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 provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 25 October 2024. Amsüss Expires 25 October 2024 [Page 1] Internet-Draft Applying Generate Random Extensions And April 2024 Copyright Notice Copyright (c) 2024 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 Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. 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. Amsüss Expires 25 October 2024 [Page 2] Internet-Draft Applying Generate Random Extensions And April 2024 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. Amsüss Expires 25 October 2024 [Page 3] Internet-Draft Applying Generate Random Extensions And April 2024 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 Amsüss Expires 25 October 2024 [Page 4] Internet-Draft Applying Generate Random Extensions And April 2024 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]: Amsüss Expires 25 October 2024 [Page 5] Internet-Draft Applying Generate Random Extensions And April 2024 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, . 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, . [RFC9170] Thomson, M. and T. Pauly, "Long-Term Viability of Protocol Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170, December 2021, . [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, . Appendix A. Open questions Do the GREASE EADs add any value that padding does not already add? Amsüss Expires 25 October 2024 [Page 6] Internet-Draft Applying Generate Random Extensions And April 2024 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. Amsüss Expires 25 October 2024 [Page 7] Internet-Draft Applying Generate Random Extensions And April 2024 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 Amsüss Expires 25 October 2024 [Page 8]