Data Integrity BBS Cryptosuites v1.0

Abstract

This specification describes a Data Integrity Cryptosuite for use when generating digital signatures using the BBS signature scheme. The Signature Suite utilizes BBS signatures to provide selective disclosure and unlinkable derived proofs.

The following algorithms describe how to use verifiable credentials with the BBS Signature Scheme [CFRG-BBS-SIGNATURE]. When using the BBS signature scheme the SHA-256 variant SHOULD be used.

Implementations SHOULD fetch and cache verification method information as early as possible when adding or verifying proofs. Parameters passed to functions in this section use information from the verification method — such as the public key size — to determine function parameters — such as the cryptographic hashing algorithm.

When the RDF Dataset Canonicalization Algorithm [RDF-CANON] is used, implementations of that algorithm will detect dataset poisoning by default, and abort processing upon detection.

The following algorithm creates a label map factory function that uses an HMAC to shuffle canonical blank node identifiers. The required input is an HMAC (previously initialized with a secret key), HMAC. A function, labelMapFactoryFunction, is produced as output.

Create a function, labelMapFactoryFunction, with one required input (a canonical node identifier map, canonicalIdMap), that will return a blank node identifier map, bnodeIdMap, as output. Set the function's implementation to:
1. Generate a new empty bnode identifier map, bnodeIdMap.
2. For each map entry, entry, in canonicalIdMap:
  1. Perform an HMAC operation on the canonical identifier from the value in entry to get an HMAC digest, digest.
  2. Generate a new string value, b64urlDigest, and initialize it to "u" followed by appending a base64url-no-pad encoded version of the digest value.
  3. Add a new entry, newEntry, to bnodeIdMap using the key from entry and b64urlDigest as the value.
3. Derive the shuffled mapping from the bnodeIdMap as follows:
  1. Set hmacIds to be the sorted array of values from the bnodeIdMap, and set bnodeKeys to be the ordered array of keys from the bnodeIdMap.
  2. For each key in bnodeKeys, replace the bnodeIdMap value for that key with the index position of the value in the hmacIds array prefixed by "b", i.e., bnodeIdMap.set(bkey, 'b' + hmacIds.indexOf(bnodeIdMap.get(bkey))).
4. Return bnodeIdMap.
Return labelMapFactoryFunction.

Note

It should be noted that step 1.2 in the above algorithm is identical to step 1.2 in Section 3.3.4 createHmacIdLabelMapFunction of [DI-ECDSA], so developers might be able to reuse the code or call the function if implementing both.

The following algorithm serializes the base proof value, including the BBS signature, HMAC key, and mandatory pointers. The required inputs are a base signature bbsSignature, an HMAC key hmacKey, and an array of mandatoryPointers. A single base proof string value is produced as output.

Initialize a byte array, proofValue, that starts with the BBS base proof header bytes 0xd9, 0x5d, and 0x02.
Initialize components to an array with five elements containing the values of: bbsSignature, bbsHeader, publicKey, hmacKey, and mandatoryPointers.
CBOR-encode components per [RFC8949] where CBOR tagging MUST NOT be used on any of the components. Append the produced encoded value to proofValue.
Initialize baseProof to a string with the multibase-base64url-no-pad-encoding of proofValue. That is, return a string starting with "u" and ending with the base64url-no-pad-encoded value of proofValue.
Return baseProof as base proof.

The following algorithm parses the components of a bbs-2023 selective disclosure base proof value. The required input is a proof value (proofValue). A single object, parsed base proof, containing five elements, using the names "bbsSignature", "bbsHeader", "publicKey", "hmacKey", and "mandatoryPointers", is produced as output.

Ensure the proofValue string starts with u (U+0075 LATIN SMALL LETTER U), indicating that it is a multibase-base64url-no-pad-encoded value, and throw an error if it does not.
Initialize decodedProofValue to the result of base64url-no-pad-decoding the substring following the leading u in proofValue.
Ensure that the decodedProofValue starts with the BBS base proof header bytes 0xd9, 0x5d, and 0x02, and throw an error if it does not.
Initialize components to an array that is the result of CBOR-decoding the bytes that follow the three-byte BBS base proof header. Ensure the result is an array of three elements.
Return an object with properties set to the five elements, using the names "bbsSignature", "bbsHeader", "publicKey", "hmacKey", and "mandatoryPointers", respectively.

The following algorithm creates data to be used to generate a derived proof. The inputs include a JSON-LD document (document), a BBS base proof (proof), an array of JSON pointers to use to selectively disclose statements (selectivePointers), an OPTIONAL BBS presentationHeader (byte array that defaults to an empty byte array if not present), and any custom JSON-LD API options (such as a document loader). A single object, disclosure data, is produced as output, which contains the bbsProof, labelMap, mandatoryIndexes, selectiveIndexes, presentationHeader, and revealDocument fields.

Initialize bbsSignature, bbsHeader, publicKey, hmacKey, and mandatoryPointers to the values of the associated properties in the object returned when calling the algorithm in Section 3.2.2 parseBaseProofValue, passing the proofValue from proof.
Initialize hmac to an HMAC API using hmacKey. The HMAC uses the same hash algorithm used in the signature algorithm, i.e., SHA-256.
Initialize labelMapFactoryFunction to the result of calling the createShuffledIdLabelMapFunction algorithm passing hmac as HMAC.
Initialize combinedPointers to the concatenation of mandatoryPointers and selectivePointers.
Initialize groupDefinitions to a map with the following entries: key of the string "mandatory" and value of mandatoryPointers; key of the string "selective" and value of selectivePointers; and key of the string "combined" and value of combinedPointers.
Initialize groups and labelMap to the result of calling the algorithm in Section 3.3.16 canonicalizeAndGroup of the [DI-ECDSA] specification, passing document labelMapFactoryFunction, groupDefinitions, and any custom JSON-LD API options. Note: This step transforms the document into an array of canonical N-Quads whose order has been shuffled based on 'hmac' applied blank node identifiers, and groups the N-Quad strings according to selections based on JSON pointers.
Compute the mandatory indexes relative to their positions in the combined statement list, i.e., find the position at which a mandatory statement occurs in the list of combined statements. One method for doing this is given below.
1. Initialize mandatoryIndexes to an empty array. Set mandatoryMatch to groups.mandatory.matching map; set combinedMatch to groups.combined.matching; and set combinedIndexes to the ordered array of just the keys of the combinedMatch map.
2. For each key in the mandatoryMatch map, find its index in the combinedIndexes array (e.g., combinedIndexes.indexOf(key)), and add this value to the mandatoryIndexes array.
Compute the selective indexes relative to their positions in the non-mandatory statement list, i.e., find the position at which a selected statement occurs in the list of non-mandatory statements. One method for doing this is given below.
1. Initialize selectiveIndexes to an empty array. Set selectiveMatch to the groups.selective.matching map; set mandatoryNonMatch to the map groups.mandatory.nonMatching; and nonMandatoryIndexes to to the ordered array of just the keys of the mandatoryNonMatch map.
2. For each key in the selectiveMatch map, find its index in the nonMandatoryIndexes array (e.g., nonMandatoryIndexes.indexOf(key)), and add this value to the selectiveIndexes array.
Initialize bbsMessages to an array of byte arrays containing the values in the nonMandatory array of strings encoded using the UTF-8 character encoding.
Set bbsProof to the value computed by the ProofGen procedure from [CFRG-BBS-SIGNATURE], i.e., ProofGen(PK, signature, header, ph, messages, disclosed_indexes), where PK is the original issuers public key, signature is the bbsSignature, header is the bbsHeader, ph is the presentationHeader messages is bbsMessages, and disclosed_indexes is selectiveIndexes.
Initialize revealDocument to the result of the "selectJsonLd" algorithm, passing document, and combinedPointers as pointers.
Run the RDF Dataset Canonicalization Algorithm [RDF-CANON] on the joined combinedGroup.deskolemizedNQuads, passing any custom options, and get the canonical bnode identifier map, canonicalIdMap. Note: This map includes the canonical blank node identifiers that a verifier will produce when they canonicalize the reveal document.
Initialize verifierLabelMap to an empty map. This map will map the canonical blank node identifiers produced by the verifier when they canonicalize the revealed document, to the blank node identifiers that were originally signed in the base proof.
For each key (inputLabel) and value (verifierLabel) in `canonicalIdMap:
1. Add an entry to verifierLabelMap, using verifierLabel as the key, and the value associated with inputLabel as a key in labelMap as the value.
Return an object with properties matching bbsProof, "verifierLabelMap" for labelMap, mandatoryIndexes, selectiveIndexes, and revealDocument.

The following algorithm compresses a label map. The required input is label map (labelMap). The output is a compressed label map.

Initialize map to an empty map.
For each entry (k, v) in labelMap:
1. Add an entry to map, with a key that is a base-10 integer parsed from the characters following the "c14n" prefix in k, and a value that is a base-10 integer parsed from the characters following the "b" prefix in v.
Return map as compressed label map.

The following algorithm decompresses a label map. The required input is a compressed label map (compressedLabelMap). The output is a decompressed label map.

Initialize map to an empty map.
For each entry (k, v) in compressedLabelMap:
1. Add an entry to map, with a key that adds the prefix "c14n" to k, and a value that adds a prefix of "b" to v.
Return map as decompressed label map.

The following algorithm serializes a derived proof value. The required inputs are a BBS proof (bbsProof), a label map (labelMap), an array of mandatory indexes (mandatoryIndexes), an array of selective indexes (selectiveIndexes), and a BBS presentation header (presentationHeader). A single derived proof value, serialized as a byte string, is produced as output.

Initialize compressedLabelMap to the result of calling the algorithm in Section 3.2.4 compressLabelMap, passing labelMap as the parameter.
Initialize a byte array, proofValue, that starts with the BBS disclosure proof header bytes 0xd9, 0x5d, and 0x03.
Initialize components to an array with five elements containing the values of bbsProof, compressedLabelMap, mandatoryIndexes, selectiveIndexes, and presentationHeader.
CBOR-encode components per [RFC8949] where CBOR tagging MUST NOT be used on any of the components. Append the produced encoded value to proofValue.
Return the derived proof as a string with the multibase-base64url-no-pad-encoding of proofValue. That is, return a string starting with "u" and ending with the base64url-no-pad-encoded value of proofValue.

The following algorithm parses the components of the derived proof value. The required input is a derived proof value (proofValue). A single derived proof value object is produced as output, which contains a set of five elements, using the names bbsProof, labelMap, mandatoryIndexes, selectiveIndexes, and presentationHeader.

Ensure the proofValue string starts with u (U+0075, LATIN SMALL LETTER U), indicating that it is a multibase-base64url-no-pad-encoded value, and throw an error if it does not.
Initialize decodedProofValue to the result of base64url-no-pad-decoding the substring that follows the leading u in proofValue.
Ensure that the decodedProofValue starts with the BBS disclosure proof header bytes 0xd9, 0x5d, and 0x03, and throw an error if it does not.
Initialize components to an array that is the result of CBOR-decoding the bytes that follow the three-byte BBS disclosure proof header. Ensure the result is an array of five elements — a byte array, a map of integers to integers, an array of integers, another array of integers, and a byte array; otherwise, throw an error.
Replace the second element in components using the result of calling the algorithm in Section 3.2.5 decompressLabelMap, passing the existing second element of components as compressedLabelMap.
Return derived proof value as an object with properties set to the five elements, using the names bbsProof, labelMap, mandatoryIndexes, selectiveIndexes, and presentationHeader, respectively.

The following algorithm creates the data needed to perform verification of a BBS-protected verifiable credential. The inputs include a JSON-LD document (document), a BBS disclosure proof (proof), and any custom JSON-LD API options (such as a document loader). A single verify data object value is produced as output containing the following fields: bbsProof, proofHash, mandatoryHash, selectedIndexes, presentationHeader, and nonMandatory.

Initialize proofHash to the result of performing RDF Dataset Canonicalization [RDF-CANON] on the proof options, i.e., the proof portion of the document with the proofValue removed. The hash used is the same as that used in the signature algorithm, i.e., SHA-256. Note: This step can be performed in parallel; it only needs to be completed before this algorithm needs to use the proofHash value.
Initialize bbsProof, labelMap, mandatoryIndexes, selectiveIndexes, and presentationHeader to the values associated with their property names in the object returned when calling the algorithm in Section 3.2.7 parseDerivedProofValue, passing proofValue from proof.
Initialize labelMapFactoryFunction to the result of calling the "createLabelMapFunction" algorithm.
Initialize nquads to the result of calling the "labelReplacementCanonicalize" algorithm of [DI-ECDSA], passing document, labelMapFactoryFunction, and any custom JSON-LD API options. Note: This step transforms the document into an array of canonical N-Quads with pseudorandom blank node identifiers based on labelMap.
Initialize mandatory to an empty array.
Initialize nonMandatory to an empty array.
For each entry (index, nq) in nquads, separate the N-Quads into mandatory and non-mandatory categories:
1. If mandatoryIndexes includes index, add nq to mandatory.
2. Otherwise, add nq to nonMandatory.
Initialize mandatoryHash to the result of calling the "hashMandatory" primitive, passing mandatory.
Return an object with properties matching baseSignature, proofHash, nonMandatory, mandatoryHash, and selectiveIndexes.

The bbs-2023 cryptographic suite takes an input document, canonicalizes the document using the Universal RDF Dataset Canonicalization Algorithm [RDF-CANON], and then applies a number of transformations and cryptographic operations resulting in the production of a data integrity proof. The algorithms in this section also include the verification of such a data integrity proof.

To generate a base proof, the algorithm in Section 4.1: Add Proof of the Data Integrity [VC-DATA-INTEGRITY] specification MUST be executed. For that algorithm, the cryptographic suite specific transformation algorithm is defined in Section 3.3.2 Base Proof Transformation (bbs-2023), the hashing algorithm is defined in Section 3.3.3 Base Proof Hashing (bbs-2023), and the proof serialization algorithm is defined in Section 3.3.5 Base Proof Serialization (bbs-2023).

The following algorithm specifies how to transform an unsecured input document into a transformed document that is ready to be provided as input to the hashing algorithm in Section 3.3.3 Base Proof Hashing (bbs-2023).

Required inputs to this algorithm are an unsecured data document (unsecuredDocument) and transformation options (options). The transformation options MUST contain a type identifier for the cryptographic suite (type), a cryptosuite identifier (cryptosuite), and a verification method (verificationMethod). The transformation options MUST contain an array of mandatory JSON pointers (mandatoryPointers) and MAY contain additional options, such as a JSON-LD document loader. A transformed data document is produced as output. Whenever this algorithm encodes strings, it MUST use UTF-8 encoding.

Initialize hmac to an HMAC API using a locally generated and exportable HMAC key. The HMAC uses the same hash algorithm used in the signature algorithm, i.e., SHA-256. Per the recommendations of [RFC2104], the HMAC key MUST be the same length as the digest size; for SHA-256, this is 256 bits or 32 bytes.
Initialize labelMapFactoryFunction to the result of calling the createShuffledIdLabelMapFunction algorithm passing hmac as HMAC.
Initialize groupDefinitions to a map with an entry with a key of the string "mandatory" and a value of mandatoryPointers.
Initialize groups to the result of calling the algorithm in Section 3.3.16 canonicalizeAndGroup of the [DI-ECDSA] specification, passing labelMapFactoryFunction, groupDefinitions, unsecuredDocument as document, and any custom JSON-LD API options. Note: This step transforms the document into an array of canonical N-Quads whose order has been shuffled based on 'hmac' applied blank node identifiers, and groups the N-Quad strings according to selections based on JSON pointers.
Initialize mandatory to the values in the groups.mandatory.matching map.
Initialize nonMandatory to the values in the groups.mandatory.nonMatching map.
Initialize hmacKey to the result of exporting the HMAC key from hmac.
Return an object with "mandatoryPointers" set to mandatoryPointers, "mandatory" set to mandatory, "nonMandatory" set to nonMandatory, and "hmacKey" set to hmacKey.

The following algorithm specifies how to cryptographically hash a transformed data document and proof configuration into cryptographic hash data that is ready to be provided as input to the algorithms in Section 3.3.5 Base Proof Serialization (bbs-2023).

The required inputs to this algorithm are a transformed data document (transformedDocument) and canonical proof configuration (canonicalProofConfig). A hash data value represented as an object is produced as output.

Initialize proofHash to the result of calling the RDF Dataset Canonicalization algorithm [RDF-CANON] on canonicalProofConfig and then cryptographically hashing the result using the same hash that is used by the signature algorithm, i.e., SHA-256. Note: This step can be performed in parallel; it only needs to be completed before this algorithm terminates, as the result is part of the return value.
Initialize mandatoryHash to the result of calling the the algorithm in Section 3.3.17 hashMandatoryNQuads of the [DI-ECDSA] specification, passing transformedDocument.mandatory and using the SHA-256 algorithm.
Initialize hashData as a deep copy of transformedDocument, and add proofHash as "proofHash" and mandatoryHash as "mandatoryHash" to that object.
Return hashData as hash data.

The following algorithm specifies how to generate a proof configuration from a set of proof options that is used as input to the base proof hashing algorithm.

The required inputs to this algorithm are proof options (options). The proof options MUST contain a type identifier for the cryptographic suite (type) and MUST contain a cryptosuite identifier (cryptosuite). A proof configuration object is produced as output.

Let proofConfig be a clone of the options object.
If proofConfig.type is not set to DataIntegirtyProof and/or proofConfig.cryptosuite is not set to bbs-2023, an INVALID_PROOF_CONFIGURATION error MUST be raised.
If proofConfig.created is set and if the value is not a valid [XMLSCHEMA11-2] datetime, an INVALID_PROOF_DATETIME error MUST be raised.
Set proofConfig.@context to unsecuredDocument.@context.
Let canonicalProofConfig be the result of applying the Universal RDF Dataset Canonicalization Algorithm [RDF-CANON] to the proofConfig.
Return canonicalProofConfig.

The following algorithm, to be called by an issuer of a BBS-protected Verifiable Credential, specifies how to create a base proof. The base proof is to be given only to the holder, who is responsible for generating a derived proof from it, exposing only selectively disclosed details in the proof to a verifier. This algorithm is designed to be used in conjunction with the algorithms defined in the Data Integrity [VC-DATA-INTEGRITY] specification, Section 4: Algorithms. Required inputs are cryptographic hash data (hashData) and proof options (options). The proof options MUST contain a type identifier for the cryptographic suite (type) and MAY contain a cryptosuite identifier (cryptosuite). A single digital proof value represented as series of bytes is produced as output.

Initialize proofHash, mandatoryPointers, mandatoryHash, nonMandatory, and hmacKey to the values associated with their property names in hashData.
Initialize bbsHeader to the concatenation of proofHash and mandatoryHash in that order.
Initialize bbsMessages to an array of byte arrays containing the values in the nonMandatory array of strings encoded using the UTF-8 character encoding.
Compute the bbsSignature using the Sign procedure of [CFRG-BBS-Signature] with appropriate key material and bbsHeader for the header and bbsMessages for the messages
Initialize `proofValue to the result of calling the algorithm in Section 3.2.1 serializeBaseProofValue, passing bbsSignature, bbsHeader, publicKey, hmacKey, and mandatoryPointers as parameters to the algorithm. Where publicKey is a byte array of the public key encoded according to [CFRG-BBS-SIGNATURE].
Return proofValue as digital proof.

The following algorithm, to be called by a holder of a bbs-2023-protected verifiable credential, creates a selective disclosure derived proof. The derived proof is to be given to the verifier. The inputs include a JSON-LD document (document), a BBS base proof (proof), an array of JSON pointers to use to selectively disclose statements (selectivePointers), an OPTIONAL BBS presentationHeader (a byte array), and any custom JSON-LD API options, such as a document loader. A single selectively revealed document value, represented as an object, is produced as output.

Initialize bbsProof, labelMap, mandatoryIndexes, selectiveIndexes, and revealDocument to the values associated with their property names in the object returned when calling the algorithm in Section 3.2.3 createDisclosureData, passing the document, proof, selectivePointers, presentationHeader, and any custom JSON-LD API options, such as a document loader.
Initialize newProof to a shallow copy of proof.
Replace proofValue in newProof with the result of calling the algorithm in Section 3.2.6 serializeDerivedProofValue, passing bbsProof, labelMap, mandatoryIndexes, and selectiveIndexes.
Set the value of the "proof" property in revealDocument to newProof.
Return revealDocument as the selectively revealed document.

The following algorithm attempts verification of a bbs-2023 derived proof. This algorithm is called by a verifier of an BBS-protected verifiable credential. The inputs include a JSON-LD document (document), a BBS disclosure proof (proof), and any custom JSON-LD API options (such as a document loader). A single boolean verification result value is produced as output.

Initialize bbsProof, proofHash, mandatoryHash, selectedIndexes, presentationHeader and nonMandatory to the values associated with their property names in the object returned when calling the algorithm in Section 3.2.8 createVerifyData, passing the document, proof, and any custom JSON-LD API options (such as a document loader).
Initialize bbsHeader to the concatenation of proofHash and mandatoryHash in that order. Initialize disclosedMessages to an array of byte arrays obtained from the UTF-8 encoding of the elements of the nonMandatory array.
Initialize verificationResult to the result of applying the verification algorithm ProofVerify(PK, proof, header, ph, disclosed_messages, disclosed_indexes) of [CFRG-BBS-SIGNATURE] with PK set as the public key of the original issuer, proof set as bbsProof, header set as bbsHeader, disclosed_messages set as disclosedMessages, ph set as presentationHeader, and disclosed_indexes set as selectiveIndexes. Return verificationResult as verification result.

This section is non-normative.

Selective disclosure permits a holder to minimize the information revealed to a verifier to achieve a particular purpose. In prescribing an overall system that enables selective disclosure, care has to be taken that additional information that was not meant to be disclosed to the verifier is minimized. Such leakage can occur through artifacts of the system. Such artifacts can come from higher layers of the system, such as in the structure of data or from the lower level cryptographic primitives.

For example the BBS signature scheme is an extremely space efficient scheme for producing a signature on multiple messages, i.e., the cryptographic signature sent to the holder is a constant size regardless of the number of messages. The holder then can selectively disclose any of these messages to a verifier, however as part of the encryption scheme, the total number of messages signed by the issuer has to be revealed to the verifier. If such information leakage needs to be avoided then it is recommended to pad the number of messages out to a common length as suggested in the privacy considerations section of [CFRG-BBS-SIGNATURE].

At the higher levels, how data gets mapped into individual statements suitable for selective disclosure, i.e., BBS messages, is a potential source of data leakage. This cryptographic suite is able to eliminate many structural artifacts used to express JSON data that might leak information (nesting, map, or array position, etc.) by using JSON-LD processing to transform inputs into RDF. RDF can then be expressed as a canonical, flat format of simple subject, property, value statements (referred to as claims in the Verifiable Credentials Data Model [VC-DATA-MODEL-2.0]). In the following, we examine RDF canonicalization, a general scheme for mapping a verifiable credential in JSON-LD format into a set of statements (BBS messages), for selective disclosure. We show that after this process is performed, there remains a possible source of information leakage, and we show how this leakage is mitigated via the use of a keyed pseudo random function (PRF).

RDF canonicalization can be used to flatten a JSON-LD VC into a set of statements. The algorithm is dependent on the content of the VC and also employs a cryptographic hash function to help in ordering the statements. In essence, how this happens is that each JSON object that represents the subject of claims within a JSON-LD document will be assigned an id, if it doesn't have an @id field defined. Such ids are known as blank node ids. These ids are needed to express claims as simple subject, property, value statements such that the subject in each claim can be differentiated. The id values are deterministically set per [RDF-CANON] and are based on the data in the document and the output of a cryptographic hash function such as SHA-256.

Below we show two slightly different VCs for a set of windsurf sails and their canonicalization into a set of statements that can be used for selective disclosure. By changing the year of the 6.1 size sail we see a major change in statement ordering between these two VCs. If the holder discloses information about just his larger sails (the 7.0 and 7.8) the verifier could tell something changed about the set of sails, i.e., information leakage.

Example 3: A VC for a set of windsurfing sails

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    {
      "@vocab": "https://windsurf.grotto-networking.com/selective#"
    }
  ],
  "type": [
    "VerifiableCredential"
  ],
  "credentialSubject": {
    "sails": [
      {
        "size": 5.5,
        "sailName": "Kihei",
        "year": 2023
      },
      {
        "size": 6.1,
        "sailName": "Lahaina",
        "year": 2023 // Will change this to see the effect on canonicalization
      },
      {
        "size": 7.0,
        "sailName": "Lahaina",
        "year": 2020
      },
      {
        "size": 7.8,
        "sailName": "Lahaina",
        "year": 2023
      }
    ]
  }
}

Canonical form of the above VC. Assignment of blank node ids, i.e., the _:c14nX labels are dependent upon the content of the VC and this also affects the ordering of the statements.

Example 4: Canonical form of the VC for a set of windsurfing sails

_:c14n0 <https://windsurf.grotto-networking.com/selective#sailName> "Lahaina" .
_:c14n0 <https://windsurf.grotto-networking.com/selective#size> "7.8E0"^^<http://www.w3.org/2001/XMLSchema#double> .
_:c14n0 <https://windsurf.grotto-networking.com/selective#year> "2023"^^<http://www.w3.org/2001/XMLSchema#integer> .
_:c14n1 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <https://www.w3.org/2018/credentials#VerifiableCredential> .
_:c14n1 <https://www.w3.org/2018/credentials#credentialSubject> _:c14n4 .
_:c14n2 <https://windsurf.grotto-networking.com/selective#sailName> "Lahaina" .
_:c14n2 <https://windsurf.grotto-networking.com/selective#size> "7"^^<http://www.w3.org/2001/XMLSchema#integer> .
_:c14n2 <https://windsurf.grotto-networking.com/selective#year> "2020"^^<http://www.w3.org/2001/XMLSchema#integer> .
_:c14n3 <https://windsurf.grotto-networking.com/selective#sailName> "Kihei" .
_:c14n3 <https://windsurf.grotto-networking.com/selective#size> "5.5E0"^^<http://www.w3.org/2001/XMLSchema#double> .
_:c14n3 <https://windsurf.grotto-networking.com/selective#year> "2023"^^<http://www.w3.org/2001/XMLSchema#integer> .
_:c14n4 <https://windsurf.grotto-networking.com/selective#sails> _:c14n0 .
_:c14n4 <https://windsurf.grotto-networking.com/selective#sails> _:c14n2 .
_:c14n4 <https://windsurf.grotto-networking.com/selective#sails> _:c14n3 .
_:c14n4 <https://windsurf.grotto-networking.com/selective#sails> _:c14n5 .
_:c14n5 <https://windsurf.grotto-networking.com/selective#sailName> "Lahaina" .
_:c14n5 <https://windsurf.grotto-networking.com/selective#size> "6.1E0"^^<http://www.w3.org/2001/XMLSchema#double> .
_:c14n5 <https://windsurf.grotto-networking.com/selective#year> "2023"^^<http://www.w3.org/2001/XMLSchema#integer> .

Updated windsurf sail collection, i.e., the 6.1 size sail has been updated to the 2024 model. This changes the ordering of statements via the assignment of blank node ids.

Example 5: A VC for a slightly updated set of windsurfing sails

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    {
      "@vocab": "https://windsurf.grotto-networking.com/selective#"
    }
  ],
  "type": [
    "VerifiableCredential"
  ],
  "credentialSubject": {
    "sails": [
      {
        "size": 5.5,
        "sailName": "Kihei",
        "year": 2023
      },
      {
        "size": 6.1,
        "sailName": "Lahaina",
        "year": 2024 // New sail to update older model, changes canonicalization
      },
      {
        "size": 7.0,
        "sailName": "Lahaina",
        "year": 2020
      },
      {
        "size": 7.8,
        "sailName": "Lahaina",
        "year": 2023
      }
    ]
  }
}

Canonical form of the previous VC. Note the difference in blank node id assignment and ordering of statements.

Example 6: Canonical form of the updated VC for a set of windsurfing sails

_:c14n0 <https://windsurf.grotto-networking.com/selective#sailName> "Lahaina" .
_:c14n0 <https://windsurf.grotto-networking.com/selective#size> "6.1E0"^^<http://www.w3.org/2001/XMLSchema#double> .
_:c14n0 <https://windsurf.grotto-networking.com/selective#year> "2024"^^<http://www.w3.org/2001/XMLSchema#integer> .
_:c14n1 <https://windsurf.grotto-networking.com/selective#sailName> "Lahaina" .
_:c14n1 <https://windsurf.grotto-networking.com/selective#size> "7.8E0"^^<http://www.w3.org/2001/XMLSchema#double> .
_:c14n1 <https://windsurf.grotto-networking.com/selective#year> "2023"^^<http://www.w3.org/2001/XMLSchema#integer> .
_:c14n2 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <https://www.w3.org/2018/credentials#VerifiableCredential> .
_:c14n2 <https://www.w3.org/2018/credentials#credentialSubject> _:c14n5 .
_:c14n3 <https://windsurf.grotto-networking.com/selective#sailName> "Lahaina" .
_:c14n3 <https://windsurf.grotto-networking.com/selective#size> "7"^^<http://www.w3.org/2001/XMLSchema#integer> .
_:c14n3 <https://windsurf.grotto-networking.com/selective#year> "2020"^^<http://www.w3.org/2001/XMLSchema#integer> .
_:c14n4 <https://windsurf.grotto-networking.com/selective#sailName> "Kihei" .
_:c14n4 <https://windsurf.grotto-networking.com/selective#size> "5.5E0"^^<http://www.w3.org/2001/XMLSchema#double> .
_:c14n4 <https://windsurf.grotto-networking.com/selective#year> "2023"^^<http://www.w3.org/2001/XMLSchema#integer> .
_:c14n5 <https://windsurf.grotto-networking.com/selective#sails> _:c14n0 .
_:c14n5 <https://windsurf.grotto-networking.com/selective#sails> _:c14n1 .
_:c14n5 <https://windsurf.grotto-networking.com/selective#sails> _:c14n3 .
_:c14n5 <https://windsurf.grotto-networking.com/selective#sails> _:c14n4 .

To prevent such information leakage from the assignment of these blank node ids and the ordering they impose on the statements, an HMAC based PRF is run on the blank node ids. The HMAC secret key is only shared between the issuer and holder and each Base Proof generated by the issuer uses a new HMAC key. An example of this can be seen in the canonical HMAC test vector of [DI-ECDSA]. As discussed in the next section, for BBS to preserve unlinkability we do not use HMAC based blank node ids but produce a shuffled version of the ordering based on the HMAC as shown in test vector Example 12. Note that this furnishes less information hiding concerning blank node ids than in the ECDSA-SD approach, since information the number of blank node ids can leak, but prevents linkage attacks via the essentially unique identifiers produced by applying an HMAC to blank node ids.

In some uses of VCs it can be important to the privacy of a holder to prevent the tracking or linking of multiple different verifier interactions. In particular we consider two important cases (i) verifier to issuer collusion, and (ii) verifier to verifier collusion. In the first case, shown in Figure 1, a verifier reports back to the original issuer of the credential on an interaction with a holder. In this situation, the issuer could track all the holder interactions with various verifiers using the issued VC. In the second situation, shown in Figure 2, multiple verifiers collude to share information about holders with whom they have interacted.

Diagram showing multiple verifiers sending data back to the issuer.
The diagram is laid out top to bottom with a circle labeled issuer at the top,
connected to a circle label holder below. From the circle labeled holder there
are multple arrows to additional circles labeled verifiers. From the circles
labeled verifiers there are dashed arrows back to the circle labeled issuer
showing collusion data flow. — Figure 1 Verifier to verifier collusion.

Diagram showing multiple verifiers sharing with each other.
The diagram is laid out top to bottom with a circle labeled issuer at the top,
connected to a circle label holder below. From the circle labeled holder there
are multple arrows to additional circles labeled verifiers. From the circles
labeled verifiers there are dashed arrows back to other circles labeled issuer
to show verifier to verifier collusion data flows. — Figure 2 Verifier to issuer collusion.

We use the term unlinkability to describe the property of a VC system to prevent such "linkage attacks" on holder privacy. Although the term unlinkability is relatively new section 3.3 of [NISTIR8053] discusses and gives a case study of Re-identification through Linkage Attacks. A systemization of knowledge on linkage attack on data privacy can be found in [Powar2023]. The most widespread use of linkage attack on user privacy occurs via the practice of web browser fingerprinting, a survey of which can be found in [Pugliese2020].

To quantify the notion of linkage, [Powar2023] introduces the idea of an anonymity set. In the VC case we are concerned with here, the anonymity set would contain the holder of a particular VC and other holders associated with a particular issuer. The smaller the anonymity set the more likely the holder can be tracked across verifiers. Since a signed VC contains a reference to a public key of the issuer, the starting size for the anonymity set for a holder possessing a VC from a particular issuer is the number of VC issued by that issuer with that particular public/private key pair. Non-malicious issuers are expected to minimize the number of public/private key pairs used to issue VCs. Note that the anonymity set idea is similar to the group privacy concept in [vc-bitstring-status-list]. When we use the term linkage here we generally mean any mechanism that results in a reduction in size of the anonymity set.

Sources of linkage in a VC system supporting selective disclosure:

Artifacts from cryptographic primitives.
Artifacts from mapping a VC into a set of statements suitable for selective disclosure.
Artifacts from Proof Options and Mandatory reveal Information in the VC.
Selectively revealed information in the VC.
External VC System Based Linkage

We discuss each of these below.

Cryptographic Hashes, HMACs, and digital signatures by their nature generate highly unique identifiers. The output of a hash function such as SHA-256, by its collision resistance properties, are guaranteed to be essentially unique given different inputs and result in a strong linkage, i.e., reduces the anonymity set size to one. Similarly deterministic signature algorithms such as Ed25519 and deterministic ECDSA will produce essentially unique outputs for different inputs and lead to strong linkages.

This implies that holders can be easily tracked across verifiers via digital signature, HMAC, or hash artifacts inside VCs and hence are vulnerable to verifier-verifier collusion and verifier-issuer collusion. Randomized signature algorithms such as some forms of ECDSA can permit the issuer to generate many distinct signatures on the same inputs and send these to the holder for use with different verifiers. Such an approach could be used to prevent verifier-verifier collusion based tracking but cannot help with verifier-issuer collusion.

To achieve unlinkability requires specially designed cryptographic signature schemes that allow the holder to generate what is called a zero knowledge proof of knowledge of a signature (ZKPKS). What this means is that the holder can take a signature from the issuer in such a scheme, compute a ZKPKS to send to a verifier. This ZKPKS cannot be linked back to the original signature, but has all the desirable properties of a signature, i.e., the verifier can use it to verify that the messages were signed by the issuers public key and that the messages have not been altered. In addition, the holder can generate as many ZKPKSs as desired for different verifiers and these are essentially independent and unlinkable. BBS is one such signature scheme that supports this capability.

Although the ZKPKS, known as a BBS proof in this document, has guaranteed unlinkability properties. BBS when used with selective disclosure has two artifacts that can contribute to linkability. These are the total number of messages originally signed, and the index values for the revealed statements. See the privacy considerations in [CFRG-BBS-SIGNATURE] for a discussion and mitigation techniques.

As mentioned in the section on Issuer's Public Keys of [CFRG-BBS-SIGNATURE] there is the potential threat that an issuer might use multiple public keys with some of those used to track a specific subset of users via verifier-issuer collusion. Since the issuers public key has to be visible to the verifier, i.e., it is referenced in the BBS proof (derived proof) this can be used as a linkage point if the issuer has many different public keys and particularly if it uses a subset of those keys with a small subset of users (holders).

We saw in the section on information leakage that RDF canonicalization uses a hash function to order statements and that a further shuffle of the order of the statements is performed based on an HMAC. This can leave a fingerprint that might allow for some linkage. How strong of a linkage is dependent on the number of blank nodes, essentially JSON objects within the VC, and the number of indexes revealed. Given n blank nodes and k disclosed indexes in the worst case this would be a reduction in the anonymity set size by a factor of C(n, k), i.e., the number combinations of size k chosen from a set of n elements. One can keep this number quite low by reducing the number of blank nodes in the VC, e.g., keep the VC short and simple.

JSON-LD is a JSON-based format for serialization of Linked Data. As such, it supports assigning a globally unambiguous @id attribute (node identifier) to each object ("node", in JSON-LD terminology) within a document. This allows for the linking of linked data, enabling information about the same entity to be correlated. This correlation can be desirable or undesirable, depending on the use case.

When using BBS for its unlinkability feature, globally unambiguous node identifiers cannot be used for individuals nor for their personally identifiable information, since the strong linkage they provide is undesirable. Note that the use of such identifiers is acceptable when expressing statements about non-personal information (e.g., using a globally unambiguous identifier to identify a large country or a concert event). Also note that JSON-LD's use of @context, which maps terms to IRIs, does not generally affect unlinkability.

In the [vc-data-integrity] specification, a number of properties of the proof attribute of a VC are given. Care has to be taken that optional fields ought not provide strong linkage across verifiers. The optional fields include: id, created, expires, domain, challenge, and nonce. For example the optional created field is a dateTimeStamp object which can specify the creation date for the proof down to an arbitrary sub-second granularity. Such information, if present, could greatly reduce the size of the anonymity set. If the issuer wants to include such information they ought to make it as coarse grained as possible, relative to the number of VCs being issued over time.

The issuer can also compel a holder to reveal certain statements to a verifier via the mandatoryPointers input used in the creation of the Base Proof. See section 3.3.2 Base Proof Transformation (bbs-2023), Example 9, and Example 10. By compel we mean that a generated Derived Proof will not verify unless these statements are revealed to the verifier. Care should be taken such that if such information is required to be disclosed, that the anonymity set remains sufficiently large.

As discussed in [Powar2023] there are many documented cases of re-identification of individuals from linkage attacks. Hence the holder is urged to reveal as little information as possible to help keep the anonymity set large. In addition, it has been shown a number of times that innocuous seeming information can be highly unique and thus leading to re-identification or tracking. See [NISTIR8053] for a walk through of a particularly famous case of a former governor of Massachusetts and [Powar2023] for further analysis and categorization of 94 such public cases.

It ought to be pointed out that maintaining unlinkability, i.e., anonymity, requires care in the systems holding and communicating the VCs. Networking artifacts such as IP address (layer 3) or Ethernet/MAC address (layer 2) are well known sources of linkage. For example, mobile phone MAC addresses can be used to track users if they revisited a particular access point, this led to mobile phone manufacturers providing a MAC address randomization feature. Public IP addresses generally provide enough information to geolocate an individual to a city or region within a country potentially greatly reducing the anonymity set.

This section is non-normative.

Demonstration of selective disclosure features including mandatory disclosure, selective disclosure, and overlap between those, requires an input credential document with more content than previous test vectors. To avoid excessively long test vectors, the starting document test vector is based on a purely fictitious windsurfing (sailing) competition scenario. In addition, we break the test vectors into two groups, based on those that would be generated by the issuer (base proof) and those that would be generated by the holder (derived proof).

To add a selective disclosure base proof to a document, the issuer needs the following cryptographic key material:

The issuer's private/public key pair, i.e., the key pair corresponding to the verification method that will be part of the proof.
An HMAC key. This is used to randomize the order of the blank node IDs to avoid potential information leakage via the blank node ID ordering. This is used only once, and is shared between issuer and holder. The HMAC in this case is functioning as a pseudorandom function (PRF).

The key material used for generating the test vectors to test add base proof is shown below. Hexadecimal representation is used for the BBS key pairs and the HMAC key.

Example 7: Private and Public keys for Signature

{
  "publicKeyHex": "a4ef1afa3da575496f122b9b78b8c24761531a8a093206ae7c45b80759c168ba4f7a260f9c3367b6c019b4677841104b10665edbe70ba3ebe7d9cfbffbf71eb016f70abfbb163317f372697dc63efd21fc55764f63926a8f02eaea325a2a888f",
  "privateKeyHex": "66d36e118832af4c5e28b2dfe1b9577857e57b042a33e06bdea37b811ed09ee0",
  "hmacKeyString": "00112233445566778899AABBCCDDEEFF00112233445566778899AABBCCDDEEFF"
}

In our scenario, a sailor is registering with a race organizer for a series of windsurfing races to be held over a number of days on Maui. The organizer will inspect the sailor's equipment to certify that what has been declared is accurate. The sailor's unsigned equipment inventory is shown below.

Example 8: Credential without Proof

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    {
      "@vocab": "https://windsurf.grotto-networking.com/selective#"
    }
  ],
  "type": [
    "VerifiableCredential"
  ],
  "issuer": "https://vc.example/windsurf/racecommittee",
  "credentialSubject": {
    "sailNumber": "Earth101",
    "sails": [
      {
        "size": 5.5,
        "sailName": "Kihei",
        "year": 2023
      },
      {
        "size": 6.1,
        "sailName": "Lahaina",
        "year": 2023
      },
      {
        "size": 7.0,
        "sailName": "Lahaina",
        "year": 2020
      },
      {
        "size": 7.8,
        "sailName": "Lahaina",
        "year": 2023
      }
    ],
    "boards": [
      {
        "boardName": "CompFoil170",
        "brand": "Wailea",
        "year": 2022
      },
      {
        "boardName": "Kanaha Custom",
        "brand": "Wailea",
        "year": 2019
      }
    ]
  }
}

In addition to letting other sailors know what kinds of equipment their competitors may be sailing on, it is mandatory that each sailor disclose the year of their most recent windsurfing board and full details on two of their sails. Note that all sailors are identified by a sail number that is printed on all their equipment. This mandatory information is specified via an array of JSON pointers as shown below.

Example 9: Mandatory Pointers

["/issuer", "/credentialSubject/sailNumber", "/credentialSubject/sails/1", "/credentialSubject/boards/0/year", "/credentialSubject/sails/2"]

The result of applying the above JSON pointers to the sailor's equipment document is shown below.

Example 10: JSON Pointers and Values

[
  {
    "pointer": "/sailNumber",
    "value": "Earth101"
  },
  {
    "pointer": "/sails/1",
    "value": {
      "size": 6.1,
      "sailName": "Lahaina",
      "year": 2023
    }
  },
  {
    "pointer": "/boards/0/year",
    "value": 2022
  },
  {
    "pointer": "/sails/2",
    "value": {
      "size": 7,
      "sailName": "Lahaina",
      "year": 2020
    }
  }
]

Transformation of the unsigned document begins with canonicalizing the document, as shown below.

Example 11: Canonical Document

[
  "_:c14n0 <https://windsurf.grotto-networking.com/selective#boardName> \"CompFoil170\" .\n",
  "_:c14n0 <https://windsurf.grotto-networking.com/selective#brand> \"Wailea\" .\n",
  "_:c14n0 <https://windsurf.grotto-networking.com/selective#year> \"2022\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:c14n1 <https://windsurf.grotto-networking.com/selective#sailName> \"Lahaina\" .\n",
  "_:c14n1 <https://windsurf.grotto-networking.com/selective#size> \"7.8E0\"^^<http://www.w3.org/2001/XMLSchema#double> .\n",
  "_:c14n1 <https://windsurf.grotto-networking.com/selective#year> \"2023\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:c14n2 <https://windsurf.grotto-networking.com/selective#boardName> \"Kanaha Custom\" .\n",
  "_:c14n2 <https://windsurf.grotto-networking.com/selective#brand> \"Wailea\" .\n",
  "_:c14n2 <https://windsurf.grotto-networking.com/selective#year> \"2019\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:c14n3 <https://windsurf.grotto-networking.com/selective#sailName> \"Lahaina\" .\n",
  "_:c14n3 <https://windsurf.grotto-networking.com/selective#size> \"7\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:c14n3 <https://windsurf.grotto-networking.com/selective#year> \"2020\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:c14n4 <https://windsurf.grotto-networking.com/selective#sailName> \"Kihei\" .\n",
  "_:c14n4 <https://windsurf.grotto-networking.com/selective#size> \"5.5E0\"^^<http://www.w3.org/2001/XMLSchema#double> .\n",
  "_:c14n4 <https://windsurf.grotto-networking.com/selective#year> \"2023\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:c14n5 <https://windsurf.grotto-networking.com/selective#boards> _:c14n0 .\n",
  "_:c14n5 <https://windsurf.grotto-networking.com/selective#boards> _:c14n2 .\n",
  "_:c14n5 <https://windsurf.grotto-networking.com/selective#sailNumber> \"Earth101\" .\n",
  "_:c14n5 <https://windsurf.grotto-networking.com/selective#sails> _:c14n1 .\n",
  "_:c14n5 <https://windsurf.grotto-networking.com/selective#sails> _:c14n3 .\n",
  "_:c14n5 <https://windsurf.grotto-networking.com/selective#sails> _:c14n4 .\n",
  "_:c14n5 <https://windsurf.grotto-networking.com/selective#sails> _:c14n6 .\n",
  "_:c14n6 <https://windsurf.grotto-networking.com/selective#sailName> \"Lahaina\" .\n",
  "_:c14n6 <https://windsurf.grotto-networking.com/selective#size> \"6.1E0\"^^<http://www.w3.org/2001/XMLSchema#double> .\n",
  "_:c14n6 <https://windsurf.grotto-networking.com/selective#year> \"2023\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:c14n7 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <https://www.w3.org/2018/credentials#VerifiableCredential> .\n",
  "_:c14n7 <https://www.w3.org/2018/credentials#credentialSubject> _:c14n5 .\n",
  "_:c14n7 <https://www.w3.org/2018/credentials#issuer> <https://vc.example/windsurf/racecommittee> .\n"
]

To prevent possible information leakage from the ordering of the blank node IDs these are processed through a PRF (i.e., the HMAC) to give the canonicalized HMAC document shown below. This represents an ordered list of statements that will be subject to mandatory and selective disclosure, i.e., it is from this list that statements are grouped.

Example 12: Canonical HMAC Document

[
  "_:b0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <https://www.w3.org/2018/credentials#VerifiableCredential> .\n",
  "_:b0 <https://www.w3.org/2018/credentials#credentialSubject> _:b3 .\n",
  "_:b0 <https://www.w3.org/2018/credentials#issuer> <https://vc.example/windsurf/racecommittee> .\n",
  "_:b1 <https://windsurf.grotto-networking.com/selective#sailName> \"Lahaina\" .\n",
  "_:b1 <https://windsurf.grotto-networking.com/selective#size> \"7.8E0\"^^<http://www.w3.org/2001/XMLSchema#double> .\n",
  "_:b1 <https://windsurf.grotto-networking.com/selective#year> \"2023\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:b2 <https://windsurf.grotto-networking.com/selective#boardName> \"CompFoil170\" .\n",
  "_:b2 <https://windsurf.grotto-networking.com/selective#brand> \"Wailea\" .\n",
  "_:b2 <https://windsurf.grotto-networking.com/selective#year> \"2022\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:b3 <https://windsurf.grotto-networking.com/selective#boards> _:b2 .\n",
  "_:b3 <https://windsurf.grotto-networking.com/selective#boards> _:b4 .\n",
  "_:b3 <https://windsurf.grotto-networking.com/selective#sailNumber> \"Earth101\" .\n",
  "_:b3 <https://windsurf.grotto-networking.com/selective#sails> _:b1 .\n",
  "_:b3 <https://windsurf.grotto-networking.com/selective#sails> _:b5 .\n",
  "_:b3 <https://windsurf.grotto-networking.com/selective#sails> _:b6 .\n",
  "_:b3 <https://windsurf.grotto-networking.com/selective#sails> _:b7 .\n",
  "_:b4 <https://windsurf.grotto-networking.com/selective#boardName> \"Kanaha Custom\" .\n",
  "_:b4 <https://windsurf.grotto-networking.com/selective#brand> \"Wailea\" .\n",
  "_:b4 <https://windsurf.grotto-networking.com/selective#year> \"2019\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:b5 <https://windsurf.grotto-networking.com/selective#sailName> \"Kihei\" .\n",
  "_:b5 <https://windsurf.grotto-networking.com/selective#size> \"5.5E0\"^^<http://www.w3.org/2001/XMLSchema#double> .\n",
  "_:b5 <https://windsurf.grotto-networking.com/selective#year> \"2023\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:b6 <https://windsurf.grotto-networking.com/selective#sailName> \"Lahaina\" .\n",
  "_:b6 <https://windsurf.grotto-networking.com/selective#size> \"6.1E0\"^^<http://www.w3.org/2001/XMLSchema#double> .\n",
  "_:b6 <https://windsurf.grotto-networking.com/selective#year> \"2023\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:b7 <https://windsurf.grotto-networking.com/selective#sailName> \"Lahaina\" .\n",
  "_:b7 <https://windsurf.grotto-networking.com/selective#size> \"7\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n",
  "_:b7 <https://windsurf.grotto-networking.com/selective#year> \"2020\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n"
]

The above canonical document gets grouped into mandatory and non-mandatory statements. The final output of the selective disclosure transformation process is shown below. Each statement is now grouped as mandatory or non-mandatory, and its index in the previous list of statements is remembered.

Example 13: Add Base Transformation

{
  "mandatoryPointers": [
    "/issuer",
    "/credentialSubject/sailNumber",
    "/credentialSubject/sails/1",
    "/credentialSubject/boards/0/year",
    "/credentialSubject/sails/2"
  ],
  "mandatory": {
    "dataType": "Map",
    "value": [
      [
        0,
        "_:b0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <https://www.w3.org/2018/credentials#VerifiableCredential> .\n"
      ],
      [
        1,
        "_:b0 <https://www.w3.org/2018/credentials#credentialSubject> _:b3 .\n"
      ],
      [
        2,
        "_:b0 <https://www.w3.org/2018/credentials#issuer> <https://vc.example/windsurf/racecommittee> .\n"
      ],
      [
        8,
        "_:b2 <https://windsurf.grotto-networking.com/selective#year> \"2022\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n"
      ],
      [
        9,
        "_:b3 <https://windsurf.grotto-networking.com/selective#boards> _:b2 .\n"
      ],
      [
        11,
        "_:b3 <https://windsurf.grotto-networking.com/selective#sailNumber> \"Earth101\" .\n"
      ],
      [
        14,
        "_:b3 <https://windsurf.grotto-networking.com/selective#sails> _:b6 .\n"
      ],
      [
        15,
        "_:b3 <https://windsurf.grotto-networking.com/selective#sails> _:b7 .\n"
      ],
      [
        22,
        "_:b6 <https://windsurf.grotto-networking.com/selective#sailName> \"Lahaina\" .\n"
      ],
      [
        23,
        "_:b6 <https://windsurf.grotto-networking.com/selective#size> \"6.1E0\"^^<http://www.w3.org/2001/XMLSchema#double> .\n"
      ],
      [
        24,
        "_:b6 <https://windsurf.grotto-networking.com/selective#year> \"2023\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n"
      ],
      [
        25,
        "_:b7 <https://windsurf.grotto-networking.com/selective#sailName> \"Lahaina\" .\n"
      ],
      [
        26,
        "_:b7 <https://windsurf.grotto-networking.com/selective#size> \"7\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n"
      ],
      [
        27,
        "_:b7 <https://windsurf.grotto-networking.com/selective#year> \"2020\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n"
      ]
    ]
  },
  "nonMandatory": {
    "dataType": "Map",
    "value": [
      [
        3,
        "_:b1 <https://windsurf.grotto-networking.com/selective#sailName> \"Lahaina\" .\n"
      ],
      [
        4,
        "_:b1 <https://windsurf.grotto-networking.com/selective#size> \"7.8E0\"^^<http://www.w3.org/2001/XMLSchema#double> .\n"
      ],
      [
        5,
        "_:b1 <https://windsurf.grotto-networking.com/selective#year> \"2023\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n"
      ],
      [
        6,
        "_:b2 <https://windsurf.grotto-networking.com/selective#boardName> \"CompFoil170\" .\n"
      ],
      [
        7,
        "_:b2 <https://windsurf.grotto-networking.com/selective#brand> \"Wailea\" .\n"
      ],
      [
        10,
        "_:b3 <https://windsurf.grotto-networking.com/selective#boards> _:b4 .\n"
      ],
      [
        12,
        "_:b3 <https://windsurf.grotto-networking.com/selective#sails> _:b1 .\n"
      ],
      [
        13,
        "_:b3 <https://windsurf.grotto-networking.com/selective#sails> _:b5 .\n"
      ],
      [
        16,
        "_:b4 <https://windsurf.grotto-networking.com/selective#boardName> \"Kanaha Custom\" .\n"
      ],
      [
        17,
        "_:b4 <https://windsurf.grotto-networking.com/selective#brand> \"Wailea\" .\n"
      ],
      [
        18,
        "_:b4 <https://windsurf.grotto-networking.com/selective#year> \"2019\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n"
      ],
      [
        19,
        "_:b5 <https://windsurf.grotto-networking.com/selective#sailName> \"Kihei\" .\n"
      ],
      [
        20,
        "_:b5 <https://windsurf.grotto-networking.com/selective#size> \"5.5E0\"^^<http://www.w3.org/2001/XMLSchema#double> .\n"
      ],
      [
        21,
        "_:b5 <https://windsurf.grotto-networking.com/selective#year> \"2023\"^^<http://www.w3.org/2001/XMLSchema#integer> .\n"
      ]
    ]
  },
  "hmacKeyString": "00112233445566778899AABBCCDDEEFF00112233445566778899AABBCCDDEEFF"
}

The next step is to create the base proof configuration and canonicalize it. This is shown in the following two examples.

Example 14: Base Proof Configuration

{
  "type": "DataIntegrityProof",
  "cryptosuite": "bbs-2023",
  "created": "2023-08-15T23:36:38Z",
  "verificationMethod": "did:key:zUC7DerdEmfZ8f4pFajXgGwJoMkV1ofMTmEG5UoNvnWiPiLuGKNeqgRpLH2TV4Xe5mJ2cXV76gRN7LFQwapF1VFu6x2yrr5ci1mXqC1WNUrnHnLgvfZfMH7h6xP6qsf9EKRQrPQ#zUC7DerdEmfZ8f4pFajXgGwJoMkV1ofMTmEG5UoNvnWiPiLuGKNeqgRpLH2TV4Xe5mJ2cXV76gRN7LFQwapF1VFu6x2yrr5ci1mXqC1WNUrnHnLgvfZfMH7h6xP6qsf9EKRQrPQ",
  "proofPurpose": "assertionMethod",
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    {
      "@vocab": "https://windsurf.grotto-networking.com/selective#"
    }
  ]
}

Example 15: Canonical Base Proof Configuration

_:c14n0 <http://purl.org/dc/terms/created> "2023-08-15T23:36:38Z"^^<http://www.w3.org/2001/XMLSchema#dateTime> .
_:c14n0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <https://w3id.org/security#DataIntegrityProof> .
_:c14n0 <https://w3id.org/security#cryptosuite> "bbs-2023"^^<https://w3id.org/security#cryptosuiteString> .
_:c14n0 <https://w3id.org/security#proofPurpose> <https://w3id.org/security#assertionMethod> .
_:c14n0 <https://w3id.org/security#verificationMethod> <did:key:zUC7DerdEmfZ8f4pFajXgGwJoMkV1ofMTmEG5UoNvnWiPiLuGKNeqgRpLH2TV4Xe5mJ2cXV76gRN7LFQwapF1VFu6x2yrr5ci1mXqC1WNUrnHnLgvfZfMH7h6xP6qsf9EKRQrPQ#zUC7DerdEmfZ8f4pFajXgGwJoMkV1ofMTmEG5UoNvnWiPiLuGKNeqgRpLH2TV4Xe5mJ2cXV76gRN7LFQwapF1VFu6x2yrr5ci1mXqC1WNUrnHnLgvfZfMH7h6xP6qsf9EKRQrPQ> .

In the hashing step, we compute the SHA-256 hash of the canonicalized proof options to produce the proofHash, and we compute the SHA-256 hash of the join of all the mandatory N-Quads to produce the mandatoryHash. These are shown below in hexadecimal format.

Example 16: Add Base Hashes

{
  "proofHash": "3a5bbf25d34d90b18c35cd2357be6a6f42301e94fc9e52f77e93b773c5614bdf",
  "mandatoryHash": "555de05f898817e31301bac187d0c3ff2b03e2cbdb4adb4d568c17de961f9a18"
}

Shown below are the computed bbsSignature in hexadecimal, and the mandatoryPointers. These are are fed to the final serialization step with the hmacKey.

Example 17: Add Base Signing

{
  "bbsSignature": "86bb8063768d4b708f9a65821ee6fe426b3d4f6fe5c2c5c9a5f80caa573fd8c20cbdf17826fe4e1a624070ba5f201d9202a0fceb55842ea9e61a72a7aa04891437fc35f6ab9ef8bf8ec3004cc46c9458",
  "mandatoryPointers": [
    "/issuer",
    "/credentialSubject/sailNumber",
    "/credentialSubject/sails/1",
    "/credentialSubject/boards/0/year",
    "/credentialSubject/sails/2"
  ]
}

Finally, the values above are run through the algorithm of Section 3.2.1 serializeBaseProofValue, to produce the proofValue which is used in the signed base document shown below.

Example 18: Signed Base Document

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    {
      "@vocab": "https://windsurf.grotto-networking.com/selective#"
    }
  ],
  "type": [
    "VerifiableCredential"
  ],
  "issuer": "https://vc.example/windsurf/racecommittee",
  "credentialSubject": {
    "sailNumber": "Earth101",
    "sails": [
      {
        "size": 5.5,
        "sailName": "Kihei",
        "year": 2023
      },
      {
        "size": 6.1,
        "sailName": "Lahaina",
        "year": 2023
      },
      {
        "size": 7,
        "sailName": "Lahaina",
        "year": 2020
      },
      {
        "size": 7.8,
        "sailName": "Lahaina",
        "year": 2023
      }
    ],
    "boards": [
      {
        "boardName": "CompFoil170",
        "brand": "Wailea",
        "year": 2022
      },
      {
        "boardName": "Kanaha Custom",
        "brand": "Wailea",
        "year": 2019
      }
    ]
  },
  "proof": {
    "type": "DataIntegrityProof",
    "cryptosuite": "bbs-2023",
    "created": "2023-08-15T23:36:38Z",
    "verificationMethod": "did:key:zUC7DerdEmfZ8f4pFajXgGwJoMkV1ofMTmEG5UoNvnWiPiLuGKNeqgRpLH2TV4Xe5mJ2cXV76gRN7LFQwapF1VFu6x2yrr5ci1mXqC1WNUrnHnLgvfZfMH7h6xP6qsf9EKRQrPQ#zUC7DerdEmfZ8f4pFajXgGwJoMkV1ofMTmEG5UoNvnWiPiLuGKNeqgRpLH2TV4Xe5mJ2cXV76gRN7LFQwapF1VFu6x2yrr5ci1mXqC1WNUrnHnLgvfZfMH7h6xP6qsf9EKRQrPQ",
    "proofPurpose": "assertionMethod",
    "proofValue": "u2V0ChVhQhruAY3aNS3CPmmWCHub-Qms9T2_lwsXJpfgMqlc_2MIMvfF4Jv5OGmJAcLpfIB2SAqD861WELqnmGnKnqgSJFDf8Nfarnvi_jsMATMRslFhYQDpbvyXTTZCxjDXNI1e-am9CMB6U_J5S936Tt3PFYUvfVV3gX4mIF-MTAbrBh9DD_ysD4svbSttNVowX3pYfmhhYYKTvGvo9pXVJbxIrm3i4wkdhUxqKCTIGrnxFuAdZwWi6T3omD5wzZ7bAGbRneEEQSxBmXtvnC6Pr59nPv_v3HrAW9wq_uxYzF_NyaX3GPv0h_FV2T2OSao8C6uoyWiqIj1ggABEiM0RVZneImaq7zN3u_wARIjNEVWZ3iJmqu8zd7v-FZy9pc3N1ZXJ4HS9jcmVkZW50aWFsU3ViamVjdC9zYWlsTnVtYmVyeBovY3JlZGVudGlhbFN1YmplY3Qvc2FpbHMvMXggL2NyZWRlbnRpYWxTdWJqZWN0L2JvYXJkcy8wL3llYXJ4Gi9jcmVkZW50aWFsU3ViamVjdC9zYWlscy8y"
  }
}

Random numbers are used, and an optional presentationHeader can be an input, for the creation of BBS proofs. To furnish a deterministic set of test vectors, we used the Mocked Random Scalars procedure from [CFRG-BBS-SIGNATURE]. The seed and presentationHeader values we used for generation of the derived proof test vectors are given in hex, below.

Example 19: seed and presentation header values

{
  "presentationHeaderHex": "113377aa",
  "pseudoRandSeedHex": "332e313431353932363533353839373933323338343632363433333833323739"
}

To create a derived proof, a holder starts with a signed document containing a base proof. The base document we will use for these test vectors is the final example from Section A.1 Base Proof, above. The first step is to run the algorithm of Section 3.2.2 parseBaseProofValue to recover bbsSignature, hmacKey, and mandatoryPointers, as shown below.

Example 20: Recovered Base Signature Data

{
  "bbsSignature": "86bb8063768d4b708f9a65821ee6fe426b3d4f6fe5c2c5c9a5f80caa573fd8c20cbdf17826fe4e1a624070ba5f201d9202a0fceb55842ea9e61a72a7aa04891437fc35f6ab9ef8bf8ec3004cc46c9458",
  "hmacKey": "00112233445566778899aabbccddeeff00112233445566778899aabbccddeeff",
  "mandatoryPointers": [
    "/issuer",
    "/credentialSubject/sailNumber",
    "/credentialSubject/sails/1",
    "/credentialSubject/boards/0/year",
    "/credentialSubject/sails/2"
  ]
}

Next, the holder needs to indicate what else, if anything, they wish to reveal to the verifiers, by specifying JSON pointers for selective disclosure. In our windsurfing competition scenario, a sailor (the holder) has just completed their first day of racing, and wishes to reveal to the general public (the verifiers) all the details of the windsurfing boards they used in the competition. These are shown below. Note that this slightly overlaps with the mandatory disclosed information which included only the year of their most recent board.

Example 21: Selective Disclosure Pointers

["/credentialSubject/boards/0", "/credentialSubject/boards/1"]

To produce the revealDocument (i.e., the unsigned document that will eventually be signed and sent to the verifier), we append the selective pointers to the mandatory pointers, and input these combined pointers along with the document without proof to the selectJsonLd algorithm of [DI-ECDSA], to get the result shown below.

Example 22: Unsigned Reveal Document

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    {
      "@vocab": "https://windsurf.grotto-networking.com/selective#"
    }
  ],
  "type": [
    "VerifiableCredential"
  ],
  "issuer": "https://vc.example/windsurf/racecommittee",
  "credentialSubject": {
    "sailNumber": "Earth101",
    "sails": [
      {
        "size": 6.1,
        "sailName": "Lahaina",
        "year": 2023
      },
      {
        "size": 7,
        "sailName": "Lahaina",
        "year": 2020
      }
    ],
    "boards": [
      {
        "year": 2022,
        "boardName": "CompFoil170",
        "brand": "Wailea"
      },
      {
        "boardName": "Kanaha Custom",
        "brand": "Wailea",
        "year": 2019
      }
    ]
  }
}

Now that we know what the revealed document looks like, we need to furnish appropriately updated information to the verifier about which statements are mandatory, and the indexes for the selected non-mandatory statements. Running step 6 of the 3.2.3 createDisclosureData yields an abundance of information about various statement groups relative to the original document. Below we show a portion of the indexes for those groups.

Example 23: Derived Group Indexes

{
  "combinedIndexes":[0,1,2,6,7,8,9,10,11,14,15,16,17,18,22,23,24,25,26,27],
  "mandatoryIndexes":[0,1,2,8,9,11,14,15,22,23,24,25,26,27],
  "nonMandatoryIndexes":[3,4,5,6,7,10,12,13,16,17,18,19,20,21],
  "selectiveIndexes":[0,1,6,7,8,9,10,16,17,18]
}

The verifier needs to be able to aggregate and hash the mandatory statements. To enable this, we furnish them with a list of indexes of the mandatory statements adjusted to their positions in the reveal document (i.e., relative to the combinedIndexes), while the selectiveIndexes need to be adjusted relative to their positions within the nonMandatoryIndexes. These "adjusted" indexes are shown below.

Example 24: Adjusted Mandatory and Selective Indexes

{
  "adjMandatoryIndexes":[0,1,2,5,6,8,9,10,14,15,16,17,18,19],
  "adjSelectiveIndexes":[3,4,5,8,9,10]
}

The last important piece of disclosure data is a mapping of canonical blank node IDs to HMAC-based shuffled IDs, the labelMap, computed according to Section 3.2.3 createDisclosureData. This is shown below along with the rest of the disclosure data minus the reveal document.

Example 25: Disclosure Data

{
  "bbsProof":"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",
  "labelMap":{"dataType":"Map","value":[["c14n0","b2"],["c14n1","b4"],["c14n2","b3"],["c14n3","b7"],["c14n4","b6"],["c14n5","b0"]]},
  "mandatoryIndexes":[0,1,2,5,6,8,9,10,14,15,16,17,18,19],
  "adjSelectiveIndexes":[3,4,5,8,9,10],
  "presentationHeader":{"0":17,"1":51,"2":119,"3":170}
}

Finally, using the disclosure data above with the algorithm of Section 3.2.6 serializeDerivedProofValue, we obtain the signed derived (reveal) document shown below.

Example 26: Signed Derived Document

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    {
      "@vocab": "https://windsurf.grotto-networking.com/selective#"
    }
  ],
  "type": [
    "VerifiableCredential"
  ],
  "issuer": "https://vc.example/windsurf/racecommittee",
  "credentialSubject": {
    "sailNumber": "Earth101",
    "sails": [
      {
        "size": 6.1,
        "sailName": "Lahaina",
        "year": 2023
      },
      {
        "size": 7,
        "sailName": "Lahaina",
        "year": 2020
      }
    ],
    "boards": [
      {
        "year": 2022,
        "boardName": "CompFoil170",
        "brand": "Wailea"
      },
      {
        "boardName": "Kanaha Custom",
        "brand": "Wailea",
        "year": 2019
      }
    ]
  },
  "proof": {
    "type": "DataIntegrityProof",
    "cryptosuite": "bbs-2023",
    "created": "2023-08-15T23:36:38Z",
    "verificationMethod": "did:key:zUC7DerdEmfZ8f4pFajXgGwJoMkV1ofMTmEG5UoNvnWiPiLuGKNeqgRpLH2TV4Xe5mJ2cXV76gRN7LFQwapF1VFu6x2yrr5ci1mXqC1WNUrnHnLgvfZfMH7h6xP6qsf9EKRQrPQ#zUC7DerdEmfZ8f4pFajXgGwJoMkV1ofMTmEG5UoNvnWiPiLuGKNeqgRpLH2TV4Xe5mJ2cXV76gRN7LFQwapF1VFu6x2yrr5ci1mXqC1WNUrnHnLgvfZfMH7h6xP6qsf9EKRQrPQ",
    "proofPurpose": "assertionMethod",
    "proofValue": "u2V0DhVkCEIW3LXS1Wq525PjphjhzUvbT4T8ZOH9ZNanzTFmqOvd4hVAe8dumdXa9JObasbHFs4kWcREsJpgsRB1PNS4byPVFESf72irSQNml1pM_RV23Qcw-edMoG8W2ERGLNjRh8ral7N1CP2t2cRaAgkZl9Q7sH1y68hnukOZs6sV1FG0aiTX3cL5tKaN2sA5OOaT6d1Xs9OtCqjur_W5IuyPpEIHwjQ0lm0ApaD0BwlN4vjxhIToJd1C4zio8CRUGGlR0BbPOWH0dgpkmn60pEDUJs-UwZ_e2B46dxmpREhkq7eNmLm2sWHbZRf0Fhj-ySbD8oC4Qq1FzZQ72ZeksPqcuq6lPyoYM1sY5U45RVvjLw7TSIvehH4N7uedrpU1YwbSsg07zOKPbS_ZFtGIhU8iX9HclX0Dk_MeRk0iuW_kDKp98CHbkemZmyp8XhnOsekG4ZEgNjoTGZVzS8OGGbe3EZ1kKK6dsKMtB89VYLgdztzeRS4NT_qTfkYoCKqWqkvSQ8LPC7fSk1VOLjQeqJTDxGIY-ZU7qqsacLAIFCcJClME72nIcc7hhC7zn5wMNFxDdUUhzGlAmx0HR2p4Gk9MrkNCbtYqOSilaMvsn9lSgPDHFbmw6-xqj8vokD1CV8x_ouV-BebxECM-WcT867GoGQJpvFIapnZkjvv2ydNPgT2-qm_MWzposT14bxtsDFZMyO6YAAgEEAgMDBwQGBQCOAAECBQYICQoODxAREhOGAwQFCAkKRBEzd6o"
  }
}

Data Integrity BBS Cryptosuites v1.0

Achieving Unlinkable Data Integrity with Pairing-based Cryptography

Abstract

Status of This Document

1. Introduction

1.1 Terminology

1.2 Conformance

2. Data Model

2.1 Verification Methods

2.1.1 Multikey

2.2 Proof Representations

2.2.1 DataIntegrityProof

3. Algorithms

3.1 Selective Disclosure Functions

3.1.1 createShuffledIdLabelMapFunction

3.2 bbs-2023 Functions

3.2.1 serializeBaseProofValue

3.2.2 parseBaseProofValue

3.2.3 createDisclosureData

3.2.4 compressLabelMap

3.2.5 decompressLabelMap

3.2.6 serializeDerivedProofValue

3.2.7 parseDerivedProofValue

3.2.8 createVerifyData

3.3 bbs-2023

3.3.1 Add Base Proof (bbs-2023)

3.3.2 Base Proof Transformation (bbs-2023)

3.3.3 Base Proof Hashing (bbs-2023)

3.3.4 Base Proof Configuration (bbs-2023)

3.3.5 Base Proof Serialization (bbs-2023)

3.3.6 Add Derived Proof (bbs-2023)

3.3.7 Verify Derived Proof (bbs-2023)

4. Security Considerations

4.1 Base Proof Security Properties

4.2 Derived Proof Security Properties

5. Privacy Considerations

5.1 Selective Disclosure and Data Leakage

5.2 Selective Disclosure and Unlinkability

5.2.1 Linkage via Cryptographic Artifacts

5.2.2 Linkage via VC Processing

5.2.3 Linkage via JSON-LD Node Identifiers

5.2.4 Linkage via Proof Options and Mandatory Reveal

5.2.5 Linkage via Holder Selective Reveal

5.2.6 External VC System Based Linkage

A. Test Vectors

A.1 Base Proof

A.2 Derived Proof

B. Acknowledgements

C. References

C.1 Normative references

C.2 Informative references