CoRE Working Group                                             C. Amsüss
Internet-Draft
Intended status: Standards Track                               M. Tiloca
Expires: 8 September 2022                                        RISE AB
                                                            7 March 2022


                            Cacheable OSCORE
                 draft-amsuess-core-cachable-oscore-04

Abstract

   Group communication with the Constrained Application Protocol (CoAP)
   can be secured end-to-end using Group Object Security for Constrained
   RESTful Environments (Group OSCORE), also across untrusted
   intermediary proxies.  However, this sidesteps the proxies' abilities
   to cache responses from the origin server(s).  This specification
   restores cacheability of protected responses at proxies, by
   introducing consensus requests which any client in a group can send
   to one server or multiple servers in the same group.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the CORE 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-cachable-oscore/.

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 8 September 2022.



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Copyright Notice

   Copyright (c) 2022 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Use cases . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  OSCORE processing without source authentication . . . . . . .   5
   3.  Deterministic Requests  . . . . . . . . . . . . . . . . . . .   7
     3.1.  Deterministic Unprotected Request . . . . . . . . . . . .   7
     3.2.  Design Considerations . . . . . . . . . . . . . . . . . .   8
     3.3.  Request-Hash  . . . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Use of Deterministic Requests . . . . . . . . . . . . . .  10
       3.4.1.  Pre-Conditions  . . . . . . . . . . . . . . . . . . .  10
       3.4.2.  Client Processing of Deterministic Request  . . . . .  11
       3.4.3.  Server Processing of Deterministic Request  . . . . .  13
       3.4.4.  Response to a Deterministic Request . . . . . . . . .  15
       3.4.5.  Deterministic Requests to Multiple Servers  . . . . .  16
   4.  Obtaining Information about the Deterministic Client  . . . .  17
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  CoAP Option Numbers Registry  . . . . . . . . . . . . . .  18
     6.2.  OSCORE Security Context Parameters Registry . . . . . . .  19
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .  23
   Appendix B.  Padding  . . . . . . . . . . . . . . . . . . . . . .  24
     B.1.  Definition of the Padding Option  . . . . . . . . . . . .  24
     B.2.  Using and processing the Padding option . . . . . . . . .  25
   Appendix C.  Simple Cacheability using Ticket Requests  . . . . .  25
   Appendix D.  Application for More Efficient End-to-End Protected
           Multicast Notifications . . . . . . . . . . . . . . . . .  27
   Appendix E.  Open questions . . . . . . . . . . . . . . . . . . .  27
   Appendix F.  Unsorted further ideas . . . . . . . . . . . . . . .  28
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  28



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] supports also
   group communication, for instance over UDP and IP multicast
   [I-D.ietf-core-groupcomm-bis].  In a group communication environment,
   exchanged messages can be secured end-to-end by using Group Object
   Security for Constrained RESTful Environments (Group OSCORE)
   [I-D.ietf-core-oscore-groupcomm].

   Requests and responses protected with the group mode of Group OSCORE
   can be read by all group members, i.e., not only by the intended
   recipient(s), thus achieving group-level confidentiality.

   This allows a trusted intermediary proxy which is also a member of
   the OSCORE group to populate its cache with responses from origin
   servers.  Later on, the proxy can possibly reply to a request in the
   group with a response from its cache, if recognized as an eligible
   server by the client.

   However, an untrusted proxy which is not member of the OSCORE group
   only sees protected responses as opaque, uncacheable ciphertext.  In
   particular, different clients in the group that originate a same
   plain CoAP request would send different protected requests, as a
   result of their Group OSCORE processing.  Such protected requests
   cannot yield a cache hit at the proxy, which makes the whole caching
   of protected responses pointless.

   This document addresses this complication and enables cacheability of
   protected responses, also for proxies that are not members of the
   OSCORE group and are unaware of OSCORE in general.  To this end, it
   builds on the concept of "consensus request" initially considered in
   [I-D.ietf-core-observe-multicast-notifications], and defines
   "deterministic request" as a convenient incarnation of such concept.

   All clients wishing to send a particular GET or FETCH request are
   able to deterministically compute the same protected request, using a
   variation on the pairwise mode of Group OSCORE.  It follows that
   cache hits become possible at the proxy, which can thus serve clients
   in the group from its cache.  Like in
   [I-D.ietf-core-observe-multicast-notifications], this requires that
   clients and servers are already members of a suitable OSCORE group.

   Cacheability of protected responses is useful also in applications
   where several clients wish to retrieve the same object from a single
   server.  Some security properties of OSCORE are dispensed with to
   gain other desirable properties.



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   In order to clearly handle the protocol's security properties, and to
   broaden applicability to group situations outside the deterministic
   case, the technical implementation is split in two halves:

   *  maintaining request-response bindings in absence of request source
      authentication, and

   *  building and processing of deterministic requests (which have no
      source authentication, and thus require the former).

1.1.  Use cases

   When firmware updates are delivered using CoAP, many similar devices
   fetch large representations at the same time.  Collecting them at a
   proxy not only keeps the traffic low, but also lets the clients ride
   single file to hide their numbers[SW-EPIV] and identities.

   When relying on intermediaries to fan out the delivery of multicast
   data protected end-to-end as in
   [I-D.ietf-core-observe-multicast-notifications], deterministic
   requests allow for a more efficient setup, by reducing the amount of
   message exchanges and enabling early population of cache entries (see
   Appendix D).

1.2.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are expected to be familiar with terms and concepts of CoAP
   [RFC7252] and its method FETCH [RFC8132], group communication for
   CoAP [I-D.ietf-core-groupcomm-bis], COSE
   [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs],
   OSCORE [RFC8613], and Group OSCORE [I-D.ietf-core-oscore-groupcomm].

   This document introduces the following new terms.

   *  Consensus Request: a CoAP request that multiple clients use to
      repeatedly access a particular resource.  In this document, it
      exclusively refers to requests protected with Group OSCORE to a
      resource hosted at one or more servers in the OSCORE group.







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      A Consensus Request has all the properties relevant to caching,
      but its transport dependent properties (e.g., Token or Message ID)
      are not defined.  Thus, different requests on the wire can be said
      to "be the same Consensus Request" even if they have different
      Tokens or source addresses.

      The Consensus Request is the reference for request-response
      binding.  In general, a client processing a response to a
      consensus request did not generate (and thus sign) the consensus
      request.  The client not only needs to decrypt the Consensus
      Request to understand a response to it (for example to tell which
      path was requested), it also needs to verify that this is the only
      Consensus Request that could elicit this response.

   *  Deterministic Client: a fictitious member of an OSCORE group,
      having no Sender Sequence Number, no asymmetric key pair, and no
      Recipient Context.

      The Group Manager sets up the Deterministic Client, and assigns it
      a unique Sender ID as for other group members.  Furthermore, the
      Deterministic Client has only the minimum common set of privileges
      shared by all group members.

   *  Deterministic Request: a Consensus Request generated by the
      Deterministic Client.  The use of Deterministic Requests is
      defined in Section 3.

   *  Ticket Request: a Consensus Request generated by the server
      itself.

      This term is not used in the main document, but is useful in
      comparison with other applications of consensus requests that are
      generated in a different way than as a Deterministic Request.  The
      prototypical Ticket Request is the Phantom Request defined in
      [I-D.ietf-core-observe-multicast-notifications].

      In Appendix C, the term is used to bridge the gap to that draft.

2.  OSCORE processing without source authentication

   The request-response binding of OSCORE is achieved by the request_kid
   / request_piv items (and, in group OSCORE, request_kid_context)
   present in the response's AAD.  Its security depends on the server
   obtaining source authentication for the request: Without, a malicious
   group member could alter a request to the server (without altering
   the request_ details above), and the client would still accept the
   response as if it were a response to its request.




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   Source authentication is thus a precondition to secure use of OSCORE.
   However, it is hard to provide when:

   *  Requests are built exclusively using shared key material (as in a
      Deterministic Client).

   *  Requests are sent without source authentication, or where the
      source authentication is not checked.  (This was part of
      [I-D.ietf-core-oscore-groupcomm] in revisions before -12).

   This document does not [ yet? ] give full guidance on how to restore
   request-response binding for the general case, but currently only
   offers suggestions:

   *  The response can contain the full request.  An option that allows
      doing that was presented in [I-D.bormann-core-responses].

   *  The response can contain a cryptographic hash of the full request.
      This is used in Section 3.3.

   *  The above details can be transported in a Class E option
      (encrypted) or a a Class I option (unencrypted but part of the
      AAD).  The latter has the advantage that it can be removed in
      transit and reconstructed at the receiver.

   *  Alternatively, the agreed-on request data can be placed in a
      different position in the AAD, or be part of the security context
      derivation.  In the latter case, care needs to be taken to never
      initialize a security context twice with the same input, as that
      would lead to nonce reuse.

   [ Suggestion for any OSCORE v2: avoid request details in the
   request's AAD as individual elements.  Rather than having
   'request_kid', 'request_piv' and (in Group OSCORE)
   'request_kid_context' as separate fields, they can better be
   something more pluggable.  This would avoid the need to make up an
   option before processing, and would allow just plugging in the hash
   or request in there replacing the request_ items. ]

   Additional care has to be taken that details not expressed in the
   request itself (like the security context from which it is assumed to
   have originated) are captured.

   Processing of requests without source authentication has to be done
   assuming only the minimal possible privilege of the requester [ which
   currently described as the authorization of the Deterministic Client,
   and may be moved up here in later versions of this document ].  If a
   response is built to such a request that contains data more sensitive



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   than that (which might be justified if the response is protected for
   an authorized group member in pairwise response mode), special
   consideration for any side channels like response size or timing is
   required.

3.  Deterministic Requests

   This section defines a method for clients starting from a same plain
   CoAP request to independently arrive at a same Deterministic Request
   protected with Group OSCORE.

3.1.  Deterministic Unprotected Request

   Clients build the unprotected Deterministic Request in a way which is
   as much reproducible as possible.  This document does not set out
   full guidelines for minimizing the variation, but considered starting
   points are:

   *  Set the inner Observe option to 0 even if no observation is
      intended (and hence no outer Observe is set).  Thus, both
      observing and non-observing requests can be aggregated into a
      single request, that is upstreamed as an observation at the latest
      when any observing request reaches a caching proxy.

      In this case, following a Deterministic Request that includes only
      an inner Observe option, servers include an inner Observe option
      (but no outer Observe option) in a successful response sent as
      reply.  Also, when receiving a response to such a Deterministic
      Request previously sent, clients have to silently ignore the inner
      Observe option in that response.

   *  Avoid setting the ETag option in requests on a whim.  Only set it
      when there was a recent response with that ETag.  When obtaining
      later blocks, do not send the known-stale ETag.

   *  In block-wise transfer, maximally sized large inner blocks (szx=6)
      should be selected.  This serves not only to align the clients on
      consistent cache entries, but also helps amortize the additional
      data transferred in the per-message signatures.

   Outer block-wise transfer can then be used if these messages excede a
   hop's efficiently usable MTU size.

   (If BERT [RFC8323] is usable with OSCORE, its use is fine as well; in
   that case, the server picks a consistent block size for all clients
   anyway).





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   *  The Padding option defined in Appendix B can be used to limit an
      adversary's ability to deduce the content and the target resource
      of Deterministic Requests from their length.  In particular, all
      Deterministic Requests of the same class (ideally, all requests to
      a particular server) can be padded to reach the same total length,
      that should be agreed on among all users of the same OSCORE
      Security Context.

   *  Clients should not send any inner Echo options [RFC9175] in
      deterministic requests.

      This limits the use of the Echo option in combination with
      deterministic requests to unprotected (outer) options, and thus is
      limited to testing the reachability of the client.  This is not
      practically limiting, as the use as an inner option would be to
      prove freshness, which is something deterministic requests simply
      cannot provide anyway.

   These only serve to ensure that cache entries are utilized; failure
   to follow them has no more severe consequences than decreasing the
   utility and effectiveness of a cache.

3.2.  Design Considerations

   The hard part is arriving at a consensus pair (key, nonce) to be used
   with the AEAD cipher for encrypting the Deterministic Request, while
   also avoiding reuse of the same (key, nonce) pair across different
   requests.

   Diversity can conceptually be enforced by applying a cryptographic
   hash function to the complete input of the encryption operation over
   the plain CoAP request (i.e., the AAD and the plaintext of the COSE
   object), and then using the result as source of uniqueness.  Any non-
   malleable cryptographically secure hash of sufficient length to make
   collisions sufficiently unlikely is suitable for this purpose.

   A tempting possibility is to use a fixed (group) key, and use the
   hash as a deterministic AEAD nonce for each Deterministic Request
   throught the Partial IV component (see Section 5.2 of [RFC8613]).
   However, the 40 bit available for the Partial IV are by far
   insufficient to ensure that the deterministic nonce is not reused
   across different Deterministic Requests.  Even if the full
   deterministic AEAD nonce could be set, the sizes used by common
   algorithms would still be too small.

   As a consequence, the proposed method takes the opposite approach, by
   considering a fixed deterministic AEAD nonce, while generating a
   different deterministic encryption key for each Deterministic



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   Request.  That is, the hash computed over the plain CoAP request is
   taken as input to the key generation.  As an advantage, this approach
   does not require to transport the computed hash in the OSCORE option.

   [ Note: This has a further positive side effect arising with version
   -11 of Group OSCORE.  That is, since the full encoded OSCORE option
   is part of the AAD, it avoids a circular dependency from feeding the
   AAD into the hash computation, which in turn needs crude workarounds
   like building the full AAD twice, or zeroing out the hash-to-be. ]

3.3.  Request-Hash

   In order to transport the hash of the plain CoAP request, a new CoAP
   option is defined, which MUST be supported by clients and servers
   that support Deterministic Requests.

   The option is called Request-Hash.  As summarized in Figure 1, the
   Request-Hash option is elective, safe to forward, part of the cache
   key and repeatable.

    +------+---+---+---+---+--------------+--------+--------+---------+
    | No.  | C | U | N | R |     Name     | Format | Length | Default |
    +------+---+---+---+---+--------------+--------+--------+---------+
    | TBD1 |   |   |   | x | Request-Hash | opaque |  any   | (none)  |
    +------+---+---+---+---+--------------+--------+--------+---------+

                       Figure 1: Request-Hash Option

   The Request-Hash option is identical in all its properties to the
   Request-Tag option defined in [RFC9175], with the following
   exceptions:

   *  It may be arbitrarily long.

      Implementations can limit its length to that of the longest output
      of the supported hash functions.

   *  It may be present in responses (TBD: Does this affect any other
      properties?).

      A response's Request-Hash is, as a matter of default value, equal
      to the request's.  The response is only valid if its Request-Hash
      is equal to the matching request's.

      Servers (including proxies) thus generally SHOULD NOT need to send
      the Request-Hash option explicitly in responses, especially as a
      matter of bandwidth efficiency.




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      A reason (and, currently, the only known) to actually send a
      Request-Hash in a response are non-traditional responses as
      described in [I-D.bormann-core-responses], which in terms of that
      document are non-matching to the request (and thus easily usable);
      the request hash in the response allows populating caches (see
      below) and decryption of the response in deterministic request
      contexts.  In the context of non-traditional responses, a matching
      request's Request-Hash can be inferred from its value in the
      response.

   *  A proxy MAY use any fresh cached response from the selected server
      to respond to a request with the same Request-Hash; this may save
      it some memory.

      A proxy can add or remove the request's Request-Tag value to /
      from a response.

   *  When used with a Deterministic Request, this option is created at
      message protection time by the sender, and used before message
      unprotection by the recipient.  Therefore, in this use case, it is
      treated as Class U for OSCORE [RFC8613] in requests.  In the same
      application, for responses, it is treated as Class I, and often
      elided from sending (but reconstructed at the receiver).  Other
      uses of this option can put it into different classes for the
      OSCORE processing.

   This option achieves request-response binding described in Section 2.

3.4.  Use of Deterministic Requests

   This section defines how a Deterministic Request is built on the
   client side and then processed on the server side.

3.4.1.  Pre-Conditions

   The use of Deterministic Requests in an OSCORE group requires that
   the interested group members are aware of the Deterministic Client in
   the group.  In particular, they need to know:

   *  The Sender ID of the Deterministic Client, to be used as 'kid'
      parameter for the Deterministic Requests.  This allows all group
      members to compute the Sender Key of the Deterministic Client.

      The Sender ID of the Deterministic Client is immutable throughout
      the lifetime of the OSCORE group.  That is, it is not relinquished
      and it does not change upon changes of the group keying material
      following a group rekeying performed by the Group Manager.




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   *  The hash algorithm to use for computing the hash of a plain CoAP
      request, when producing the associated Deterministic Request.

   Group members have to obtain this information from the Group Manager.
   A group member can do that, for instance, when obtaining the group
   keying material upon joining the OSCORE group, or later on as an
   active member by sending a request to a dedicated resource at the
   Group Manager.

   The joining process based on the Group Manager defined in
   [I-D.ietf-ace-key-groupcomm-oscore] can be easily extended to support
   the provisioning of information about the Deterministic Client.  Such
   an extension is defined in Section 4 of this document.

3.4.2.  Client Processing of Deterministic Request

   In order to build a Deterministic Request, the client protects the
   plain CoAP request using the pairwise mode of Group OSCORE (see
   Section 9 of [I-D.ietf-core-oscore-groupcomm]), with the following
   alterations.

   1.  When preparing the OSCORE option, the external_aad and the AEAD
       nonce:

       *  The used Sender ID is the Deterministic Client's Sender ID.

       *  The used Partial IV is 0.

       When preparing the external_aad, the element 'sender_public_key'
       in the aad_array takes the empty CBOR byte string.

   2.  The client uses the hash function indicated for the Deterministic
       Client, and computes a hash H over the following input: the
       Sender Key of the Deterministic Client, concatenated with the
       external_aad from step 1, concatenated with the COSE plaintext.

       Note that the payload of the plain CoAP request (if any) is not
       self-delimiting, and thus hash functions are limited to non-
       malleable ones.

   3.  The client derives the deterministic Pairwise Sender Key K as
       defined in Section 2.3.1 of [I-D.ietf-core-oscore-groupcomm],
       with the following differences:

       *  The Sender Key of the Deterministic Client is used as first
          argument of the HKDF.





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       *  The hash H from step 2 is used as second argument of the HKDF,
          i.e., as a pseudo IKM-Sender computable by all the group
          members.

          Note that an actual IKM-Sender cannot be obtained, since there
          is no authentication credential (and public key included
          therein) associated with the Deterministic Client, to be used
          as Sender Authentication Credential and for computing an
          actual Diffie-Hellman Shared Secret.

       *  The Sender ID of the Deterministic Client is used as value for
          the 'id' element of the 'info' parameter used as third
          argument of the HKDF.

   4.  The client includes a Request-Hash option in the request to
       protect, with value set to the hash H from Step 2.

   5.  The client MAY include an inner Observe option set to 0 to be
       protected with OSCORE, even if no observation is intended (see
       Section 3.1).

   6.  The client protects the request using the pairwise mode of Group
       OSCORE as defined in Section 9.3 of
       [I-D.ietf-core-oscore-groupcomm], using the AEAD nonce from step
       1, the deterministic Pairwise Sender Key K from step 3 as AEAD
       encryption key, and the finalized AAD.

   7.  The client MUST NOT include an unprotected (outer) Observe option
       if no observation is intended, even in case an inner Observe
       option was included at step 5 above.

   8.  The client sets FETCH as the outer code of the protected request
       to make it usable for a proxy's cache, even if no observation is
       requested [RFC7641].

   The result is the Deterministic Request to be sent.

   Since the encryption key K is derived using material from the whole
   plain CoAP request, this (key, nonce) pair is only used for this very
   message, which is deterministically encrypted unless there is a hash
   collision between two Deterministic Requests.

   The deterministic encryption requires the used AEAD algorithm to be
   deterministic in itself.  This is the case for all the AEAD
   algorithms currently registered with COSE in [COSE.Algorithms].  For
   future algorithms, a flag in the COSE registry is to be added.





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   Note that, while the process defined above is based on the pairwise
   mode of Group OSCORE, no information about the server takes part to
   the key derivation or is included in the AAD.  This is intentional,
   since it allows for sending a deterministic request to multiple
   servers at once (see Section 3.4.5).  On the other hand, it requires
   later checks at the client when verifying a response to a
   Deterministic Request (see Section 3.4.4).

3.4.3.  Server Processing of Deterministic Request

   Upon receiving a Deterministic Request, a server performs the
   following actions.

   A server that does not support Deterministic Requests would not be
   able to create the necessary Recipient Context, and thus will fail
   decrypting the request.

   1.  If not already available, the server retrieves the information
       about the Deterministic Client from the Group Manager, and
       derives the Sender Key of the Deterministic Client.

   2.  The server actually recognizes the request to be a Deterministic
       Request, due to the presence of the Request-Hash option and to
       the 'kid' parameter of the OSCORE option set to the Sender ID of
       the Deterministic Client.

       If the 'kid' parameter of the OSCORE option specifies a different
       Sender ID than the one of the Deterministic Client, the server
       MUST NOT take the following steps, and instead processes the
       request as per Section 9.4 of [I-D.ietf-core-oscore-groupcomm].

   3.  The server retrieves the hash H from the Request-Hash option.

   4.  The server derives a Recipient Context for processing the
       Deterministic Request.  In particular:

       *  The Recipient ID is the Sender ID of the Deterministic Client.

       *  The Recipient Key is derived as the key K in step 3 of
          Section 3.4.2, with the hash H retrieved at the previous step.

   5.  The server verifies the request using the pairwise mode of Group
       OSCORE, as defined in Section 9.4 of
       [I-D.ietf-core-oscore-groupcomm], using the Recipient Context
       from step 4, with the difference that the server does not perform
       replay checks against a Replay Window (see below).





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   In case of successful verification, the server MUST also perform the
   following actions, before possibly delivering the request to the
   application.

   *  Starting from the recovered plain CoAP request, the server MUST
      recompute the same hash that the client computed at step 2 of
      Section 3.4.2.

      If the recomputed hash value differs from the value retrieved from
      the Request-Hash option at step 3, the server MUST treat the
      request as invalid and MAY reply with an unprotected 4.00 (Bad
      Request) error response.  The server MAY set an Outer Max-Age
      option with value zero.  The diagnostic payload MAY contain the
      string "Decryption failed".

      This prevents an attacker that guessed a valid authentication tag
      for a given Request-Hash value to poison caches with incorrect
      responses.

   *  The server MUST verify that the unprotected request is safe to be
      processed in the REST sense, i.e., that it has no side effects.
      If verification fails, the server MUST discard the message and
      SHOULD reply with a protected 4.01 (Unauthorized) error response.

      Note that some CoAP implementations may not be able to prevent
      that an application produces side effects from a safe request.
      This may incur checking whether the particular resource handler is
      explicitly marked as eligible for processing deterministic
      requests.  An implementation may also have a configured list of
      requests that are known to be side effect free, or even a pre-
      built list of valid hashes for all sensible requests for them, and
      reject any other request.

      These checks replace the otherwise present requirement that the
      server needs to check the Replay Window of the Recipient Context
      (see step 5 above), which is inapplicable with the Recipient
      Context derived at step 4 from the value of the Request-Hash
      option.  The reasoning is analogous to the one in
      [I-D.amsuess-lwig-oscore] to treat the potential replay as
      answerable, if the handled request is side effect free.











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3.4.4.  Response to a Deterministic Request

   When treating a response to a deterministic request, the Request-Hash
   option is treated as a Class I option (but usually not sent).  This
   creates the request-response binding ensuring that no mismatched
   responses can be successfully unprotected (see Section 2).  The
   client MUST reject responses with a Request-Hash not matching the one
   it sent in the request.

   When preparing the response, the server performs the following
   actions.

   1.  The server sets a non-zero Max-Age option, thus making the
       Deterministic Request usable for the proxy cache.

   2.  The server preliminarily sets the Request-Hash option with the
       full request hash.

   3.  If the Deterministic Request included an inner Observe option but
       not an outer Observe option, the server MUST include an inner
       Observe option in the response.

   4.  The server MUST protect the response using the group mode of
       Group OSCORE, as defined in Section 8.3 of
       [I-D.ietf-core-oscore-groupcomm].  This is required to ensure
       that the client can verify source authentication of the response,
       since the "pairwise" key used for the Deterministic Request is
       actually shared among all the group members.

       Note that the Request-Hash option is treated as Class I here.

   5.  The server MUST use its own Sender Sequence Number as Partial IV
       to protect the response, and include it as Partial IV in the
       OSCORE option of the response.  This is required since the server
       does not perform replay protection on the Deterministic Request
       (see Section 3.4.4).

   6.  The server uses 2.05 (Content) as outer code even though it is
       not necessarily an Observe notification [RFC7641], in order to
       make the response cacheable.

   7.  The server SHOULD remove the Request-Hash option from the message
       before sending as per the general option mechanism of
       Section 3.3.

   8.  If the Deterministic Request included an inner Observe option but
       not an outer Observe option, the server MUST NOT include an outer
       Observe option in the response.



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   Upon receiving the response, the client performs the following
   actions.

   1.  In case the response includes a 'kid' in the OSCORE option and
       unless responses from multiple servers are expected (see
       Section 3.4.5), the client MUST verify it to be exactly the 'kid'
       of the server to which the Deterministic Request was sent.

   2.  The client sets the Request-Hash option with the full request
       hash on the reponse.  If an option of a different value is
       already present, it rejects the response.

   3.  The client verifies the response using the group mode of Group
       OSCORE, as defined in Section 8.4 of
       [I-D.ietf-core-oscore-groupcomm].  In particular, the client
       verifies the counter signature in the response, based on the
       'kid' of the server it sent the request to.  When verifying the
       response, the Request-Hash option is treated as a Class I option.

   4.  If the Deterministic Request included an inner Observe option but
       not an outer Observe option (see Section 3.1), the client MUST
       silently ignore the inner Observe option in the response and MUST
       NOT stop processing the response.

   [ Note: This deviates from Section 4.1.3.5.2 of RFC 8613, but it is
   limited to a very specific situation, where the client and server
   both know exactly what happens.  This does not affect the use of
   OSCORE in other situations. ]

3.4.5.  Deterministic Requests to Multiple Servers

   A Deterministic Request _can_ be sent to a CoAP group, e.g., over UDP
   and IP multicast [I-D.ietf-core-groupcomm-bis], thus targeting
   multiple servers at once.

   To simplify key derivation, such a Deterministic Request is still
   created in the same way as a one-to-one request and still protected
   with the pairwise mode of Group OSCORE, as defined in Section 3.4.2.

   [ Note: If it was protected with the group mode, the request hash
   would need to be fed into a group key derivation just for this corner
   case.  Furthermore, there would need to be a signature in spite of no
   authentication credential (and public key included therein)
   associated with the Deterministic Client. ]







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   When a server receives a request from the Deterministic Client as
   addressed to a CoAP group, the server proceeds as defined in
   Section 3.4.3, with the difference that it MUST include its own
   Sender ID in the response, as 'kid' parameter of the OSCORE option.

   Although it is normally optional for the server to include its Sender
   ID when replying to a request protected in pairwise mode, it is
   required in this case for allowing the client to retrieve the
   Recipient Context associated with the server originating the
   response.

4.  Obtaining Information about the Deterministic Client

   This section extends the Joining Process defined in
   [I-D.ietf-ace-key-groupcomm-oscore], and based on the ACE framework
   for Authentication and Authorization [I-D.ietf-ace-oauth-authz].
   Upon joining the OSCORE group, this enables a new group member to
   obtain from the Group Manager the required information about the
   Deterministic Client (see Section 3.4.1).

   With reference to the 'key' parameter of the Joining Response defined
   in Section 6.4 of [I-D.ietf-ace-key-groupcomm-oscore], the
   Group_OSCORE_Input_Material object specified as its value contains
   also the two additional parameters 'det_senderId' and 'det_hash_alg'.
   These are defined in Section 6.2 of this document.  In particular:

   *  The 'det_senderId' parameter, if present, has as value the OSCORE
      Sender ID assigned to the Deterministic Client by the Group
      Manager.  This parameter MUST be present if the OSCORE group uses
      deterministic requests as defined in this document.  Otherwise,
      this parameter MUST NOT be present.

   *  The 'det_hash_alg' parameter, if present, has as value the hash
      algorithm to use for computing the hash of a plain CoAP request,
      when producing the associated Deterministic Request.  This
      parameter takes values from the "Value" column of the "COSE
      Algorithms" Registry [COSE.Algorithms].  This parameter MUST be
      present if the OSCORE group uses deterministic requests as defined
      in this document.  Otherwise, this parameter MUST NOT be present.

   The same extension above applies also to the 'key' parameter when
   included in a Key Distribution Response (see Sections 8.1 and 8.2 of
   [I-D.ietf-ace-key-groupcomm-oscore]) and in a Signature Verification
   Data Response (see Section 13 of
   [I-D.ietf-ace-key-groupcomm-oscore]).






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5.  Security Considerations

   The same security considerations from [RFC7252][I-D.ietf-core-groupco
   mm-bis][RFC8613][I-D.ietf-core-oscore-groupcomm] hold for this
   document.

   Compared to Group OSCORE, deterministic requests dispense with some
   of OSCORE's security properties by just so much as to make caching
   possible:

   *  Receiving a response to a deterministic request does not mean that
      the response was generated after the request was sent.

      It still contains two freshness statements, though:

      -  It is more recent than any other response from the same group
         member that has a smaller sequence number.

      -  It is more recent than the original creation of the
         deterministic security context's key material.

   *  Request privacy is limited.

      An intermediary can determine that two requests from different
      clients are identical, and associate the different responses
      generated for them.  A server producing responses of varying size
      to a request can use the Padding option to hide when the response
      is changing.

   *  Source authentication for requests is lost.

      Instead, the server must verify that the request (precisely: its
      handler) is side effect free.  The distinct semantics of the CoAP
      request codes can help the server make that assessment.

   [ More on the verification of the Deterministic Request ]

6.  IANA Considerations

   This document has the following actions for IANA.

6.1.  CoAP Option Numbers Registry

   IANA is asked to enter the following option numbers to the "CoAP
   Option Numbers" registry defined in [RFC7252] within the "CoRE
   Parameters" registry.





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               +--------+--------------+-------------------+
               | Number |     Name     |     Reference     |
               +--------+--------------+-------------------+
               |  TBD1  | Request-Hash | [[this document]] |
               +--------+--------------+-------------------+
               |  TBD2  | Padding      | [[this document]] |
               +--------+--------------+-------------------+

                       Figure 2: CoAP Option Numbers

   [

   For the Request-Hash option, the number suggested to IANA is 548.

   For the Padding option, the option number is picked to be the highest
   number in the Experts Review range; the high option number allows it
   to follow practically all other options, and thus to be set when the
   final unpadded message length including all options is known.
   Therefore, the number suggested to IANA is 64988.

   Applications that make use of the "Experimental use" range and want
   to preserve that property are invited to pick the largest suitable
   experimental number (65532)

   Note that unless other high options are used, this means that padding
   a message adds an overhead of at least 3 bytes, i.e., 1 byte for
   option delta/length and two more bytes of extended option delta.
   This is considered acceptable overhead, given that the application
   has already chosen to prefer the privacy gains of padding over wire
   transfer length.

   ]

6.2.  OSCORE Security Context Parameters Registry

   IANA is asked to register the following entries in the "OSCORE
   Security Context Parameters" Registry defined in Section 9.4 of
   [I-D.ietf-ace-oscore-profile].

   *  Name: det_senderId

   *  CBOR Label: TBD3

   *  CBOR Type: bstr

   *  Registry: -





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   *  Description: OSCORE Sender ID assigned to the Deterministic Client
      of an OSCORE group

   *  Reference: [[this document]] (Section 4)


   *  Name: det_hash_alg

   *  CBOR Label: TBD4

   *  CBOR Type: int / tstr

   *  Registry: -

   *  Description: Hash algorithm to use in an OSCORE group when
      producing a Deterministic Request

   *  Reference: [[this document]] (Section 4)

7.  References

7.1.  Normative References

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", Work in
              Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
              05, 25 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              groupcomm-bis-05>.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
              and J. Park, "Group OSCORE - Secure Group Communication
              for CoAP", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-groupcomm-13, 25 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              oscore-groupcomm-13>.

   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              rfc8152bis-struct-15>.






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   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              rfc8152bis-algs-12>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7252>.

   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/rfc/rfc8132>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/rfc/rfc8613>.

   [COSE.Algorithms]
              IANA, "COSE Algorithms",
              <https://www.iana.org/assignments/cose/
              cose.xhtml#algorithms>.

7.2.  Informative References

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/rfc/rfc7641>.

   [RFC9175]  Amsüss, C., Preuß Mattsson, J., and G. Selander,
              "Constrained Application Protocol (CoAP): Echo, Request-
              Tag, and Token Processing", RFC 9175,
              DOI 10.17487/RFC9175, February 2022,
              <https://www.rfc-editor.org/rfc/rfc9175>.



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   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", Work in Progress, Internet-
              Draft, draft-ietf-ace-key-groupcomm-oscore-12, 25 October
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              ace-key-groupcomm-oscore-12>.

   [I-D.amsuess-lwig-oscore]
              Amsüss, C., "OSCORE Implementation Guidance", Work in
              Progress, Internet-Draft, draft-amsuess-lwig-oscore-00, 29
              April 2020, <https://datatracker.ietf.org/doc/html/draft-
              amsuess-lwig-oscore-00>.

   [I-D.ietf-core-observe-multicast-notifications]
              Tiloca, M., Höglund, R., Amsüss, C., and F. Palombini,
              "Observe Notifications as CoAP Multicast Responses", Work
              in Progress, Internet-Draft, draft-ietf-core-observe-
              multicast-notifications-02, 25 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              observe-multicast-notifications-02>.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
              draft-ietf-ace-oauth-authz-46, 8 November 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ace-
              oauth-authz-46>.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE Profile of the Authentication and Authorization
              for Constrained Environments Framework", Work in Progress,
              Internet-Draft, draft-ietf-ace-oscore-profile-19, 6 May
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              ace-oscore-profile-19>.

   [SW-EPIV]  Lucas, G., "Star Wars", Lucasfilm Ltd. , 1977.

   [I-D.bormann-core-responses]
              Bormann, C. and C. Amsüss, "CoAP: Non-traditional response
              forms", Work in Progress, Internet-Draft, draft-bormann-
              core-responses-01, 3 February 2022,
              <https://datatracker.ietf.org/doc/html/draft-bormann-core-
              responses-01>.





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   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/rfc/rfc8323>.

Appendix A.  Change log

   Since -03:

   *  Processing steps in case only inner Observe is included.

   *  Clarified preserving/eliding the Request-Hash option in responses.

   *  Clarified limited use of the Echo option.

   *  Clarifications on using the Padding option.

   Since -02:

   *  Separate parts needed to respond to unauthenticated requests from
      the remaining deterministic response part.  (Currently this is
      mainly an addition; the document will undergo further refactoring
      if that split proves helpful).

   *  Inner Observe is set unconditionally in deterministic requests.

   *  Clarifications around padding and security considerations.

   Since -01:

   *  Not meddling with request_kid any more.

      Instead, Request-Hash in responses is treated as Class I, but
      typically elided.

      In requests, this removes the need to compute the external_aad
      twice.

   *  Derivation of the hash now uses the external_aad, rather than the
      full AAD.  This is good enough because AAD is a function only of
      the external_aad, and the external_aad is easier to get your hands
      on if COSE manages all the rest.

   *  The Sender ID of the Deterministic Client is immutable throughout
      the group lifetime.  Hence, no need for any related expiration/
      creation time and mechanisms to perform its update in the group.




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   *  Extension to the ACE Group Manager of ace-key-groupcomm-oscore to
      provide required info about the Deterministic Client to new group
      members when joining the group.

   *  Alignment with changes in core-oscore-groupcomm-12.

   *  Editorial improvements.

   Since -00:

   *  More precise specification of the hashing (guided by first
      implementations)

   *  Focus shifted to deterministic requests (where it should have been
      in the first place; all the build-up of Token Requests was moved
      to a motivating appendix)

   *  Aligned with draft-tiloca-core-observe-responses-multicast-05 (not
      submitted at the time of submission)

   *  List the security properties lost compared to OSCORE

Appendix B.  Padding

   As discussed in Section 5, information can be leaked by the length of
   a response or, in different contexts, of a request.

   In order to hide such information and mitigate the impact on privacy,
   the following Padding option is defined, to allow increasing a
   message's length without changing its meaning.

   The option can be used with any CoAP transport, but is especially
   useful with OSCORE as that does not provide any padding of its own.

   Before choosing to pad a message by using the Padding option,
   application designers should consider whether they can arrange for
   common message variants to have the same length by picking a suitable
   content representation; the canonical example here is expressing
   "yes" and "no" with "y" and "n", respectively.

B.1.  Definition of the Padding Option

   As summarized in Figure 3, the Padding option is elective, safe to
   forward and not part of the cache key; these follow from the usage
   instructions.  The option may be repeated, as that may be the only
   way to achieve a certain total length for the padded message.





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      +------+---+---+---+---+---------+--------+--------+---------+
      | No.  | C | U | N | R |  Name   | Format | Length | Default |
      +------+---+---+---+---+---------+--------+--------+---------+
      | TBD2 |   |   | x | x | Padding | opaque | any    | (none)  |
      +------+---+---+---+---+---------+--------+--------+---------+

                          Figure 3: Padding Option

   When used with OSCORE, the Padding option is of Class E, this makes
   it indistinguishable from other Class E options or the payload to
   third parties.

B.2.  Using and processing the Padding option

   When a server produces different responses of different length for a
   given class of requests but wishes to produce responses of consistent
   length (typically to hide the variation from anyone but the intended
   recipient), the server can pick a length that all possible responses
   can be padded to, and set the Padding option with a suitable all-zero
   option value in all responses to that class of requests.

   Likewise, a client can decide on a class of requests that it pads to
   consistent length.  This has considerably less efficacy and
   applicability when applied to Deterministic Requests.  That is: an
   external observer can group requests even if they are of the same
   length; and padding would hinder convergence on a single Consensus
   Request, thus requiring all users of the same OSCORE Security Context
   to agree on the same total length in advance.

   Any party receiving a Padding option MUST ignore it.  In particular,
   a server MUST NOT make its choice of padding dependent on any padding
   present in the request.  (An option to coordinate response padding
   driven by the client is out of scope for this document).

   Proxies that see a padding option MAY discard it.

Appendix C.  Simple Cacheability using Ticket Requests

   Building on the concept of Phantom Requests and Informative Responses
   defined in [I-D.ietf-core-observe-multicast-notifications], basic
   caching is already possible without building a Deterministic Request.

   The approach discussed in this appendix is not provided for
   application.  In fact, it is efficient only when dealing with very
   large representations and no OSCORE inner Block-Wise mode (which is
   inefficient for other reasons), or when dealing with observe
   notifications (which are already well covered in
   [I-D.ietf-core-observe-multicast-notifications]).



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   Rather, it is more provided as a "mental exercise" for the authors
   and interested readers to bridge the gap between this document and
   [I-D.ietf-core-observe-multicast-notifications].

   That is, instead of replying to a client with a regular response, a
   server can send an Informative Response, defined as a protected 5.03
   (Service Unavailable) error message.  The payload of the Informative
   Response contains the Phantom Request, which is a Ticket Request in
   this document's broader terminology.

   Unlike a Deterministic Request, a Phantom Request is protected in
   Group Mode.  Instead of verifying a hash, the client can see from the
   signature that this was indeed the request the server is answering.
   The client also verifies that the request URI is identical between
   the original request and the Ticket Request.

   The remaining exchange largely plays out like in
   [I-D.ietf-core-observe-multicast-notifications]'s "Example with a
   Proxy and Group OSCORE": The client sends the Phantom Request to the
   proxy (but, lacking a tp_info, without a Listen-To-Multicast-
   Responses option), which forwards it to the server for lack of the
   option.

   The server then produces a regular response and includes a non-zero
   Max-Age option as an outer CoAP option.  Note that there is no point
   in including an inner Max-Age option, as the client could not pin it
   in time.

   When a second, different client later asks for the same resource at
   the same server, its new request uses a different 'kid' and 'Partial
   IV' than the first client's.  Thus, the new request produces a cache
   miss at the proxy and is forwarded to the server, which responds with
   the same Ticket Request provided to the first client.  After that,
   when the second client sends the Ticket Request, a cache hit at the
   proxy will be produced, and the Ticket Request can be served from the
   proxy's cache.

   When multiple proxies are in use, or the response has expired from
   the proxy's cache, the server receives the Ticket Request multiple
   times.  It is a matter of perspective whether the server treats that
   as an acceptable replay (given that this whole mechansim only makes
   sense on requests free of side effects), or whether it is
   conceptualized as having an internal proxy where the request produces
   a cache hit.







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Appendix D.  Application for More Efficient End-to-End Protected
             Multicast Notifications

   [I-D.ietf-core-observe-multicast-notifications] defines how a CoAP
   server can serve all clients observing a same resource at once, by
   sending notifications over multicast.  The approach supports the
   possible presence of intermediaries such as proxies, also if Group
   OSCORE is used to protect notifications end-to-end.

   However, comparing the "Example with a Proxy" in Appendix A of
   [I-D.ietf-core-observe-multicast-notifications] and the "Example with
   a Proxy and Group OSCORE" in Appendix B of
   [I-D.ietf-core-observe-multicast-notifications] shows that, when
   using Group OSCORE, more requests need to hit the server.  This is
   because every client originally protects its Observation request
   individually, and thus needs a custom response served to obtain the
   Phantom Request as a Ticket Request.

   If the clients send their requests as the same deterministic request,
   the server can use these requests as Ticket Requests as well.  Thus,
   there is no need for the server to provide a same Phantom Request to
   each client.

   Instead, the server can send a single unprotected Informative
   Response - very much like in the example without Group OSCORE - hence
   setting the proxy up and optionally providing also the latest
   notification along the way.

   The proxy can thus be configured by the server following the first
   request from the clients, after which it has an active observation
   and a fresh cache entry in time for the second client to arrive.

Appendix E.  Open questions

   *  Is "deterministic encryption" something worthwhile to consider in
      COSE?

      COSE would probably specify something more elaborate for the KDF
      (the current KDF round is the pairwise mode's; COSE would probably
      run through KDF with a KDF context structure).

      COSE would give a header parameter name to the Request-Hash (which
      for the purpose of OSCORE deterministic requests would put back
      into Request-Hash by extending the option compression function
      across the two options).

      Conceptually, they should align well, and the implementation
      changes are likely limited to how the KDF is run.



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   *  An unprotection failure from a mismatched hash will not be part of
      the ideally constant-time code paths that otherwise lead to AEAD
      unprotect failures.  Is that a problem?

      After all, it does tell the attacker that they did succeed in
      producing a valid MAC (it's just not doing it any good, because
      this key is only used for deterministic requests and thus also
      needs to pass the Request-Hash check).

Appendix F.  Unsorted further ideas

   *  All or none of the deterministic requests should have an inner
      observe option.  Preferably none -- that makes messages shorter,
      and clients need to ignore that option either way when checking
      whether a Consensus Request matches their intended request.

   *  We could allows clients to elide all other options than Request-
      Hash, and elide the payload, if it has reason to believe that it
      can produce a cache hit with the abbreviated request alone.

      This may prove troublesome in terms of cache invalidation (the
      server would have to use short-lived responses to indicate that it
      does need the full request, or we'd need special handling for
      error responses, or criteria by which proxies don't even forward
      these if they don't have a response at hand).

      That may be more trouble than it's worth without a strong use case
      (say, of complex but converging FETCH requests).

      Hashes could also be used in truncated form for that.

Acknowledgments

   The authors sincerely thank Michael Richardson, Jim Schaad and Goeran
   Selander for their comments and feedback.

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home
   (Grant agreement 952652).

Authors' Addresses

   Christian Amsüss
   Austria
   Email: christian@amsuess.com






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   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   SE-16440 Stockholm Kista
   Sweden
   Email: marco.tiloca@ri.se













































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