COSE Receipts with CCF
draft-birkholz-cose-receipts-ccf-profile-05

TBD                                                          H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                      A. Delignat-Lavaud
Expires: 17 May 2026                                          C. Fournet
                                                             A. Chamayou
                                                      Microsoft Research
                                                        13 November 2025

                         COSE Receipts with CCF
              draft-birkholz-cose-receipts-ccf-profile-05

Abstract

   This document defines a new verifiable data structure type for COSE
   Signed Merkle Tree Proofs specifically designed for transaction
   ledgers produced via Trusted Execution Environments (TEEs), such as
   the Confidential Consortium Framework ([CCF]) to provide stronger
   tamper-evidence guarantees.

Discussion Venues

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

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-scitt/draft-birkholz-cose-cometre-ccf-
   profile.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 17 May 2026.

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   3
   2.  Description of the CCF Ledger Verifiable Data Structure . . .   3
     2.1.  Merkle Tree Shape . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Transaction Components  . . . . . . . . . . . . . . . . .   4
   3.  CCF Inclusion Proofs  . . . . . . . . . . . . . . . . . . . .   5
     3.1.  CCF Inclusion Proof Signature . . . . . . . . . . . . . .   6
     3.2.  Inclusion Proof Verification Algorithm  . . . . . . . . .   6
   4.  Usage in COSE Receipts  . . . . . . . . . . . . . . . . . . .   7
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     6.1.  Trusted Execution Environments  . . . . . . . . . . . . .   8
     6.2.  Operators . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Additions to Existing Registries  . . . . . . . . . . . .   9
       7.1.1.  Tree Algorithms . . . . . . . . . . . . . . . . . . .   9
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The COSE Receipts document [I-D.ietf-cose-merkle-tree-proofs] defines
   a common framework for defining different types of proofs, such as
   proof of inclusion, about verifiable data structures (VDS).  For
   instance, inclusion proofs guarantee to a verifier that a given
   serializable element is recorded at a given state of the VDS, while
   consistency proofs are used to establish that an inclusion proof is
   still consistent with the new state of the VDS at a later time.

   In this document, we define a new type of VDS, associated with an
   application of the Confidential Consortium Framework (CCF) ledger
   that implements the SCITT Architecture defined in

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   [I-D.ietf-scitt-architecture].  This VDS carries indexed transaction
   information in a binary Merkle Tree, where new transactions are
   appended to the right, so that the binary decomposition of the index
   of a transaction can be interpreted as the position in the tree if 0
   represents the left branch and 1 the right branch.  Compared to
   [RFC9162], the leaves of CCF trees carry additional internal
   information for the following purposes:

   1.  To bind the full details of the transaction executed, which is a
       super-set of what is exposed in the proof and captures internal
       information details useful for detailed system audit, but not for
       application purposes.

   2.  To allow the distributed system executing the application logic
       in Trusted Excecution Environments to persist signatures to
       storage early, but only enable receipt production once they are
       fully committed by the consensus protocol.

1.1.  Requirements Notation

   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.

2.  Description of the CCF Ledger Verifiable Data Structure

   This documents extends the verifiable data structure registry of
   [I-D.ietf-cose-merkle-tree-proofs] with the following value:

   +===================+===============+==================+===========+
   | Name              | Value         | Description      | Reference |
   +===================+===============+==================+===========+
   | CCF_LEDGER_SHA256 | TBD_1         | Historical       | RFCthis   |
   |                   | (requested    | transaction      |           |
   |                   | assignment 2) | ledgers, such as |           |
   |                   |               | the CCF ledger   |           |
   +-------------------+---------------+------------------+-----------+

              Table 1: Verifiable Data Structure Algorithms

   This document defines inclusion proofs for CCF ledgers.

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2.1.  Merkle Tree Shape

   A CCF ledger is a binary Merkle Tree constructed from a hash function
   H, which is defined from the log type.  For instance, the hash
   function for CCF_LEDGER_SHA256 is SHA256, whose HASH_SIZE is 32
   bytes.

   The Merkle tree encodes an ordered list of n transactions T_n =
   {T[0], T[1], ..., T[n-1]}. We define the Merkle Tree Hash (MTH)
   function, which takes as input a list of serialized transactions (as
   byte strings), and outputs a single HASH_SIZE byte string called the
   Merkle root hash, by induction on the list.

   This function is defined as follows:

   The hash of an empty list is the hash of an empty string:

   MTH({}) = HASH().

   The hash of a list with one entry (also known as a leaf hash) is:

   MTH({d[0]}) = HASH(d[0]).

   For n > 1, let k be the largest power of two smaller than n (i.e., k
   < n <= 2k).  The Merkle Tree Hash of an n-element list D_n is then
   defined recursively as:

   MTH(D_n) = HASH(MTH(D[0:k]) || MTH(D[k:n])),

   where:

   *  || denotes concatenation

   *  : denotes concatenation of lists

   *  D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] =
      d[k1+1], ..., d'[k2-k1-1] = d[k2-1]} of length (k2 - k1).

2.2.  Transaction Components

   Each leaf in a CCF ledger carries the following components:

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   ccf-leaf = [
     ; Byte string of size HASH_SIZE(32)
     internal-transaction-hash: bstr .size 32

     ; Text string of at most 1024 bytes
     internal-evidence: tstr .size (1..1024)

     ; Byte string of size HASH_SIZE(32)
     data-hash: bstr .size 32
   ]

   The internal-transaction-hash and internal-evidence byte strings are
   internal to the CCF implementation.  They can be safely ignored by
   receipt Verifiers, but they commit the TS to the whole tree contents
   and may be used for additional, CCF-specific auditing.

   internal-transaction-hash is a hash over the complete entry in the
   [CCF-Ledger-Format], and internal-evidence is a revealable
   [CCF-Commit-Evidence] value that allows early persistence of ledger
   entries before distributed consensus can be established.  This
   mechanism is useful to implement high-throughput transparency
   applications in Trusted Execution Environments that only provide a
   limited amount of memory, while maintaining high availability
   afforded by distributed consensus.

   data-hash summarises the application data included in the ledger at
   this transaction, which is a Signed Statement as defined by
   [I-D.ietf-scitt-architecture].

3.  CCF Inclusion Proofs

   CCF inclusion proofs consist of a list of digests tagged with a
   single left-or-right bit.

   ccf-proof-element = [
     ; Position of the element
     left: bool

     ; Hash of the proof element: byte string of size HASH_SIZE(32)
     hash: bstr .size 32
   ]

   ccf-inclusion-proof = bstr .cbor {
     &(leaf: 1) => ccf-leaf
     &(path: 2) => [+ ccf-proof-element]
   }

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   Unlike some other tree algorithms, the index of the element in the
   tree is not explicit in the inclusion proof, but the list of left-or-
   right bits can be treated as the binary decomposition of the index,
   from the least significant (leaf) to the most significant (root).

3.1.  CCF Inclusion Proof Signature

   The proof signature for a CCF inclusion proof is a COSE signature
   (encoded with the COSE_Sign1 CBOR type) which includes the following
   additional requirements for protected and unprotected headers.
   Please note that there may be additional header parameters defined by
   the application.

   The protected header parameters for the CCF inclusion proof signature
   MUST include the following:

   *  verifiable-data-structure: int/tstr.  This header MUST be set to
      the verifiable data structure algorithm identifier for ccf-ledger
      (TBD_1).

   *  label: int.  This header MUST be set to the value of the inclusion
      proof type in the IANA registry of Verifiable Data Structure Proof
      Type (-1).

   The unprotected header for a CCF inclusion proof signature MUST
   include the following:

   *  inclusion-proof: bstr .cbor ccf-inclusion-proof.  This contains
      the serialized CCF inclusion proof, as defined above.

   The payload of the signature is the CCF ledger Merkle root digest,
   and MUST be detached in order to force verifiers to recompute the
   root from the inclusion proof in the unprotected header.  This
   provides a safeguard against implementation errors that use the
   payload of the signature but do not recompute the root from the
   inclusion proof.

3.2.  Inclusion Proof Verification Algorithm

   CCF uses the following algorithm to verify an inclusion receipt:

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   compute_root(proof):
     h := proof.leaf.internal-transaction-hash
          || HASH(proof.leaf.internal-evidence)
          || proof.leaf.data-hash

     for [left, hash] in proof:
         h := HASH(hash + h) if left
              HASH(h + hash) else
     return h

   verify_inclusion_receipt(inclusion_receipt):
     let label = INCLUSION_PROOF_LABEL
     assert(label in inclusion_receipt.unprotected_header)
     let proof = inclusion_receipt.unprotected_header[label]
     assert(inclusion_receipt.payload == nil)
     let payload = compute_root(proof)

     # Use the Merkle Root as the detached payload
     return verify_cose(inclusion_receipt, payload)

   A description can also be found at [CCF-Receipt-Verification].

4.  Usage in COSE Receipts

   A COSE Receipt with a CCF inclusion proof is described by the
   following CDDL definition:

   protected-header-map = {
     &(alg: 1) => int
     &(vds: 395) => 2
     * cose-label => cose-value
   }

   *  alg (label: 1): REQUIRED.  Signature algorithm identifier.  Value
      type: int.

   *  vds (label: 395): REQUIRED. verifiable data structure algorithm
      identifier.  Value type: int.

   The unprotected header for an inclusion proof signature is described
   by the following CDDL definition:

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   inclusion-proof = ccf-inclusion-proof

   inclusion-proofs = [ + inclusion-proof ]

   verifiable-proofs = {
     &(inclusion-proof: -1) => inclusion-proofs
   }

   unprotected-header-map = {
     &(vdp: 396) => verifiable-proofs
     * cose-label => cose-value
   }

5.  Privacy Considerations

   See the privacy considerations section of:

   *  [I-D.ietf-cose-merkle-tree-proofs]

6.  Security Considerations

   The security consideration of [I-D.ietf-cose-merkle-tree-proofs]
   apply.

6.1.  Trusted Execution Environments

   CCF networks of nodes rely on executing in Trusted Execution
   Environments to secure their function, in particular:

   1.  The evaluation of registration policies

   2.  The creation and usage of receipt signing keys

   A compromise in the Trusted Execution Environment platform used to
   execute the network may allow an attacker to produce invalid and
   incompatible ledger branches.  Clients can mitigate this risk in two
   ways: by regularly auditing the consistency of the CCF ledger; and by
   regularly fetching attestation information about the TEE instances,
   available in the ledger and from the network itself, and confirming
   that the nodes composing the network are running up-to-date, trusted
   platform components.

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6.2.  Operators

   The operator of a CCF network has the ability to start successor
   networks, with a distinct identity, which endorse the receipts
   produced by a previous instance.  This functionality is important to
   provide service continuity in the case of a catastrophic failure of a
   majority of nodes, but allows a potentially malicious operator to
   start from a prefix of an earlier ledger.  Clients can mitigate this
   risk by auditing the successor ledger and its attestation
   information, as described above.  In particular, clients can check
   that the latest receipt they hold is present in the successor ledger
   before they begin making use of it.

7.  IANA Considerations

7.1.  Additions to Existing Registries

7.1.1.  Tree Algorithms

   This document requests IANA to add the following new value to the
   'COSE Verifiable Data Structures' registry:

   *  Name: CCF_LEDGER_SHA256

   *  Value: 2 (requested assignment)

   *  Description: Append-only logs that are integrity-protected by a
      Merkle Tree and signatures produced via Trusted Execution
      Environments containing a mix of public and confidential
      information, as specified by the Confidential Consortium
      Framework.

   *  Reference: This document

8.  Normative References

   [CCF]      "Confidential Consortium Framework", n.d.,
              <https://github.com/microsoft/ccf>.

   [CCF-Commit-Evidence]
              "CCF Commit Evidence", n.d.,
              <https://microsoft.github.io/CCF/main/use_apps/
              verify_tx.html#commit-evidence>.

   [CCF-Ledger-Format]
              "CCF Ledger Format", n.d.,
              <https://microsoft.github.io/CCF/main/architecture/
              ledger.html>.

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   [CCF-Receipt-Verification]
              "CCF Receipt Verification", n.d.,
              <https://microsoft.github.io/CCF/main/use_apps/
              verify_tx.html#receipt-verification>.

   [I-D.ietf-cose-merkle-tree-proofs]
              Steele, O., Birkholz, H., Delignat-Lavaud, A., and C.
              Fournet, "COSE (CBOR Object Signing and Encryption)
              Receipts", Work in Progress, Internet-Draft, draft-ietf-
              cose-merkle-tree-proofs-17, 10 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              merkle-tree-proofs-17>.

   [I-D.ietf-scitt-architecture]
              Birkholz, H., Delignat-Lavaud, A., Fournet, C., Deshpande,
              Y., and S. Lasker, "An Architecture for Trustworthy and
              Transparent Digital Supply Chains", Work in Progress,
              Internet-Draft, draft-ietf-scitt-architecture-22, 10
              October 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-scitt-architecture-22>.

   [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>.

   [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>.

   [RFC9162]  Laurie, B., Messeri, E., and R. Stradling, "Certificate
              Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
              December 2021, <https://www.rfc-editor.org/rfc/rfc9162>.

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt
   Germany
   Email: henk.birkholz@ietf.contact

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   Antoine Delignat-Lavaud
   Microsoft Research
   21 Station Road
   Cambridge
   CB1 2FB
   United Kingdom
   Email: antdl@microsoft.com

   Cedric Fournet
   Microsoft Research
   21 Station Road
   Cambridge
   CB1 2FB
   United Kingdom
   Email: fournet@microsoft.com

   Amaury Chamayou
   Microsoft Research
   21 Station Road
   Cambridge
   CB1 2FB
   United Kingdom
   Email: amaury.chamayou@microsoft.com

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