One document matched: draft-josefsson-ssh-curves-01.xml

<?xml version="1.0"?>

<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
<!ENTITY rfc2119 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml">
<!ENTITY rfc4250 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4250.xml">
<!ENTITY rfc4251 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4251.xml">
<!ENTITY rfc4253 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4253.xml">
<!ENTITY rfc5656 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5656.xml">
<!ENTITY CURVES PUBLIC '' 'http://xml2rfc.ietf.org/public/rfc/bibxml3/reference.I-D.irtf-cfrg-curves.xml'>
]>

<?rfc compact="yes"?>
<?rfc toc="yes"?>
<?rfc symrefs="yes"?>

<rfc category="info" ipr="trust200902"
     docName="draft-josefsson-ssh-curves-01">

  <front>

    <title abbrev="Curve25519/448 for SSH">
      Secure Shell (SSH) Key Exchange Method using Curve25519 and
      Curve448
    </title>
    
    <author initials="A." surname="Adamantiadis" fullname="Aris Adamantiadis">
      <organization>libssh</organization>
      <address>
	<email>aris@badcode.be</email>
      </address>
    </author>

    <author initials="S." surname="Josefsson" fullname="Simon Josefsson">
      <organization>SJD AB</organization>
      <address>
	<email>simon@josefsson.org</email>
      </address>
    </author>

    <date month="November" year="2015"/>

    <abstract>

      <t>
	How to implement the Curve25519 and Curve448 key exchange
	methods in the Secure Shell (SSH) protocol is described.
      </t>

    </abstract>

  </front>
  
  <middle>

    <section title="Introduction">

      <t>
	In <xref target="Curve25519" />, a new elliptic curve function
	for use in cryptographic applications was introduced.  In
	<xref target="Ed448-Goldilocks" /> the Ed448-Goldilocks curve
	(also known as Curve448) is described.  In <xref
	target="I-D.irtf-cfrg-curves"/>, the Diffie-Hellman functions
	using Curve25519 and Curve448 are specified.
      </t>

      <t>
	Secure Shell (SSH) <xref target="RFC4251"/> is a secure remote
	login protocol.  The key exchange key exchange protocol
	described in <xref target="RFC4253"/> supports an extensible
	set of methods.  In <xref target="RFC5656"/> it is described
	how elliptic curves are integrated in SSH, and this document
	reuses those protocol messages.
      </t>

      <t>
	This document describe how key exchange based on Curve25519
	and Curve448 is achieved in SSH.  For Curve25519 what we
	described is identical to an already implemented (in libssh
	and OpenSSH) and widely deployed proposal registered in the
	private namespace ("curve25519-sha256@libssh.org").  The
	Curve448 key exchange method is novel but similar in spirit.
      </t>
      
      <t>
	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 <xref target="RFC2119">RFC 2119</xref>.
      </t>

    </section>

    <section title="Key Exchange Methods">

      <t>
	The key exchange procedure is similar to the ECDH method
	described in chapter 4 of <xref target="RFC5656" />, though
	with a different wire encoding used for public values and the
	final shared secret.  Public ephemeral keys are transmitted
	over SSH encapsulated into standard SSH strings.
      </t>

      <t>
	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 with chapter 4 of <xref
	target="RFC5656" />.
      </t>

      <t>
	The method names registered by this document are
	"curve25519-sha256" and "curve448-sha256".
      </t>

      <t>
	The whole method is based on the Curve25519 and Curve448
	scalar multiplication, as described in <xref
	target="I-D.irtf-cfrg-curves"/>.  Private and public keys are
	generated as described therein, and no special validation is
	required beyond what is discussed there.  Public keys are
	defined as strings of 32 bytes for Curve25519 and 56 bytes for
	Curve448.  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 <xref
	target="I-D.irtf-cfrg-curves"/>.
      </t>

      <t>
	The shared secret, k, is defined in SSH specifications to be a
	multiple precision integer (mpint).  Curve25519/448 outputs a
	binary string.  The binary string is converted into a number
	as follows.  This step differs from <xref target="RFC5656"/>
	which do not need this conversion.  X is the 32 or 56 bytes
	point obtained by the scalar multiplication of the other
	side's public key and the local private key scalar.  The whole
	32 or 56 bytes of the number X is then converted into a
	multiple precision integer (mpint) k.  This conversion follows
	the network byte order.  Since the SSH mpint encoding use the
	most significant bit to signal negative integers, this means
	that if the most significant bit of the derived secret is set,
	a zero byte is prepended to encode the correct value.
      </t>

    </section>

    <section title="Acknowledgements">

      <t>
	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.
      </t>

      <t>
	Thanks to the following people for review and comments: Denis
	Bider, Damien Miller.
      </t>

    </section>

    <section title="Security Considerations">

      <t>
	The security considerations of <xref target="RFC4251"/>, <xref
	target="RFC5656"/>, and <xref target="I-D.irtf-cfrg-curves"/>
	are inherited.
      </t>

      <t>
	The way the derived binary secret string is encoded into a
	mpint before it is hashed (i.e., adding a zero byte when the
	most significant bit of the secret is set) raise the potential
	for a side-channel attack determining the length of what is
	hashed which would leak the most significant bit of the
	derived secret.
      </t>
      
    </section>

    <section anchor="iana-considerations" title="IANA Considerations">

      <t>
	IANA is requested to add "curve25519-sha256" and
	"curve448-sha256" to the "Key Exchange Method Names" registry
	for SSH that was created in <xref target="RFC4250">RFC 4250
	section 4.10</xref>.
      </t>

    </section>

  </middle>

  <back>

    <references title="Normative References">

      &rfc2119;
      &rfc4250;
      &rfc4251;
      &rfc4253;
      &rfc5656;
      &CURVES;

    </references>

    <references title="Informative References">

     <reference anchor="Curve25519"
		target="http://dx.doi.org/10.1007/11745853_14">
       <front>
         <title>
           Curve25519: New Diffie-Hellman Speed Records
         </title>
         <author surname="Bernstein" initials="J."
                 fullname="Daniel J. Bernstein">
           <organization></organization>
         </author>
         <date month="February" year="2006"/>
       </front>
       <seriesInfo name="LNCS" value="3958, pp. 207-228" />
     </reference>

     <reference anchor="Ed448-Goldilocks"
		target="https://eprint.iacr.org/2015/625">
       <front>
         <title>
           Ed448-Goldilocks, a new elliptic curve
         </title>
         <author surname="Hamburg" fullname="Mike Hamburg">
           <organization></organization>
         </author>
         <date month="June" year="2015"/>
       </front>
     </reference>

    </references>

    <section title="Copying conditions">

      <t>
	Regarding this entire document or any portion of it, the
	authors makes no guarantees and is not responsible for any
	damage resulting from its use.  The authors grants 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.
      </t>

    </section>

  </back>

</rfc>