Network Working Group D. Benjamin
Internet-Draft Google
Intended status: Informational January 18, 2017
Expires: July 22, 2017
Applying GREASE to TLS Extensibility
draft-ietf-tls-grease-00
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 by clients to ensure servers correctly handle
unknown values.
Status of This Memo
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This Internet-Draft will expire on July 22, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. GREASE Values . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 3
4. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The TLS protocol [RFC5246] includes several points of extensibility,
including the list of cipher suites and the list of extensions. The
values in these lists identify implementation capabilities. TLS
follows a model where clients advertise capabilities and servers
select them. It is required that servers ignore unknown values so
that new capabilities may be introduced to the ecosystem while
maintaining interoperability.
However, bugs may cause a server to reject unknown values. These
broken servers will interoperate with existing clients, so the
mistake may spread through the ecosystem unnoticed. Later, when new
values are defined, updated clients 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 clients to advertise at random. Correct server
implementations will ignore these values and interoperate. Servers
that do not tolerate unknown values will fail to interoperate with
existing clients, revealing the mistake before it is widespread.
This document reserves such values in the TLS cipher suite,
extension, named group [RFC4492], and ALPN [RFC7301] registries.
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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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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 cipher suite values:
{Values TBD}
The following values are reserved as both GREASE extension values and
GREASE named group values:
{Values TBD}
[[TODO: Depending on which of this or TLS 1.3 happens first, also
reserve SignatureScheme values. (The same number scheme will work
fine there too.)]]
Note that these correspond to the reserved cipher suites when treated
as big-endian 16-bit integers.
Finally, this document reserves all ALPN identifiers beginning with
the prefix "ignore/". This corresponds to the seven-octet prefix:
0x69, 0x67, 0x6e, 0x6f, 0x72, 0x65, 0x2f.
3. Client Behavior
When sending a ClientHello, a client which implements GREASE behaves
as follows:
o A client MAY select one or more random GREASE cipher suite values
and advertise them in the ClientHello.cipher_suites field.
o A client MAY select one or more random GREASE named group values
and advertise them in the supported_groups extension, if sent.
o A client MAY select one or more random GREASE extension values and
advertise corresponding extensions with varying length and
contents in the ClientHello.extensions field.
o A client MAY select one or more random GREASE ALPN identifiers and
advertise them in the application_layer_protocol_negotiation
extension, if sent.
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Clients 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.
Clients MUST reject GREASE values when negotiated by the server.
When processing a ServerHello containing a GREASE value in the
ServerHello.cipher_suite or ServerHello.extensions fields, the client
MUST fail the connection. When processing an ECParameters structure
with a GREASE value in the ECParameter.namedcurve field, the client
MUST fail the connection.
Note that this requires no special processing on the client. Clients
are already required to reject unknown values selected by the server.
4. Server Behavior
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.
Note that these requirements are restatements or corollaries of
existing server requirements in TLS.
5. 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 | Reference |
+-------------------+-------------+---------+-----------------+
| {TBD} {0x0A,0x0A} | Reserved | Y | (this document) |
| {TBD} {0x1A,0x1A} | Reserved | Y | (this document) |
| {TBD} {0x2A,0x2A} | Reserved | Y | (this document) |
| {TBD} {0x3A,0x3A} | Reserved | Y | (this document) |
| {TBD} {0x4A,0x4A} | Reserved | Y | (this document) |
| {TBD} {0x5A,0x5A} | Reserved | Y | (this document) |
| {TBD} {0x6A,0x6A} | Reserved | Y | (this document) |
| {TBD} {0x7A,0x7A} | Reserved | Y | (this document) |
| {TBD} {0x8A,0x8A} | Reserved | Y | (this document) |
| {TBD} {0x9A,0x9A} | Reserved | Y | (this document) |
| {TBD} {0xAA,0xAA} | Reserved | Y | (this document) |
| {TBD} {0xBA,0xBA} | Reserved | Y | (this document) |
| {TBD} {0xCA,0xCA} | Reserved | Y | (this document) |
| {TBD} {0xDA,0xDA} | Reserved | Y | (this document) |
| {TBD} {0xEA,0xEA} | Reserved | Y | (this document) |
| {TBD} {0xFA,0xFA} | Reserved | Y | (this document) |
+-------------------+-------------+---------+-----------------+
Additions to the TLS Cipher Suite Registry
The cipher suite numbers listed in the first column are numbers used
for cipher suite 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 | Reference |
+-------------+-------------+---------+-----------------+
| {TBD} 2570 | Reserved | Y | (this document) |
| {TBD} 6682 | Reserved | Y | (this document) |
| {TBD} 10794 | Reserved | Y | (this document) |
| {TBD} 14906 | Reserved | Y | (this document) |
| {TBD} 19018 | Reserved | Y | (this document) |
| {TBD} 23130 | Reserved | Y | (this document) |
| {TBD} 27242 | Reserved | Y | (this document) |
| {TBD} 31354 | Reserved | Y | (this document) |
| {TBD} 35466 | Reserved | Y | (this document) |
| {TBD} 39578 | Reserved | Y | (this document) |
| {TBD} 43690 | Reserved | Y | (this document) |
| {TBD} 47802 | Reserved | Y | (this document) |
| {TBD} 51914 | Reserved | Y | (this document) |
| {TBD} 56026 | Reserved | Y | (this document) |
| {TBD} 60138 | Reserved | Y | (this document) |
| {TBD} 64250 | Reserved | Y | (this document) |
+-------------+-------------+---------+-----------------+
Additions to the Supported Groups Registry
The named group numbers listed in the first column are numbers used
for cipher suite 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 name | Reference |
+-------------+----------------+-----------------+
| {TBD} 2570 | Reserved | (this document) |
| {TBD} 6682 | Reserved | (this document) |
| {TBD} 10794 | Reserved | (this document) |
| {TBD} 14906 | Reserved | (this document) |
| {TBD} 19018 | Reserved | (this document) |
| {TBD} 23130 | Reserved | (this document) |
| {TBD} 27242 | Reserved | (this document) |
| {TBD} 31354 | Reserved | (this document) |
| {TBD} 35466 | Reserved | (this document) |
| {TBD} 39578 | Reserved | (this document) |
| {TBD} 43690 | Reserved | (this document) |
| {TBD} 47802 | Reserved | (this document) |
| {TBD} 51914 | Reserved | (this document) |
| {TBD} 56026 | Reserved | (this document) |
| {TBD} 60138 | Reserved | (this document) |
| {TBD} 64250 | Reserved | (this document) |
+-------------+----------------+-----------------+
Additions to the ExtensionType Values registry
The extension numbers listed in the first column are numbers used for
cipher suite interoperability testing and it's suggested that IANA
use these values for assignment.
[[TODO: How do I write IANA instructions to reserve all ALPN
identifiers that begin with "ignore/"? Perhaps it would be better to
reserve a concrete handful of identifiers instead.]]
6. 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.
7. 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.
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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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492,
DOI 10.17487/RFC4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[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, <http://www.rfc-editor.org/info/rfc7301>.
Author's Address
David Benjamin
Google
355 Main St
Cambridge, MA 02142
USA
Email: davidben@google.com
Benjamin Expires July 22, 2017 [Page 8]
