TBD H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track A. Delignat-Lavaud
Expires: 15 July 2024 C. Fournet
Microsoft Research
12 January 2024
A Transaction Ledger Verifiable Structure for COSE Merkle Tree Proofs
draft-birkholz-cose-cometre-ccf-profile-01
Abstract
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) to provide
stronger tamper-evidence guarantees.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. Format of CCF Ledger . . . . . . . . . . . . . . . . . . . . 3
2.1. Merkle Tree Shape . . . . . . . . . . . . . . . . . . . . 3
2.2. Transaction Components . . . . . . . . . . . . . . . . . 4
3. CCF Inclusion Proofs . . . . . . . . . . . . . . . . . . . . 4
3.1. Inclusion Proof Verification Algorithm . . . . . . . . . 5
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6.1. Additions to Existing Registries . . . . . . . . . . . . 5
6.1.1. Tree Algorithms . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Attic . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The Concise Encoding of Signed Merkle Tree Proofs (CoMeTre)
[I-D.ietf-cose-merkle-tree-proofs] defines a common framework for
defining different types of proofs about verifiable data structures
(also abbreviated as "logs" in this document). For instance,
inclusion proofs guarantee to a verifier that a given serializable
element is recorded at a given state of the log, while consistency
proofs are used to establish that an inclusion proof is still
consistent with the new state of the log at a later time.
In this document, we define a new type of log, associated with the
Confidential Consortium Framework (CCF) ledger. This log 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 opaque information that is used 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.
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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. Format of CCF Ledger
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 | Historical | This |
| | (requested | transaction | document |
| | assignment 2) | ledgers, such as | |
| | | the CCF ledger | |
+-------------------+---------------+------------------+-----------+
Table 1: Verifiable Data Structure Algorithms
This document defines inclusion proofs and consistency proof formats
for CCF ledgers. Verifiers MUST reject all other proof types.
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]}. 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:
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]).
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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 transaction in a CCF ledger carries the following
components:
CCF-leaf = [
internal-hash: bstr ; a string of HASH_SIZE bytes;
internal-data: bstr; a string of at most 1024 bytes; and
data_hash: bstr ; the serialization of the element stored at this leaf.
]
The internal_hash and internal_data byte strings are internal to the
CCF implementation. Similarly, the auxiliary tree entries are
internal to CCF. They are opaque to receipt Verifiers, but they
commit the TS to the whole tree contents and may be used for
additional, CCF-specific auditing.
3. CCF Inclusion Proofs
CCF inclusion proofs consist of a list of digests tagged with a
single left-or-right bit.
CCF-inclusion-proof: [
leaf: CCF-leaf ;
path: [+ ccf-proof-element] ;
]
ccf-proof-element = [
left: bool
hash: bstr
]
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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).
3.1. Inclusion Proof Verification Algorithm
CCF uses the following algorithm to validate an inclusion receipt:
compute_root(proof):
let h = proof.leaf.internal-hash
|| HASH(proof.leaf.internal-data)
|| 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):
let proofs = inclusion_receipt.unprotected_headers[-222] or fail
let payload = nil
assert(inclusion_receipt.payload == nil)
for proof in proofs
let root = compute_root(proof)
if payload = nil then payload := root
else assert(root == payload)
# Use the Merkle Root as the detached payload
return verif_cose(inclusion_receipt, payload)
4. Privacy Considerations
Privacy Considerations
5. Security Considerations
Security Considerations
6. IANA Considerations
6.1. Additions to Existing Registries
6.1.1. Tree Algorithms
This document requests IANA to add the following new value to the
'COSE Verifiable Data Structures' registry:
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* Name: CCF_LEDGER_SHA256
* Value: TBD_1 (requested assignment 2)
* Description: Historical transaction ledgers, such as the CCF
ledger
* Reference: This document
7. References
7.1. 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,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, .
7.2. Informative References
[I-D.ietf-cose-merkle-tree-proofs]
Steele, O., Birkholz, H., Delignat-Lavaud, A., and C.
Fournet, "Concise Encoding of Signed Merkle Tree Proofs",
Work in Progress, Internet-Draft, draft-ietf-cose-merkle-
tree-proofs-03, 11 December 2023,
.
Appendix A. Attic
Not ready to throw these texts into the trash bin yet.
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@sit.fraunhofer.de
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Antoine Delignat-Lavaud
Microsoft Research
21 Station Road
Cambridge
CB1 2FB
United Kingdom
Email: antdl@microsoft.com
Cedric Fournet
Microsoft Research
21 Station Road
Cambridge
CB1 2FB
United Kingdom
Email: fournet@microsoft.com
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