Network Working Group S. Josefsson
Internet-Draft SJD AB
Intended status: Standards Track June 8, 2015
Expires: December 10, 2015
EdDSA and Ed25519 for Transport Layer Security (TLS)
draft-josefsson-tls-eddsa2-00
Abstract
This document introduce the public-key signature algorithm EdDSA for
use in Transport Layer Security (TLS). By using a new
SignatureAlgorithm value, defined in this document, together with the
NamedCurve and ECPointFormat assignments from the Curve25519 ECDHE
key exchange mechanism, we describe how EdDSA is used for digital
signatures in the existing ECDSA cipher suites. This is intended to
work with any version of TLS and Datagram TLS.
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1. Introduction
TLS [RFC5246] and DTLS [RFC6347] support different key exchange
algorithms and authentication mechanisms. In ECC in TLS [RFC4492],
key exchange and authentication using ECC is specified, where the
NamedCurve and ECPointFormat registries and associated TLS extensions
are introduced.
In [I-D.josefsson-tls-curve25519] support for ECDHE key exchange with
the Curve25519 curve is added. That document introduces a new
NamedCurve value for Curve25519, and a new ECPointFormat value to
correspond to the public-key encoding.
This document describes how to use EdDSA and Ed25519
[I-D.josefsson-eddsa-ed25519] as a new authentication mechanism in
TLS. It reuses the newly registered NamedCurve and ECPointFormat
values, and define a new SignatureAlgorithm value to indicate EdDSA.
The goal is that all existing ECDSA cipher suites will, when the
EdDSA SignatureAlgorithm is negotiated, use EdDSA instead of the
traditional ECDSA signature algorithm.
This document is a self-contained alternative to draft-josefsson-tls-
eddsa. This document reuse the ECDSA cipher suites for EdDSA,
whereas draft-josefsson-tls-eddsa specify new cipher suites for
EdDSA. It is an open issue which approach is to be prefered.
1.1. Requirements Terminology
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 [RFC2119].
2. EdDSA SignatureAlgorithm
Negotiation of the authentication mechanism is signalled by sending a
SignatureAlgorithm value. Here we extend this enumeration for EdDSA.
enum {
eddsa(4)
} SignatureAlgorithm;
EdDSA is suitable for use with TLS [RFC5246] and DTLS [RFC6347].
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3. Using EdDSA in a handshake
The following describe how EdDSA is used in a handshake. For ease of
explanation, we assume a full handshake. [RFC4492] describes the
semantics of ECC in TLS, including how resumed handshakes work, and
this document refer to it for a complete discussion.
A client that wish to negotiate use of EdDSA in a handshake MUST
offer a ECDSA key exchange algorithm and send, in the same way as is
done for other Signature Algorithms in ECDSA, a Signature Algorithm
extension that include the "eddsa" SignatureAlgorithm value
The HashAlgorithm value to specify for EdDSA MUST be "none" as the
EdDSA signature algorithm does not hash the input before signing.
The client MUST also include a Supported Elliptic Curves Extension
("NamedCurve") containing the "Curve25519" value defined in
[I-D.josefsson-tls-curve25519]. This is used to indicate request for
Ed25519. Use of other curves with EdDSA is not defined here.
The client MUST also include a Supported Point Formats Extension
("ECPointFormat") that contains the "montgomery_x_le" value defined
in [I-D.josefsson-tls-curve25519].
This document RECOMMENDS use of ECDHE_ECDSA key exchange algorithm,
in particular with Curve25519, rather than the ECDH_ECDSA key
exchange algorithm.
4. IANA Considerations
EdDSA should be registered in the Transport Layer Security (TLS)
Parameters [IANA-TLS] registry under "SignatureAlgorithm" as follows.
+-------+-------------+---------+-----------+
| Value | Description | DTLS-OK | Reference |
+-------+-------------+---------+-----------+
| 4 | eddsa | Y | This doc |
+-------+-------------+---------+-----------+
5. Security Considerations
The security considerations of TLS [RFC5246], DTLS [RFC6347], ECC in
TLS [RFC4492] Curve25519 in TLS [I-D.josefsson-tls-curve25519], and
EdDSA and Ed25519 [I-D.josefsson-eddsa-ed25519] are inherited.
As with all cryptographic algorithms, the reader should stay informed
about new research insights into the security of the algorithms
involved.
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While discussed in the EdDSA/Ed25519 specification and papers, we
would like to stress the significance of secure implementation of
EdDSA/Ed25519. For example, implementations ought to be constant-
time to avoid side-channel attacks.
6. Acknowledgements
Thanks to Yoav Nir for suggesting re-use of ECDSA cipher suites with
EdDSA, to reduce the cartesian product cipher suite explosion.
Thanks to Klaus Hartke and Nicolas Williams for fixes to the
document.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[I-D.josefsson-tls-curve25519]
Josefsson, S. and M. Pegourie-Gonnard, "Curve25519 for
ephemeral key exchange in Transport Layer Security (TLS)",
draft-josefsson-tls-curve25519-06 (work in progress),
September 2014.
[I-D.josefsson-eddsa-ed25519]
Josefsson, S. and N. Moller, "EdDSA and Ed25519", draft-
josefsson-eddsa-ed25519-02 (work in progress), February
2015.
7.2. Informative References
[IANA-TLS]
Internet Assigned Numbers Authority, "Transport Layer
Security (TLS) Parameters",
<http://www.iana.org/assignments/tls-parameters/
tls-parameters.xml>.
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Author's Address
Simon Josefsson
SJD AB
Email: simon@josefsson.org
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