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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)


   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.

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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   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 11, 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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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of

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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",
   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].

   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 {
         } 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
   appears on the wire as follows (including the length byte of

         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

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 {
      } 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

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

9.2.  Informative References

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

Author's Address

   Simon Josefsson

   Email: simon@josefsson.org

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