CCF Profile for COSE Receipts
draft-ietf-scitt-receipts-ccf-profile-04

SCITT                                                        H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                      A. Delignat-Lavaud
Expires: 26 December 2026                                     C. Fournet
                                                             A. Chamayou
                                                      Microsoft Research
                                                            24 June 2026

                     CCF Profile for COSE Receipts
                draft-ietf-scitt-receipts-ccf-profile-04

Abstract

   This document defines a new verifiable data structure (VDS) type for
   COSE Receipts and inclusion proofs specifically designed for append-
   only logs produced by the Confidential Consortium Framework (CCF) to
   provide stronger tamper-evidence guarantees.

Status of This Memo

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   This Internet-Draft will expire on 26 December 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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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 Confidential Consortium Framework 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  . . . . . . . . .   7
   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.  COSE Verifiable Data Structure Algorithms . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The COSE Receipts document [I-D.ietf-cose-merkle-tree-proofs] defines
   a common framework for expressing different types of proofs about
   verifiable data structures (VDS), providing a standardized way to
   convey trust-relevant evidence.  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 and inclusion proof
   associated with an application of the Confidential Consortium
   Framework (CCF) ledger that implements the SCITT Architecture defined
   in [I-D.ietf-scitt-architecture].  This VDS carries indexed

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   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
       superset of what is exposed in the proof and captures internal
       details useful for detailed system audit, but not for application
       purposes.

   2.  To allow the distributed system executing the application logic
       in Trusted Execution Environments (TEEs) to persist signatures to
       storage early.  Receipt production is only enabled once
       transactions 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 Confidential Consortium Framework Ledger
    Verifiable Data Structure

   This document extends the "COSE Verifiable Data Structure Algorithms"
   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: COSE Verifiable Data Structure Algorithms

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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().

                  Figure 1: Merkle Tree Hash of Empty List

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

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

                Figure 2: Merkle Tree Hash of a Single Entry

   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])),

                    Figure 3: Recursive Merkle Tree Hash

   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
   ]

                          Figure 4: CCF Leaf CDDL

   The internal-transaction-hash and internal-evidence values are
   internal to the CCF implementation.  They can be safely ignored by
   receipt Verifiers, but they commit the transparency service (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 (TEEs) that only
   provide a limited amount of memory, while maintaining high
   availability afforded by distributed consensus.  Using a secure one-
   way function f to publish an f(x) commitment to an x value that can
   be revealed at a later time is a common feature of distributed
   protocols ([COIN-FLIPPING]).  The hash function used for internal-
   transaction-hash is the same hash function H as in the Merkle Tree
   construction for the selected verifiable data structure algorithm.
   For CCF_LEDGER_SHA256, this function is SHA256.

   data-hash summarizes the application data included in the ledger at
   this transaction, which is a Signed Statement as defined by
   [I-D.ietf-scitt-architecture].  The hash function used for data-hash
   is also the same hash function H as in the Merkle Tree construction
   for the selected verifiable data structure algorithm.  For
   CCF_LEDGER_SHA256, this function is SHA256.

3.  CCF Inclusion Proofs

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

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   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]
   }

                     Figure 5: CCF Inclusion Proof CDDL

   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_SHA256 (TBD_1).

   *  label: int.  This header MUST be set to the value of the inclusion
      proof type in the IANA "COSE Verifiable Data Structure Proofs"
      registry (-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

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   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:

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

     for [left, hash] in proof.path:
         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)

                Figure 6: CCF Inclusion Receipt Verification

   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
   }

                    Figure 7: Protected Header Map CDDL

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   *  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:

   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
   }

                   Figure 8: Unprotected Header Map CDDL

5.  Privacy Considerations

   See the privacy considerations section of:

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

6.  Security Considerations

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

6.1.  Trusted Execution Environments

   CCF networks of nodes rely on executing in TEEs 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 TEE platform used to execute the network may
   allow an attacker to produce invalid and divergent ledger branches.
   Clients can mitigate this risk in two ways: by regularly auditing the
   consistency of the CCF ledger; and by regularly fetching attestation

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   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.

6.2.  Operators

   An operator has the ability to start successor networks with a
   distinct identity.  The operator of a CCF network can recover the
   service by starting a successor network, for example a new CCF
   network with its own service identity, that endorses the ledger state
   of the previous instance.  This provides service continuity after a
   catastrophic failure of a majority of the nodes.  However, a
   malicious operator could exploit this mechanism and truncate the
   ledger’s history by initializing the successor network from an
   earlier ledger prefix, thereby omitting some later entries.  Clients
   can mitigate this risk by auditing the successor ledger and verifying
   that their latest known receipts from the prior service are included
   in the successor’s ledger.

7.  IANA Considerations

7.1.  Additions to Existing Registries

7.1.1.  COSE Verifiable Data Structure Algorithms

   This document requests IANA to add the following new value to the
   "COSE Verifiable Data Structure Algorithms" 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: RFCthis

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

8.  References

8.1.  Normative References

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   [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-18, 2 December 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              merkle-tree-proofs-18>.

   [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>.

8.2.  Informative 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>.

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

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   [COIN-FLIPPING]
              "Coin Flipping By Telephone - A Protocol For Solving
              Impossible Problems", DOI 10.1145/1323293.1294280, n.d.,
              <https://dl.acm.org/doi/epdf/10.1145/1008908.1008911>.

Authors' Addresses

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

   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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