CCF Profile for COSE Receipts

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.¶

2.1. 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]}. Each transaction T[i] is serialized to a byte string d[i] as defined in Section 2.2, giving the list D_n = {d[0], d[1], ..., d[n-1]}. We define the Merkle Tree Hash (MTH) function, which takes as input such a list of serialized transactions, 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:¶

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 ccf-leaf array is the representation of these components in an inclusion proof (Section 3). It is not what is hashed into the tree. The serialized transaction d[i] that is input to MTH in Section 2.1 is the byte string of length 3 * HASH_SIZE obtained by concatenating the internal transaction hash, the hash of the internal evidence, and the data hash:¶

d[i] = internal-transaction-hash
       || HASH(internal-evidence)
       || data-hash

where HASH(internal-evidence) is the digest of the UTF-8 encoding of the internal-evidence text string. The leaf hash MTH({d[i]}) = HASH(d[i]) is therefore HASH(internal-transaction-hash || HASH(internal-evidence) || data-hash), which is the value computed by the first step of compute_root in Figure 7. Note that the proof carries internal-evidence itself rather than its digest: revealing the evidence is what demonstrates that the transaction was committed (see below).¶

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 is the hash of the bytes of the Signed Statement as registered: the COSE_Sign1 encoding with CBOR tag 18 retained (Section 6.1 of [RFC9943]) and an empty unprotected header map (Section 6.3 of [RFC9943]), other elements as submitted. 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.¶

data-hash captures the Signed Statement, not the Transparent Statement of Section 7 of [RFC9943], which adds Receipts to that unprotected header. A Verifier holding a Transparent Statement recovers the preimage as follows:¶

1. Take the COSE_Sign1 of the Transparent Statement.¶

2. Replace its unprotected header with an empty map (Section 6.3 of [RFC9943]).¶

3. Keep tag 18, and keep the encoded values of the protected header, payload and signature as received.¶

4. Hash the resulting byte string with H (SHA256 for CCF_LEDGER_SHA256).¶

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. These follow the conventions of Section 5.2.1 of [RFC9942]; the corresponding CDDL is given in Section 5. 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:¶

• vds (label 395): int. This header MUST be set to the verifiable data structure algorithm identifier for CCF_LEDGER_SHA256 (TBD_1).¶

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

• vdp (label 396): map. This header conveys the verifiable data structure proofs, keyed by proof type. It MUST contain the inclusion-proof (-1) key, whose value is an array of one or more ccf-inclusion-proof values as defined above. The proof type is identified solely by this key; no other header parameter is used to convey it.¶

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 proofs 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. When the array contains more than one inclusion proof, every proof MUST compute to the same root.¶

3.2. Inclusion Proof Verification Algorithm

After decoding each ccf-inclusion-proof byte string, the Verifier MUST validate that the decoded value is well constructed with regard to the schema defined in Figure 6, including Figure 4 for the leaf, and MUST reject the proof before compute_root is called if any check fails. In particular:¶

• the byte string is valid CBOR (Section 5.3.1 of [RFC8949]), in particular the map has no duplicate keys;¶

• the map has the keys leaf (1) and path (2);¶

• internal-transaction-hash is a byte string of exactly HASH_SIZE (32) bytes;¶

• internal-evidence is a text string of 1 to 1024 bytes of valid UTF-8;¶

• data-hash is a byte string of exactly HASH_SIZE (32) bytes;¶

• path is an array, each element of which is an array of exactly two items: a boolean and a byte string of exactly HASH_SIZE (32) bytes.¶

Without these checks, since leaf and interior node hashes are not domain separated, an oversized path element lets an existing leaf be recomputed as if it were an interior node, so that a proof for a leaf and data-hash of the attacker's choosing still recomputes a validly signed root (a second-preimage-style forgery).¶

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, statement_digest):
  assert(VDP_LABEL in inclusion_receipt.unprotected_header)
  let vdp = inclusion_receipt.unprotected_header[VDP_LABEL]
  assert(INCLUSION_PROOF_LABEL in vdp)
  let proofs = vdp[INCLUSION_PROOF_LABEL]
  assert(len(proofs) > 0)
  assert(inclusion_receipt.payload == nil)

  for proof in proofs:
      assert(validate_ccf_inclusion_proof(proof))
      assert(proof.leaf.data-hash == statement_digest)
      # Use the Merkle Root as the detached payload
      let payload = compute_root(proof)
      assert(verify_cose(inclusion_receipt, payload))
  return true

VDP_LABEL is the vdp header parameter label (396) and INCLUSION_PROOF_LABEL is the inclusion-proof proof type label (-1), both defined by [RFC9942]. Each element of proofs is a ccf-inclusion-proof, i.e., a byte string wrapping a CBOR-encoded map, which is decoded and validated against the schema defined in Figure 6 before compute_root is applied.¶

statement_digest is the data-hash of the candidate Signed Statement, computed by the Verifier from the bytes of that Signed Statement as described in Section 2.2; it MUST NOT be taken from the receipt. Every proof in the receipt MUST have a data-hash equal to statement_digest. This defines how the inclusion proofs of this profile are consumed (Section 4.4.1 of [RFC9942]), by applying them to the bytes of a candidate entry (Section 5.2.1 of [RFC9942]). If the receipt also carries consistency proofs, they MUST recompute the same root (Section 5).¶

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

4.1. CCF Consistency Proof Signature

The proof signature for a CCF consistency proof is a COSE_Sign1 with the same protected header requirements as an inclusion proof signature (Section 3.1). Its unprotected header MUST include:¶

• vdp (label 396): map. It MUST contain the consistency-proof (-2) key, whose value is an array of one or more ccf-consistency-proof values, each relating one older root to the newer root.¶

The payload is the newer root R_n, and MUST be detached. When the array contains more than one consistency proof, every proof MUST compute to the same newer root, and a Verifier MUST reject the receipt if any proof computes to a different one. The older root R_m is not carried by the receipt: the verifier already holds it, typically as the root recomputed from an inclusion receipt (Figure 7), and checks that at least one proof recomputes it. Other proofs in the same array may start from other older roots.¶

4.2. Consistency Proof Verification Algorithm

Before applying this algorithm, the Verifier MUST have validated the vdp map as required in Section 5. After decoding each ccf-consistency-proof byte string, the Verifier MUST validate that the decoded value is well constructed with regard to the schema defined in Figure 8, and MUST reject the proof before compute_roots is called if any check fails. In particular:¶

• the byte string is valid CBOR (Section 5.3.1 of [RFC8949]), in particular the map has no duplicate keys;¶

• the map has the keys anchor (1) and path (2);¶

• anchor is a byte string of exactly HASH_SIZE (32) bytes;¶

• path is a non-empty array, each element of which is an array of exactly two items: a boolean and a byte string of exactly HASH_SIZE (32) bytes.¶

These checks ensure that compute_roots is only applied to well-formed proofs.¶

A consistency receipt is verified against a trusted older_root as follows:¶

compute_roots(proof):
  older := proof.anchor
  newer := proof.anchor

  for [left, hash] in proof.path:
      if left:
          older := HASH(hash || older)
          newer := HASH(hash || newer)
      else:
          newer := HASH(newer || hash)
  return older, newer

verify_consistency_receipt(consistency_receipt, older_root):
  assert(VDP_LABEL in consistency_receipt.unprotected_header)
  let vdp = consistency_receipt.unprotected_header[VDP_LABEL]
  assert(CONSISTENCY_PROOF_LABEL in vdp)
  let proofs = vdp[CONSISTENCY_PROOF_LABEL]
  assert(len(proofs) > 0)
  assert(consistency_receipt.payload == nil)

  let payload = nil
  let older_root_found = false
  for proof in proofs:
      assert(validate_ccf_consistency_proof(proof))
      # The first sibling of the anchor is in the newer tree only
      assert(proof.path[0].left == false)
      let older, newer = compute_roots(proof)
      # Every proof must compute to the same newer root
      assert(payload == nil or newer == payload)
      payload := newer
      if older == older_root:
          older_root_found := true
  # At least one proof must start from older_root
  assert(older_root_found)

  # Use the common newer Merkle Root as the detached payload
  assert(verify_cose(consistency_receipt, payload))
  return true

CONSISTENCY_PROOF_LABEL is the consistency-proof proof type label (-2) defined by [RFC9942]. Each element of proofs is a ccf-consistency-proof, i.e., a byte string wrapping a CBOR-encoded map, which is decoded and validated against the schema defined in Figure 8 before compute_roots is applied. All proofs are folded before the signature is checked, so that a receipt in which any proof computes to a different newer root is rejected, whichever proof the Verifier is interested in. If the receipt also carries inclusion proofs, they MUST recompute the same root (Section 5). The comparison with older_root binds the receipt to a state the verifier already trusts: a receipt in which no proof recomputes older_root proves nothing about that state. It also confirms that the anchor is a node of that state, but not that it is the anchor required in Section 4, which is not needed for the guarantee the receipt provides (Section 7.3).¶

The signature attests that payload is the root of a state of the ledger; verification establishes that the state with root older_root is a prefix of it, not that the signed value is itself a Merkle Tree Hash of Section 2.1 (Section 7.3).¶

7.1. 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, for instance by verifying consistency receipts (Figure 9) from roots they hold to the current root; 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.¶

7.2. 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.¶

7.3. Consistency Receipts

A consistency receipt relates two roots and establishes nothing about ledger contents on its own. Verifiers MUST compare the recomputed older root with a root they have already verified; which signing key they accept, including that of a successor network (Section 7.2), is their trust policy as for any Receipt of [RFC9942]. Consistency receipts also cannot provide guarantees about the correct application of Registration Policies, which requires additional material as outlined in Section 5.1.1.2 of [RFC9943].¶

The receipt carries no tree sizes and the signature covers only the newer root, so sizes cannot be authenticated; none is needed. Because the older root is trusted and the newer root is signed, every node a passing proof folds is, barring a second preimage of H, a node of one of the two states, and every transaction of the older state is at the same position in the signed one. This holds whether or not the anchor is the canonical node of Section 4.¶

Verification does not establish that the signed value is itself a Merkle Tree Hash of Section 2.1; that is attested by the Transparency Service, and a Transparency Service signing values that are not roots of its ledger is the misbehaviour addressed in Section 7.1 and Section 7.2. A Verifier that needs m or n MUST obtain them from a source it trusts independently of the receipt; this document does not define one.¶

8.1. Additions to Existing Registries

The registries in this section are defined by [RFC9942].¶

9.1. Normative References

[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>.

[RFC8949]

Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, December 2020, <https://www.rfc-editor.org/rfc/rfc8949>.

[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>.

[RFC9942]

Steele, O., Birkholz, H., Delignat-Lavaud, A., and C. Fournet, "CBOR Object Signing and Encryption (COSE) Receipts", RFC 9942, DOI 10.17487/RFC9942, June 2026, <https://www.rfc-editor.org/rfc/rfc9942>.

[RFC9943]

Birkholz, H., Delignat-Lavaud, A., Fournet, C., Deshpande, Y., and S. Lasker, "An Architecture for Trustworthy and Transparent Digital Supply Chains", RFC 9943, DOI 10.17487/RFC9943, June 2026, <https://www.rfc-editor.org/rfc/rfc9943>.

9.2. 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>.

[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>.