Network Working Group S. Josefsson
Internet-Draft SJD AB
Intended status: Standards Track June 16, 2015
Expires: December 18, 2015
EdDSA and Ed25519 for Transport Layer Security (TLS)
draft-josefsson-tls-eddsa2-02
Abstract
This document introduce the public-key signature algorithm EdDSA for
use in Transport Layer Security (TLS). By defining new
SignatureAlgorithm and NamedCurve enumeration values, we describe how
EdDSA and Ed25519 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, and define the
SignatureAlgorithm enumeration for different signature algorithms.
In TLS-ECC [RFC4492], key exchange and authentication using ECC is
specified, where the NamedCurve registry and associated TLS
extensions are introduced.
This document describes how to use EdDSA and Ed25519
[I-D.josefsson-eddsa-ed25519] as a new authentication mechanism in
TLS. It define new SignatureAlgorithm and NamedCurve enumeration
values and describe how these are used to negotiate EdDSA-based
signatures.
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(TBD1)
} SignatureAlgorithm;
EdDSA is suitable for use with TLS [RFC5246] and DTLS [RFC6347].
Note that EdDSA merely signals which signature algorithm to use, it
does not imply any curve or hash parameter choice.
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For TLS 1.0 and 1.1 the DigitallySigned structure did not convey the
signature algorithm to use, and hence implementations need to inspect
the certificate to find out the signature algorithm to use.
3. Ed25519 NamedCurve
Negotiation the elliptic curve is signalled with the Supported
Elliptic Curves Extension extension. Here we extend the NamedCurve
enumeration for EdDSA with the Ed25519 parameter choice.
enum {
ed25519(TBD3)
} NamedCurve;
4. 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 and NamedCurve types are also used, but depend on the
actual EdDSA parameter choices.
The HashAlgorithm value to specify for Ed25519 MUST be "sha512", to
indicate use of Ed25519-SHA-512 as defined in
[I-D.josefsson-eddsa-ed25519]. The meaning of using a "eddsa"
SignatureAlgorithm with other HashAlgorithms is not defined here.
The Supported Elliptic Curves Extension ("NamedCurve") must contain
the relevant curve. Currently the only curve applicable to EdDSA is
the "ed25519" NamedCurve value defined in this document. This is
used to indicate request for Ed25519.
Where applicable, the Supported Point Formats Extension
("ECPointFormat") value to use for EdDSA is "uncompressed". The
format of EdDSA public-keys, e.g. Ed25519-SHA-512 public-keys,
follows from the NamedCurve and HashAlgorithm value.
5. IANA Considerations
IANA is requested to assign a number for EdDSA described in Section 2
to the Transport Layer Security (TLS) Parameters [IANA-TLS] registry
under "SignatureAlgorithm" as follows.
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+-------+-------------+---------+-----------+
| Value | Description | DTLS-OK | Reference |
+-------+-------------+---------+-----------+
| TBD1 | eddsa | Y | This doc |
+-------+-------------+---------+-----------+
IANA is requested to assign a number for Ed25519 described in
Section 3 to the Transport Layer Security (TLS) Parameters registry
EC Named Curve [IANA-TLS] as follows.
+-------+-------------+---------+-----------+
| Value | Description | DTLS-OK | Reference |
+-------+-------------+---------+-----------+
| TBD2 | ed25519 | Y | This doc |
+-------+-------------+---------+-----------+
6. Security Considerations
The security considerations of TLS [RFC5246], DTLS [RFC6347], TLS-ECC
[RFC4492], and EdDSA and Ed25519 [I-D.josefsson-eddsa-ed25519] are
inherited.
As with all cryptographic algorithms and security protocols, the
reader should stay informed about new research insights into the
security of the algorithms and protocols involved.
While discussed in the EdDSA/Ed25519 specification and papers, we
would like to stress the significance of EdDSA/Ed25519 implementation
security. In particular, implementations must avoid side-channel
attacks by, for example, being constant-time.
7. 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 numerous fixes to the
document. Other people who contributed include Ilari Liusvaara and
Martin Thomson.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[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-eddsa-ed25519]
Josefsson, S. and N. Moller, "EdDSA and Ed25519", draft-
josefsson-eddsa-ed25519-02 (work in progress), February
2015.
8.2. Informative References
[IANA-TLS]
Internet Assigned Numbers Authority, "Transport Layer
Security (TLS) Parameters",
<http://www.iana.org/assignments/tls-parameters/
tls-parameters.xml>.
Author's Address
Simon Josefsson
SJD AB
Email: simon@josefsson.org
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