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

  • D. Benjamin
Informational
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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.

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:

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:

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>:

Table 1: Additions to the TLS Cipher Suites Registry
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]

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

Table 2: Additions to the TLS Supported Groups Registry
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]

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

Table 3: Additions to the TLS ExtensionType Values Registry
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]

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

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

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, , <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, , <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, , <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, , <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, , <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, , <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
RFC 8701: Applying Generate Random Extensions And Sustain Extensibility (GREASE) to TLS Extensibility
Informational