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Versions: 00 01 02                                                      
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)


   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
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   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
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   include Simplified BSD License text as described in Section 4.e of

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Internet-Draft          EdDSA and Ed25519 for TLS              June 2015

   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",
   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 {
      } 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

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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

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.

              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.

              Josefsson, S. and N. Moller, "EdDSA and Ed25519", draft-
              josefsson-eddsa-ed25519-02 (work in progress), February

7.2.  Informative References

              Internet Assigned Numbers Authority, "Transport Layer
              Security (TLS) Parameters",

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Author's Address

   Simon Josefsson

   Email: simon@josefsson.org

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