TBD O. Steele
Internet-Draft Transmute
Intended status: Standards Track H. Birkholz
Expires: 18 February 2024 Fraunhofer SIT
A. Delignat-Lavaud
C. Fournet
Microsoft
17 August 2023
Concise Encoding of Signed Merkle Tree Proofs
draft-ietf-cose-merkle-tree-proofs-00
Abstract
This specification describes verifiable data structures and
associated proof types for use with COSE. The extensibility of the
approach is demonstrated by providing CBOR encodings for RFC9162.
Status of This Memo
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This Internet-Draft will expire on 18 February 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Verifiable Data Structures in CBOR . . . . . . . . . . . . . 4
3.1. Algorithms Registry . . . . . . . . . . . . . . . . . . . 4
3.1.1. Registration Requirements . . . . . . . . . . . . . . 4
4. Proof Types in CBOR . . . . . . . . . . . . . . . . . . . . . 5
4.1. Proof Types Registry . . . . . . . . . . . . . . . . . . 5
4.2. Inclusion Proof . . . . . . . . . . . . . . . . . . . . . 5
4.3. Consistency Proof . . . . . . . . . . . . . . . . . . . . 6
5. RFC9162_SHA256 as a Verifiable Data Structure . . . . . . . . 6
5.1. Algorithm Definition . . . . . . . . . . . . . . . . . . 6
5.2. Inclusion Proof Definition . . . . . . . . . . . . . . . 6
5.2.1. Inclusion Proof Signature . . . . . . . . . . . . . . 7
5.3. Consistency Proof Definition . . . . . . . . . . . . . . 8
5.3.1. Consistency Proof Signature . . . . . . . . . . . . . 9
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10
6.1. Leaf Blinding . . . . . . . . . . . . . . . . . . . . . . 11
6.1.1. Blinding Example . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7.1. Hash Function Agility . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8.1. Additions to Existing Registries . . . . . . . . . . . . 11
8.1.1. New Entries to the COSE Header Parameters Registry . 11
8.1.2. Verifiable Data Structures . . . . . . . . . . . . . 12
8.1.3. Verifiable Data Structure Proof Types . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Implementation Status . . . . . . . . . . . . . . . 15
A.1. Implementer . . . . . . . . . . . . . . . . . . . . . . . 15
A.2. Implementation Name . . . . . . . . . . . . . . . . . . . 15
A.3. Implementation URL . . . . . . . . . . . . . . . . . . . 15
A.4. Maturity . . . . . . . . . . . . . . . . . . . . . . . . 16
A.5. Coverage and Version Compatibility . . . . . . . . . . . 16
A.6. License . . . . . . . . . . . . . . . . . . . . . . . . . 16
A.7. Implementation Dependencies . . . . . . . . . . . . . . . 16
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A.8. Contact . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
Merkle trees are one of many verifiable data structures that enable
tamper evident secure information storage, through their ability to
protect the integrity of batches of documents or collections of
statements.
Merkle trees can be constructed from simple operations such as
concatenation and digest via a cryptographic hash function, however,
more advanced constructions enable proofs of different properties of
the underlying verifiable data structure.
Verifiable data structure proofs can be used to prove a document is
in a database (proof of inclusion), that a database is append only
(proof of consistency), that a smaller set of statements are
contained in a large set of statements (proof of disclosure, a
special case of proof of inclusion), or proof that certain data is
not yet present in a database (proofs of non inclusion).
Differences in the representation of verifiable data structures, and
verifiable data structure proof types, can increase the burden for
implementers, and create interoperability challenges for transparency
services.
This document describes how to convey verifiable data structures, and
associated proof types in COSE envelopes.
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. Terminology
Verifiable Data Structure: A data structure which supports one or
more Proof Types.
Proof Type: A verifiable process, that proves properties of one or
more Verifiable Data Structures.
Proof Value: An encoding of a Proof Type in CBOR.
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Proof Signature: A COSE Sign1 encoding of a specific Proof Type for
a specific Verifiable Data Structure.
3. Verifiable Data Structures in CBOR
This section describes representations of verifiable data structure
proofs structures in CBOR.
Different verifiable data structures support the same proof types,
but the representations of the proofs varies greatly.
For example, construction of a merkle tree leaf, or an inclusion
proof from a leaf to a merkle root, might have several different
representations, depending on the verifiable data structure used.
Some differences in representations are necessary to support
efficient verification of different kinds of proofs and for
compatibility with specific implementations.
Some proof types benefit from standard envelope formats for signing
and encryption, whilst others require no further cryptographic
intervention at all.
In order to improve interoperability we define two extension points
for enabling verifiable data structures with COSE, and we provide
concrete examples for the structures and proofs defined in [RFC9162].
3.1. Algorithms Registry
This document establishes a registry of verifiable data structure
algorithms, with the following initial contents:
+============+================+===========+
| Identifier | Algorithm | Reference |
+============+================+===========+
| 0 | N/A | |
+------------+----------------+-----------+
| 1 | RFC9162_SHA256 | [RFC9162] |
+------------+----------------+-----------+
Table 1: Verifiable Data Structure
Alogrithms
3.1.1. Registration Requirements
Each specification MUST define how to encode the algorithm and proof
types in CBOR.
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Each specification MUST define how to produce and consume the
supported proof types.
See Section 5 as an example.
4. Proof Types in CBOR
Proof types are specific to their associated "verifiable data
structure", for example, different Merkle trees might support
different representations of "inclusion proof" or "consistency
proof".
Implementers should not expect interoperability accross "verifiable
data structures", but they should expect conceptually similar
properties across registered proof types.
For example, 2 different merkle tree based verifiable data structures
might both support proofs of inclusion. Protocols requiring proof of
inclusion ought to be able to preserve their functionality, while
switching from one verifiable data structure to another, so long as
both structures support the same proof types.
4.1. Proof Types Registry
This document establishes a registry of verifiable data structure
proof types tags, with the following initial contents:
+============+=============+=============+
| Identifier | Proof Type | Reference |
+============+=============+=============+
| 0 | N/A | |
+------------+-------------+-------------+
| TBD_2 | inclusion | Section 4.2 |
+------------+-------------+-------------+
| TBD_3 | consistency | Section 4.3 |
+------------+-------------+-------------+
Table 2: Verifiable Data Structure
Proof Types
Editors note: The registry requirements needs to address the case of
multiple proofs of a given type.
4.2. Inclusion Proof
Inclusion proofs provide a mechanism for a verifier to validate set
membership.
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The integer identifier for this Proof Type is TBD_2. The string
identifier for this Proof Type is "inclusion".
Section 5.2 provides a concrete example.
4.3. Consistency Proof
Consistency proofs provide a mechanism for a verifier to validate the
consistency of a verifiable data structure.
The integer identifier for this Proof Type is TBD_3. The string
identifier for this Proof Type is "consistency".
Section 5.3 provides a concrete example.
5. RFC9162_SHA256 as a Verifiable Data Structure
This section defines how the data structures described in [RFC9162]
are mapped to the terminology defined in this document, using cbor
and cose.
RFC9162_SHA256 requires the following:
* TBD_1 (verifiable-data-structure): 1, the integer representing the
RFC9162_SHA256 verifiable data structure algorithm.
* TBD_2 (inclusion-proof): a bstr representing the RFC9162_SHA256
inclusion proof
* TBD_3 (consistency-proof): a bstr representing the RFC9162_SHA256
consistency proof
5.1. Algorithm Definition
The integer identifier for this Verifiable Data Structure is 1. The
string identifier for this Verifiable Data Structure is
"RFC9162_SHA256".
See Section 3.1.
See [RFC9162], 2.1.1. Definition of the Merkle Tree, for a complete
description of this verifiable data structure.
5.2. Inclusion Proof Definition
See [RFC9162], 2.1.3.1. Generating an Inclusion Proof, for a
complete description of this verifiable data structure proof type.
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The cbor representation of an inclusion proof for RFC9162_SHA256 is:
inclusion-proof = #TBD_2([
tree-size: int
leaf-index: int
inclusion-path: [+ bstr]
])
5.2.1. Inclusion Proof Signature
In a signed inclusion proof, the previous merkle tree root, maps to
tree-size-1, and is a detached payload.
Other specifications refer to signed inclusion proofs as "receipts",
profiles of proof signatures are encouraged to make additional
protected header parameters mandatory.
TODO: reference to scitt receipts.
The protected header for an RFC9162_SHA256 inclusion proof signature
is:
* alg (label: 1): REQUIRED. Signature algorithm identifier. Value
type: int / tstr.
* verifiable-data-structure (label: TBD_1): REQUIRED. verifiable
data structure algorithm identifier. Value type: int / tstr.
* crit (label: 2): OPTIONAL. Criticality marker. Value type: [
+label ]
Editors note: Recommend removing crit and mandating kid. See issue
21 (https://github.com/ietf-scitt/draft-steele-cose-merkle-tree-
proofs/issues/21).
The unprotected header for an RFC9162_SHA256 inclusion proof
signature is:
* inclusion-proof (label: TBD_2): REQUIRED. proof type identifier.
Value type: bstr.
The payload of an RFC9162_SHA256 inclusion proof signature is:
A previous Merkle tree hash as defined in [RFC9162].
The payload MUST be detached.
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Detaching the payload forces verifiers to recompute the root from the
inclusion proof signature, this protects against implementation
errors where the signature is verified but to root does not match the
inclusion proof.
The following example needs to be converted to proper CDDL:
# COSE_Sign1
18([
# Protected Header
h'a2012604588368747470733a2f2f73636974742e78797a2f75726e3a696574663a706172616d733a7472616e733a696e636c7573696f6e3a726663393136325f7368613235363a303a65343263333764326638306361613464323035353635376534303463386538363838313534346136663264313731356530663564616435643436343833633531',
# {
# "alg" : "ES256",
# 1 : -7,
# "verifiable-data-structure" : "RFC9162_SHA256",
# TBD_1 : 1,
# }
# Unprotected Header
{
# "inclusion-proof" : "h'3133312c322c302c3132392c3231362c36342c38382c33322c3235342c3132382c33392c34392c3131382c312c3230352c38372c3235332c3136312c31332c3136312c38352c3139302c3133322c3234312c3137332c34352c3132372c32302c35302c35342c31332c3134342c33332c3233372c3234382c3132382c32332c3138392c3133352c3932'"
TBD_2 : h'3133312c322c302c3132392c3231362c36342c38382c33322c3235342c3132382c33392c34392c3131382c312c3230352c38372c3235332c3136312c31332c3136312c38352c3139302c3133322c3234312c3137332c34352c3132372c32302c35302c35342c31332c3134342c33332c3233372c3234382c3132382c32332c3138392c3133352c3932'
},
# Detached Payload
# Signature
h'4862c1dced27ceeb1f7a6277d13be127a8969a7171ae000ffa90ef5757b817ca8ee61d57645d1a087251a97f06eb61aec46ecf958e4a0fb94ae37f410c7f77ea'
])
5.3. Consistency Proof Definition
See [RFC9162], 2.1.4.1. Generating a Consistency Proof, for a
complete description of this verifiable data structure proof type.
The cbor representation of a consistency proof for RFC9162_SHA256 is:
consistency-proof = #TBD_3([
tree-size-1: int ; size of the tree, when the previous root was produced.
tree-size-2: int ; size of the tree, when the latest root was produced.
consistency-path: [+ bstr] ; consistency path, from previous root to latest root.
])
Editors note: tree-size-1, could be ommited, if an inclusion-proof is
always present, since the inclusion proof contains, tree-size-1.
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5.3.1. Consistency Proof Signature
In a signed consistency proof, the latest merkle tree root, maps to
tree-size-2, and is an attached payload.
The protected header for an RFC9162_SHA256 consistency proof
signature is:
* alg (label: 1): REQUIRED. Signature algorithm identifier. Value
type: int / tstr.
* verifiable-data-structure (label: TBD_1): REQUIRED. verifiable
data structure algorithm identifier. Value type: int / tstr.
* crit (label: 2): OPTIONAL. Criticality marker. Value type: [
+label ]
Editors note: Recommend removing crit and mandating kid. See issue
21 (https://github.com/ietf-scitt/draft-steele-cose-merkle-tree-
proofs/issues/21).
The unprotected header for an RFC9162_SHA256 consistency proof
signature is:
* consistency-proof (label: TBD_2): REQUIRED. proof type identifier.
Value type: bstr.
The payload of an RFC9162_SHA256 consistency proof signature is:
The latest Merkle tree hash as defined in [RFC9162].
The payload MUST be attached.
The following example needs to be converted to proper CDDL:
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# COSE_Sign1
18([
# Protected Header
h'a2012604588568747470733a2f2f73636974742e78797a2f75726e3a696574663a706172616d733a7472616e733a636f6e73697374656e63793a726663393136325f7368613235363a303a66653830323733313736303163643537666461313064613135356265383466316164326437663134333233363064393032316564663838303137626438373563',
# {
# "alg" : "ES256",
# 1 : -7,
# "verifiable-data-structure" : "RFC9162_SHA256",
# TBD_1 : 1,
# }
# Unprotected Header
{
# "consistency-proof" : "h'3133312c312c312c3132392c3231362c36342c38382c33322c3235342c3132382c33392c34392c3131382c312c3230352c38372c3235332c3136312c31332c3136312c38352c3139302c3133322c3234312c3137332c34352c3132372c32302c35302c35342c31332c3134342c33332c3233372c3234382c3132382c32332c3138392c3133352c3932'"
TBD_3 : h'3133312c312c312c3132392c3231362c36342c38382c33322c3235342c3132382c33392c34392c3131382c312c3230352c38372c3235332c3136312c31332c3136312c38352c3139302c3133322c3234312c3137332c34352c3132372c32302c35302c35342c31332c3134342c33332c3233372c3234382c3132382c32332c3138392c3133352c3932'
},
# Protected Payload
h'fe8027317601cd57fda10da155be84f1ad2d7f1432360d9021edf88017bd875c',
# Signature
h'fe476fcddb783805fe344fc96837f4a5531c2e5a56d6f6353831e84e17ac69d4407a5a0d6eadf27f3a570bcf604181fd11b4692d3ac17b116c6226ba43726113'
])
6. Privacy Considerations
See the privacy considerations section of:
* [RFC9162]
* [RFC9053]
Although the word transparency implies to some degree read access, it
is important to note that transparency logs might include sensitive
information.
Depending on the verifiable data structure used, a service provider
might be able to count unique entries.
In the case that an entry is produced from a cose sign 1 envelope,
adding information to the unprotected header can be used to produce a
unique entry.
However, this could impact privacy, and some transparency service
operators might prefer only integrity protected content be made
transparent.
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6.1. Leaf Blinding
In cases where a single merkle root and multiple inclusion paths are
used to prove inclusion for multiple payloads. There is a risk that
an attacker may be able to learn the content of undisclosed payloads,
by brute forcing the values adjacent to the disclosed payloads
through application of the cryptographic hash function and comparison
to the the disclosed inclusion paths. This kind of attack can be
mitigated by including a cryptographic nonce in the construction of
the leaf, however this nonce must then disclosed along side an
inclusion proof which increases the size of multiple payload signed
inclusion proofs.
Tree algorithm designers are encouraged to comment on this property
of their leaf construction algorithm.
6.1.1. Blinding Example
Implementers wishing to leverage multiple inclusion proofs to support
selective disclosure, can prepend each payload with extra data before
computing the inclusion proof, where extra data is a cryptographic
nonce.
7. Security Considerations
See the security considerations section of:
* [RFC9162]
* [RFC9053]
7.1. Hash Function Agility
The choice of cryptographic hash function is the primary primitive
impacting the security of authenticating payload inclusion in a
merkle root. Tree algorithm designers should review the latest
guidance on selecting a suitable cryptographic hash function.
8. IANA Considerations
8.1. Additions to Existing Registries
8.1.1. New Entries to the COSE Header Parameters Registry
This document requests IANA to add new values to the 'COSE
Algorithms' and to the 'COSE Header Algorithm Parameters' registries
in the 'Standards Action With Expert Review category.
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8.1.1.1. COSE Header Algorithm Parameters
* Name: verifiable-data-structure
* Label: TBD_1 (requested assignment 12)
* Value type: int / tstr
* Value registry: See Table 1
* Description: Tag indicating the Verifiable Data Structure, see
Section 3.
Editors note: Authors are discussing how to avoid flooding the cose
header parameters registry with new proof types.
* Name: inclusion-proof
* Label: TBD_2 (requested assignment 13)
* Value type: bstr
* Value registry: See Table 2
* Description: Tag indicating the "inclusion" Proof Type, see
Section 4.2.
* Name: consistency-proof
* Label: TBD_2 (requested assignment 14)
* Value type: bstr
* Value registry: See Table 2
* Description: Tag indicating the "consistency" Proof Type, see
Section 4.3.
8.1.2. Verifiable Data Structures
IANA will be asked to establish a registry of tree algorithm
identifiers, named "Verifiable Data Structures" to be administered
under a Specification Required policy [RFC8126].
Template:
* Identifier: The two-byte identifier for the algorithm
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* Algorithm: The name of the data structure
* Reference: Where the data structure is defined
Initial contents: Provided in Table 1
8.1.3. Verifiable Data Structure Proof Types
IANA will be asked to establish a registry of tree algorithm
identifiers, named "Verifiable Data Structures Proof Types" to be
administered under a Specification Required policy [RFC8126].
Template:
* Identifier: The two-byte identifier for the algorithm
* Algorithm: The name of the proof type algorithm
* Reference: Where the algorithm is defined
Initial contents: Provided in Table 2
9. References
9.1. Normative References
[BCP205] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
.
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[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, .
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, .
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
.
[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, .
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, .
9.2. Informative References
[I-D.ietf-cose-countersign]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Countersignatures", Work in Progress, Internet-Draft,
draft-ietf-cose-countersign-10, 20 September 2022,
.
[I-D.ietf-scitt-architecture]
Birkholz, H., Delignat-Lavaud, A., Fournet, C., Deshpande,
Y., and S. Lasker, "An Architecture for Trustworthy and
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Transparent Digital Supply Chains", Work in Progress,
Internet-Draft, draft-ietf-scitt-architecture-02, 10 July
2023, .
Appendix A. Implementation Status
Note to RFC Editor: Please remove this section as well as references
to [BCP205] before AUTH48.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [BCP205].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [BCP205], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
A.1. Implementer
An open-source implementation was initiated and is maintained by the
Transmute Industries Inc. - Transmute.
A.2. Implementation Name
An application demonstrating the concepts is available at
https://scitt.xyz (https://scitt.xyz).
A.3. Implementation URL
An open-source implementation is available at:
* https://github.com/transmute-industries/cose
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A.4. Maturity
The code's level of maturity is considered to be "prototype".
A.5. Coverage and Version Compatibility
The current version ('main') implements the tree algorithm, inclusion
proof and consistency proof concepts of this draft.
A.6. License
The project and all corresponding code and data maintained on GitHub
are provided under the Apache License, version 2.
A.7. Implementation Dependencies
The implementation builds on concepts described in SCITT
[I-D.ietf-scitt-architecture] (https://scitt.io/).
The implementation uses the Concise Binary Object Representation
[RFC7049] (https://cbor.io/).
The implementation uses the CBOR Object Signing and Encryption
[RFC9053], maintained at: - https://github.com/erdtman/cose-js
The implementation uses an implementation of [RFC9162], maintained
at:
* https://github.com/transmute-industries/rfc9162/tree/main/src/
CoMETRE
A.8. Contact
Orie Steele (orie@transmute.industries)
Authors' Addresses
Orie Steele
Transmute
United States
Email: orie@transmute.industries
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
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Email: henk.birkholz@sit.fraunhofer.de
Antoine Delignat-Lavaud
Microsoft
United Kingdom
Email: antdl@microsoft.com
Cedric Fournet
Microsoft
United Kingdom
Email: fournet@microsoft.com
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