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RFC 8422

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Proposed STD
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Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS) Versions 1.2 and Earlier

Part 1 of 3, p. 1 to 9
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Obsoletes:    4492


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Internet Engineering Task Force (IETF)                            Y. Nir
Request for Comments: 8422                                   Check Point
Obsoletes: 4492                                             S. Josefsson
Category: Standards Track                                         SJD AB
ISSN: 2070-1721                                      M. Pegourie-Gonnard
                                                                     ARM
                                                             August 2018


            Elliptic Curve Cryptography (ECC) Cipher Suites
      for Transport Layer Security (TLS) Versions 1.2 and Earlier

Abstract

   This document describes key exchange algorithms based on Elliptic
   Curve Cryptography (ECC) for the Transport Layer Security (TLS)
   protocol.  In particular, it specifies the use of Ephemeral Elliptic
   Curve Diffie-Hellman (ECDHE) key agreement in a TLS handshake and the
   use of the Elliptic Curve Digital Signature Algorithm (ECDSA) and
   Edwards-curve Digital Signature Algorithm (EdDSA) as authentication
   mechanisms.

   This document obsoletes RFC 4492.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8422.

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Copyright Notice

   Copyright (c) 2018 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
   (https://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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   4
   2.  Key Exchange Algorithm  . . . . . . . . . . . . . . . . . . .   4
     2.1.  ECDHE_ECDSA . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  ECDHE_RSA . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.3.  ECDH_anon . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.4.  Algorithms in Certificate Chains  . . . . . . . . . . . .   7
   3.  Client Authentication . . . . . . . . . . . . . . . . . . . .   8
     3.1.  ECDSA_sign  . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  TLS Extensions for ECC  . . . . . . . . . . . . . . . . . . .   9
   5.  Data Structures and Computations  . . . . . . . . . . . . . .  10
     5.1.  Client Hello Extensions . . . . . . . . . . . . . . . . .  10
       5.1.1.  Supported Elliptic Curves Extension . . . . . . . . .  11
       5.1.2.  Supported Point Formats Extension . . . . . . . . . .  13
       5.1.3.  The signature_algorithms Extension and EdDSA  . . . .  13
     5.2.  Server Hello Extension  . . . . . . . . . . . . . . . . .  14
     5.3.  Server Certificate  . . . . . . . . . . . . . . . . . . .  15
     5.4.  Server Key Exchange . . . . . . . . . . . . . . . . . . .  16
       5.4.1.  Uncompressed Point Format for NIST Curves . . . . . .  19
     5.5.  Certificate Request . . . . . . . . . . . . . . . . . . .  20
     5.6.  Client Certificate  . . . . . . . . . . . . . . . . . . .  21
     5.7.  Client Key Exchange . . . . . . . . . . . . . . . . . . .  22
     5.8.  Certificate Verify  . . . . . . . . . . . . . . . . . . .  23
     5.9.  Elliptic Curve Certificates . . . . . . . . . . . . . . .  24
     5.10. ECDH, ECDSA, and RSA Computations . . . . . . . . . . . .  24
     5.11. Public Key Validation . . . . . . . . . . . . . . . . . .  26
   6.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . . . .  26
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  27
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     10.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Equivalent Curves (Informative)  . . . . . . . . . .  32
   Appendix B.  Differences from RFC 4492  . . . . . . . . . . . . .  33
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

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1.  Introduction

   This document describes additions to TLS to support ECC that are
   applicable to TLS versions 1.0 [RFC2246], 1.1 [RFC4346], and 1.2
   [RFC5246].  The use of ECC in TLS 1.3 is defined in [TLS1.3] and is
   explicitly out of scope for this document.  In particular, this
   document defines:

   o  the use of the ECDHE key agreement scheme with ephemeral keys to
      establish the TLS premaster secret, and

   o  the use of ECDSA and EdDSA signatures for authentication of TLS
      peers.

   The remainder of this document is organized as follows.  Section 2
   provides an overview of ECC-based key exchange algorithms for TLS.
   Section 3 describes the use of ECC certificates for client
   authentication.  TLS extensions that allow a client to negotiate the
   use of specific curves and point formats are presented in Section 4.
   Section 5 specifies various data structures needed for an ECC-based
   handshake, their encoding in TLS messages, and the processing of
   those messages.  Section 6 defines ECC-based cipher suites and
   identifies a small subset of these as recommended for all
   implementations of this specification.  Section 8 discusses security
   considerations.  Section 9 describes IANA considerations for the name
   spaces created by this document's predecessor.  Appendix B provides
   differences from [RFC4492], the document that this one replaces.

   Implementation of this specification requires familiarity with TLS,
   TLS extensions [RFC4366], and ECC.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Key Exchange Algorithm

   This document defines three new ECC-based key exchange algorithms for
   TLS.  All of them use Ephemeral ECDH (ECDHE) to compute the TLS
   premaster secret, and they differ only in the mechanism (if any) used
   to authenticate them.  The derivation of the TLS master secret from
   the premaster secret and the subsequent generation of bulk
   encryption/MAC keys and initialization vectors is independent of the
   key exchange algorithm and not impacted by the introduction of ECC.

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   Table 1 summarizes the new key exchange algorithms.  All of these key
   exchange algorithms provide forward secrecy if and only if fresh
   ephemeral keys are generated and used, and also destroyed after use.

     +-------------+------------------------------------------------+
     | Algorithm   | Description                                    |
     +-------------+------------------------------------------------+
     | ECDHE_ECDSA | Ephemeral ECDH with ECDSA or EdDSA signatures. |
     | ECDHE_RSA   | Ephemeral ECDH with RSA signatures.            |
     | ECDH_anon   | Anonymous ephemeral ECDH, no signatures.       |
     +-------------+------------------------------------------------+

                   Table 1: ECC Key Exchange Algorithms

   These key exchanges are analogous to DHE_DSS, DHE_RSA, and DH_anon,
   respectively.

   With ECDHE_RSA, a server can reuse its existing RSA certificate and
   easily comply with a constrained client's elliptic curve preferences
   (see Section 4).  However, the computational cost incurred by a
   server is higher for ECDHE_RSA than for the traditional RSA key
   exchange, which does not provide forward secrecy.

   The anonymous key exchange algorithm does not provide authentication
   of the server or the client.  Like other anonymous TLS key exchanges,
   it is subject to man-in-the-middle attacks.  Applications using TLS
   with this algorithm SHOULD provide authentication by other means.

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          Client                                        Server
          ------                                        ------
          ClientHello          -------->
                                                   ServerHello
                                                  Certificate*
                                            ServerKeyExchange*
                                          CertificateRequest*+
                               <--------       ServerHelloDone
          Certificate*+
          ClientKeyExchange
          CertificateVerify*+
          [ChangeCipherSpec]
          Finished             -------->
                                            [ChangeCipherSpec]
                               <--------              Finished
          Application Data     <------->      Application Data

               * message is not sent under some conditions
               + message is not sent unless client authentication
                 is desired

            Figure 1: Message Flow in a Full TLS 1.2 Handshake

   Figure 1 shows all messages involved in the TLS key establishment
   protocol (aka full handshake).  The addition of ECC has direct impact
   only on the ClientHello, the ServerHello, the server's Certificate
   message, the ServerKeyExchange, the ClientKeyExchange, the
   CertificateRequest, the client's Certificate message, and the
   CertificateVerify.  Next, we describe the ECC key exchange algorithm
   in greater detail in terms of the content and processing of these
   messages.  For ease of exposition, we defer discussion of client
   authentication and associated messages (identified with a '+' in
   Figure 1) until Section 3 and of the optional ECC-specific extensions
   (which impact the Hello messages) until Section 4.

2.1.  ECDHE_ECDSA

   In ECDHE_ECDSA, the server's certificate MUST contain an ECDSA- or
   EdDSA-capable public key.

   The server sends its ephemeral ECDH public key and a specification of
   the corresponding curve in the ServerKeyExchange message.  These
   parameters MUST be signed with ECDSA or EdDSA using the private key
   corresponding to the public key in the server's Certificate.

   The client generates an ECDH key pair on the same curve as the
   server's ephemeral ECDH key and sends its public key in the
   ClientKeyExchange message.

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   Both client and server perform an ECDH operation (see Section 5.10)
   and use the resultant shared secret as the premaster secret.

2.2.  ECDHE_RSA

   This key exchange algorithm is the same as ECDHE_ECDSA except that
   the server's certificate MUST contain an RSA public key authorized
   for signing and the signature in the ServerKeyExchange message must
   be computed with the corresponding RSA private key.

2.3.  ECDH_anon

   NOTE: Despite the name beginning with "ECDH_" (no E), the key used in
   ECDH_anon is ephemeral just like the key in ECDHE_RSA and
   ECDHE_ECDSA.  The naming follows the example of DH_anon, where the
   key is also ephemeral but the name does not reflect it.

   In ECDH_anon, the server's Certificate, the CertificateRequest, the
   client's Certificate, and the CertificateVerify messages MUST NOT be
   sent.

   The server MUST send an ephemeral ECDH public key and a specification
   of the corresponding curve in the ServerKeyExchange message.  These
   parameters MUST NOT be signed.

   The client generates an ECDH key pair on the same curve as the
   server's ephemeral ECDH key and sends its public key in the
   ClientKeyExchange message.

   Both client and server perform an ECDH operation and use the
   resultant shared secret as the premaster secret.  All ECDH
   calculations are performed as specified in Section 5.10.

2.4.  Algorithms in Certificate Chains

   This specification does not impose restrictions on signature schemes
   used anywhere in the certificate chain.  The previous version of this
   document required the signatures to match, but this restriction,
   originating in previous TLS versions, is lifted here as it had been
   in RFC 5246.

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3.  Client Authentication

   This document defines a client authentication mechanism named after
   the type of client certificate involved: ECDSA_sign.  The ECDSA_sign
   mechanism is usable with any of the non-anonymous ECC key exchange
   algorithms described in Section 2 as well as other non-anonymous
   (non-ECC) key exchange algorithms defined in TLS.

   Note that client certificates with EdDSA public keys also use this
   mechanism.

   The server can request ECC-based client authentication by including
   this certificate type in its CertificateRequest message.  The client
   must check if it possesses a certificate appropriate for the method
   suggested by the server and is willing to use it for authentication.

   If these conditions are not met, the client SHOULD send a client
   Certificate message containing no certificates.  In this case, the
   ClientKeyExchange MUST be sent as described in Section 2, and the
   CertificateVerify MUST NOT be sent.  If the server requires client
   authentication, it may respond with a fatal handshake failure alert.

   If the client has an appropriate certificate and is willing to use it
   for authentication, it must send that certificate in the client's
   Certificate message (as per Section 5.6) and prove possession of the
   private key corresponding to the certified key.  The process of
   determining an appropriate certificate and proving possession is
   different for each authentication mechanism and is described below.

   NOTE: It is permissible for a server to request (and the client to
   send) a client certificate of a different type than the server
   certificate.

3.1.  ECDSA_sign

   To use this authentication mechanism, the client MUST possess a
   certificate containing an ECDSA- or EdDSA-capable public key.

   The client proves possession of the private key corresponding to the
   certified key by including a signature in the CertificateVerify
   message as described in Section 5.8.

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4.  TLS Extensions for ECC

   Two TLS extensions are defined in this specification: (i) the
   Supported Elliptic Curves Extension and (ii) the Supported Point
   Formats Extension.  These allow negotiating the use of specific
   curves and point formats (e.g., compressed vs. uncompressed,
   respectively) during a handshake starting a new session.  These
   extensions are especially relevant for constrained clients that may
   only support a limited number of curves or point formats.  They
   follow the general approach outlined in [RFC4366]; message details
   are specified in Section 5.  The client enumerates the curves it
   supports and the point formats it can parse by including the
   appropriate extensions in its ClientHello message.  The server
   similarly enumerates the point formats it can parse by including an
   extension in its ServerHello message.

   A TLS client that proposes ECC cipher suites in its ClientHello
   message SHOULD include these extensions.  Servers implementing ECC
   cipher suites MUST support these extensions, and when a client uses
   these extensions, servers MUST NOT negotiate the use of an ECC cipher
   suite unless they can complete the handshake while respecting the
   choice of curves specified by the client.  This eliminates the
   possibility that a negotiated ECC handshake will be subsequently
   aborted due to a client's inability to deal with the server's EC key.

   The client MUST NOT include these extensions in the ClientHello
   message if it does not propose any ECC cipher suites.  A client that
   proposes ECC cipher suites may choose not to include these
   extensions.  In this case, the server is free to choose any one of
   the elliptic curves or point formats listed in Section 5.  That
   section also describes the structure and processing of these
   extensions in greater detail.

   In the case of session resumption, the server simply ignores the
   Supported Elliptic Curves Extension and the Supported Point Formats
   Extension appearing in the current ClientHello message.  These
   extensions only play a role during handshakes negotiating a new
   session.


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