Internet DRAFT - draft-ietf-curdle-ssh-curves
draft-ietf-curdle-ssh-curves
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
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 March 7, 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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Adamantiadis, et al. Expires March 7, 2020 [Page 1]
Internet-Draft Curve25519/448 for SSH September 2019
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.
Adamantiadis, et al. Expires March 7, 2020 [Page 2]
Internet-Draft Curve25519/448 for SSH September 2019
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.
Adamantiadis, et al. Expires March 7, 2020 [Page 3]
Internet-Draft Curve25519/448 for SSH September 2019
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.
Adamantiadis, et al. Expires March 7, 2020 [Page 4]
Internet-Draft Curve25519/448 for SSH September 2019
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>.
Adamantiadis, et al. Expires March 7, 2020 [Page 5]
Internet-Draft Curve25519/448 for SSH September 2019
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
Adamantiadis, et al. Expires March 7, 2020 [Page 6]
