CCF Profile for COSE Receipts

Introduction The COSE Receipts document 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 . 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 , the leaves of CCF trees carry additional internal information for the following purposes:

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.
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.

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 when, and only when, they appear in all capitals, as shown here.

Description of the Confidential Consortium Framework Ledger Verifiable Data Structure This document extends the VDS registry of with the following value: Verifiable Data Structure Algorithms

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

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

Transaction Components Each leaf in a CCF ledger carries the following components: 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 , and internal-evidence is a revealable 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 (). 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 . 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.

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

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 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.

Inclusion Proof Verification Algorithm CCF uses the following algorithm to verify an inclusion receipt: A description can also be found at .

Usage in COSE Receipts A COSE Receipt with a CCF inclusion proof is described by the following CDDL definition: 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: inclusion-proofs } unprotected-header-map = { &(vdp: 396) => verifiable-proofs * cose-label => cose-value } ]]>

Privacy Considerations See the privacy considerations section of:

Security Considerations The security considerations of apply.

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

The evaluation of registration policies
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 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.

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.

IANA Considerations

Additions to Existing Registries

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: RFCthis
Related information: