]> CBOR Object Signing and Encryption (COSE) Receipts Tradeverifyd
United States of America orie@or13.io
Fraunhofer SIT
Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@ietf.contact
Microsoft
United Kingdom antdl@microsoft.com
Microsoft
United Kingdom fournet@microsoft.com
SEC cose COSE CBOR Object Signing and Encryption (COSE) Receipts prove properties of a Verifiable Data Structure (VDS) to a verifier. VDSs 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 Concise Binary Object Representation (CBOR) and COSE. The extensibility of the approach is demonstrated by providing CBOR encodings for Merkle inclusion and consistency proofs.
Introduction COSE Receipts are signed proofs that include metadata about certain states of a Verifiable Data Structure (VDS) that are true when the COSE Receipt was issued. COSE Receipts can include proofs 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 that certain data is not yet present in a database (proof of non-inclusion). Different VDSs can produce different Verifiable Data Structure Proofs (VDPs). The combination of representations of various VDSs and VDP can significantly increase the 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.
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 when, and only when, they appear in all capitals, as shown here.
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.
394:
A COSE header parameter named "receipts" with a value type of array where the array contains one or more COSE Receipts as specified in this document.
395:
A COSE header parameter named "vds" (for Verifiable Data Structure), which conveys the algorithm identifier for a VDS. Correspondingly, see for a registry defining the integers used to identify VDSs.
396:
A COSE header parameter named "vdp" (for VDPs), which conveys a map containing VDPs organized by Proof Type. Correspondingly, see for a registry defining the integers used to identify VDP Proof Types.
Terminology The terms "header" and "payload" are defined in . Additionally, this document uses the following terminology:
CDDL:
Concise Data Definition Language (CDDL) is defined in .
EDN:
CBOR Extended Diagnostic Notation (EDN) is defined in , where it is referred to as "diagnostic notation", and is revised in .
Entry:
An entry in a VDS for which proofs can be derived.
Proof Type:
A property that can be obtained by verifying a given proof over one or more entries in a VDS. For example, a VDS, such as a binary Merkle Tree, can support inclusion proofs where each proof confirms that a given entry is included in a Merkle Tree root.
Proof Value:
An encoding of a Proof Type in CBOR .
Receipt:
A COSE Single Signer Data Object, as defined in RFC 9052 , containing the header parameters necessary to convey one or more VDP for an associated VDS.
Verifiable Data Structure (VDS):
A data structure that supports one or more VDP Proof Types. This property describes an algorithm used to maintain a VDS, for example, a binary Merkle Tree algorithm.
Verifiable Data Structure Proofs (VDPs):
A data structure used to convey Proof Types for proving different 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).
VDSs in CBOR This section describes representations of VDPs in . For example, construction of a Merkle Tree leaf or an inclusion proof from a leaf to a Merkle Tree root might have several different representations, depending on the VDS 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 VDSs with COSE and provides concrete examples for the structures and proofs defined in and . The design of these structures is influenced by the conventions established for COSE Keys.
Structures Similar to COSE Key Types , different VDSs support different algorithms. This document establishes a registry of VDS algorithms; see for details.
Proofs As is the case for COSE Key Type Parameters , EC2 keys (1: 2) require and give meaning to specific parameters, such as -1 (crv), -2 (x), -3 (y), and -4 (d). RFC9162_SHA256 (395: 1) supports both (-1) inclusion and (-2) consistency proofs. This document establishes a registry of VDPs; see for details. Proof Types are specific to their associated "VDS"; for example, different Merkle Trees might support different representations of inclusion proof or consistency proof. Implementers should not expect interoperability across "VDSs". Security analysis MUST be conducted prior to migrating to new structures to ensure the new security and privacy assumptions are acceptable for the use case.
Usage This document registers a new COSE header parameter "receipts" (394) to enable Receipts to be conveyed in the protected and unprotected headers of Enveloped COSE Structures. When the "receipts" header parameter is present, the verifier MUST confirm that the associated VDS and VDPs match entries present in the registries established in this specification, including values added in subsequent registrations. Receipts MUST be tagged as COSE_Sign1. The following definition from is provided:
CDDL for a COSE_Sign1 with Attached Receipts cose-values } Unprotected_Header = { &(receipts: 394) => [+ bstr .cbor Receipt] * cose-label => cose-values } COSE_Sign1 = [ protected : bstr .cbor Protected_Header, unprotected : Unprotected_Header, payload : bstr / nil, signature : bstr ] Receipt = Receipt_For_Inclusion / Receipt_For_Consistency ; Note the proof formats shown here are for RFC9162_SHA256. ; Other VDSs may have different proof formats. Receipt_For_Inclusion = /6.18(Signed_Inclusion_Proof) Signed_Inclusion_Proof = [ protected : bstr .cbor RFC9162_SHA256_Inclusion_Protected_Header, unprotected : RFC9162_SHA256_Inclusion_Unprotected_Header, payload : bstr / nil, signature : bstr ] RFC9162_SHA256_Inclusion_Protected_Header = { &(alg: 1) => int &(vds: 395) => int * cose-label => cose-values } RFC9162_SHA256_Inclusion_Unprotected_Header = { &(vdp: 396) => RFC9162_SHA256_Verifiable_Inclusion_Proofs * cose-label => cose-values } RFC9162_SHA256_Verifiable_Inclusion_Proofs = { &(inclusion-proof: -1) => RFC9162_SHA256_Inclusion_Proofs } RFC9162_SHA256_Inclusion_Proofs = [ + RFC9162_SHA256_Inclusion_Proof ] RFC9162_SHA256_Inclusion_Proof = bstr .cbor [ tree_size: uint, leaf_index: uint, inclusion_path: [ + bstr ] ] Receipt_For_Consistency = /6.18(Signed_Consistency_Proof) Signed_Consistency_Proof = [ protected : bstr .cbor RFC9162_SHA256_Consistency_Protected_Header, unprotected : RFC9162_SHA256_Consistency_Unprotected_Header, payload : bstr / nil, ; Newer Merkle Tree root signature : bstr ] RFC9162_SHA256_Consistency_Protected_Header = { &(alg: 1) => int, &(vds: 395) => int, * cose-label => cose-values } RFC9162_SHA256_Consistency_Unprotected_Header = { &(vdp: 396) => RFC9162_SHA256_Verifiable_Consistency_Proofs * cose-label => cose-values } RFC9162_SHA256_Verifiable_Consistency_Proofs = { &(consistency-proof: -2) => RFC9162_SHA256_Consistency_Proofs } RFC9162_SHA256_Consistency_Proofs = [ + RFC9162_SHA256_Consistency_Proof ] RFC9162_SHA256_Consistency_Proof = bstr .cbor [ tree_size_1: uint, tree_size_2: uint, consistency_path: [ + bstr ] ] ]]>
The following informative EDN is provided:
An Example COSE Signature with Multiple Receipts >, / unprotected / { / receipts / 394 : [ <>, / unprotected / { / proofs / 396 : { / inclusion / -1 : [ <<[ / size / 9, / leaf / 8, / inclusion path / h'7558a95f...e02e35d6' ]>> ], }, }, / payload / null, / signature / h'02d227ed...ccd3774f' ])>>, <>, / 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' ]) ]]>
The specific structure of COSE Receipts is dependent on the structure of the COSE_Sign1 payload and the VDPs contained in the COSE_Sign1 unprotected header. The CDDL definition for VDPs is specific to each VDS. This document describes proofs for RFC9162_SHA256 in the following sections.
Profiles New VDSs can require the definition of a profile. The payload in such definitions SHOULD be detached. Detached payloads force verifiers to recompute the root from the proof and protect against implementation errors where the signature is verified but the payload is incompatible with the proof. Profiles of proof signatures that define additional protected header parameters are encouraged to make their presence 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 and the similar guidance provided in .
Registration Requirements Each VDS specification applying for inclusion in this registry MUST define how to encode the VDS identifier and its Proof Types in CBOR. Each specification MUST define how to produce and consume the supported Proof Types. See as an example. Where a specification supports a choice of hash algorithm, a separate IANA registration must be made for each supported algorithm. For example, to provide support for SHA256 and SHA3_256 with Merkle inclusion proofs and Merkle consistency proofs defined, respectively, in and , both "RFC9162_SHA256" and "RFC9162_SHA3_256" require entries in the relevant IANA registries. This document only defines "RFC9162_SHA256".
RFC9162_SHA256 This section defines how the data structure described in is mapped to the terminology defined in this document, using and .
Verifiable Data Structure The integer identifier for this VDS is 1. The string identifier for this VDS is "RFC9162_SHA256", a Merkle Tree where SHA256 is used as the hash algorithm (see ). See for a complete description of this VDS.
Inclusion Proof See for a complete description of this VDP Proof Type. The CBOR representation of an inclusion proof for RFC9162_SHA256 is:
CBOR-Encoded Inclusion Proof for RFC9162_SHA256
The term leaf-index is used for alignment with the use established in . Note that defines inclusion proofs only for leaf nodes, and that:
If leaf_index is greater than or equal to tree_size, then fail the proof verification.
The identifying index of a leaf node is relative to all nodes in the tree size for which the proof was obtained.
Receipt of Inclusion In a signed proof, the payload is the Merkle Tree root that corresponds to the log at size tree-size. The protected header for an RFC9162_SHA256 inclusion proof signature is:
Protected Header for a Receipt of Inclusion int &(vds: 395) => int * cose-label => cose-value }]]>
alg (label: 1):
REQUIRED. Signature algorithm identifier. Value type: int.
vds (label: 395):
REQUIRED. VDS algorithm identifier. Value type: int.
The unprotected header for an RFC9162_SHA256 inclusion proof signature is:
A VDP in an Unprotected Header inclusion-proofs } unprotected-header-map = { &(vdp: 396) => verifiable-proofs * cose-label => cose-value }]]>
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 . An EDN example for a Receipt containing an inclusion proof for RFC9162_SHA256 with a detached payload (see ) is:
Receipt of Inclusion >, / 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' ])]]>
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 verifier 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 Tree root, which is then attached as the COSE_Sign1 payload. Second, the verifier checks the signature of the COSE_Sign1. If the resulting signature can be verified, the Receipt has proved inclusion of the entry in the VDS. If the resulting signature cannot be verified, the signature may have been tampered with.
Consistency Proof See for a complete description of this VDP Proof Type. The cbor representation of a consistency proof for RFC9162_SHA256 is:
CBOR-Encoded Consistency Proof for RFC9162_SHA256
Receipt of Consistency In a signed consistency proof, the newer Merkle Tree root (proven to be consistent with an older Merkle Tree root) is a detached payload and corresponds to the log at size tree-size-2. The protected header for an RFC9162_SHA256 consistency proof signature is:
Protected Header for a Receipt of Consistency int &(vds: 395) => int * cose-label => cose-value }]]>
alg (label: 1):
REQUIRED. Signature algorithm identifier. Value type: int.
vds (label: 395):
REQUIRED. VDS algorithm identifier. Value type: int.
The unprotected header for an RFC9162_SHA256 consistency proof signature is: consistency-proofs } unprotected-header-map = { &(vdp: 396) => verifiable-proofs * cose-label => cose-value }]]>
vdp (label: 396):
REQUIRED. VDPs. 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 . An EDN example for a Receipt containing a consistency proof for RFC9162_SHA256 with a detached payload (see ) is:
Example Consistency Receipt >, / 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' ])]]>
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 VDS is not assured. This approach is specific to RFC9162_SHA256; different VDSs 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.
Privacy Considerations
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.
Header Parameters Additional header parameters can reveal information about the transparency service or its log entries. The receipt producer MUST perform a privacy analysis for all mandatory fields in profiles based on this specification.
Security Considerations See the Security Considerations sections of:
Choice of Signature Algorithms A security analysis ought to 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 VDS used; for example, both RFC9162_SHA256 and ES256 depend on the SHA256 hash function.
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 for one way to accomplish this. The details of expressing validity periods are out of scope for this document.
Status Updates In some cases, receipts should be "revocable" or "suspendable" after being issued, regardless of their validity period. The details of expressing statuses are out of scope for this document.
IANA Considerations
COSE Header Parameter IANA has added the COSE header parameters defined in , and as listed in , to the "COSE Header Parameters" subregistry in the "CBOR Object Signing and Encryption (COSE)" registry group. These COSE header parameters fall in the 'Integer values from 256 to 65535' range (with a Specification Required registration procedure (see )). The Value Registry listed for "vds" is the "COSE Verifiable Data Structure Algorithm" subregistry. The map labels in the "vdp" are assigned from the "COSE Verifiable Data Structure Proofs" subregistry. Newly Registered COSE Header Parameters
Name Label Value Type Value Registry Description Reference
receipts 394 array   Priority ordered sequence of CBOR encoded Receipts RFC 9942,
vds 395 int COSE Verifiable Data Structure Algorithm identifier for Verifiable Data Structures that is used to produce Verifiable Data Structure Proofs RFC 9942,
vdp 396 map map key in COSE Verifiable Data Structure Proofs Location for Verifiable Data Structure Proofs in COSE Header Parameters RFC 9942,
VDS Registries IANA has established the "COSE Verifiable Data Structure Algorithms" and "COSE Verifiable Data Structure Proofs" subregistries under a Specification Required policy as described in .
Expert Review Expert reviewers (see ) should take into consideration the following points:
  • Experts are advised to assign the next available positive integer for VDSs.
  • 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 .
  • It is not permissible to assign points in the "COSE Verifiable Data Structure Algorithms" registry for which no corresponding entry in the "COSE Verifiable Data Structure Proofs" registry exists, and vice versa.
  • The change controller for related registrations of structures and proofs should be the same.
Templates and Initial Contents
COSE Verifiable Data Structure Algorithms Registry
Registration Template:
Name:
This is a descriptive name for the VDS that enables easier reference to the item.
Value:
This is the value used to identify the VDS.
Description:
This field contains a brief description of the VDS.
Reference:
This contains a pointer to the public specification for the VDS.
Change Controller:
For Standards Track RFCs, list the "IETF". For others, give the name of the responsible party. Other details (e.g., postal address, email address, home page URI) may also be included.
COSE Verifiable Data Structure Algorithms Registry Initial Contents
Name Value Description Change Controller Reference
Reserved 0 Reserved RFC 9942
RFC9162_SHA256 1 SHA256 Binary Merkle Tree IETF
COSE Verifiable Data Structure Proofs Registry
Registration Template:
Verifiable Data Structure:
This value used identifies the related VDS.
Name:
This is a descriptive name for the Proof Type that enables easier reference to the item.
Label:
This is the value used to identify the VDP Proof Type.
CBOR Type:
This contains the CBOR type for the value portion of the label.
Description:
This field contains a brief description of the Proof Type.
Reference:
This contains a pointer to the public specification for the Proof Type.
Change Controller:
For Standards Track RFCs, list the "IETF". For others, give the name of the responsible party. Other details (e.g., postal address, email address, home page URI) may also be included.
COSE Verifiable Data Structure Proofs Registry Initial Contents
Verifiable Data Structure Name Label CBOR Type Description Change Controller Reference
1 inclusion proofs -1 array (of bstr) Proof of inclusion IETF RFC 9942,
1 consistency proofs -2 array (of bstr) Proof of append-only property IETF RFC 9942,
References Normative References COSE Header Parameters IANA Informative References
Acknowledgements We would like to thank , , , , , and for their contributions (some of which substantial) to this document and to the initial set of implementations.
Contributors Microsoft
United Kingdom amaury.chamayou@microsoft.com
stevenlasker@hotmail.com
MITRE Corporation
United States of America ramartin@mitre.org
United States of America mwiseman32@acm.org
United States of America roywill@msn.com