Applying Generate Random Extensions And Sustain Extensibility (GREASE) to TLS Extensibility
RFC 8701



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: davidben@google.com