TLS Working Group                                             P. Gutmann
Internet-Draft                                    University of Auckland
Intended status: Standards Track                            May 10, 2011
Expires: November 11, 2011

                 Standardised ECC Cipher Suites for TLS


   This document describes a set of standard ECC cipher suites for TLS
   that simplify the complex selection procedure described in the
   existing ECC RFC, simplifying implementation and easing
   interoperability problems.

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   This Internet-Draft will expire on November 11, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  Cipher Suites  . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Discussion . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   5.  Normative References . . . . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   [TLS-ECC] provides an extremely flexible, and by extension extremely
   complex means of specifying a large number of options involving the
   use of ECC algorithms for [TLS].  As such the "cipher suites" in
   [TLS-ECC] aren't suites in the conventional TLS sense but more an
   indication of intent to negotiate a Chinese menu, with details to be
   decided on later via various TLS extensions and parameter settings.
   This makes deciding on a particular suite nondeterministic, since
   later parameter choices and settings can negate the initial "cipher
   suite" choice, requiring returning to the suite list to try with
   another Chinese-menu suite in the hope that later parameter choices
   allow it to be used.

   In practice no deployed implementation actually does this, either
   dropping the connection or aborting the handshake with a handshake-
   failure if the expected parameters aren't present throughout the
   various locations in the TLS handshake in which ECC parameters can be
   specified.  This means that establishing a TLS connection using ECC
   often requires trial-and-error probing to ascertain what the other
   side is expecting to see before a connection can be established.

   Experience with deployed implementations indicates that all of them
   appear to implement a common subset of fixed ECC parameters that work
   in all cases (alongside the more obscure options), representing a de
   facto profile of standard cipher suites rather than Chinese-menu
   selection options.  This document standardises this de facto usage by
   defining a small number of standard ECC cipher suites with
   unambiguous parameters and settings.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.  Cipher Suites

   The table below defines standard ECC cipher suites with fixed,
   unambiguous parameters, based on the de facto profiles of suites seen
   in use in practice.  Since the form of these suites match the
   existing non-ECC suites, they follow the existing suites in the {
   0x00, 0xXX } range rather than being placed with the Chinese-menu
   suites at { 0xC0, 0xXX }.

  CipherSuite TLS_ECDHE_ECDSA_P256_WITH_AES_128_CBC_SHA = { 0x00, 0xXX }
  CipherSuite TLS_ECDHE_ECDSA_P256_WITH_AES_256_CBC_SHA = { 0x00, 0xXX }
  CipherSuite TLS_ECDHE_ECDSA_P384_WITH_AES_128_CBC_SHA = { 0x00, 0xXX }
  CipherSuite TLS_ECDHE_ECDSA_P384_WITH_AES_256_CBC_SHA = { 0x00, 0xXX }

  CipherSuite TLS_ECDHE_ECDSA_P256_WITH_AES_128_CBC_SHA256 =
    { 0x00, 0xXX }
  CipherSuite TLS_ECDHE_ECDSA_P256_WITH_AES_256_CBC_SHA256 =
    { 0x00, 0xXX }
  CipherSuite TLS_ECDHE_ECDSA_P384_WITH_AES_128_CBC_SHA384 =
    { 0x00, 0xXX }
  CipherSuite TLS_ECDHE_ECDSA_P384_WITH_AES_256_CBC_SHA384 =
    { 0x00, 0xXX }

  CipherSuite TLS_ECDHE_ECDSA_P256_WITH_AES_128_GCM_SHA256 =
    { 0x00, 0xXX }
  CipherSuite TLS_ECDHE_ECDSA_P256_WITH_AES_256_GCM_SHA256 =
    { 0x00, 0xXX }
  CipherSuite TLS_ECDHE_ECDSA_P384_WITH_AES_128_GCM_SHA384 =
    { 0x00, 0xXX }
  CipherSuite TLS_ECDHE_ECDSA_P384_WITH_AES_256_GCM_SHA384 =
    { 0x00, 0xXX }

   In the above lists, the first set of suites allows use with TLS 1.0
   and 1.1, the second set allows use with TLS 1.2, and the third set
   allows use with Suite B.

   [[[At least one major implementation, Microsoft's SChannel, already
   does this, see
   For example it lists 'suites' like
   TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256_P256.  The above choices
   happen to coincide with the Microsoft ones not because of any
   explicit attempt to copy the Microsoft options but because they
   represent the obvious, logical choices]]].

   For each cipher suite with their ECC parameters denoted 'P256' or
   'P384', the ECC parameters are:

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   o  ECDH key agreement in Server Key Exchange/Client Key Exchange
      message: NIST P-256/X9.62 p256r1/SECG p256r1 or NIST P-384/SECG
      p384r1 curve with uncompressed points as indicated in the suite
   o  ECDSA signature in Server Key Exchange message: P256 or P384 curve
      as for ECDH with uncompressed points and SHA1, SHA256 or SHA384 as
      indicated in the suite name.
   o  Client authentication in Certificate Request/Certificate Verify
      messages: SHA1, SHA256, or SHA384 as indicated in the suite name.

   If no additional Chinese-menu ECC suites are used, implementations
   SHOULD NOT send the Supported Elliptic Curves or Supported Point
   Formats extensions since these parameters are fully specified by the
   suite choice.  If additional Chinese-menu suites are used,
   implementations MUST send the Supported Elliptic Curves and Supported
   Point Formats extensions as per [TLS-ECC].  The parameters specified
   in these extensions apply only to the Chinese-menu suites, not the
   fixed suites defined above.

   [TLS] states that if the client does not send the
   signature_algorithms extension then the hash algorithm defaults to
   SHA1.  This is required in order to provide a fall-back default if no
   other means of specifying the hash algorithm to be used is available.
   Since this document makes the use of the hash algorithm explicit in
   the cipher suite, the fall- back to the SHA1 default is never

   Note that the suites defined in this document augment, rather than
   supplant, the existing Chinese-menu suites options.  Anyone requiring
   the use of more unusual ECC parameters and options can use the
   Chinese-menu capability to specify and select any parameters that
   they require.

2.1.  Discussion

   The issue that this document is intended to address may be more
   easily seen by considering how the parts of the Client Hello are
   processed.  For standard cipher suites the server iterates through a
   list of suites proposed by the client and selects the most cromulent
   one.  For example a server may have a list of suite IDs and
   parameters sorted in order of preference and select the lowest-ranked
   suite in the list from the ones proposed by the client.

   For the Chinese-menu suites on the other hand, the server sees a
   Chinese menu selector sent by the client and then has to skip the
   remaining suites and other parts of the hello and process the
   extensions to see whether what's in there matches up with that the
   Chinese-menu selector requested.  For example if the Chinese menu

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   said TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 but the supported-curves
   says P256 then the server has to either hope that the other side does
   the special-case X9.62 handling for hash truncation and gets it right
   (experience with current implementations indicates that they don't
   even support this capability, let alone get it right), or not take
   the gamble and go back to the cipher suites and look for another
   Chinese-menu option, and then skip the rest of the hello and process
   the extensions again to see if things work out this time, and if that
   doesn't work either then go back ...

   In practice with currently-deployed implementations it's hard enough
   just trying to figure out which basic combinations of parameters they
   support (the usual response is a dropped connection or aborted
   handshake, requiring the use of trial-and-error probing to find out
   what's possible), and even getting to the point of being able to
   interop-test any of the more exotic combinations like hash truncation
   becomes more or less impossible.  So the purpose of this document is
   to try and identify the common combinations of parameters that
   everyone seems to implement anyway and list them as conventional
   cipher suites, with no further parameterisation required.

   An additional problem with the Chinese-menu selection process is the
   fact that although it allows the specification of arbitrary numbers
   of handshake parameters, it never nails down how and where these
   parameters should be applied.  Practical experience with
   implementations indicates that only the most straightforward
   combinations of algorithm parameters are likely to work.  For example
   although it's possible to specify both P256 and P384 as acceptable
   curves, what this tends to mean in practice is that { ECDH P256 +
   ECDSA P256 } or { ECDH P384 + ECDSA P384 } are acceptable but { ECDH
   P256 + ECDSA P384 } or { ECDH P384 + ECDSA P256 } aren't.  In the
   interests of interoperability it's recommended that, despite the
   apparent flexibility implied by the Chinese menu, implementations
   stick to the most straightforward application of algorithm
   parameters, using the same algorithm or parameters throughout the
   handshake even if it's implied by the Chinese-menu that mix-and-match
   combinations are possible.  For example if the overall cipher suite
   is TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 then use SHA256 everywhere
   a hash function is used; if the curve types are P256 or P384 then use
   either P256 everywhere or P384 everywhere.  This design principle is
   captured in the requirements given in Section 2.

   The term "Chinese menu" comes from the US, where Chinese restaurants
   traditionally had columns for ordering food, and orders were put
   together in a mix-and-match manner by ordering an item from column A,
   two from column B, and so on.  Any process that involves picking a
   selection from different columns has become described as a "Chinese
   menu system".

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3.  Security Considerations

   This document is a profile of, and simplifcation of, [TLS-ECC].  No
   further security considerations are introduced beyond those present
   in [TLS-ECC] .

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4.  IANA Considerations

   This document defines new cipher suites for TLS [to be allocated in
   the currently unallocated range { 0x00, 0xC6 } - { 0x00, 0xD1 }].

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5.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [TLS-ECC]  Blake-Wilson, S., Bolyard, N., Gupta, V., and C. Hawk,
              "Elliptic Curve Cryptography (ECC) Cipher Suites for
              Transport Layer Security (TLS)", RFC 4492, May 2006.

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Author's Address

   Peter Gutmann
   University of Auckland
   Department of Computer Science
   New Zealand


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