COSE O. Steele
Internet-Draft Transmute
Intended status: Standards Track H. Birkholz
Expires: 25 August 2025 Fraunhofer SIT
A. Delignat-Lavaud
C. Fournet
Microsoft
21 February 2025
COSE Receipts
draft-ietf-cose-merkle-tree-proofs-08
Abstract
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 RFC9162.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 25 August 2025.
Copyright Notice
Copyright (c) 2025 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
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. New COSE Header Parameters . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Verifiable Data Structures in CBOR . . . . . . . . . . . . . 5
4.1. Structures . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Parameters . . . . . . . . . . . . . . . . . . . . . . . 6
4.3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3.1. Registration Requirements . . . . . . . . . . . . . . 10
5. RFC9162_SHA256 . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Verifiable Data Structure . . . . . . . . . . . . . . . . 11
5.2. Inclusion Proof . . . . . . . . . . . . . . . . . . . . . 11
5.2.1. Receipt of Inclusion . . . . . . . . . . . . . . . . 11
5.3. Consistency Proof . . . . . . . . . . . . . . . . . . . . 13
5.3.1. Receipt of Consistency . . . . . . . . . . . . . . . 14
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 16
6.1. Log Length . . . . . . . . . . . . . . . . . . . . . . . 17
6.2. Header Parameters . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7.1. Choice of Signature Algorithms . . . . . . . . . . . . . 17
7.2. Validity Period . . . . . . . . . . . . . . . . . . . . . 17
7.3. Status Updates . . . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9.1. COSE Header Parameter . . . . . . . . . . . . . . . . . . 18
9.2. COSE Verifiable Data Structures . . . . . . . . . . . . . 19
9.2.1. Expert Review . . . . . . . . . . . . . . . . . . . . 19
9.3. COSE Verifiable Data Structure Parameters . . . . . . . . 20
9.3.1. Expert Review . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Implementation Status . . . . . . . . . . . . . . . 23
A.1. Transmute Prototype . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
COSE Receipts are signed proofs that include metadata about about
certain states of a verifiable data structure (VDS) that are true
when the COSE Receipt was issued. COSE Receipts can include proves
that 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). Different VDS can produce different verifiable data
structure proofs (VDP). The combination of representations of
various VDS and VDP can significantly increase burden for
implementers and create interoperability challenges for transparency
services. This document describes how to convey VDS and associated
VDP types in unified 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. New COSE Header Parameters
This document defines three new COSE header parameters, which are
introduced up-front in this Section and elaborated on later in this
document.
TBD_0 (requested assignment 394): A COSE header parameter named
receipts with a value type of array where the array contains one
ore more COSE Receipts as specified in this document.
TBD_1 (requested assignment 395): A COSE header parameter named vds
with a value type of integer where the integer is a verifiable
data structure (VDS) algorithm identifier as specified in this
document. Correspondingly, this document introduces a new
verifiable data structure algorithm identifier registry that
registers the integers used as values for this COSE header
parameter.
TBD_2 (requested assignment 396): A COSE header parameter named vdp
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with a value type of map where the map contains verifiable data
structure proofs (VDP) as specified in this this document.
Correspondingly, this document introduces a new verifiable data
structure proof registry that registers the integers that are used
as labels in the map of this COSE header parameter.
The other codepoints in this document are assigned from the
registries established in this draft, they are therefore not marked
TBD.
3. Terminology
CDDL: Concise Data Definition Language (CDDL) is defined in
[RFC8610].
EDN: CBOR Extended Diagnostic Notation (EDN) is defined in
[RFC8949], where it is referred to as "diagnostic notation", and
is revised in [I-D.draft-ietf-cbor-edn-literals].
Verifiable Data Structure (VDS): A data structure which supports one
or more Proof Types. This property is conceptually similar to
"alg" (1), it described an algorithm used to maintain the
verifiable data structure, for example a binary merkle tree
algorithm.
Verifiable Data Structure Parameters (VDP): Parameters to a
verifiable data structure that are used to prove properties, such
as authentication, inclusion, consistency, and freshness.
Parameters can include multiple proofs of a given type, or
multiple types of proof (inclusion and consistency). This
property is conceptually similar to COSE Header Parameter "epk"
(-1) or CBOR Web Token (CWT) claim "cnf" (8), it is applied to a
verifiable data structure, to confirm a property. For example an
encrypted message might be decrypted using epk and a private key,
a digital signature for authentication might be verified using cnf
and the (CWT) claim "nonce" and "audience", and an inclusion proof
for a binary merkle tree might be verified with VDP and some entry
that is being tested or inclusion in the tree.
Proof Type: A verifiable process, that proves properties of a
Verifiable Data Structure. For example, a VDS, such as a binary
merkle tree, can support multiple proofs of type "inclusion" where
each proof confirms that a given entry is included in a merkle
root.
Proof Value: An encoding of a Proof Type in CBOR [RFC7049].
Entry: An entry in a verifiable data structure for which proofs can
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be derived.
Receipt: A COSE object, as defined in [RFC9052], containing the
header parameters necessary to convey VDP for an associated VDS.
4. Verifiable Data Structures in CBOR
This section describes representations of verifiable data structure
proofs in [RFC7049]. 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. Differences in representations are necessary to
support efficient verification, unique security or privacy
properties, and for compatibility with specific implementations.
This document defines two extension points for enabling verifiable
data structures with COSE and provides concrete examples for the
structures and proofs defined in [RFC9162]. The design of these
structures is influenced by the conventions established for COSE
Keys.
During testing and development the experimental range SHOULD be used,
unless early assignment for a provisional entry has been completed.
4.1. Structures
Similar to COSE Key Types (https://www.iana.org/assignments/cose/
cose.xhtml#key-type), different verifiable data structures support
different algorithms. As EC2 keys (1: 2) support both digital
signature and key agreement algorithms, RFC9162_SHA256 (TBD_1 : 1)
supports both inclusion and consistency proofs.
This document establishes a registry of verifiable data structure
algorithms, with the following initial contents:
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+================+=======+===========================+===========+
| Name | Value | Description | Reference |
+================+=======+===========================+===========+
| N/A | 0 | N/A | N/A |
+----------------+-------+---------------------------+-----------+
| RFC9162_SHA256 | 1 | SHA256 Binary Merkle Tree | [RFC9162] |
+----------------+-------+---------------------------+-----------+
| EXPERIMENTAL | 11 | Unknown | RFC XXXX |
+----------------+-------+---------------------------+-----------+
| EXPERIMENTAL | 22 | Unknown | RFC XXXX |
+----------------+-------+---------------------------+-----------+
| EXPERIMENTAL | 33 | Unknown | RFC XXXX |
+----------------+-------+---------------------------+-----------+
Table 1: COSE Verifiable Data Structures
When designing new verifiable data structures, please request the
next available positive integer as your requested assignment, for
example:
+================+================+===============+===============+
| Name | Value | Description | Reference |
+================+================+===============+===============+
| N/A | 0 | N/A | N/A |
+----------------+----------------+---------------+---------------+
| RFC9162_SHA256 | 1 | SHA256 Binary | [RFC9162] |
| | | Merkle Tree | |
+----------------+----------------+---------------+---------------+
| Your name | TBD (requested | tbd | Your |
| | assignment 2) | | specification |
+----------------+----------------+---------------+---------------+
Table 2: How to register new structures
4.2. Parameters
Similar to COSE Key Type Parameters
(https://www.iana.org/assignments/cose/cose.xhtml#key-type-
parameters), as EC2 keys (1: 2) keys require and give meaning to
specific parameters, such as -1 (crv), -2 (x), -3 (y), -4 (d),
RFC9162_SHA256 (TBD_1 : 1) supports both (-1) inclusion and (-2)
consistency proofs.
This document establishes a registry of verifiable data structure
algorithms, with the following initial contents:
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+============+=============+=====+=======+=============+===========+
| Verifiable | Name |Label| CBOR | Description | Reference |
| Data | | | Type | | |
| Structure | | | | | |
+============+=============+=====+=======+=============+===========+
| 1 | inclusion |-1 | array | Proof of | Section |
| | proofs | | (of | inclusion | 5.2 |
| | | | bstr) | | |
+------------+-------------+-----+-------+-------------+-----------+
| 1 | consistency |-2 | array | Proof of | Section |
| | proofs | | (of | append only | 5.3 |
| | | | bstr) | property | |
+------------+-------------+-----+-------+-------------+-----------+
| 11 | unknown |-1 | array | Unknown | RFC XXXX |
| | | | (of | | |
| | | | bstr) | | |
+------------+-------------+-----+-------+-------------+-----------+
| 22 | unknown |-1 | array | Unknown | RFC XXXX |
| | | | (of | | |
| | | | bstr) | | |
+------------+-------------+-----+-------+-------------+-----------+
| 33 | unknown |-1 | array | Unknown | RFC XXXX |
| | | | (of | | |
| | | | bstr) | | |
+------------+-------------+-----+-------+-------------+-----------+
Table 3: COSE Verifiable Data Structure Parameters
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 across
"verifiable data structures", but they should expect conceptually
similar properties across the different registered proof types. For
example, 2 different merkle tree based verifiable data structures
might both support proofs of inclusion. Security analysis SHOULD be
conducted prior to migrating to new structures to ensure the new
security and privacy assumptions are acceptable for the use case.
When designing new verifiable data structure parameters (or proof
types), please start with -1, and count down for each proof type
supported by your verifiable data structure:
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+==========+===========+=====+=====+===========+==================+
|Verifiable|Name |Label|CBOR |Description|Reference |
|Data | | |Type | | |
|Structure | | | | | |
+==========+===========+=====+=====+===========+==================+
|1 |inclusion |-1 |array|Proof of |Section 5.2 |
| |proofs | |(of |inclusion | |
| | | |bstr)| | |
+----------+-----------+-----+-----+-----------+------------------+
|1 |consistency|-2 |array|Proof of |Section 5.3 |
| |proofs | |(of |append only| |
| | | |bstr)|property | |
+----------+-----------+-----+-----+-----------+------------------+
|TBD |new proof |-1 |tbd |tbd |Your_Specification|
|(requested|type | | | | |
|assignment| | | | | |
|2) | | | | | |
+----------+-----------+-----+-----+-----------+------------------+
|TBD |new proof |-2 |tbd |tbd |Your_Specification|
|(requested|type | | | | |
|assignment| | | | | |
|2) | | | | | |
+----------+-----------+-----+-----+-----------+------------------+
|TBD |new proof |-3 |tbd |tbd |Your_Specification|
|(requested|type | | | | |
|assignment| | | | | |
|2) | | | | | |
+----------+-----------+-----+-----+-----------+------------------+
Table 4: How to register new parameters
4.3. Usage
This document registered a new COSE Header Parameter receipts (394)
to enable this Receipts to be conveyed in the protected and
unprotected headers of COSE Objects.
When the receipts header parameter is present, the associated
verifiable data structure and verifiable data structure proofs MUST
match entries present in the registries established in this
specification.
The following informative CDDL is provided:
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Receipt = #6.18(COSE_Sign1)
Protected_Header = {
* cose-label => cose-value
}
Unprotected_Header = {
&(receipts: 394) => [+ Receipt]
* cose-label => cose-value
}
COSE_Sign1 = [
protected : bstr .cbor Protected_Header,
unprotected : Unprotected_Header,
payload : bstr / nil,
signature : bstr
]
Figure 1: CDDL for a COSE Sign1 with attached receipts
The following informative EDN is provided:
/ cose-sign1 / 18([
/ protected / <<{
/ key / 4 : "vCl7UcS0ZZY99VpRthDc-0iUjLdfLtnmFqLJ2-Tt8N4",
/ algorithm / 1 : -7, # ES256
}>>,
/ unprotected / {
/ receipts / 394 : {
<</ cose-sign1 / 18([
/ protected / <<{
/ key / 4 : "mxA4KiOkQFZ-dkLebSo3mLOEPR7rN8XtxkJe45xuyJk",
/ algorithm / 1 : -7, # ES256
/ vds / 395 : 1, # RFC9162 SHA-256
}>>,
/ unprotected / {
/ proofs / 396 : {
/ inclusion / -1 : [
<<[
/ size / 9, / leaf / 8,
/ inclusion path /
h'7558a95f...e02e35d6'
]>>
],
},
},
/ payload / null,
/ signature / h'02d227ed...ccd3774f'
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])>>,
<</ cose-sign1 / 18([
/ protected / <<{
/ key / 4 : "ajOkeBTJou_wPrlExLMw7L9OTCD5ZIOBYc-O6LESe9c",
/ algorithm / 1 : -7, # ES256
/ vds / 395 : 1, # RFC9162 SHA-256
}>>,
/ unprotected / {
/ proofs / 396 : {
/ inclusion / -1 : [
<<[
/ size / 6, / leaf / 5,
/ inclusion path /
h'9352f974...4ffa7ce0',
h'54806f32...f007ea06'
]>>
],
},
},
/ payload / null,
/ signature / h'36581f38...a5581960'
])>>
},
},
/ payload / h'0167c57c...deeed6d4',
/ signature / h'2544f2ed...5840893b'
])
Figure 2: A COSE Signature with multiple receipts
4.3.1. Registration Requirements
Each specification MUST define how to encode the verifiable data
structure and its parameters (also called proof types) in CBOR. Each
specification MUST define how to produce and consume the supported
proof types. See Section 5 as an example.
Where a specification supports a choice of hash algorithm, an IANA
registration must be made for each individually supported algorithm.
For example, to provide for both SHA256 and SHA3_256 with [RFC9162],
both "RFC9162_SHA256" and "RFC9162_SHA3_256" require entries in the
relevant IANA registries.
5. RFC9162_SHA256
This section defines how the data structures described in [RFC9162]
are mapped to the terminology defined in this document, using
[RFC7049] and [RFC9053].
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5.1. Verifiable Data Structure
The integer identifier for this Verifiable Data Structure is 1. The
string identifier for this Verifiable Data Structure is
"RFC9162_SHA256". See Table 1. See [RFC9162], 2.1.1. Definition of
the Merkle Tree, for a complete description of this verifiable data
structure.
5.2. Inclusion Proof
See [RFC9162], 2.1.3.1. Generating an Inclusion Proof, for a
complete description of this verifiable data structure proof type.
The CBOR representation of an inclusion proof for RFC9162_SHA256 is:
inclusion-proof = bstr .cbor [
; tree size at current merkle root
tree-size: uint
; index of leaf in tree
leaf-index: uint
; path from leaf to current merkle root
inclusion-path: [ + bstr ]
]
Figure 3: CBOR Encoded RFC9162 Inclusion Proof
The term leaf-index is used for alignment with the use established in
[RFC9162]
Note that [RFC9162] defines that verification MUST fail if leaf-index
is >= tree-size, and inclusion proofs are defined only for leaf
nodes. The identifying index of a leaf node is relative to all nodes
in the tree size for which the proof was obtained.
5.2.1. Receipt of Inclusion
In a signed inclusion proof, the payload is the merkle tree root
which corresponds to the log at size tree-size. Specifications are
encouraged to make payloads detached when possible, forcing
validation-time comparison. Profiles of proof signatures are
encouraged to make additional protected header parameters mandatory,
to ensure that claims are processed with their intended semantics.
One way to include this information in the COSE structure is use of
the typ (type) Header Parameter, see
[I-D.ietf-cose-typ-header-parameter] and the similar guidance
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provided in [I-D.ietf-cose-cwt-claims-in-headers]. The protected
header for an RFC9162_SHA256 inclusion proof signature is:
protected-header-map = {
&(alg: 1) => int
&(vds: 395) => int
* cose-label => cose-value
}
Figure 4: Protected Header for a Receipt of Inclusion
* 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 RFC9162_SHA256 inclusion proof
signature is:
inclusion-proofs = [ + inclusion-proof ]
verifiable-proofs = {
&(inclusion-proof: -1) => inclusion-proofs
}
unprotected-header-map = {
&(vdp: 396) => verifiable-proofs
* cose-label => cose-value
}
Figure 5: A Verifiable Data Structure Proofs in an Unprotected Header
* vdp (label: 396): REQUIRED. Verifiable data structure proofs.
Value type: Map.
* inclusion-proof (label: -1): REQUIRED. Inclusion proofs. Value
type: Array of bstr.
The payload of an RFC9162_SHA256 inclusion proof signature is the
Merkle tree hash as defined in [RFC9162]. The payload SHOULD 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 merkle root does not
match the inclusion proof. The EDN for a Receipt containing an
inclusion proof for RFC9162_SHA256 is:
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/ cose-sign1 / 18([
/ protected / <<{
/ algorithm / 1 : -7, # ES256
/ vds / 395 : 1, # RFC9162 SHA-256
}>>,
/ unprotected / {
/ proofs / 396 : {
/ inclusion / -1 : [
<<[
/ size / 20, / leaf / 17,
/ inclusion path /
h'fc9f050f...221c92cb',
h'bd0136ad...6b28cf21',
h'd68af9d6...93b1632b'
]>>
],
},
},
/ payload / null,
/ signature / h'de24f0cc...9a5ade89'
])
Figure 6: Receipt of Inclusion
The VDS in the protected header is necessary to understand the
inclusion proof structure in the unprotected header.
The inclusion proof and signature are verified in order. First the
verifiers applies the inclusion proof to a possible entry (set
member) bytes. If this process fails, the inclusion proof may have
been tampered with. If this process succeeds, the result is a merkle
root, which in the attached as the COSE Sign1 payload. 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.
5.3. Consistency Proof
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:
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consistency-proof = bstr .cbor [
; older merkle root tree size
tree-size-1: uint
; newer merkle root tree size
tree-size-2: uint
; path from older merkle root to newer merkle root.
consistency-path: [ + bstr ]
]
Figure 7: CBOR Encoded RFC9162 Consistency Proof
5.3.1. Receipt of Consistency
In a signed consistency proof, the newer merkle tree root (proven to
be consistent with an older merkle tree root) is an attached payload
and corresponds to the log at size tree-size-2.
The protected header for an RFC9162_SHA256 consistency proof
signature is:
protected-header-map = {
&(alg: 1) => int
&(vds: 395) => int
* cose-label => cose-value
}
Figure 8: Protected Header for a Receipt of Consistency
* alg (label: 1): REQUIRED. Signature algorithm identifier. Value
type: int.
* vds (label: TBD_1): REQUIRED. Verifiable data structure algorithm
identifier. Value type: int.
The unprotected header for an RFC9162_SHA256 consistency proof
signature is:
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consistency-proofs = [ + consistency-proof ]
verifiable-proofs = {
&(consistency-proof: -2) => consistency-proofs
}
unprotected-header-map = {
&(vdp: 396) => verifiable-proofs
* cose-label => cose-value
}
* vdp (label: 396): REQUIRED. Verifiable data structure proofs.
Value type: Map.
* consistency-proof (label: -2): REQUIRED. Consistency proofs.
Value type: Array of bstr.
The payload of an RFC9162_SHA256 consistency proof signature is: The
newer Merkle tree hash as defined in [RFC9162]. The payload SHOULD
be detached. Detaching the payload forces verifiers to recompute the
root from the consistency proof, this protects against implementation
errors where the signature is verified but the merkle root does not
match the proof.
The EDN for a Receipt containing a consistency proof for
RFC9162_SHA256 is:
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/ cose-sign1 / 18([
/ protected / <<{
/ algorithm / 1 : -7, # ES256
/ vds / 395 : 1, # RFC9162 SHA-256
}>>,
/ unprotected / {
/ proofs / 396 : {
/ consistency / -2 : [
<<[
/ old / 20, / new / 104,
/ consistency path /
h'e5b3e764...c4a813bc',
h'87e8a084...4f529f69',
h'f712f76d...92a0ff36',
h'd68af9d6...93b1632b',
h'249efab6...b7614ccd',
h'85dd6293...38914dc1'
]>>
],
},
},
/ payload / null,
/ signature / h'94469f73...52de67a1'
])
Figure 9: Example consistency receipt
The VDS in the protected header is necessary to understand the
consistency proof structure in the unprotected header.
The signature and consistency proof are verified in order.
First the verifier checks the signature on the COSE Sign1. If the
verification fails, the consistency proof is not checked. Second the
consistency proof is checked by applying a previous inclusion proof,
to the consistency proof. If the verification fails, the append only
property of the verifiable data structure is not assured. This
approach is specific to RFC9162_SHA256, different verifiable data
structures may not support consistency proofs. 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.
6. Privacy Considerations
See the privacy considerations section of:
* [RFC9162]
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* [RFC9053]
6.1. Log Length
Some structures and proofs leak the size of the log at the time of
inclusion. In the case that a log only stores certain kinds of
information, this can reveal details that could impact reputation.
For example, if a transparency log only stored breach notices, a
receipt for a breach notice would reveal the number of previous
breaches at the time the notice was made transparent.
6.2. Header Parameters
Additional header parameters can reveal information about the
transparency service or its log entries. A privacy analysis MUST be
performed for all mandatory fields in profiles based on this
specification.
7. Security Considerations
See the security considerations section of:
* [RFC9162]
* [RFC9053]
7.1. Choice of Signature Algorithms
A security analysis MUST be performed to ensure that the digital
signature algorithm alg has the appropriate strength to secure
receipts.
It is recommended to select signature algorithms that share
cryptographic components with the verifiable data structure used, for
example: Both RFC9162_SHA256 and ES256 depend on the sha-256 hash
function.
7.2. Validity Period
In some cases, receipts MAY include strict validity periods, for
example, activation not too far in the future, or expiration, not too
far in the past. See the iat, nbf, and exp claims in [RFC8392], for
one way to accomplish this. The details of expressing validity
periods are out of scope for this document.
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7.3. Status Updates
In some cases, receipts should be "revocable" or "suspendible", after
being issued, regardless of their validity period. The details of
expressing statuses are out of scope for this document.
8. Acknowledgements
We would like to thank Maik Riechert, Jon Geater, Mike Jones, Mike
Prorock, Ilari Liusvaara, for their contributions (some of which
substantial) to this draft and to the initial set of implementations.
9. IANA Considerations
9.1. COSE Header Parameter
IANA is requested to add the COSE header parameters defined in
Section 2, as listed in Table 5, to the "COSE Header Parameters"
registry [IANA.cose_header-parameters] in the 'Integer values from
256 to 65535' range ('Specification Required' Registration
Procedure). All new entries use
https://www.iana.org/assignments/cose/cose.xhtml#header-parameters as
the value for the "Value Registry" column.
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+==========+=============+=======+======================+===========+
| Name | Label | Value | Description | Reference |
| | | Type | | |
+==========+=============+=======+======================+===========+
| receipts | TBD_0 | array | Priority ordered | RFCthis, |
| | (requested | | sequence of CBOR | Section 2 |
| | assignment: | | encoded Receipts | |
| | 394) | | | |
+----------+-------------+-------+----------------------+-----------+
| vds | TBD_1 | int | Algorithm | RFCthis, |
| | (requested | | identifier for | Section 2 |
| | assignment: | | verifiable data | |
| | 395) | | structure, used | |
| | | | to produce | |
| | | | verifiable data | |
| | | | structure proofs | |
+----------+-------------+-------+----------------------+-----------+
| vdp | TBD_2 | map | Location for | RFCthis, |
| | (requested | | verifiable data | Section 2 |
| | assignment: | | structure proofs | |
| | 396) | | in COSE Header | |
| | | | Parameters | |
+----------+-------------+-------+----------------------+-----------+
Table 5: Newly registered COSE Header Parameters
9.2. COSE Verifiable Data Structures
IANA will be asked to establish a registry of verifiable data
structure identifiers, named "COSE Verifiable Data Structures" to be
administered under a Specification Required policy [RFC8126].
Template:
* Name: The name of the verifiable data structure
* Value: The identifier for the verifiable data structure
* Description: A brief description of the verifiable data structure
* Reference: Where the verifiable data structure is defined
Initial contents: Provided in Table 1
9.2.1. Expert Review
This IANA registries is established under a Specification Required
policy.
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This section gives some general guidelines for what the experts
should be looking for, but they are being designated as experts for a
reason, so they should be given substantial latitude.
Expert reviewers should take into consideration the following points:
* Point squatting should be discouraged. Reviewers are encouraged
to get sufficient information for registration requests to ensure
that the usage is not going to duplicate one that is already
registered, and that the point is likely to be used in
deployments.
* Specifications are required for all point assignments. Early
Allocation is permissible, see Section 2 of [RFC7120].
Provisional assignments to expired drafts MUST be removed from the
registry.
* Points assigned in this registry MUST have references that match
the COSE Verifiable Data Structure Parameters registry. It is not
permissible to assign points in this registry, for which no
Verifiable Data Structure Parameters entries exist.
9.3. COSE Verifiable Data Structure Parameters
IANA will be asked to establish a registry of verifiable data
structure parameters, named "COSE Verifiable Data Structure
Parameters" to be administered under a Specification Required policy
[RFC8126].
Template:
* Verifiable Data Structure: The identifier for the verifiable data
structure
* Name: The name of the proof type
* Label: The integer of the proof type
* CBOR Type: The cbor data type of the proof
* Description: The description of the proof type
* Reference: Where the proof type is defined
Initial contents: Provided in Table 3
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9.3.1. Expert Review
This IANA registries is established under a Specification Required
policy.
This section gives some general guidelines for what the experts
should be looking for, but they are being designated as experts for a
reason, so they should be given substantial latitude.
Expert reviewers should take into consideration the following points:
* Point squatting should be discouraged. Reviewers are encouraged
to get sufficient information for registration requests to ensure
that the usage is not going to duplicate one that is already
registered, and that the point is likely to be used in
deployments.
* Specifications are required for all point assignments. Early
Allocation is permissible, see Section 2 of [RFC7120].
Provisional assignments to expired drafts MUST be removed from the
registry.
* Points assigned in this registry MUST have references that match
the COSE Verifiable Data Structures registry. It is not
permissible to assign points in this registry, for which no
Verifiable Data Structure entry exists.
10. References
10.1. Normative References
[IANA.cose_header-parameters]
IANA, "COSE Header Parameters",
<https://www.iana.org/assignments/cose>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://doi.org/10.17487/RFC2119>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://doi.org/10.17487/RFC7049>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://doi.org/10.17487/RFC8174>.
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[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://doi.org/10.17487/RFC9053>.
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, <https://doi.org/10.17487/RFC9162>.
10.2. Informative 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,
<https://doi.org/10.17487/RFC7942>.
[I-D.draft-ietf-cbor-edn-literals]
Bormann, C., "CBOR Extended Diagnostic Notation (EDN)",
Work in Progress, Internet-Draft, draft-ietf-cbor-edn-
literals-16, 8 January 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
edn-literals-16>.
[I-D.ietf-cose-cwt-claims-in-headers]
Looker, T. and M. B. Jones, "CBOR Web Token (CWT) Claims
in COSE Headers", Work in Progress, Internet-Draft, draft-
ietf-cose-cwt-claims-in-headers-10, 29 November 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
cwt-claims-in-headers-10>.
[I-D.ietf-cose-typ-header-parameter]
Jones, M. B. and O. Steele, "COSE "typ" (type) Header
Parameter", Work in Progress, Internet-Draft, draft-ietf-
cose-typ-header-parameter-05, 3 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
typ-header-parameter-05>.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://doi.org/10.17487/RFC7120>.
[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,
<https://doi.org/10.17487/RFC8126>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://doi.org/10.17487/RFC8392>.
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[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://doi.org/10.17487/RFC8610>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://doi.org/10.17487/RFC8949>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://doi.org/10.17487/RFC9052>.
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. Transmute Prototype
An open-source implementation was initiated and is maintained by the
Transmute Industries Inc. - Transmute. An application demonstrating
the concepts is available at COSE SCITT Receipts (https://github.com/
transmute-industries/cose?tab=readme-ov-file#transparent-statement)
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Implementation URL: https://github.com/transmute-industries/cose
Maturity: The code's level of maturity is considered to be
"prototype". Coverage and Version Compatibility: The current version
('main') implements the verifiable data structure algorithm,
inclusion proof and consistency proof concepts of this draft.
License: The project and all corresponding code and data maintained
on GitHub are provided under the Apache License, version 2. 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
Email: henk.birkholz@ietf.contact
Antoine Delignat-Lavaud
Microsoft
United Kingdom
Email: antdl@microsoft.com
Cedric Fournet
Microsoft
United Kingdom
Email: fournet@microsoft.com
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