Internet Engineering Task Force (IETF)                       D. Benjamin
Request for Comments: 8701                                    Google LLC
Category: Informational                                     January 2020
ISSN: 2070-1721


Applying Generate Random Extensions And Sustain Extensibility (GREASE)
                         to TLS Extensibility

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 to ensure peers correctly handle unknown values.

Status of This Memo

  This document is not an Internet Standards Track specification; it is
  published for informational purposes.

  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).  Not all documents
  approved by the IESG are candidates for any level of Internet
  Standard; see 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/rfc8701.

Copyright Notice

  Copyright (c) 2020 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.

Table of Contents

  1.  Introduction
    1.1.  Requirements Language
  2.  GREASE Values
  3.  Client-Initiated Extension Points
    3.1.  Client Behavior
    3.2.  Server Behavior
  4.  Server-Initiated Extension Points
    4.1.  Server Behavior
    4.2.  Client Behavior
  5.  Sending GREASE Values
  6.  IANA Considerations
  7.  Security Considerations
  8.  Normative References
  Acknowledgments
  Author's Address

1.  Introduction

  The TLS protocol [RFC8446] includes several points of extensibility,
  including the list of cipher suites and several lists of extensions.
  The values transmitted in these lists identify implementation
  capabilities.  TLS follows a model where one side, usually the
  client, advertises capabilities, and the peer, usually the server,
  selects them.  The responding side must ignore unknown values so that
  new capabilities may be introduced to the ecosystem while maintaining
  interoperability.

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

  To avoid this problem, this document reserves some currently unused
  values for TLS implementations to advertise at random.  Correctly
  implemented peers will ignore these values and interoperate.  Peers
  that do not tolerate unknown values will fail to interoperate,
  revealing the mistake before it is widespread.

  In keeping with the rusted joint metaphor, this technique is called
  "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", "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.  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 values for cipher suites
  and Application-Layer Protocol Negotiation (ALPN) [RFC7301]
  identifiers:

     {0x0A,0x0A}

     {0x1A,0x1A}

     {0x2A,0x2A}

     {0x3A,0x3A}

     {0x4A,0x4A}

     {0x5A,0x5A}

     {0x6A,0x6A}

     {0x7A,0x7A}

     {0x8A,0x8A}

     {0x9A,0x9A}

     {0xAA,0xAA}

     {0xBA,0xBA}

     {0xCA,0xCA}

     {0xDA,0xDA}

     {0xEA,0xEA}

     {0xFA,0xFA}

  The following values are reserved as GREASE values for extensions,
  named groups, signature algorithms, and versions:

     0x0A0A

     0x1A1A

     0x2A2A

     0x3A3A

     0x4A4A

     0x5A5A

     0x6A6A

     0x7A7A

     0x8A8A

     0x9A9A

     0xAAAA

     0xBABA

     0xCACA

     0xDADA

     0xEAEA

     0xFAFA

  The values allocated above are thus no longer available for use as
  TLS or DTLS [RFC6347] version numbers.

  The following values are reserved as GREASE values for
  PskKeyExchangeModes:

     0x0B

     0x2A

     0x49

     0x68

     0x87

     0xA6

     0xC5

     0xE4

3.  Client-Initiated Extension Points

  Most extension points in TLS are offered by the client and selected
  by the server.  This section details client and server behavior
  around GREASE values for these.

3.1.  Client Behavior

  When sending a ClientHello, a client MAY behave as follows:

  *  A client MAY select one or more GREASE cipher suite values and
     advertise them in the "cipher_suites" field.

  *  A client MAY select one or more GREASE extension values and
     advertise them as extensions with varying length and contents.

  *  A client MAY select one or more GREASE named group values and
     advertise them in the "supported_groups" extension, if sent.  It
     MAY also send KeyShareEntry values for a subset of those selected
     in the "key_share" extension.  For each of these, the
     "key_exchange" field MAY be any value.

  *  A client MAY select one or more GREASE signature algorithm values
     and advertise them in the "signature_algorithms" or
     "signature_algorithms_cert" extensions, if sent.

  *  A client MAY select one or more GREASE version values and
     advertise them in the "supported_versions" extension, if sent.

  *  A client MAY select one or more GREASE PskKeyExchangeMode values
     and advertise them in the "psk_key_exchange_modes" extension, if
     sent.

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

  Clients MUST reject GREASE values when negotiated by the server.  In
  particular, the client MUST fail the connection if a GREASE value
  appears in any of the following:

  *  The "version" value in a ServerHello or HelloRetryRequest

  *  The "cipher_suite" value in a ServerHello

  *  Any ServerHello extension

  *  Any HelloRetryRequest, EncryptedExtensions, or Certificate
     extension in TLS 1.3

  *  The "namedcurve" value in a ServerKeyExchange for an Ephemeral
     Elliptic Curve Diffie-Hellman (ECDHE) cipher in TLS 1.2 [RFC5246]
     or earlier

  *  The signature algorithm in a ServerKeyExchange signature in TLS
     1.2 or earlier

  *  The signature algorithm in a server CertificateVerify signature in
     TLS 1.3

  Note that this can be implemented without special processing on the
  client.  The client is already required to reject unknown server-
  selected values, so it may leave GREASE values as unknown and reuse
  the existing logic.

3.2.  Server Behavior

  When processing a ClientHello, 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.  (There may not be any known
  parameters remaining, in which case parameter negotiation will fail.)

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

4.  Server-Initiated Extension Points

  Some extension points are offered by the server and selected by the
  client.  This section details client and server behavior around
  GREASE values for these.

4.1.  Server Behavior

  When sending a CertificateRequest in TLS 1.3, a server MAY behave as
  follows:

  *  A server MAY select one or more GREASE extension values and
     advertise them as extensions with varying length and contents.

  *  A server MAY select one or more GREASE signature algorithm values
     and advertise them in the "signature_algorithms" or
     "signature_algorithms_cert" extensions, if present.

  When sending a NewSessionTicket message in TLS 1.3, a server MAY
  select one or more GREASE extension values and advertise them as
  extensions with varying length and contents.

  Servers MUST reject GREASE values when negotiated by the client.  In
  particular, the server MUST fail the connection if a GREASE value
  appears in any of the following:

  *  Any Certificate extension in TLS 1.3

  *  The signature algorithm in a client CertificateVerify signature

  Note that this can be implemented without special processing on the
  server.  The server is already required to reject unknown client-
  selected values, so it may leave GREASE values as unknown and reuse
  the existing logic.

4.2.  Client Behavior

  When processing a CertificateRequest or NewSessionTicket, clients
  MUST NOT treat GREASE values differently from any unknown value.
  Clients MUST NOT negotiate any GREASE value when offered by the
  server.  Clients MUST correctly ignore unknown values offered by the
  server and attempt to negotiate with one of the remaining parameters.
  (There may not be any known parameters remaining, in which case
  parameter negotiation will fail.)

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

5.  Sending GREASE Values

  Implementations advertising GREASE values SHOULD select them at
  random.  This is intended to encourage implementations to ignore all
  unknown values rather than any individual value.  Implementations
  MUST honor protocol specifications when sending GREASE values.  For
  instance, Section 4.2 of [RFC8446] forbids duplicate extension types
  within a single extension block.  Implementations sending multiple
  GREASE extensions in a single block must therefore ensure the same
  value is not selected twice.

  Implementations SHOULD balance diversity in GREASE advertisements
  with determinism.  For example, a client that 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 that 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.

6.  IANA Considerations

  This document updates the "TLS Cipher Suites" registry, available at
  <https://www.iana.org/assignments/tls-parameters>:

    +-------------+-------------+---------+-------------+-----------+
    |    Value    | Description | DTLS-OK | Recommended | Reference |
    +=============+=============+=========+=============+===========+
    | {0x0A,0x0A} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0x1A,0x1A} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0x2A,0x2A} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0x3A,0x3A} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0x4A,0x4A} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0x5A,0x5A} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0x6A,0x6A} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0x7A,0x7A} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0x8A,0x8A} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0x9A,0x9A} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0xAA,0xAA} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0xBA,0xBA} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0xCA,0xCA} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0xDA,0xDA} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0xEA,0xEA} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+
    | {0xFA,0xFA} |   Reserved  |    Y    |      N      | [RFC8701] |
    +-------------+-------------+---------+-------------+-----------+

           Table 1: Additions to the TLS Cipher Suites Registry

  This document updates the "TLS Supported Groups" registry, available
  at <https://www.iana.org/assignments/tls-parameters>:

       +-------+-------------+---------+-------------+-----------+
       | Value | Description | DTLS-OK | Recommended | Reference |
       +=======+=============+=========+=============+===========+
       |  2570 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       |  6682 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 10794 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 14906 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 19018 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 23130 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 27242 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 31354 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 35466 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 39578 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 43690 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 47802 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 51914 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 56026 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 60138 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+
       | 64250 |   Reserved  |    Y    |      N      | [RFC8701] |
       +-------+-------------+---------+-------------+-----------+

         Table 2: Additions to the TLS Supported Groups Registry

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

   +-------+----------------+-------------+-------------+-----------+
   | Value | Extension Name |   TLS 1.3   | Recommended | Reference |
   +=======+================+=============+=============+===========+
   |  2570 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   |  6682 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 10794 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 14906 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 19018 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 23130 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 27242 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 31354 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 35466 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 39578 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 43690 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 47802 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 51914 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 56026 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 60138 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+
   | 64250 |    Reserved    | CH, CR, NST |      N      | [RFC8701] |
   +-------+----------------+-------------+-------------+-----------+

      Table 3: Additions to the TLS ExtensionType Values Registry

  This document updates the "TLS Application-Layer Protocol Negotiation
  (ALPN) Protocol IDs" registry, available at
  <https://www.iana.org/assignments/tls-extensiontype-values>:

           +----------+-------------------------+-----------+
           | Protocol | Identification Sequence | Reference |
           +==========+=========================+===========+
           | Reserved |        0x0A 0x0A        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0x1A 0x1A        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0x2A 0x2A        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0x3A 0x3A        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0x4A 0x4A        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0x5A 0x5A        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0x6A 0x6A        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0x7A 0x7A        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0x8A 0x8A        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0x9A 0x9A        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0xAA 0xAA        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0xBA 0xBA        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0xCA 0xCA        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0xDA 0xDA        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0xEA 0xEA        | [RFC8701] |
           +----------+-------------------------+-----------+
           | Reserved |        0xFA 0xFA        | [RFC8701] |
           +----------+-------------------------+-----------+

               Table 4: Additions to the TLS Application-
             Layer Protocol Negotiation (ALPN) Protocol IDs
                                Registry

7.  Security Considerations

  GREASE values cannot 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.  Implementations SHOULD NOT retry with GREASE disabled on
  connection failure.  While allowing an attacker to disable GREASE is
  unlikely to have immediate security consequences, such a fallback
  would prevent GREASE from defending against extensibility failures.

  If an implementation does not select GREASE values at random, it is
  possible it will allow for fingerprinting of the implementation or
  perhaps even of individual users.  This can result in a negative
  impact to a user's privacy.

8.  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,
             <https://www.rfc-editor.org/info/rfc2119>.

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

  [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
             Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
             January 2012, <https://www.rfc-editor.org/info/rfc6347>.

  [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, <https://www.rfc-editor.org/info/rfc7301>.

  [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

  [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
             Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
             <https://www.rfc-editor.org/info/rfc8446>.

Acknowledgments

  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.

Author's Address

  David Benjamin
  Google LLC

  Email: [email protected]

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