Network Working Group                                        D. Benjamin
Internet-Draft                                                    Google
Intended status: Informational                         September 2, 2016
Expires: March 6, 2017


                  Applying GREASE to TLS Extensibility
                      draft-davidben-tls-grease-01

Abstract

   This document describes GREASE (Generate Random Extensions And
   Sustain Extensibility), a mechanism to prevent extensibility failures
   in the TLS ecosystem.  It reserves a set of TLS protocol values that
   may be advertised by clients to ensure servers correctly handle
   unknown values.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on March 6, 2017.

Copyright Notice

   Copyright (c) 2016 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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  GREASE Values . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

.  Introduction

   The TLS protocol [RFC5246] includes several points of extensibility,
   including the list of cipher suites and the list of extensions.  The
   values in these lists identify implementation capabilities.  TLS
   follows a model where clients advertise capabilities and servers
   select them.  It is required that servers ignore unknown values so
   that new capabilities may be introduced to the ecosystem while
   maintaining interoperability.

   However, bugs may cause a server to reject unknown values.  These
   broken servers will interoperate with existing clients, so the
   mistake may spread through the ecosystem unnoticed.  Later, when new
   values are defined, updated clients will discover that the
   metaphorical joint in the protocol has rusted shut and that the new
   values cannot be deployed without interoperability failures.

   To avoid this problem, this document reserves some currently unused
   values for clients to advertise at random.  Correct server
   implementations will ignore these values and interoperate.  Servers
   that do not tolerate unknown values will fail to interoperate with
   existing clients, revealing the mistake before it is widespread.
   This document reserves such values in the TLS cipher suite,
   extension, named group [RFC4492], and ALPN [RFC7301] registries.

   In keeping with the rusted joint metaphor, this technique is named
   GREASE (Generate Random Extensions And Sustain Extensibility).

1.1.  Requirements Language

   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 RFC 2119 [RFC2119].





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.  GREASE Values

   This document reserves a number of TLS protocol values, referred to
   as GREASE values.  These values were allocated sparsely to discourage
   server implementations from conditioning on them.  For convenience,
   they were also chosen so all types share a number scheme with a
   consistent pattern while avoiding collisions with any existing
   applicable registries in TLS.

   The following values are reserved as GREASE cipher suite values:

      {Values TBD}

   The following values are reserved as both GREASE extension values and
   GREASE named group values:

      {Values TBD}

   [[TODO: Depending on which of this or TLS 1.3 happens first, also
   reserve SignatureScheme values.  (The same number scheme will work
   fine there too.)]]

   Note that these correspond to the reserved cipher suites when treated
   as big-endian 16-bit integers.

   Finally, this document reserves all ALPN identifiers beginning with
   the prefix "ignore/".  This corresponds to the seven-octet prefix:
   0x69, 0x67, 0x6e, 0x6f, 0x72, 0x65, 0x2f.

.  Client Behavior

   When sending a ClientHello, a client which implements GREASE behaves
   as follows:

   o  A client MAY select one or more random GREASE cipher suite values
      and advertise them in the ClientHello.cipher_suites field.

   o  A client MAY select one or more random GREASE named group values
      and advertise them in the supported_groups extension, if sent.

   o  A client MAY select one or more random GREASE extension values and
      advertise corresponding extensions with varying length and
      contents in the ClientHello.extensions field.

   o  A client MAY select one or more random GREASE ALPN identifiers and
      advertise them in the application_layer_protocol_negotiation
      extension, if sent.




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   Clients SHOULD balance diversity in GREASE advertisements with
   determinism.  For example, a client which randomly varies GREASE
   value positions for each connection may only fail against a broken
   server with some probability.  This risks the failure being masked by
   automatic retries.  A client which positions GREASE values
   deterministically over a period of time (such as a single software
   release) stresses fewer cases but is more likely to detect bugs from
   those cases.

   Clients MUST reject GREASE values when negotiated by the server.
   When processing a ServerHello containing a GREASE value in the
   ServerHello.cipher_suite or ServerHello.extensions fields, the client
   MUST fail the connection.  When processing an ECParameters structure
   with a GREASE value in the ECParameter.namedcurve field, the client
   MUST fail the connection.

   Note that this requires no special processing on the client.  Clients
   are already required to reject unknown values selected by the server.

.  Server Behavior

   Servers MUST NOT treat GREASE values differently from any unknown
   value.  Servers MUST NOT negotiate any GREASE value when offered in a
   ClientHello.  Servers MUST correctly ignore unknown values in a
   ClientHello and attempt to negotiate with one of the remaining
   parameters.

   Note that these requirements are restatements or corollaries of
   existing server requirements in TLS.

.  IANA Considerations

   This document updates the TLS Cipher Suite Registry, available from
   <https://www.iana.org/assignments/tls-parameters>:

















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      +-------------------+-------------+---------+-----------------+
      |       Value       | Description | DTLS-OK |    Reference    |
      +-------------------+-------------+---------+-----------------+
      | {TBD} {0x0A,0x0A} |   Reserved  |    Y    | (this document) |
      | {TBD} {0x1A,0x1A} |   Reserved  |    Y    | (this document) |
      | {TBD} {0x2A,0x2A} |   Reserved  |    Y    | (this document) |
      | {TBD} {0x3A,0x3A} |   Reserved  |    Y    | (this document) |
      | {TBD} {0x4A,0x4A} |   Reserved  |    Y    | (this document) |
      | {TBD} {0x5A,0x5A} |   Reserved  |    Y    | (this document) |
      | {TBD} {0x6A,0x6A} |   Reserved  |    Y    | (this document) |
      | {TBD} {0x7A,0x7A} |   Reserved  |    Y    | (this document) |
      | {TBD} {0x8A,0x8A} |   Reserved  |    Y    | (this document) |
      | {TBD} {0x9A,0x9A} |   Reserved  |    Y    | (this document) |
      | {TBD} {0xAA,0xAA} |   Reserved  |    Y    | (this document) |
      | {TBD} {0xBA,0xBA} |   Reserved  |    Y    | (this document) |
      | {TBD} {0xCA,0xCA} |   Reserved  |    Y    | (this document) |
      | {TBD} {0xDA,0xDA} |   Reserved  |    Y    | (this document) |
      | {TBD} {0xEA,0xEA} |   Reserved  |    Y    | (this document) |
      | {TBD} {0xFA,0xFA} |   Reserved  |    Y    | (this document) |
      +-------------------+-------------+---------+-----------------+

                Additions to the TLS Cipher Suite Registry

   The cipher suite numbers listed in the first column are numbers used
   for cipher suite interoperability testing and it's suggested that
   IANA use these values for assignment.

   This document updates the Supported Groups Registry, available from
   <https://www.iana.org/assignments/tls-parameters>:






















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         +-------------+-------------+---------+-----------------+
         |    Value    | Description | DTLS-OK |    Reference    |
         +-------------+-------------+---------+-----------------+
         |  {TBD} 2570 |   Reserved  |    Y    | (this document) |
         |  {TBD} 6682 |   Reserved  |    Y    | (this document) |
         | {TBD} 10794 |   Reserved  |    Y    | (this document) |
         | {TBD} 14906 |   Reserved  |    Y    | (this document) |
         | {TBD} 19018 |   Reserved  |    Y    | (this document) |
         | {TBD} 23130 |   Reserved  |    Y    | (this document) |
         | {TBD} 27242 |   Reserved  |    Y    | (this document) |
         | {TBD} 31354 |   Reserved  |    Y    | (this document) |
         | {TBD} 35466 |   Reserved  |    Y    | (this document) |
         | {TBD} 39578 |   Reserved  |    Y    | (this document) |
         | {TBD} 43690 |   Reserved  |    Y    | (this document) |
         | {TBD} 47802 |   Reserved  |    Y    | (this document) |
         | {TBD} 51914 |   Reserved  |    Y    | (this document) |
         | {TBD} 56026 |   Reserved  |    Y    | (this document) |
         | {TBD} 60138 |   Reserved  |    Y    | (this document) |
         | {TBD} 64250 |   Reserved  |    Y    | (this document) |
         +-------------+-------------+---------+-----------------+

                Additions to the Supported Groups Registry

   The named group numbers listed in the first column are numbers used
   for cipher suite interoperability testing and it's suggested that
   IANA use these values for assignment.

   This document updates the ExtensionType Values registry, available
   from <https://www.iana.org/assignments/tls-extensiontype-values>:






















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            +-------------+----------------+-----------------+
            |    Value    | Extension name |    Reference    |
            +-------------+----------------+-----------------+
            |  {TBD} 2570 |    Reserved    | (this document) |
            |  {TBD} 6682 |    Reserved    | (this document) |
            | {TBD} 10794 |    Reserved    | (this document) |
            | {TBD} 14906 |    Reserved    | (this document) |
            | {TBD} 19018 |    Reserved    | (this document) |
            | {TBD} 23130 |    Reserved    | (this document) |
            | {TBD} 27242 |    Reserved    | (this document) |
            | {TBD} 31354 |    Reserved    | (this document) |
            | {TBD} 35466 |    Reserved    | (this document) |
            | {TBD} 39578 |    Reserved    | (this document) |
            | {TBD} 43690 |    Reserved    | (this document) |
            | {TBD} 47802 |    Reserved    | (this document) |
            | {TBD} 51914 |    Reserved    | (this document) |
            | {TBD} 56026 |    Reserved    | (this document) |
            | {TBD} 60138 |    Reserved    | (this document) |
            | {TBD} 64250 |    Reserved    | (this document) |
            +-------------+----------------+-----------------+

              Additions to the ExtensionType Values registry

   The extension numbers listed in the first column are numbers used for
   cipher suite interoperability testing and it's suggested that IANA
   use these values for assignment.

   [[TODO: How do I write IANA instructions to reserve all ALPN
   identifiers that begin with "ignore/"?  Perhaps it would be better to
   reserve a concrete handful of identifiers instead.]]

.  Security Considerations

   GREASE values may not be negotiated, so they do not directly impact
   the security of TLS connections.

   Historically, when interoperability problems arise in deploying new
   TLS features, implementations have used a fallback retry on error
   with the feature disabled.  This allows an active attacker to
   silently disable the new feature.  By preventing a class of such
   interoperability problems, GREASE reduces the need for this kind of
   fallback.

.  Acknowledgements

   The author would like to thank Adam Langley, Nick Harper, and Steven
   Valdez for their feedback and suggestions.  In addition, the rusted
   joint metaphor is originally due to Adam Langley.



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.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              DOI 10.17487/RFC4492, May 2006,
              <http://www.rfc-editor.org/info/rfc4492>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <http://www.rfc-editor.org/info/rfc7301>.

Author's Address

   David Benjamin
   Google
   355 Main St
   Cambridge, MA  02142
   USA

   Email: davidben@google.com



















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