Internet DRAFT - draft-ietf-tls-grease
draft-ietf-tls-grease
Network Working Group D. Benjamin
Internet-Draft Google LLC
Intended status: Informational August 21, 2019
Expires: February 22, 2020
Applying GREASE to TLS Extensibility
draft-ietf-tls-grease-04
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 22, 2020.
Copyright Notice
Copyright (c) 2019 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. GREASE Values . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Client-Initiated Extension Points . . . . . . . . . . . . . . 4
3.1. Client Behavior . . . . . . . . . . . . . . . . . . . . . 4
3.2. Server Behavior . . . . . . . . . . . . . . . . . . . . . 6
4. Server-Initiated Extension Points . . . . . . . . . . . . . . 6
4.1. Server Behavior . . . . . . . . . . . . . . . . . . . . . 6
4.2. Client Behavior . . . . . . . . . . . . . . . . . . . . . 7
5. Sending GREASE Values . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
9. Normative References . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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).
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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][RFC2119] RFC 8174 [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.
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
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{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:
o A client MAY select one or more GREASE cipher suite values and
advertise them in the "cipher_suites" field.
o A client MAY select one or more GREASE extension values and
advertise them as extensions with varying length and contents.
o 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
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in the "key_share" extension. For each of these, the
"key_exchange" field MAY be any value.
o 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.
o A client MAY select one or more GREASE version values and
advertise them in the "supported_versions" extension, if sent.
o A client MAY select one or more GREASE PskKeyExchangeMode values
and advertise them in the "psk_key_exchange_modes" extension, if
sent.
o 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:
o The "version" value in a ServerHello or HelloRetryRequest
o The "cipher_suite" value in a ServerHello
o Any ServerHello extension
o Any HelloRetryRequest, EncryptedExtensions, or Certificate
extension in TLS 1.3
o The "namedcurve" value in a ServerKeyExchange for an ECDHE cipher
in TLS 1.2 [RFC5246] or earlier
o The signature algorithm in a ServerKeyExchange signature in TLS
1.2 or earlier
o 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.
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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:
o A server MAY select one or more GREASE extension values and
advertise them as extensions with varying length and contents.
o 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:
o Any Certificate extension in TLS 1.3
o 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.
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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>:
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+---------------+-------------+---------+-------------+-------------+
| Value | Description | DTLS-OK | Recommended | Reference |
+---------------+-------------+---------+-------------+-------------+
| {TBD} | Reserved | Y | N | (this |
| {0x0A,0x0A} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0x1A,0x1A} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0x2A,0x2A} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0x3A,0x3A} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0x4A,0x4A} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0x5A,0x5A} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0x6A,0x6A} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0x7A,0x7A} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0x8A,0x8A} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0x9A,0x9A} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0xAA,0xAA} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0xBA,0xBA} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0xCA,0xCA} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0xDA,0xDA} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0xEA,0xEA} | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| {0xFA,0xFA} | | | | document) |
+---------------+-------------+---------+-------------+-------------+
Additions to the TLS Cipher Suite Registry
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>:
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+------------+-------------+---------+-------------+----------------+
| Value | Description | DTLS-OK | Recommended | Reference |
+------------+-------------+---------+-------------+----------------+
| {TBD} 2570 | Reserved | Y | N | (this |
| | | | | document) |
| {TBD} 6682 | Reserved | Y | N | (this |
| | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 10794 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 14906 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 19018 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 23130 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 27242 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 31354 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 35466 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 39578 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 43690 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 47802 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 51914 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 56026 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 60138 | | | | document) |
| {TBD} | Reserved | Y | N | (this |
| 64250 | | | | document) |
+------------+-------------+---------+-------------+----------------+
Additions to the Supported Groups Registry
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>:
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+----------+--------------+-----------+-------------+---------------+
| Value | Extension | TLS 1.3 | Recommended | Reference |
| | name | | | |
+----------+--------------+-----------+-------------+---------------+
| {TBD} | Reserved | CH, CR, | N | (this |
| 2570 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 6682 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 10794 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 14906 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 19018 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 23130 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 27242 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 31354 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 35466 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 39578 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 43690 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 47802 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 51914 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 56026 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 60138 | | NST | | document) |
| {TBD} | Reserved | CH, CR, | N | (this |
| 64250 | | NST | | document) |
+----------+--------------+-----------+-------------+---------------+
Additions to the ExtensionType Values registry
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
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<https://www.iana.org/assignments/tls-extensiontype-values/tls-
extensiontype-values>:
+----------+-------------------------+-----------------+
| Protocol | Identification Sequence | Reference |
+----------+-------------------------+-----------------+
| Reserved | {TBD} 0x0A 0x0A | (this document) |
| Reserved | {TBD} 0x1A 0x1A | (this document) |
| Reserved | {TBD} 0x2A 0x2A | (this document) |
| Reserved | {TBD} 0x3A 0x3A | (this document) |
| Reserved | {TBD} 0x4A 0x4A | (this document) |
| Reserved | {TBD} 0x5A 0x5A | (this document) |
| Reserved | {TBD} 0x6A 0x6A | (this document) |
| Reserved | {TBD} 0x7A 0x7A | (this document) |
| Reserved | {TBD} 0x8A 0x8A | (this document) |
| Reserved | {TBD} 0x9A 0x9A | (this document) |
| Reserved | {TBD} 0xAA 0xAA | (this document) |
| Reserved | {TBD} 0xBA 0xBA | (this document) |
| Reserved | {TBD} 0xCA 0xCA | (this document) |
| Reserved | {TBD} 0xDA 0xDA | (this document) |
| Reserved | {TBD} 0xEA 0xEA | (this document) |
| Reserved | {TBD} 0xFA 0xFA | (this document) |
+----------+-------------------------+-----------------+
Additions to the 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.
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8. 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.
9. 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>.
Author's Address
David Benjamin
Google LLC
Email: davidben@google.com
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