Network Working Group S. Josefsson
Internet-Draft SJD AB
Intended status: Standards Track June 9, 2015
Expires: December 11, 2015
EdDSA and Ed25519 for Transport Layer Security (TLS)
draft-josefsson-tls-eddsa2-01
Abstract
This document introduce the public-key signature algorithm EdDSA for
use in Transport Layer Security (TLS). By defining new
SignatureAlgorithm, NamedCurve and ECPointFormat enumerations, 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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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 and ECPointFormat registries 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, NamedCurve and ECPointFormat
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(4)
} 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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3. EdDSA Public-key format
This section defines a new point format suitable to encode EdDSA
public keys, as well as an identifier to negotiate this new format in
TLS, and includes guidance on their use.
The curves defined in [RFC4492] define a public key as a point on the
curve. In order to exchange public keys, the points are serialized
as a string of bytes. Since EdDSA public keys already are string of
bytes, no serialization is needed.
We define the following ECPointFormat value for raw binary EdDSA
public keys.
enum {
eddsa_public_key(4),
} ECPointFormat;
When included in a ServerKeyExchange or ClientKeyExchange message,
the public key is wrapped in an ECPoint structure as defined in
[RFC4492], whose payload is as described above.
For example, a Ed25519 public key with value
D75A980182B10AB7D54BFED3C964073A0EE172F3DAA62325AF021A68F707511A
appears on the wire as follows (including the length byte of
ECPoint.point).
20 D7 5A 98 01 82 B1 0A B7 D5 4B FE D3 C9 64 07
3A 0E E1 72 F3 DA A6 23 25 AF 02 1A 68 F7 07 51
1A
4. 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(5)
} NamedCurve;
5. 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.
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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, NamedCurve and ECPointFormat types are also required,
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 will require additional
specification to specify the meaning.
The Supported Elliptic Curves Extension ("NamedCurve") must also
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.
The client MUST also include a Supported Point Formats Extension
("ECPointFormat") that contains the "eddsa_public_key" value.
6. 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.
+-------+-------------+---------+-----------+
| Value | Description | DTLS-OK | Reference |
+-------+-------------+---------+-----------+
| 4 | eddsa | Y | This doc |
+-------+-------------+---------+-----------+
IANA is requested to assign a number for Ed25519 described in
Section 4 to the Transport Layer Security (TLS) Parameters registry
EC Named Curve [IANA-TLS] as follows.
+-------+-------------+---------+-----------+
| Value | Description | DTLS-OK | Reference |
+-------+-------------+---------+-----------+
| 5 | ed25519 | Y | This doc |
+-------+-------------+---------+-----------+
IANA is requested to assign a number for eddsa_public_key described
in Section 3 to the Transport Layer Security (TLS) Parameters
registry EC Point Format [IANA-TLS] as follows.
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+-------+------------------+---------+-----------+
| Value | Description | DTLS-OK | Reference |
+-------+------------------+---------+-----------+
| 4 | eddsa_public_key | Y | This doc |
+-------+------------------+---------+-----------+
7. 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.
8. 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.
9. References
9.1. 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.
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[I-D.josefsson-eddsa-ed25519]
Josefsson, S. and N. Moller, "EdDSA and Ed25519", draft-
josefsson-eddsa-ed25519-02 (work in progress), February
2015.
9.2. 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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