TBD H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track A. Delignat-Lavaud
Expires: 17 April 2025 C. Fournet
A. Chamayou
Microsoft Research
14 October 2024
COSE Receipts Profile and Tree Algorithm for the Confidential Consortium
Framework
draft-birkholz-cose-receipts-ccf-profile-00
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), such as
the Confidential Consortium Framework ([CCF]) to provide stronger
tamper-evidence guarantees.
Status of This Memo
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. Description of the CCF Ledger Verifiable Data Structure . . . 3
2.1. Merkle Tree Shape . . . . . . . . . . . . . . . . . . . . 3
2.2. Transaction Components . . . . . . . . . . . . . . . . . 4
3. CCF Inclusion Proofs . . . . . . . . . . . . . . . . . . . . 4
3.1. CCF Inclusion Proof Signature . . . . . . . . . . . . . . 5
3.2. Inclusion Proof Verification Algorithm . . . . . . . . . 6
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6.1. Additions to Existing Registries . . . . . . . . . . . . 6
6.1.1. Tree Algorithms . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Attic . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
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:
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.
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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.
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. Description of the CCF Ledger Verifiable Data Structure
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 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:
This function is defined as follows:
The hash of an empty list is the hash of an empty string:
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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:
* || 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 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.
3. CCF Inclusion Proofs
CCF inclusion proofs consist of a list of digests tagged with a
single left-or-right bit.
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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).
3.1. 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.
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3.2. Inclusion Proof Verification Algorithm
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].
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:
* Name: CCF_LEDGER_SHA256
* Value: TBD_1 (requested assignment 2)
* Description: Historical transaction ledgers produced by Trusted
Execution Environments, such as the CCF ledger
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* Reference: This document
7. Normative References
[CCF] "Confidential Consortium Framework", n.d.,
.
[CCF-Commit-Evidence]
"CCF Commit Evidence", n.d.,
.
[CCF-Ledger-Format]
"CCF Ledger Format", n.d.,
.
[CCF-Receipt-Verification]
"CCF Receipt Verification", n.d.,
.
[I-D.IETF-cose-merkle-tree-proofs]
"*** BROKEN REFERENCE ***".
[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, .
Appendix A. Attic
Not ready to throw these texts into the trash bin yet.
Authors' Addresses
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Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@sit.fraunhofer.de
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
Amaury Chamayou
Microsoft Research
21 Station Road
Cambridge
CB1 2FB
United Kingdom
Email: amaury.chamayou@microsoft.com
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