Network Working Group D. Benjamin
Internet-Draft Google
Intended status: Informational June 6, 2018
Expires: December 8, 2018
Applying GREASE to TLS Extensibility
draft-ietf-tls-grease-01
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 December 8, 2018.
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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 . . . . . . . . . . . . . . . . . . . . . 5
4. Server-Initiated Extension Points . . . . . . . . . . . . . . 6
4.1. Server Behavior . . . . . . . . . . . . . . . . . . . . . 6
4.2. Client Behavior . . . . . . . . . . . . . . . . . . . . . 6
5. Sending GREASE Values . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
9. Normative References . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The TLS protocol [I-D.ietf-tls-tls13] 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 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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:
{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
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{TBD} 0xBABA
{TBD} 0xCACA
{TBD} 0xDADA
{TBD} 0xEAEA
{TBD} 0xFAFA
Future versions of TLS or DTLS [RFC6347] MUST NOT use any of the
above values as versions.
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
Finally, this document reserves all ALPN identifiers [RFC7301]
beginning with the prefix "ignore/". This corresponds to the seven-
octet prefix: 0x69, 0x67, 0x6e, 0x6f, 0x72, 0x65, 0x2f.
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 corresponding 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
in the "key_share" extension. For each of these, the
"key_exchange" field MAY be any value.
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o A client MAY select one or more GREASE signature algorithm values
and advertise them in the "signature_algorithms" extension, 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.
Specifically, 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 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 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.
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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 corresponding 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" extension.
When sending a NewSessionTicket message in TLS 1.3, a server MAY
select one or more GREASE extension values and advertise
corresponding extensions with varying length and contents.
Servers MUST reject GREASE values when negotiated by the client.
Specifically, 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 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.
Note that these requirements are restatements or corollaries of
existing client requirements in TLS.
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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, implementations sending multiple GREASE values as
extensions MUST NOT send the same GREASE value 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
[[TODO: Update IANA considerations for TLS 1.3 and rebase over draft-
ietf-tls-iana-registry-updates.]]
This document updates the TLS Cipher Suite Registry, available from
<https://www.iana.org/assignments/tls-parameters>:
+-------------------+-------------+---------+-----------------+
| 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
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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>:
+-------------+-------------+---------+-----------------+
| 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.]]
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.
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.
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9. Normative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
[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>.
[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>.
Author's Address
David Benjamin
Google
355 Main St
Cambridge, MA 02142
USA
Email: davidben@google.com
Benjamin Expires December 8, 2018 [Page 10]
