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Microsoft Research

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Microsoft Research

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Security TBD Internet-Draft 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 () to provide stronger tamper-evidence guarantees.

Introduction The COSE Receipts document 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 , 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 verify that elements are only written by the Trusted Execution Environment, which addresses the persistence of committed transactions that happen between new signatures of the Merkle Tree root.

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 CCF Ledger Verifiable Data Structure This documents extends the verifiable data structure 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	This document

This document defines inclusion proofs for CCF ledgers. Verifiers MUST reject all other proof types

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 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 , and internal-evidence is a revealable 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 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 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.

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

Privacy Considerations Privacy Considerations

Security Considerations Security Considerations

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: TBD_1 (requested assignment 2)
• Description: Historical transaction ledgers produced by Trusted Execution Environments, such as the CCF ledger
• Reference: This document

Normative References This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. This document obsoletes RFC 6962. It also specifies a new TLS extension that is used to send various CT log artifacts. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. n.d. n.d. n.d. n.d. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

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