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.

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 http://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 December 10, 2015.

Copyright Notice

Copyright (c) 2015 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 (http://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.

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].

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.

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.

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

7.2. Informative References

Author's Address

Table of Contents

• 1. Introduction
• 1.1. Requirements Terminology
• 2. EdDSA SignatureAlgorithm
• 3. Using EdDSA in a handshake
• 4. IANA Considerations
• 5. Security Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address