Curve25519 for ephemeral key exchange in Transport Layer Security (TLS)
draft-ietf-tls-curve25519-00

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Document Type Active Internet-Draft (tls WG)
Last updated 2015-06-12
Replaces draft-josefsson-tls-curve25519
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Network Working Group                                       S. Josefsson
Internet-Draft                                                    SJD AB
Updates: 4492, 5246 (if approved)                    M. Pegourie-Gonnard
Intended status: Informational                    Independent / PolarSSL
Expires: December 11, 2015                                  June 9, 2015

Curve25519 for ephemeral key exchange in Transport Layer Security (TLS)
                      draft-ietf-tls-curve25519-00

Abstract

   This document specifies the use of Curve25519 for ephemeral key
   exchange in the Transport Layer Security (TLS) protocol, as well as
   its DTLS variant.  It updates RFC 5246 (TLS 1.2) and RFC 4492
   (Elliptic Curve Cryptography for 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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   Internet-Drafts are draft documents valid for a maximum of six months
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   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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   described in the Simplified BSD License.

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1.  Introduction

   In [Curve25519] [I-D.irtf-cfrg-curves], a new elliptic curve function
   for use in cryptographic applications was specified.  Curve25519 is a
   Diffie-Hellman function designed with performance and security in
   mind.

   [RFC4492] defines the usage of elliptic curves for authentication and
   key agreement in TLS 1.0 and TLS 1.1, and these mechanisms are also
   applicable to TLS 1.2 [RFC5246].  The use of ECC curves for key
   exchange requires the definition and assignment of additional
   NamedCurve IDs.  This document specify that value for Curve25519, as
   well as the minor changes in key selection and representation that
   are required to accommodate for Curve25519's slightly different
   nature.

   This document only describes usage of Curve25519 for ephemeral key
   exchange (ECDHE).  It does not define its use for signature, since
   the primitive considered here is a Diffie-Hellman function; the
   related signature scheme, Ed25519, is outside the scope of this
   document.  The use of Curve25519 with long-term keys embedded in
   X.509 certificates is also out of scope here.

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.  Data Structures and Computations

2.1.  Cryptographic computations

   All cryptographic computations are done using the Curve25519 function
   defined in [Curve25519] [I-D.irtf-cfrg-curves].  In this memo, this
   function is considered as a black box that takes as input a (secret
   key, public key) pair and outputs a public key.  Public keys are
   defined as strings of 32 bytes.  Secret keys are defined as 255 bits
   numbers such as the high-order bit (bit 254) is set, and the three
   lowest-order bits are unset.  In addition, a common public key,
   denoted by G, is shared by all users.

   An ECDHE key exchange using Curve25519 goes as follows.  Each party
   picks a secret key d uniformly at random and computes the
   corresponding public key x = Curve25519(d, G).  Parties exchange
   their public keys (see Section 2.3) and compute a shared secret as
   x_S = Curve25519(d, x_peer).  This shared secret is used directly as

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   the premaster secret, which is always exactly 32 bytes when ECDHE
   with Curve25519 is used.

   A complete description of the Curve25519 function, as well as a few
   implementation notes, are provided in Appendix A.
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