draft-ietf-curdle-ssh-curves-05.txt   draft-ietf-curdle-ssh-curves-06.txt 
Internet Engineering Task Force A. Adamantiadis Internet Engineering Task Force A. Adamantiadis
Internet-Draft libssh Internet-Draft libssh
Intended status: Standards Track S. Josefsson Intended status: Standards Track S. Josefsson
Expires: November 11, 2017 SJD AB Expires: May 16, 2018 SJD AB
M. Baushke M. Baushke
Juniper Networks, Inc. Juniper Networks, Inc.
May 10, 2017 November 12, 2017
Secure Shell (SSH) Key Exchange Method using Curve25519 and Curve448 Secure Shell (SSH) Key Exchange Method using Curve25519 and Curve448
draft-ietf-curdle-ssh-curves-05 draft-ietf-curdle-ssh-curves-06
Abstract Abstract
This document describes the conventions for using Curve25519 and This document describes the conventions for using Curve25519 and
Curve448 key exchange methods in the Secure Shell (SSH) protocol. Curve448 key exchange methods in the Secure Shell (SSH) protocol.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on November 11, 2017. This Internet-Draft will expire on May 16, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Key Exchange Methods . . . . . . . . . . . . . . . . . . . . 2 2. Key Exchange Methods . . . . . . . . . . . . . . . . . . . . 3
2.1. Shared Secret Encoding . . . . . . . . . . . . . . . . . 3 2.1. Shared Secret Encoding . . . . . . . . . . . . . . . . . 3
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5 6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 5 6.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Copying conditions . . . . . . . . . . . . . . . . . 6 Appendix A. Copying conditions . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction 1. Introduction
Secure Shell (SSH) [RFC4251] is a secure remote login protocol. The Secure Shell (SSH) [RFC4251] is a secure remote login protocol. The
key exchange protocol described in [RFC4253] supports an extensible key exchange protocol described in [RFC4253] supports an extensible
skipping to change at page 2, line 35 skipping to change at page 2, line 35
This document describes how to implement key exchange based on This document describes how to implement key exchange based on
Curve25519 and Ed448-Goldilocks [RFC7748] in SSH. For Curve25519 Curve25519 and Ed448-Goldilocks [RFC7748] in SSH. For Curve25519
with SHA-256 [RFC6234], the algorithm we describe is equivalent to with SHA-256 [RFC6234], the algorithm we describe is equivalent to
the privately defined algorithm "curve25519-sha256@libssh.org", which the privately defined algorithm "curve25519-sha256@libssh.org", which
is currently implemented and widely deployed in libssh and OpenSSH. is currently implemented and widely deployed in libssh and OpenSSH.
The Curve448 key exchange method is novel but similar in spirit, and 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 we chose to couple it with SHA-512 [RFC6234] to further separate it
from the Curve25519 alternative. from the Curve25519 alternative.
This document provide Curve25519 as the prefered choice, but suggests This document provide Curve25519 as the preferred choice, but
that the fall back option Curve448 is implemented to provide an hedge suggests that the fall back option Curve448 is implemented to provide
against unforseen analytical advances against Curve25519 and SHA-256. an hedge against unforeseen analytical advances against Curve25519
Due to different implementation status of these two curves (high- and SHA-256. Due to different implementation status of these two
quality free implementations of Curve25519 has been in deployed use curves (high-quality free implementations of Curve25519 has been in
for several years, while Curve448 implementations are slowly deployed use for several years, while Curve448 implementations are
appearing), it is accepted that adoption of Curve448 will be slower. slowly appearing), it is accepted that adoption of Curve448 will be
slower.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in RFC 2119 [RFC2119]. "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.
2. Key Exchange Methods 2. Key Exchange Methods
The key exchange procedure is similar to the ECDH method described in The key exchange procedure is similar to the ECDH method described in
chapter 4 of [RFC5656], though with a different wire encoding used chapter 4 of [RFC5656], though with a different wire encoding used
for public values and the final shared secret. Public ephemeral keys for public values and the final shared secret. Public ephemeral keys
are encoded for transmission as standard SSH strings. are encoded for transmission as standard SSH strings.
The protocol flow, the SSH_MSG_KEX_ECDH_INIT and The protocol flow, the SSH_MSG_KEX_ECDH_INIT and
SSH_MSG_KEX_ECDH_REPLY messages, and the structure of the exchange SSH_MSG_KEX_ECDH_REPLY messages, and the structure of the exchange
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is used. The encodings of all values are defined in [RFC7748]. The is used. The encodings of all values are defined in [RFC7748]. The
hash used is SHA-256 for Curve25519 and SHA-512 for Curve448. hash used is SHA-256 for Curve25519 and SHA-512 for Curve448.
2.1. Shared Secret Encoding 2.1. Shared Secret Encoding
The following step differs from [RFC5656], which uses a different The following step differs from [RFC5656], which uses a different
conversion. This is not intended to modify that text generally, but conversion. This is not intended to modify that text generally, but
only to be applicable to the scope of the mechanism described in this only to be applicable to the scope of the mechanism described in this
document. document.
The shared secret, K, is defined in [RFC4253] as a multiple precision The shared secret, K, is defined in [RFC4253] and [RFC5656] as an
integer (mpint). Curve25519/448 outputs a binary string X, which is integer encoded as a multiple precision integer (mpint).
the 32 or 56 byte point obtained by scalar multiplication of the Curve25519/448 outputs a binary string X, which is the 32 or 56 byte
other side's public key and the local private key scalar. The 32 or point obtained by scalar multiplication of the other side's public
56 bytes of X are converted into K by interpreting the bytes as an key and the local private key scalar. The 32 or 56 bytes of X are
unsigned fixed-length integer encoded in network byte order. This converted into K by interpreting the octets as an unsigned fixed-
conversion follows the normal "mpint" process as described in section length integer encoded in network byte order.
5 of [RFC4251].
To clarify a corner-case in this conversion, when X is encoded as an
mpint K, in order to calculate the exchange hash, it may vary as
follows:
o Trim all leading zero-bytes of X. If X is all zero-bytes, then
the key exchange MUST fail.
o If the high bit of X is set, the mpint format requires a zero byte
to be prepended.
o The length of the encoded K may not be the same as the original
length of X due to trimming or prepending zero-bytes as needed for
"mpint" format.
Or, as pseudo code:
k := x;
while (k.length() > 0 && k[0] == 0) k = k[1:];
assert(k.length() > 0);
if 0 != (k[0] & 0x80) k = '\0' .. k;
Figure 1 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 When performing the X25519 or X448 operations, the integer values
there will be encoded into byte strings by doing a fix-length there will be encoded into byte strings by doing a fixed-length
unsigned litle-endian conversion, per [RFC7748]. It is only later unsigned little-endian conversion, per [RFC7748]. It is only later
when these byte strings are then passed to the ECDH code in SSH that 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 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 integer value K, and then later that K value is encoded as a
variable-length signed "mpint" before being fed to the hash algorithm variable-length signed "mpint" before being fed to the hash algorithm
used for key generation. 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.
3. Acknowledgements 3. Acknowledgements
The "curve25519-sha256" key exchange method is identical to the The "curve25519-sha256" key exchange method is identical to the
"curve25519-sha256@libssh.org" key exchange method created by Aris "curve25519-sha256@libssh.org" key exchange method created by Aris
Adamantiadis and implemented in libssh and OpenSSH. Adamantiadis and implemented in libssh and OpenSSH.
Thanks to the following people for review and comments: Denis Bider, Thanks to the following people for review and comments: Denis Bider,
Damien Miller, Niels Moeller, Matt Johnston, Eric Rescorla, Ron Damien Miller, Niels Moeller, Matt Johnston, Eric Rescorla, Ron
Frederick, Stefan Buehler. Frederick, Stefan Buehler.
skipping to change at page 4, line 43 skipping to change at page 4, line 33
4. Security Considerations 4. Security Considerations
The security considerations of [RFC4251], [RFC5656], and [RFC7748] The security considerations of [RFC4251], [RFC5656], and [RFC7748]
are inherited. are inherited.
Curve25519 provide strong security and is efficient on a wide range Curve25519 provide strong security and is efficient on a wide range
of architectures, and has properties that allows better of architectures, and has properties that allows better
implementation properties compared to traditional elliptic curves. implementation properties compared to traditional elliptic curves.
Curve448 with SHA-512 is similar, but has not received the same Curve448 with SHA-512 is similar, but has not received the same
cryptographic review as Curve25519, and is slower, but it is provided cryptographic review as Curve25519, and is slower, but it is provided
as an hedge to combat unforseen analytical advances against as an hedge to combat unforeseen analytical advances against
Curve25519 and SHA-256. Curve25519 and SHA-256.
The way the derived binary secret string is encoded into a mpint The way the derived binary secret string is encoded into a mpint
before it is hashed (i.e., adding or removing zero-bytes for before it is hashed (i.e., adding or removing zero-bytes for
encoding) raises the potential for a side-channel attack which could encoding) raises the potential for a side-channel attack which could
determine the length of what is hashed. This would leak the most determine the length of what is hashed. This would leak the most
significant bit of the derived secret, and/or allow detection of when significant bit of the derived secret, and/or allow detection of when
the most significant bytes are zero. For backwards compatibility the most significant bytes are zero. For backwards compatibility
reasons it was decided not to adress this potential problem. reasons it was decided not to address this potential problem.
5. IANA Considerations 5. IANA Considerations
IANA is requested to add "curve25519-sha256" and "curve448-sha512" to IANA is requested to add "curve25519-sha256" and "curve448-sha512" to
the "Key Exchange Method Names" registry for SSH [IANA-KEX] that was the "Key Exchange Method Names" registry for SSH [IANA-KEX] that was
created in RFC 4250 section 4.10 [RFC4250]. created in RFC 4250 section 4.10 [RFC4250].
6. References 6. References
6.1. Normative References 6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH) [RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250, Protocol Assigned Numbers", RFC 4250,
DOI 10.17487/RFC4250, January 2006, DOI 10.17487/RFC4250, January 2006,
<http://www.rfc-editor.org/info/rfc4250>. <https://www.rfc-editor.org/info/rfc4250>.
[RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
January 2006, <http://www.rfc-editor.org/info/rfc4251>. January 2006, <https://www.rfc-editor.org/info/rfc4251>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) [RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <http://www.rfc-editor.org/info/rfc4253>. January 2006, <https://www.rfc-editor.org/info/rfc4253>.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm [RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer", Integration in the Secure Shell Transport Layer",
RFC 5656, DOI 10.17487/RFC5656, December 2009, RFC 5656, DOI 10.17487/RFC5656, December 2009,
<http://www.rfc-editor.org/info/rfc5656>. <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>.
6.2. Informative References 6.2. Informative References
[IANA-KEX] [IANA-KEX]
Internet Assigned Numbers Authority (IANA), "Secure Shell Internet Assigned Numbers Authority (IANA), "Secure Shell
(SSH) Protocol Parameters: Key Exchange Method Names", (SSH) Protocol Parameters: Key Exchange Method Names",
March 2017, <http://www.iana.org/assignments/ssh- March 2017, <http://www.iana.org/assignments/ssh-
parameters/ssh-parameters.xhtml#ssh-parameters-16>. parameters/ssh-parameters.xhtml#ssh-parameters-16>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, (SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011, DOI 10.17487/RFC6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>. <https://www.rfc-editor.org/info/rfc6234>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <http://www.rfc-editor.org/info/rfc7748>. 2016, <https://www.rfc-editor.org/info/rfc7748>.
Appendix A. Copying conditions Appendix A. Copying conditions
Regarding this entire document or any portion of it, the authors make Regarding this entire document or any portion of it, the authors make
no guarantees and are not responsible for any damage resulting from no guarantees and are not responsible for any damage resulting from
its use. The authors grant irrevocable permission to anyone to use, its use. The authors grant irrevocable permission to anyone to use,
modify, and distribute it in any way that does not diminish the modify, and distribute it in any way that does not diminish the
rights of anyone else to use, modify, and distribute it, provided rights of anyone else to use, modify, and distribute it, provided
that redistributed derivative works do not contain misleading author that redistributed derivative works do not contain misleading author
or version information. Derivative works need not be licensed under or version information. Derivative works need not be licensed under
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