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Security TBD Internet-Draft This document defines a new verifiable data structure type for COSE Receipts specifically for use with ledgers based on post-order traversal binary Merkle trees and which are designed for high throughput, ease of replication and compatibility with commodity cloud storage. Post-order traversal binary Merkle trees, also known as history trees, are more commonly known as Merkle Mountain Ranges.

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: a post ordered binary merkle tree is, logically, the unique series of perfect binary merkle trees required to commit its leaves. Example, This illustrates MMR(8), which is comprised of two perfect trees rooted at 6 and 7. 7 is the root of a tree comprised of a single element. The peaks of the perfect trees form the accumulator. The storage of a tree maintained in this way is addressed as a linear array, and additions to the tree are always appends. Proving and verifying are defined in terms of the cryptographic asynchronous accumulator described by ReyzinYakoubov. The technical advantages of post-order traversal binary Merkle trees are discussed in CrosbyWallachStorage and PostOrderTlog.

Conventions and Definitions 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.

• A complete MMR(n) defines an mmr with n nodes where no equal height sibling trees exist.
• i shall be the index of any node, including leaf nodes, in the MMR
• g shall be the zero based height of a node in the tree.
• H(x) shall be the SHA-256 digest of any value x
• || shall mean concatenation of raw byte representations of the referenced values.

In this specification, all numbers are unsigned 64 bit integers. The maximum height of a single tree is 64 (which will have g=63 for its peak).

Description of the Verifiable Data Structure This documents extends the verifiable data structure registry of with the following value: Verifiable Data Structure Algorithms

Name	Value	Description	Reference
MMR_SHA256	TBD_1 (requested assignment 3)	Linearly addressed, position committing, MMR implementations, such as the MMR ledger	This document

Inclusion Proofs The CBOR representation of an inclusion proof is Note that the inclusion path for the index leads to a single permanent node in the tree. This node will initially be a peak in the accumulator, as the tree grows it will eventually be "buried" by a new peak.

inclusion_proof_path inclusion_proof_path(i, c) is used to produce the verification paths for inclusion proofs and consistency proofs. Given:

• c the index of the last node in any tree which contains i.
• i the index of the mmr node whose verification path is required.

And the methods:

• index_height which obtains the zero based height g of any node.

And the constraints:

• i <= c

We define inclusion_proof_path as g: # The witness to the right sibling is offset behind i isibling = i - siblingoffset + 1 # The parent of a right sibling is stored immediately # after i += 1 else: # The witness to a left sibling is offset ahead of i isibling = i + siblingoffset - 1 # The parent of a left sibling is stored immediately after # its right sibling i += siblingoffset # When the computed sibling exceeds the range of MMR(C+1), # we have completed the path if isibling > c: return path path.append(isibling) # Set g to the height of the next item in the path. g += 1 ]]>

COSE Receipt of Inclusion The cbor representation of an inclusion proof is: int &(vds: 395) => 3 * 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: inclusion-proofs } unprotected-header-map = { &(vdp: 396) => verifiable-proofs * cose-label => cose-value } ]]> The payload of an inclusion proof signature is the tree peak committing to the nodes inclusion, or the node itself where the proof path is empty. The algorithm included_root obtains this value. The payload MUST be detached. Detaching the payload forces verifiers to recompute the root from the inclusion proof, this protects against implementation errors where the signature is verified but the payload merkle root does not match the inclusion proof.

Verifying the Receipt of inclusion The inclusion proof and signature are verified in order. First the verifiers applies the inclusion proof to a possible entry (set member) bytes. The result is the merkle root implied by the inclusion proof path for the candidate value. The COSE Sign1 payload MUST be set to this value. Second the verifier checks the signature of the COSE Sign1. If the resulting signature verifies, the Receipt has proved inclusion of the entry in the verifiable data structure. If the resulting signature does not verify, the signature may have been tampered with. It is recommended that implementations return a single boolean result for Receipt verification operations, to reduce the chance of accepting a valid signature over an invalid inclusion proof. As the proof must be processed prior to signature verification the implementation SHOULD check the lengths of the proof paths are appropriate for the provided tree sizes.

included_root The algorithm included_root calculates the accumulator peak for the provided proof and node value. Given:

• i is the index the nodeHash is to be shown at
• nodehash the value whose inclusion is to be shown
• proof is the path of sibling values committing i.

And the methods:

• index_height which obtains the zero based height g of any node.
• hash_pospair64 which applies H to the new node position and its children.

We define included_root as g: # The parent of a right sibling is stored immediately after i = i + 1 # Set `root` to `H(i+1 || sibling || root)` root = hash_pospair64(i + 1, sibling, root) else: # The parent of a left sibling is stored immediately after # its right sibling. i = i + (2 << g) # Set `root` to `H(i+1 || root || sibling)` root = hash_pospair64(i + 1, root, sibling) # Set g to the height of the next item in the path. g = g + 1 # If the path length was zero, the original nodehash is returned return root ]]>

Consistency Proof A consistency proof shows that the accumulator, defined in ReyzinYakoubov, for tree-size-1 is a prefix of the accumulator for tree-size-2. The signature is over the complete accumulator for tree-size-2 obtained using the proof and the, supplied, possibly empty, list of right-peaks which complete the accumulator for tree-size-2. The receipt of consistency is defined so that a chain of cumulative consistency proofs can be verified together. The cbor representation of a consistency proof is:

consistency_proof_path Produces the verification paths for inclusion of the peaks of tree-size-1 under the peaks of tree-size-2. right-peaks are obtained by invoking peaks(tree-size-2 - 1), and discarding length(proofs) from the left. Given:

• ifrom is the last index of tree-size-1
• ito is the last index of tree-size-2

And the methods:

• inclusion_proof_path

And the constraints:

• ifrom <= ito

We define consistency_proof_paths as

COSE Receipt of Consistency The cbor representation of an inclusion proof is: int &(vds: 395) => 3 * 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: consistency-proof } unprotected-header-map = { &(vdp: 396) => verifiable-proofs * cose-label => cose-value } ]]> The payload MUST be detached. Detaching the payload forces verifiers to recompute the roots from the consistency proofs. This protects against implementation errors where the signature is verified but the payload is not genuinely produced by the included proof.

Verifying the Receipt of consistency Verification accommodates verifying the result of a cumulative series of consistency proofs. Perform the following for each consistency-proof in the list, verifying the signature with the output of the last.

1. Initialize current proof as the first consistency-proof.
2. Initialize accumulatorfrom to the peaks of tree-size-1 in the current proof.
3. Initialize ifrom to tree-size-1 - 1 from the current proof.
4. Initialize proofs to the consistency-paths from the current proof.
5. Apply the algorithm consistent_roots
6. Apply the peaks algorithm to obtain the accumulator for tree-size-2
7. From the peaks for tres-size-2, discard from the left the number of roots returned by consistent_roots.
8. Create the consistent accumulator by appending the remaining peaks to the consistent roots.
9. If there are no remaining proofs, use the consistent accumulator as the detached payload and verify the signature of the COSE Sign1.

It is recommended that implementations return a single boolean result for Receipt verification operations, to reduce the chance of accepting a valid signature over an invalid consistency proof. As the proof must be processed prior to signature verification the implementation SHOULD check the lengths of the proof paths are appropriate for the provided tree sizes.

consistent_roots consistent_roots returns the descending height ordered list of elements from the accumulator for the consistent future state. Implementations MUST require that the number of peaks returned by (ifrom) equals the number of entries in accumulatorfrom. Given:

• ifrom the last index in the complete MMR from which consistency was proven.
• accumulatorfrom the node values corresponding to the peaks of the accumulator for tree-size-1
• proofs the inclusion proofs for each node in accumulatorfrom for tree-size-2

And the methods:

• included_root

We define consistent_roots as ERROR roots = [] for i in range(len(accumulatorfrom)): root = included_root( frompeaks[i], accumulatorfrom[i], proofs[i]) if roots and roots[-1] == root: continue roots.append(root) return roots ]]>

Appending a leaf An algorithm for appending to a tree maintained in post order layout is provided.

add_leaf_hash When a new node is appended, if its height matches the height of its immediate predecessor, then the two equal height siblings MUST be merged. Merging is defined as the append of a new node which takes the adjacent peaks as its left and right children. This process MUST proceed until there are no more completable sub trees. add_leaf_hash(f) adds the leaf hash value f to the tree. Given:

• f the leaf value resulting from H(x) for the caller defined leaf value x
• db an interface supporting append(entry) -> index and get(index) -> entry methods.

And the methods:

• index_height

We define add_leaf_hash as g: # Set ileft to the index of the left child of i, # which is i - 2^(g+1) ileft = i - (2 << g) # Set iright to the index of the the right child of i, # which is i - 1 iright = i - 1 # Set v to H(i + 1 || Get(ileft) || Get(iright)) # Set i to the result of invoking Append(v) i = db.append( hash_pospair64(i+1, db.get(ileft), db.get(iright))) # Set g to the height of the new i, which is g + 1 g += 1 return i ]]>

Node values Interior nodes in the tree MUST prefix the value provided to H(x) with pos. The value v for any interior node MUST be H(pos || Get(LEFT_CHILD) || Get(RIGHT_CHILD)) The algorithm for leaf addition is provided the result of H(x) directly.

hash_pospair64 Returns H(pos || a || b), which is the value for the node identified by index pos - 1 Editors note: How this draft accommodates hash alg agility is tbd. Given:

• pos the size of the MMR whose last node index is pos - 1
• a the first value to include in the hash after pos
• b the second value to include in the hash after pos

And the constraints:

• pos < 2^64
• a and b MUST be hashes produced by the appropriate hash alg.

We define hash_pospair64 as

Essential supporting algorithms

index_height index_height(i) returns the zero based height g of the node index i Given:

• i the index of any mmr node.

We define index_height as int: pos = i + 1 while not all_ones(pos): pos = pos - most_sig_bit(pos) + 1 return bit_length(pos) - 1 ]]>

peaks peaks(i) returns the peak indices for MMR(i+1), which is also its accumulator. Assumes MMR(i+1) is complete, implementations can check for this condition by testing the height of i+1 Given:

• i the index of any mmr node.

We define peaks

Security Considerations See the security considerations section of:

Detection of improper inclusion A receipt of inclusion shows only that the element is included in the ledger. Defining whether that inclusion was legitimate, or in some way valid, is out of scope for this document.

Misbehaving Ledgers A ledger can misbehave in several ways. Examples include the following: failing to incorporate a leaf entry in the MMR; presenting different, conflicting views of the MMR at different times and/or to different parties. Detection of a failure to include items in the first place is out of scope for this document. Having included an element, ledger implementations using this draft MUST use consistency proofs as the basis for proving entries are not moved, modified or excluded in future states of the MMR. Similarly, consistency proofs MUST be the basis for proving the unequivocal history of additions.

IANA Considerations Editors note: Hash agility is desired. We can start with SHA-256. Two of the referenced implementations use BLAKE2b-256, We would like to add support for SHA3-256, SHA3-512, and possibly Keccak and Pedersen.

Additions to Existing Registries Editors note: todo registry requests

New Registries

Normative References Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol (RFC 9052). This document, along with RFC 9052, obsoletes RFC 8152. Tradeverifyd Fraunhofer SIT Microsoft Microsoft COSE (CBOR Object Signing and Encryption) Receipts prove properties of a verifiable data structure to a verifier. Verifiable data structures and associated proof types enable security properties, such as minimal disclosure, transparency and non-equivocation. Transparency helps maintain trust over time, and has been applied to certificates, end to end encrypted messaging systems, and supply chain security. This specification enables concise transparency oriented systems, by building on CBOR (Concise Binary Object Representation) and COSE. The extensibility of the approach is demonstrated by providing CBOR encodings for Merkle inclusion and consistency proofs. 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.

References

Informative References

• ReyzinYakoubov
• CrosbyWallach
• CrosbyWallachStorage 3.3 Storing the log on secondary storage
• PostOrderTlog
• PeterTodd
• KnuthTBT 2.3.1 Traversing Binary Trees
• BNT

Assumed bit primitives

log2floor Returns the floor of log base 2 x

most_sig_bit Returns the mask for the the most significant bit in pos int: return 1 << (pos.bit_length() - 1) ]]> The following primitives are assumed for working with bits as they commonly have library or hardware support.

bit_length The minimum number of bits to represent pos. b011 would be 2, b010 would be 2, and b001 would be 1.

all_ones Tests if all bits, from the most significant that is set, are 1, b0111 would be true, b0101 would be false. bool: msb = most_sig_bit(pos) mask = (1 << (msb + 1)) - 1 return pos == mask ]]>

ones_count Count of set bits. For example ones_count(b101) is 2

trailing_zeros

Acknowledgments