One document matched: draft-josefsson-ssh-curves-02.txt
Differences from draft-josefsson-ssh-curves-01.txt
Network Working Group A. Adamantiadis
Internet-Draft libssh
Intended status: Informational S. Josefsson
Expires: May 21, 2016 SJD AB
November 18, 2015
Secure Shell (SSH) Key Exchange Method using Curve25519 and Curve448
draft-josefsson-ssh-curves-02
Abstract
How to implement the Curve25519 and Curve448 key exchange methods in
the Secure Shell (SSH) protocol is described.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 21, 2016.
Copyright Notice
Copyright (c) 2015 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Key Exchange Methods . . . . . . . . . . . . . . . . . . . . 2
2.1. Shared Secret Encoding . . . . . . . . . . . . . . . . . 3
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. Normative References . . . . . . . . . . . . . . . . . . 4
6.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Copying conditions . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
In [Curve25519], a new elliptic curve function for use in
cryptographic applications was introduced. In [Ed448-Goldilocks] the
Ed448-Goldilocks curve (also known as Curve448) is described. In
[I-D.irtf-cfrg-curves], the Diffie-Hellman functions using Curve25519
and Curve448 are specified.
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 in SSH. For Curve25519, 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.
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].
2. 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].
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The method names registered by this document are "curve25519-sha256"
and "curve448-sha256".
The methods are based on Curve25519 and Curve448 scalar
multiplication, as described in [I-D.irtf-cfrg-curves]. 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. Clients and servers MUST verify the length of the received
public keys, but no further validation is required beyond what is
discussed in [I-D.irtf-cfrg-curves]. 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 [I-D.irtf-cfrg-curves].
2.1. 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 this document.
The shared secret, K, is defined in [RFC4253] 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 bytes as an
unsigned fixed-length integer encoded in network byte order.
When K is encoded as mpint in order to calculate the exchange hash,
its encoding will often be identical to X, but will vary as follows:
o If the high bit of X is set, the mpint format requires a zero byte
to be prepended. In this case, encoded K is larger than encoded
X.
o If X has leading zero bytes, the mpint format requires such bytes
to be skipped. In this case, encoded K is smaller than encoded X.
3. 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.
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4. Security Considerations
The security considerations of [RFC4251], [RFC5656], and
[I-D.irtf-cfrg-curves] are inherited.
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.
5. IANA Considerations
IANA is requested to add "curve25519-sha256" and "curve448-sha256" to
the "Key Exchange Method Names" registry for SSH that was created in
RFC 4250 section 4.10 [RFC4250].
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250, DOI 10.17487/
RFC4250, January 2006,
<http://www.rfc-editor.org/info/rfc4250>.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[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>.
[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>.
[I-D.irtf-cfrg-curves]
Langley, A. and M. Hamburg, "Elliptic Curves for
Security", draft-irtf-cfrg-curves-10 (work in progress),
October 2015.
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6.2. Informative References
[Curve25519]
Bernstein, J., "Curve25519: New Diffie-Hellman Speed
Records", LNCS 3958, pp. 207-228, February 2006,
<http://dx.doi.org/10.1007/11745853_14>.
[Ed448-Goldilocks]
Hamburg, , "Ed448-Goldilocks, a new elliptic curve", June
2015, <https://eprint.iacr.org/2015/625>.
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
libssh
Email: aris@badcode.be
Simon Josefsson
SJD AB
Email: simon@josefsson.org
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