]>

Austria christian@amsuess.com

LAKE This document applies the extensibility mechanism GREASE (Generate Random Extensions And Sustain Extensibility), which was pioneered for TLS, to the ecosystem of Ephemeral Diffie-Hellman Over COSE (EDHOC). It reserves a set of External Authorization Data (EAD) labels and unusable cipher suites that may be included in messages to ensure peers correctly handle unknown values. Discussion Venues Discussion of this document takes place on the Lightweight Authenticated Key Exchange Working Group mailing list (lake@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction This document applies the extensibility mechanism GREASE (Generate Random Extensions And Sustain Extensibility), which was pioneered for TLS in , to Ephemeral Diffie-Hellman Over COSE (EDHOC, ) ecosystem. The introduction of and provide comprehensive motivation for adding such extensions; provides additional background that influenced this document. The extension points of the EDHOC protocol are cipher suites, methods, EAD (External Authorization Data) items and COSE header parameters. This document utilizes the cipher suite and EAD extension points. 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 additional data.

Variability in other extension points If the selected method is unsupported by the Responder, 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 in 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, allows only the numeric identifier form where that is possible). Where variation is allowed, e.g., in padding or in the ordering of EAD items, applications are encouraged to exercise it. The extension point of COSE header parameters (identifying other ID_CRED_x types) is beyond the scope of this document, and might be addressed orthogonally in the "COSE Header Parameters" registry.

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

The GREASE EAD labels This document registers the following EAD labels for use with GREASE EAD items: 160, 41120, 43690, 44975 These EAD labels can be used in any EDHOC message for non-critical EAD items (see ). The numbers cover the different lengths of encoding available in CBOR for the registry's range (except the 23 precious small values). It is expected that future documents register additional values with the same semantics.

Use of GREASE EAD items by message senders A sender of an EDHOC message MAY include an arbitrary number of GREASE EAD items, with any or no ead_value (that is, with or without a byte string of any usable length). 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, they might use it less frequently when the added size causes fragmentation of the message). On networks where the data added by the grease EAD items does not significantly impact the network, senders SHOULD irregularly send arbitrary (possibly random) GREASE EAD items with their messages to ensure that errors resulting from the use of GREASE are detected. The GREASE EAD items MAY be used as an alternative form of padding.

Pattern for limited fingerprinting A method of applying 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.

Running EDHOC already requires the presence of a cryptographically secure random number generator. Implementers can use that same source here to avoid any privacy implications from insufficiently initialized faster sources of randomness.

Use of GREASE EAD items by message recipients A party receiving a GREASE EAD item MUST NOT alter its behavior in any way that would allow random GREASE EAD items to alter the security context that gets established. It MAY alter its behavior in other ways; in particular, it SHOULD randomly insert GREASE EAD items in later messages of a session in which unprocessed EAD items (including GREASE EAD items) were present. Implementations SHOULD NOT attempt to recognize GREASE EAD items, and SHOULD apply the default processing rules.

GREASE cipher suites This document registers the following GREASE cipher suites: 160, 41120, -41121, 43690 The numbers cover the different lengths of encoding available in CBOR for the registry's range (except the 46 precious small values), and both available signs. 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 GREASE cipher suites, and MUST treat them like any other cipher suite it does not support. Thus, a GREASE cipher suite never occurs as the selected cipher suite, i.e., it is never specified as the last cipher suite in EDHOC message_1. An Initiator whose offer of a GREASE cipher suite is accepted through cipher suite negotiation () needs to discontinue the protocol.

Processing of GREASE-related failures It is RECOMMENDED that any counters or statistics about successful and failed sessions distinguish between sessions 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 . On constrained devices, one suitable operator feedback channel is CORECONF ; no general YANG model is available for that purpose. Whether logging of GREASE-related failed session details is appropriate depends on the privacy policies of the application.

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 , 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.

Security Considerations The use of GREASE has no impact on security in a correct EDHOC implementation. As the application of GREASE contributes to an ecosystem that can have security updates deployed in the future, implementers of EDHOC are strongly encouraged to apply GREASE regularly whenever their operational constraints permit it.

IANA considerations

EDHOC EAD labels IANA is requested to register four new entries into the EDHOC External Authorization Data Registry established in : 160, 41120, 43690, 44975 All share the name "GREASE", the description "Arbitrary data to ensure extensibility", and this document as a reference.

EDHOC cipher suites IANA is requested to register four new values into the EDHOC Cipher Suites Registry established in : 160, 41120, -41121, 43690 All share the array N/A, the description "Unimplementable GREASE cipher suite to ensure extensibility", and this document as a reference.

References Normative References This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios, and a main use case is to establish an Object Security for Constrained RESTful Environments (OSCORE) security context. By reusing CBOR Object Signing and Encryption (COSE) for cryptography, Concise Binary Object Representation (CBOR) for encoding, and Constrained Application Protocol (CoAP) for transport, the additional code size can be kept very low. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References 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. The ability to change protocols depends on exercising the extension and version-negotiation mechanisms that support change. This document explores how regular use of new protocol features can ensure that it remains possible to deploy changes to a protocol. Examples are given where lack of use caused changes to be more difficult or costly. Cloudflare Apple Armidale Consulting Active use and maintenance of network protocols is an important way to ensure that protocols remain interoperable and extensible over time. Techniques such as intentionally exercising extension points with non-meaningful values (referred to as "grease") or adding variability to how protocol elements are used help generate this active use. Grease and variability are used across various protocols developed by the IETF. This document discusses considerations when designing and deploying grease and variability mechanisms, and provides advice for making them as effective as possible. Trilliant Networks Inc. consultant IMT Atlantique YumaWorks Universität Bremen TZI This document describes a network management interface for constrained devices and networks, called CoAP Management Interface (CORECONF). The Constrained Application Protocol (CoAP) is used to access datastore and data node resources specified in YANG, or SMIv2 converted to YANG. CORECONF uses the YANG to CBOR mapping and converts YANG identifier strings to numeric identifiers for payload size reduction. CORECONF extends the set of YANG based protocols, NETCONF and RESTCONF, with the capability to manage constrained devices and networks.

Using extension points beyond successful EDHOC runs Some ways of using the extension points, in particular the use of critical GREASE EAD items and placing a GREASE cipher suite in the selected position do not result in the successful continuation of the EDHOC session. They can be useful during testing (e.g., to verify that a peer does indeed implement the correct behavior of not silently tolerating critical EAD items it can not process), particularly when they allow a testing system to provoke an error response from the implementation under test. However, this document is concerned with test performed during successful operation, therefore that application is out of scope.

Change log Since draft-ietf-lake-edhoc-grease-01: Address WGLC comments.

• seccons: Stronly encourage use of GREASE
• Explicit SHOULD on applying default processing rules (not just by exclusion of SHOULD NOT attempt)
• Point to EDHOC CS-RNG for fingerprint resistance
• Point to CORECONF as example of how to report
• Add remark on why numeric values were chosen
• Elaboration on cipher suite selection
• Added BCP14 boilerplate Terminology section
• Updated reference from edm-protocol-greasing-02 to iab-protocol-greasing (currently at -01)
• Editorial fixes; highlights:
  • Consistency around "EAD items" and "EAD labels"
  • EAD items can be critical/non-critical, not labels

Since draft-ietf-lake-edhoc-grease-00: Resolve all open issues.

• Question on "is this better than padding" removed. (There are currently implementations of EDHOC that can't use all EAD values but can do padding).
• Question of COSE header extension deferred to COSE maintenance.
• Use of GREASE values in critical form is out of scope, but appendix illustrates that it can make sense to do, and emphasizes that indeed those options do cause errors when used with negative sign.

Since draft-amsuess-lake-edhoc-grease-01:

• Document was adopted in LAKE.
• Instead of discouraging GREASE around fragmentation limits wholesale, suggest reduced frequency.
• Editorial fix to fingerprinting section.

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, and provided many general improvements. Göran Selander provided input to reduce mistakable text. Meiling Chen and Shujaatali Badami found places where the text did not provide the right guidance to readers.