of OpenBSD, "The OpenSSH Project", September
              2019, <https://www.openssh.com/>.

   [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>.

   [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>.

Authors' Addresses

   Aris Adamantiadis
   libssh

   Email: aris@badcode.be


   Simon Josefsson
   SJD AB

   Email: simon@josefsson.org


   Mark D. Baushke
   Juniper Networks, Inc.

   Email: mdb@juniper.net



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