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RFC 8701	Applying Generate Random Extensions And Sustain Extensibility (GREASE) to TLS Extensibility	January 2020
Benjamin	Informational	[Page]

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 athttps://www.rfc-editor.org/info/rfc8701.¶

Copyright Notice

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.¶

1.Introduction

The TLS protocol[RFC8446] includes several points ofextensibility, including the list of cipher suites and several lists of extensions.The values transmitted in these lists identify implementation capabilities. TLS followsa model where one side, usually the client, advertises capabilities, and thepeer, usually the server, selects them. The responding side must ignore unknownvalues so that new capabilities may be introduced to the ecosystem whilemaintaining 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 forTLS implementations to advertise at random. Correctly implemented peers will ignorethese values and interoperate. Peers that do not tolerate unknown values willfail to interoperate, revealing the mistake before it iswidespread.¶

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 bythe server. This section details client and server behavior aroundGREASE values for these.¶

3.1.Client Behavior

When sending a ClientHello, a clientMAY behave as follows:¶

A clientMAY select one or more GREASE cipher suite values and advertise them in the "cipher_suites" field.¶
A clientMAY select one or more GREASE extension values and advertise them as extensions with varying length and contents.¶
A clientMAY select one or more GREASE named group values and advertise them in the "supported_groups" extension, if sent. ItMAY also send KeyShareEntry values for a subset of those selected in the "key_share" extension. For each of these, the "key_exchange" fieldMAY be any value.¶
A clientMAY select one or more GREASE signature algorithm values and advertise them in the "signature_algorithms" or "signature_algorithms_cert" extensions, if sent.¶
A clientMAY select one or more GREASE version values and advertise them in the "supported_versions" extension, if sent.¶
A clientMAY select one or more GREASE PskKeyExchangeMode values and advertise them in the "psk_key_exchange_modes" extension, if sent.¶
A clientMAY select one or more GREASE ALPN identifiers and advertise them in the "application_layer_protocol_negotiation" extension, if sent.¶

ClientsMUST reject GREASE values when negotiated by the server. In particular, the clientMUST 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, serversMUST NOT treat GREASE values differently from any unknown value.ServersMUST NOT negotiate any GREASE value when offered in a ClientHello.ServersMUST 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 theclient. This section details client and server behavior around GREASEvalues for these.¶

4.1.Server Behavior

When sending a CertificateRequest in TLS 1.3, a serverMAY behave as follows:¶

A serverMAY select one or more GREASE extension values and advertise them as extensions with varying length and contents.¶
A serverMAY 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 serverMAY select one or more GREASE extension values and advertise them as extensions with varying length and contents.¶

ServersMUST reject GREASE values when negotiated by the client.In particular, the serverMUST fail the connection if a GREASEvalue 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, clientsMUST NOT treat GREASE values differently from any unknown value.ClientsMUST NOT negotiate any GREASE value when offered by the server.ClientsMUST 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 valuesSHOULD select them at random.This is intended to encourage implementations to ignore all unknown valuesrather than any individual value. ImplementationsMUST honor protocolspecifications when sending GREASE values. For instance,Section 4.2 of [RFC8446] forbids duplicate extension types within a singleextension block. Implementations sending multiple GREASE extensions in a singleblock must therefore ensure the same value is not selected twice.¶

ImplementationsSHOULD 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. ImplementationsSHOULD 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>.