Applying Generate Random Extensions And Sustain Extensibility (GREASE) to EDHOC Extensibility

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Table of Contents

1. Introduction

This document applies the extensibility mechanism GREASE (Generate Random Extensions And Sustain Extensibility), which was pioneered for TLS in [RFC8701], to the EDHOC [RFC9528] ecosystem.¶

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.¶

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.¶

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.¶

3. GREASE cipher suites

This document registers the following cipher suites:¶

160, 41120, -41121, 43690¶

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

Appendix A. Open questions

Do the GREASE EADs add any value that padding does not already add?¶

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-lake-edhoc-grease-00:¶

• Expanded introduction section to just point to the abstract any more.¶

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.¶

Acknowledgements

Marco Tiloca pointed out a critical error in the numeric constructions. Göran Selander provided input to reduce mistakable text.¶

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