TBD H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track A. Delignat-Lavaud
Expires: 11 January 2024 C. Fournet
Microsoft Research
10 July 2023
A CoMETRE Profile and Tree Algorithm for the Confidential Consortium
Framework
draft-birkholz-cose-cometre-ccf-profile-00
Abstract
This document defines a new verifiable data structure type for COSE
Signed Merkle Tree Proofs specifically designed for implementations
that rely on Trusted Execution Environments (TEEs) to provide
stronger tamper-evidence guarantees.
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. Description of the CCF Ledger Verifiable Data Structure . . . 3
2.1. Tree Shape . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Leaf Components . . . . . . . . . . . . . . . . . . . . . 4
3. CCF Inclusion Proofs . . . . . . . . . . . . . . . . . . . . 4
3.1. CCF Inclusion Proof Signature . . . . . . . . . . . . . . 5
3.2. Inclusion Proof Verification Algorithm . . . . . . . . . 5
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6.1. Additions to Existing Registries . . . . . . . . . . . . 6
6.1.1. COSE Header Parameters registry . . . . . . . . . . . 6
6.1.2. Tree Algorithms . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Attic . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The Concise Encoding of Signed Merkle Tree Proofs (CoMeTre)
[I-D.steele-cose-merkle-tree-proofs] defines a common framework for
defining different types of proofs, such as proof of inclusion, 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.
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In this document, we define a new type of log, associated with the
Confidential Consortium Framework (CCF) ledger. 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.
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.steele-cose-merkle-tree-proofs] with the following value:
+============+============+===============+
| Identifier | Algorithm | Reference |
+============+============+===============+
| TBD_1 | CCF_LEDGER | This document |
+------------+------------+---------------+
Table 1: Verifiable Data Structure
Algorithms
2.1. Tree Shape
The input of the Merkle Tree Hash (MTH) function is a list of n byte
strings, written D_n = {d[0], d[1], ..., d[n-1]}. The output is a
single HASH_SIZE byte string, also called the Merkle root hash.
This function is defined as follows:
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. Leaf Components
Each leaf 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: [+ proof-element],
proof-element = [
left: bool
hash: bstr
]
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).
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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).
* proof-type: int. This header MUST be set to the value of the
inclusion proof type in the IANA registry of Verifiable Data
Structure Proof Type.
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.
* leaf (label TBD_2): bstr .cbor CCF-leaf. This contains the CCF-
specific serialization of the leaf element
The payload of the signature is the CCF ledger Markle 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.
3.2. Inclusion Proof Verification Algorithm
CCF uses the following algorithm to recompute the payload of the
signature based on the inclusion-proof header:
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compute_root(leaf, proof):
h := leaf.internal-hash
|| HASH(leaf.internal-data)
|| leaf.data-hash
for [left, hash] in proof:
h := HASH(hash + h) if left
HASH(h + hash) else
return h
verify_inclusion_proof(signed_proof):
leaf := signed_proof.unprotected_headers[LEAF_LABEL] or fail
proof := signed_proof.unprotected_headers[INCLUSION_PROOF_LABEL] or fail
payload := compute_root(leaf, proof)
return verif_cose_detached(signed_proof, payload)
4. Privacy Considerations
Privacy Considerations
5. Security Considerations
Security Considerations
6. IANA Considerations
6.1. Additions to Existing Registries
6.1.1. COSE Header Parameters registry
This document requests IANA to add the following new value to the
'COSE Header Parameters' registry:
* Label: TBD_2
* Value type: bstr
* Reference: This document
6.1.2. Tree Algorithms
This document requests IANA to add the following new value to the
'Tree Algorithms' registry:
* Identifier: TBD_1 (requested assignment 2)
* Tree Algorithm: ccf-ledger
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* 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.steele-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-steele-cose-
merkle-tree-proofs-01, 10 July 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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