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.
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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