| Internet-Draft | COSE Receipts with CCF | December 2024 |
| Birkholz, et al. | Expires 13 June 2025 | [Page] |
This document defines a new verifiable data structure type for COSE Signed Merkle Tree Proofs specifically designed for transaction ledgers produced by Trusted Execution Environments (TEEs), such as the Confidential Consortium Framework ([CCF]) to provide stronger tamper-evidence guarantees.¶
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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 the Confidential Consortium Framework (CCF) ledger. 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:¶
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.¶
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 (requested assignment 2) | Historical transaction ledgers, such as the CCF ledger | RFCthis |
This document defines inclusion proofs for CCF ledgers. Corresponding CCF Verifiers MUST reject proof types they do not support.¶
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:¶
Each leaf in a CCF ledger carries the following components:¶
ccf-leaf = [ internal-transaction-hash: bstr .size 32 ; a string of HASH_SIZE(32) bytes internal-evidence: tstr .size (1..1024) ; a string of at most 1024 bytes data-hash: bstr .size 32 ; a string of HASH_SIZE(32) bytes ]¶
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.¶
data-hash summarises the subject of the proof: the data which is included in the ledger at this transaction.¶
CCF inclusion proofs consist of a list of digests tagged with a single left-or-right bit.¶
ccf-proof-element = [
left: bool ; position of the element
hash: bstr .size 32; hash of the proof element (string of HASH_SIZE(32) bytes)
]
ccf-inclusion-proof = bstr .cbor {
&(leaf: 1) => ccf-leaf
&(path: 2) => [+ ccf-proof-element]
}
¶
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).¶
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 headers defined by the application.¶
The protected headers 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.¶
CCF uses the following algorithm to verify an inclusion receipt:¶
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 proof = inclusion_receipt.unprotected_headers[INCLUSION_PROOF_LABEL] or fail
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].¶
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
}
¶
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
}
¶
Maybe a list of precursors that are specific to CCF VDS goes here (e.g., trade-offs, pro/con, use of TEE).¶
This document requests IANA to add the following new value to the 'COSE Verifiable Data Structures' registry:¶