Internet DRAFT - draft-amsuess-core-edhoc-grease
draft-amsuess-core-edhoc-grease
CoRE C. Amsüss
Internet-Draft 22 October 2023
Intended status: Informational
Expires: 24 April 2024
Applying Generate Random Extensions And Sustain Extensibility (GREASE)
to EDHOC Extensibility
draft-amsuess-core-edhoc-grease-01
Abstract
This document applies the extensibility mechanism GREASE (Generate
Random Extensions And Sustain Extensibility), which was pioneered for
TLS, to the EDHOC ecosystem. It reserves a set of non-critical EAD
labels and unusable cipher suites that may be included in messages to
ensure peers correctly handle unknown values.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Constrained RESTful
Environments Working Group mailing list (core@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/core/.
Source for this draft and an issue tracker can be found at
https://gitlab.com/chrysn/core-edhoc-grease.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 24 April 2024.
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Copyright Notice
Copyright (c) 2023 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The GREASE EAD labels . . . . . . . . . . . . . . . . . . . . 3
2.1. Use of GREASE EADs by message senders . . . . . . . . . . 3
2.1.1. Pattern for limited fingerprinting . . . . . . . . . 3
2.2. Use of GREASE EADs by message recipients . . . . . . . . 4
3. GREASE cipher suites . . . . . . . . . . . . . . . . . . . . 4
4. Privacy considerations . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 5
6.1. EDHOC EADs . . . . . . . . . . . . . . . . . . . . . . . 5
6.2. EDHOC cipher suites . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Open questions . . . . . . . . . . . . . . . . . . . 5
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 6
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
[ See abstract ]
The introduction of [RFC8701] provides comprehensive motivation for
adding such extensions.
The extension points of the EDHOC protocol ([I-D.ietf-lake-edhoc])
are cipher suites, methods, EADs (External Authorization Data items)
and COSE headers. Of these, EADs and cipher suites can be used in
such a way that even in the presence of an unknown value, a
connection can still be established.
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Unlike in TLS GREASE, EDHOC is operating on tight bandwidth and
message size budget, with some messages just barely fitting within
relevant networks' fragmentation limits. Thus, more than with TLS
GREASE, it is up to implementations to decide whether in their
particular use case they can afford to send addtional data.
2. The GREASE EAD labels
This document registers the following EAD labels as GREASE EADs:
160, 41120, 43690, 44975
These EADs are available in all EDHOC messages. The EADs are only
used in their positive (non-critical) form.
2.1. Use of GREASE EADs by message senders
A sender of an EDHOC message MAY send a GREASE EAD using the non-
critical (positive) form at any time, with any or no EAD value (that
is, with or without a byte string of any usable length), in any
message.
Senders SHOULD consider the properties of the network their messages
are sent over, and refrain from adding GREASE when its use would be
detrimental to the network (for example, when the added size causes
fragmentation of the message).
On networks where the data added by the grease EADs does not
significantly impact the network, senders SHOULD irregularly send
arbitrary (possibly random) GREASE EADs with their messages to ensure
that errors resulting from the use of GREASE are detected.
The GREASE EADs MAY be used as an alternative form of padding.
2.1.1. Pattern for limited fingerprinting
A method of deciding how to apply GREASE is suggested as follows:
* For every message, use GREASE with a random probability of 1 in
64.
* Pick a random GREASE label out of the uniform distribution of
available options.
* Pick a random length from the uniformly distributed interval 9 to
40 (inclusive).
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* Add the selected GREASE label with a value of the selected length,
filled with random bytes.
2.2. Use of GREASE EADs by message recipients
A party receiving a GREASE EAD MUST NOT alter its behavior in any way
that would allow random GREASE EADs to alter the security context
that gets established.
It MAY alter its behavior in other ways; in particular, it SHOULD
randomly insert GREASE EADs in later messages of an exchange in which
any were received.
If it does not alter its behavior, it is RECOMMENDED that
implementations make no attempt to recognize GREASE EADs, and apply
the default processing -- that is, to ignoring any unknown non-
critical EADs.
3. GREASE cipher suites
This document registers the following cipher suites:
160, 41120, -41121, 43690
An initiator may insert a GREASE cipher suite at any position in its
sequence of preferred cipher suites.
A responder MUST NOT support any of these cipher suites, and MUST
treat them like any other cipher suite it does not support.
Thus, these cipher suites never occur as the selected cipher suite.
An initiator whose choice of a GREASE cipher suite is accepted needs
to discontinue the protocol.
4. Privacy considerations
The way in which GREASE is applied can contribute to identifying
which implementation of EDHOC is being used. Implementers of EDHOC
are encouraged to use the algorithm described in Section 2.1.1, both
to reduce the likelihood of their implementation to be identified
through the use of GREASE and to increase the anonymity set of other
users of the same algorithm.
5. Security Considerations
The use of the GREASE option has no impact on security in a correct
EDHOC implementation.
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6. IANA considerations
6.1. EDHOC EADs
IANA is requested to register four new entries into the EDHOC
External Authorization Data Registry established in
[I-D.ietf-lake-edhoc]:
160, 41120, 43690, 44975
All share the name "GREASE", the description "Arbitrary data to
ensure extensibility", and this document as a reference.
6.2. EDHOC cipher suites
IANA is requested to register four new values into the EDHOC Cipher
Suites Registry established in [I-D.ietf-lake-edhoc]:
160, 41120, -41121, 43690
All share the name "GREASE", the array N/A, the description
"Unimplementable cipher suite to ensure extensibility", and this
document as a reference.
7. References
7.1. Normative References
[I-D.ietf-lake-edhoc]
Selander, G., Mattsson, J. P., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
Progress, Internet-Draft, draft-ietf-lake-edhoc-22, 25
August 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-lake-edhoc-22>.
7.2. Informative References
[RFC8701] Benjamin, D., "Applying Generate Random Extensions And
Sustain Extensibility (GREASE) to TLS Extensibility",
RFC 8701, DOI 10.17487/RFC8701, January 2020,
<https://www.rfc-editor.org/rfc/rfc8701>.
Appendix A. Open questions
Do the GREASE EADs add any value that padding does not already add?
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Probably yes, because padding is "special enough" that it could be
handled in a hard-coded fashion. (Then again, there's nothing but
the effort stopping anyone else from doing the same with the GREASE
EADs, right?)
Can anything be done about extra methods and COSE headers?
They would not result in successful operations, but maybe there is
still some value in registering one or two -- using them would mean
sacrificing the full connection, but it may still be possible to
conclude that the extension points are in order from watching the
EDHOC exchange fail in the predicted way.
Appendix B. Change log
Since -00:
* Fixed a mix-up between positivity and criticality of options.
* Adjusted numbers accordingly to once more fit in the 0xa. pattern
(actually they're using 0x.a, but that doesn't work the same way
with CBOR).
* Text improvements around recipient side processing.
Acknowledgements
Marco Tiloca pointed out a critical error in the numeric
constructions. Göran Selander provided input to reduce mistakable
text.
Author's Address
Christian Amsüss
Austria
Email: christian@amsuess.com
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