Applying Generate Random Extensions And Sustain Extensibility (GREASE) to EDHOC Extensibility
draft-amsuess-lake-edhoc-grease-00

LAKE                                                           C. Amsüss
Internet-Draft                                             23 April 2024
Intended status: Informational                                          
Expires: 25 October 2024

 Applying Generate Random Extensions And Sustain Extensibility (GREASE)
                         to EDHOC Extensibility
                   draft-amsuess-lake-edhoc-grease-00

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
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   This Internet-Draft will expire on 25 October 2024.

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Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Variability in other extension points . . . . . . . . . .   3
   2.  The GREASE EAD labels . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Use of GREASE EADs by message senders . . . . . . . . . .   3
       2.1.1.  Pattern for limited fingerprinting  . . . . . . . . .   4
     2.2.  Use of GREASE EADs by message recipients  . . . . . . . .   4
   3.  GREASE cipher suites  . . . . . . . . . . . . . . . . . . . .   4
   4.  Processing of GREASE related failures . . . . . . . . . . . .   5
   5.  Privacy considerations  . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  EDHOC EADs  . . . . . . . . . . . . . . . . . . . . . . .   5
     7.2.  EDHOC cipher suites . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Open questions . . . . . . . . . . . . . . . . . . .   6
   Appendix B.  Change log . . . . . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [ See abstract ]

   The introduction of [RFC8701] and Section 3.3 of [RFC9170] provide
   comprehensive motivation for adding such extensions;
   [I-D.edm-protocol-greasing-02] provides additional background that
   influenced this document.

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   The extension points of the EDHOC protocol ([RFC9528]) are cipher
   suites, methods, EADs (External Authorization Data items) and COSE
   headers.  This document utilizes the cipher suite and EAD extension
   points.

   Unlike in TLS GREASE [RFC8701], 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.

1.1.  Variability in other extension points

   If the selected method or the used COSE heades are unsupported by the
   peer, EDHOC does not conclude successfully.  While values could be
   reserved for these for use as GREASE, these failed attempts would not
   be verified between the EDHOC participants without maintaining state
   between attempted EDHOC sessions.  Such an addition is considered
   impractical for constrained devices, and thus out of scope for this
   document.

   Recommendations for GREASE Section 4 of
   [I-D.edm-protocol-greasing-02] also include varying other aspects of
   the protocol, such as varying sequences of elements.  EDHOC has
   little known variability, and intentionally limits choice at times
   (for example, Section 3.3.2 of [RFC9528] allows only the numeric
   identifier form where that is possible).  Where variation is allowed,
   e.g. in padding or in the ordering of EAD options, applications are
   encouraged to exercise it.

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.

   It is expected that future documents register additional values with
   the same semantics.

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.

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   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).

   *  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
   unprocessed EADs (including GREASE EADs) were present.

   Implementations SHOULD NOT attempt to recognize GREASE EADs, and
   apply the default processing rules.

3.  GREASE cipher suites

   This document registers the following cipher suites:

   160, 41120, -41121, 43690

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   It is expected that future documents register additional values with
   the same semantics.

   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.  Processing of GREASE related failures

   It is RECOMMENDED that any counters or statistics about successful
   and failed connections distinguish between connections in which
   GREASE was applied and those in which it was not applied.  Any
   operator feedback channel, be it immediately to the user or through
   network monitoring, SHOULD warn the operator if there are errors that
   were determined to originate from the use of GREASE or that are
   significantly likely to originate from there.  This provides a
   feedback path as described in Section 4.4 of [RFC9170].

   Whether logging of GREASE related failed connection details is
   appropriate depends on the privacy policies of the application.

5.  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.

6.  Security Considerations

   The use of the GREASE option has no impact on security in a correct
   EDHOC implementation.

7.  IANA considerations

7.1.  EDHOC EADs

   IANA is requested to register four new entries into the EDHOC
   External Authorization Data Registry established in [RFC9528]:

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   160, 41120, 43690, 44975

   All share the name "GREASE", the description "Arbitrary data to
   ensure extensibility", and this document as a reference.

7.2.  EDHOC cipher suites

   IANA is requested to register four new values into the EDHOC Cipher
   Suites Registry established in [RFC9528]:

   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.

8.  References

8.1.  Normative References

   [RFC9528]  Selander, G., Preuß Mattsson, J., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", RFC 9528,
              DOI 10.17487/RFC9528, March 2024,
              <https://www.rfc-editor.org/rfc/rfc9528>.

8.2.  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>.

   [RFC9170]  Thomson, M. and T. Pauly, "Long-Term Viability of Protocol
              Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170,
              December 2021, <https://www.rfc-editor.org/rfc/rfc9170>.

   [I-D.edm-protocol-greasing-02]
              Pardue, L., "Maintaining Protocols Using Grease and
              Variability", Work in Progress, Internet-Draft, draft-edm-
              protocol-greasing-02, 10 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-edm-protocol-
              greasing-02>.

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 draft-amsuess-core-edhoc-grease-01:

   *  Update references to RFC9528 🎉

   *  Change target WG to LAKE, renaming to draft-amsuess-lake-edhoc-
      grease

   *  Process RFC9170

      -  Add a section on failure processing

      -  Reference where appropriate

   *  Process draft-edm-protocol-greasing-02

      -  Variability outside of extension points

      -  Be firmer against recognizing GREASE values

      -  Point out that future options may be registered (instead of the
         suggested algorithmic registrations)

   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.

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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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