Applying GREASE to TLS Extensibility
draft-ietf-tls-grease-03

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

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This Internet-Draft will expire on January 30, 2020.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

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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. Acknowledgements
• 9. Normative References
• 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 that 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 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", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 RFC 2119[RFC2119] RFC 8174 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.

RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH. The values prefaced with {TBD} are suggested values and subject to change prior to final allocation by IANA.

The following values are reserved as GREASE values for cipher suites and ALPN [RFC7301] identifiers:

• {TBD} {0x0A,0x0A}
• {TBD} {0x1A,0x1A}
• {TBD} {0x2A,0x2A}
• {TBD} {0x3A,0x3A}
• {TBD} {0x4A,0x4A}
• {TBD} {0x5A,0x5A}
• {TBD} {0x6A,0x6A}
• {TBD} {0x7A,0x7A}
• {TBD} {0x8A,0x8A}
• {TBD} {0x9A,0x9A}
• {TBD} {0xAA,0xAA}
• {TBD} {0xBA,0xBA}
• {TBD} {0xCA,0xCA}
• {TBD} {0xDA,0xDA}
• {TBD} {0xEA,0xEA}
• {TBD} {0xFA,0xFA}

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

• {TBD} 0x0A0A
• {TBD} 0x1A1A
• {TBD} 0x2A2A
• {TBD} 0x3A3A
• {TBD} 0x4A4A
• {TBD} 0x5A5A
• {TBD} 0x6A6A
• {TBD} 0x7A7A
• {TBD} 0x8A8A
• {TBD} 0x9A9A
• {TBD} 0xAAAA
• {TBD} 0xBABA
• {TBD} 0xCACA
• {TBD} 0xDADA
• {TBD} 0xEAEA
• {TBD} 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.

• {TBD} 0x0B
• {TBD} 0x2A
• {TBD} 0x49
• {TBD} 0x68
• {TBD} 0x87
• {TBD} 0xA6
• {TBD} 0xC5
• {TBD} 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 any in 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 ECDHE cipher in TLS 1.2 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 requires no special processing on the client. Clients are already required to reject unknown values selected by the server.

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 any in the following:

• Any Certificate extension in TLS 1.3
• The signature algorithm in a client CertificateVerify signature

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

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 thus must ensure the same value is not selected twice.

Implementations 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.

6. IANA Considerations

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

RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH. The cipher suite numbers listed in the first column are numbers used for 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>:

RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH. The named group numbers listed in the first column are numbers used for 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>:

RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH. The extension numbers listed in the first column are numbers used for interoperability testing and it's suggested that IANA use these values for assignment.

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

7. 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.

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. 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.

9. Normative References

Author's Address