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This specification describes mechanisms for ensuring the authenticity and integrity of Verifiable Credentials and similar types of constrained digital documents using cryptography, especially through the use of digital signatures and related mathematical proofs.
This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.
This document was published by the Verifiable Credentials Working Group as a Working Draft using the Recommendation track.
Publication as a Working Draft does not imply endorsement by W3C and its Members.
This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
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This document is governed by the 2 November 2021 W3C Process Document.
This section is non-normative.
This specification describes mechanisms for ensuring the authenticity and integrity of Verifiable Credentials and similar types of constrained digital documents using cryptography, especially through the use of digital signatures and related mathematical proofs. Cryptographic proofs enable functionality that is useful to implementors of distributed systems. For example, proofs can be used to:
This section is non-normative.
The operation of Data Integrity is conceptually simple. To create a cryptographic proof, the following steps are performed: 1) Transformation, 2) Hashing, and 3) Proof Generation.
Transformation is a process described by a transformation algorithm that takes input data and prepares it for the hashing process. One example of a possible transformation is to take a record of people's names that attended a meeting, sort the list alphabetically by the individual's family name, and rewrite the names on a piece of paper, one per line, in sorted order. Possible transformations include canonicalization and binary-to-text encoding.
Hashing is a process described by a hashing algorithm that calculates an identifier for the transformed data using a cryptographic hash function. This process is conceptually similar to how a phone address book functions, where one takes a person's name (the input data) and maps that name to that individual's phone number (the hash). Possible cryptographic hash functions include SHA-3 and BLAKE-3.
Proof Generation is a process described by a proof generation algorithm that calculates a value that protects the integrity of the input data from modification or otherwise proves a certain desired threshold of trust. This process is conceptually similar to the way a wax seal can be used on an envelope containing a letter to establish trust in the sender and show that the letter has not been tampered with in transit. Possible proof generation functions include digital signatures and proofs of stake.
To verify a cryptographic proof, the following steps are performed: 1) Transformation, 2) Hashing, and 3) Proof Verification.
During verification, the transformation and hashing steps are conceptually the same as described above.
Proof Verification is a process that is described by a proof verification algorithm that applies a cryptographic proof verification function to see if the input data can be trusted. Possible proof verification functions include digital signatures and proofs of stake.
This specification details how cryptographic software architects and implementers can package these processes together into things called cryptographic suites and provide them to application developers for the purposes of protecting the integrity of application data in transit and at rest.
This section is non-normative.
This specification optimizes for the following design goals:
While this specification primarily focuses on Verifiable Credentials, the design of this technology is generalized, such that it can be used for non-Verifiable Credential use cases. In these instances, implementers are expected to perform their own due diligence and expert review as to the applicability of the technology to their use case.
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words MAY, MUST, MUST NOT, OPTIONAL, RECOMMENDED, and SHOULD in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This section is non-normative.
This section defines the terms used in this specification. A link to these terms is included whenever they appear in this specification.
example.com
, an
ad-hoc value such as mycorp-level3-access
, or a very
specific transaction value like 8zF6T8J34qP3mqP
. A signer could
include a domain in its digital proof to restrict its use
to particular target, identified by the specified domain.
id
property in a controller document.
Anything can be a subject: person, group, organization, physical thing, digital
thing, logical thing, etc.
A set of parameters that can be used together with a process to independently verify a proof. For example, a cryptographic public key can be used as a verification method with respect to a digital signature; in such usage, it verifies that the signer possessed the associated cryptographic private key.
"Verification" and "proof" in this definition are intended to apply broadly. For example, a cryptographic public key might be used during Diffie-Hellman key exchange to negotiate a shared symmetric key for encryption. This guarantees the integrity of the key agreement process. It is thus another type of verification method, even though descriptions of the process might not use the words "verification" or "proof."
An expression of the relationship between the subject and a verification method. An example of a verification relationship is 2.3.2.1 Authentication.
This section specifies the data model that is used for expressing data integrity proofs and verification methods.
A data integrity proof provides information about the proof mechanism, parameters required to verify that proof, and the proof value itself. All of this information is provided using Linked Data vocabularies such as the [SECURITY-VOCABULARY].
The following attributes are defined for use in a data integrity proof:
urn:uuid:6a1676b8-b51f-11ed-937b-d76685a20ff5
).
The usage of this property is further explained in Section
2.1.2 Proof Chains.
DataIntegrityProof
and Ed25519Signature2020
.
Proof types determine what other fields are required to secure and
verify the proof.
assertionMethod
) during a login process
(authentication
) which would then result in the creation of a
Verifiable Credential they never meant to create instead of the intended action,
which was to merely logging into a website.
domain
parameter is useful in challenge-response
protocols where the verifier is operating from within a security domain known to
the creator of the proof. Examples of a domain parameter include:
domain.example
(DNS domain), https://domain.example:8443
(full Web origin),
mycorp-intranet
(well-known text string), and
b31d37d4-dd59-47d3-9dd8-c973da43b63a
(UUID).
domain
is specified.
The value is used once for a particular domain and window of time. This
value is used to mitigate replay attacks. Examples of a challenge value include:
1235abcd6789
, 79d34551-ae81-44ae-823b-6dadbab9ebd4
, and ruby
.
verificationMethod
specified. The contents of the value MUST be a
multibase-encoded binary value. Alternative fields with different encodings MAY
be used to encode the data necessary to verify the digital proof instead of this
value. The contents of this value are determined by a cryptosuite and set to the
proof value generated by the
Add Proof Algorithm.
All of the terms above map to URLs. The vocabulary where these terms are defined is the [SECURITY-VOCABULARY].
A proof can be added to a JSON document like the following:
{
"title": "Hello world!"
};
by adding the parameters outlined in this section:
{
"title": "Hello world!",
"proof": {
"type": "DataIntegrityProof",
"cryptosuite": "example-signature-2022",
"created": "2020-11-05T19:23:24Z",
"verificationMethod": "https://di.example/issuer#z6MkjLrk3gKS2nnkeWcmcxi
ZPGskmesDpuwRBorgHxUXfxnG",
"proofPurpose": "assertionMethod",
"proofValue": "zQeVbY4oey5q2M3XKaxup3tmzN4DRFTLVqpLMweBrSxMY2xHX5XTYV8nQA
pmEcqaqA3Q1gVHMrXFkXJeV6doDwLWx"
}
}
The example proof above suggests a ficticious
example-signature-2022
cryptography suite that produces a
verifiable digital proof by presumptively transforming the input data using the
JSON Canonicalization Scheme [RFC8785] and then digitally signing it using an
Ed25519 elliptic curve signature.
Similarly, a proof can be added to a JSON-LD data document like the following:
{
"@context": {"title": "https://schema.org/title"},
"title": "Hello world!"
};
by adding the parameters outlined in this section:
{
"@context": [
{"title": "https://schema.org/title"},
"https://w3id.org/security/data-integrity/v1"
],
"title": "Hello world!",
"proof": {
"type": "DataIntegrityProof",
"cryptosuite": "eddsa-2022",
"created": "2020-11-05T19:23:24Z",
"verificationMethod": "https://ldi.example/issuer#z6MkjLrk3gKS2nnkeWcmcxi
ZPGskmesDpuwRBorgHxUXfxnG",
"proofPurpose": "assertionMethod",
"proofValue": "z4oey5q2M3XKaxup3tmzN4DRFTLVqpLMweBrSxMY2xHX5XTYVQeVbY8nQA
VHMrXFkXJpmEcqdoDwLWxaqA3Q1geV6"
}
}
The proof example above uses the Ed25519Signature2020
proof
type to produce a verifiable digital proof by canonicalizing the input data
using the RDF Dataset Canonicalization algorithm [RDF-DATASET-C14N] and then
digitally signing it using an Ed25519 elliptic curve signature.
Ed25519Signature2020
definition to a single supported
mechanism; specify its identifier as a URL. In order to make it easy to
specify a variety of combinations of algorithms, introduce a core
type DataIntegrityProof
that allows for easy filtering/discover of
proof nodes, but that type on its own doesn't specify any default
proof or hash algorithms, those need to be given via other properties in the
nodes.The pattern that Data Integrity Signatures use presently leads to a proliferation in signature types and JSON-LD Contexts. This proliferation can be avoided without any loss of the security characteristics of tightly binding a cryptography suite version to one or more acceptable public keys. The following signature suites are currently being contemplated: eddsa-2022, nist-ecdsa-2022, koblitz-ecdsa-2022, rsa-2022, pgp-2022, bbs-2022, eascdsa-2022, ibsa-2022, and jws-2022.
{
"@context": ["https://w3id.org/security/data-integrity/v1"],
"type": "DataIntegrityProof",
"cryptosuite": "ecdsa-2022",
"created": "2022-11-29T20:35:38Z",
"verificationMethod": "did:example:123456789abcdefghi#keys-1",
"proofPurpose": "assertionMethod",
"proofValue": "z2rb7doJxczUFBTdV5F5pehtbUXPDUgKVugZZ99jniVXCUpojJ9PqLYV
evMeB1gCyJ4HqpnTyQwaoRPWaD3afEZboXCBTdV5F5pehtbUXPDUgKVugUpoj"
}
The Data Integrity specification supports the concept of multiple proofs in a single document. There are two types of multi-proof approaches that are identified: Proof Sets (un-ordered) and Proof Chains (ordered).
A proof set is useful when the same data needs to be secured by multiple
entities, but where the order of proofs does not matter,
such as in the case of a set of signatures on a contract. A proof set,
which has no order, is represented by associating a set of proofs
with the proof
key in a document.
{
"@context": [
{"title": "https://schema.org/title"},
"https://w3id.org/security/data-integrity/v1"
],
"title": "Hello world!",
"proof": [{
"type": "DataIntegrityProof",
"cryptosuite": "eddsa-2022",
"created": "2020-11-05T19:23:24Z",
"verificationMethod": "https://ldi.example/issuer/1#z6MkjLrk3gKS2nnkeWcmcxi
ZPGskmesDpuwRBorgHxUXfxnG",
"proofPurpose": "assertionMethod",
"proofValue": "z4oey5q2M3XKaxup3tmzN4DRFTLVqpLMweBrSxMY2xHX5XTYVQeVbY8nQA
VHMrXFkXJpmEcqdoDwLWxaqA3Q1geV6"
}, {
"type": "DataIntegrityProof",
"cryptosuite": "eddsa-2022",
"created": "2020-11-05T13:08:49Z",
"verificationMethod": "https://pfps.example/issuer/2#z6MkGskxnGjLrk3gKS2mes
DpuwRBokeWcmrgHxUXfnncxiZP",
"proofPurpose": "assertionMethod",
"proofValue": "z5QLBrp19KiWXerb8ByPnAZ9wujVFN8PDsxxXeMoyvDqhZ6Qnzr5CG9876
zNht8BpStWi8H2Mi7XCY3inbLrZrm95"
}]
}
A proof chain is useful when the same data needs to be signed by
multiple entities and the order of when the proofs occurred matters,
such as in the case of a notary counter-signing a proof that had been
created on a document. A proof chain, where proof order needs to be preserved,
is expressed by providing at least one proof with an id
, such as a UUID as a URN,
and another proof with a previousProof
value that identifies the
previous proof.
{
"@context": [
{"title": "https://schema.org/title"},
"https://w3id.org/security/data-integrity/v1"
],
"title": "Hello world!",
"proof": [{
"id": "urn:uuid:60102d04-b51e-11ed-acfe-2fcd717666a7",
"type": "DataIntegrityProof",
"cryptosuite": "eddsa-2022",
"created": "2020-11-05T19:23:42Z",
"verificationMethod": "https://ldi.example/issuer/1#z6MkjLrk3gKS2nnkeWcmcxi
ZPGskmesDpuwRBorgHxUXfxnG",
"proofPurpose": "assertionMethod",
"proofValue": "zVbY8nQAVHMrXFkXJpmEcqdoDwLWxaqA3Q1geV64oey5q2M3XKaxup3tmzN4
DRFTLVqpLMweBrSxMY2xHX5XTYVQe"
}, {
"type": "DataIntegrityProof",
"cryptosuite": "eddsa-2022",
"created": "2020-11-05T21:28:14Z",
"verificationMethod": "https://pfps.example/issuer/2#z6MkGskxnGjLrk3gKS2mes
DpuwRBokeWcmrgHxUXfnncxiZP",
"proofPurpose": "assertionMethod",
"proofValue": "z6Qnzr5CG9876zNht8BpStWi8H2Mi7XCY3inbLrZrm955QLBrp19KiWXerb8
ByPnAZ9wujVFN8PDsxxXeMoyvDqhZ",
"previousProof": "urn:uuid:60102d04-b51e-11ed-acfe-2fcd717666a7"
}]
}
A proof that describes its purpose helps prevent it from being misused for some other purpose.
Add a mention of JWK's key_ops
parameter and WebCrypto's
KeyUsage
restrictions; explain that Proof Purpose serves a
different goal and allows for finer-grained restrictions.
Dave Longley suggested that proof purposes enable verifiers to know what the
proof creator's intent was so the message can't be accidentally abused for
another purpose, e.g., a message signed for the purpose of merely making an
assertion (and thus perhaps intended to be widely shared) being abused as a
message to authenticate to a service or take some action (invoke a capability).
It's a goal to keep the number of them limited to as few categories as are
really needed to accomplish this goal.
The following is a list of commonly used proof purpose values.
Note: The Authorization Capabilities [ZCAP] specification defines additional
proof purposes for that use case, such as capabilityInvocation
and
capabilityDelegation
.
A controller document is a set of data that specifies one or more relationships between a controller and a set of data, such as a set of public cryptographic keys. The controller document SHOULD contain verification relationships that explicitly permit the use of certain verification methods for specific purposes.
A controller document can express verification methods, such as cryptographic public keys, which can be used to authenticate or authorize interactions with the controller or associated parties. For example, a cryptographic public key can be used as a verification method with respect to a digital signature; in such usage, it verifies that the signer could use the associated cryptographic private key. Verification methods might take many parameters. An example of this is a set of five cryptographic keys from which any three are required to contribute to a cryptographic threshold signature.
The verificationMethod
property is OPTIONAL. If present, the value
MUST be a set of verification
methods, where each verification method is expressed using a map. The verification method map MUST include the id
,
type
, controller
, and specific verification material
properties that are determined by the value of type
and are defined
in 2.3.1.1 Verification Material. A verification method MAY
include additional properties. Verification methods SHOULD be registered
in the Data Integrity Specification Registries [TBD - DIS-REGISTRIES].
The value of the id
property for a verification
method MUST be a string that conforms to the
conforms to the [URL] syntax.
type
property MUST be a string that references exactly one verification
method type. In order to maximize global interoperability, the
verification method type SHOULD be registered in the Data Integrity Specification
Registries [TBD -- DIS-REGISTRIES].
controller
property MUST be a string that conforms to the [URL] syntax.
{
"@context": [
"https://www.w3.org/ns/did/v1",
"https://w3id.org/security/data-integrity/v1"
]
"id": "did:example:123456789abcdefghi",
...
"verificationMethod": [{
"id": ...,
"type": ...,
"controller": ...,
"publicKeyJwk": ...
}, {
"id": ...,
"type": ...,
"controller": ...,
"publicKeyMultibase": ...
}]
}
The semantics of the controller
property are the same when the
subject of the relationship is the controller document as when the subject of
the relationship is a verification method, such as a cryptographic public
key. Since a key can't control itself, and the key controller cannot be inferred
from the controller document, it is necessary to explicitly express the identity
of the controller of the key. The difference is that the value of
controller
for a verification method is not
necessarily a controller. controllers are expressed
using the `controller` property at the highest level of the
controller document.
Verification material is any information that is used by a process that applies
a verification method. The type
of a verification
method is expected to be used to determine its compatibility with such
processes. Examples of verification material properties are
`publicKeyJwk or
publicKeyMultibase`. A
cryptographic suite specification is responsible for specifying the
verification method type
and its associated verification
material. For example, see JSON
Web Signature 2020 and Ed25519 Signature 2020.
For all registered verification method types and associated verification
material available for controllers, please see the Data Integrity
Specification Registries [TBD - DIS-REGISTRIES].
Ensuring that cryptographic suites are versioned and tightly scoped to a very small set of possible key types and signature schemes (ideally one key type and size and one signature output type) is a design goal for most Data Integrity cryptographic suites. Historically, this has been done by defining both the key type and the cryptographic suite that uses the key type in the same specification. The downside of doing so, however, is that there might be a proliferation of different key types in multikey that result in different cryptosuites defining the same key material differently. For example, one cryptosuite might use compressed Curve P-256 keys while another uses uncompressed values. If that occurs, it will harm interoperability. It will be important in the coming months to years to ensure that this does not happen by fully defining the multikey format in a separate specification so cryptosuite specifications, such as this one, can refer to the multikey specification, thus reducing the chances of multikey type proliferation and improving the chances of maximum interoperability for the multikey format.
To increase the likelihood of interoperable implementations, this specification limits the number of formats for expressing verification material in a controller document. The fewer formats that implementers have to implement, the more likely it will be that they will support all of them. This approach attempts to strike a delicate balance between ease of implementation and supporting formats that have historically had broad deployment. Two supported verification material properties are listed below:
The publicKeyJwk
property is OPTIONAL. If present, the value MUST
be a map representing a JSON Web Key that
conforms to [RFC7517]. The map MUST NOT
contain "d", or any other members of the private information class as described
in Registration
Template. It is RECOMMENDED that verification methods that use JWKs
[RFC7517] to represent their public keys use the value of kid
as
their fragment identifier. It is RECOMMENDED that JWK
kid
values are set to the public key fingerprint [RFC7638]. See
the first key in Example 9 for
an example of a public key with a compound key identifier.
The publicKeyMultibase
property is OPTIONAL. This feature is
non-normative. If present, the value MUST be a string representation of a [MULTIBASE] encoded
public key.
Note that the [MULTIBASE] specification is not yet a standard and is
subject to change. There might be some use cases for this data format
where `publicKeyMultibase is defined, to allow for
expression of public keys, but
privateKeyMultibase`
is not defined, to protect against accidental leakage of secret keys.
A verification method MUST NOT contain multiple verification material
properties for the same material. For example, expressing key material in a
verification method using both publicKeyJwk
and
publicKeyMultibase
at the same time is prohibited.
An example of a controller document containing verification methods using both properties above is shown below.
{ "@context": [ "https://www.w3.org/ns/did/v1", "https://w3id.org/security/suites/jws-2020/v1", "https://w3id.org/security/multikey/v1" ] "id": "did:example:123456789abcdefghi", ... "verificationMethod": [{ "id": "did:example:123#_Qq0UL2Fq651Q0Fjd6TvnYE-faHiOpRlPVQcY_-tA4A", "type": "JsonWebKey2020", // external (property value) "controller": "did:example:123", "publicKeyJwk": { "crv": "Ed25519", // external (property name) "x": "VCpo2LMLhn6iWku8MKvSLg2ZAoC-nlOyPVQaO3FxVeQ", // external (property name) "kty": "OKP", // external (property name) "kid": "_Qq0UL2Fq651Q0Fjd6TvnYE-faHiOpRlPVQcY_-tA4A" // external (property name) } }, { "id": "did:example:123456789abcdefghi#keys-1", "type": "Multikey", // external (property value) "controller": "did:example:pqrstuvwxyz0987654321", "publicKeyMultibase": "z6MkmM42vxfqZQsv4ehtTjFFxQ4sQKS2w6WR7emozFAn5cxu" }], ... }
The Multikey data model is a specific type of verification method that
utilizes the [MULTICODEC] specification to encode key types into a single
binary stream that is then encoded using the [MULTIBASE] specification.
To encode a Multikey, the verification method type
MUST be set to
Multikey
and the publicKeyMultibase
value MUST be a [MULTIBASE] encoded
[MULTICODEC] value. An example of a Multikey is provided below:
{ "@context": ["https://w3id.org/security/multikey/v1"], "id": "did:example:123456789abcdefghi#keys-1", "type": "Multikey", "controller": "did:example:123456789abcdefghi", "publicKeyMultibase": "z6MkmM42vxfqZQsv4ehtTjFFxQ4sQKS2w6WR7emozFAn5cxu" }
In the example above, the publicKeyMultibase
value starts with the letter z
,
which is the [MULTIBASE] header that conveys that the binary data is
base58-encoded using the Bitcoin base-encoding alphabet. The decoded binary data
[MULTICODEC] header is 0xed
, which specifies that the remaining data
is a 32-byte raw Ed25519 public key.
The Multikey data model is also capable of encoding secret keys, sometimes referred to as private keys.
{ "@context": ["https://w3id.org/security/suites/secrets/v1"], "id": "did:example:123456789abcdefghi#keys-1", "type": "Multikey", "controller": "did:example:123456789abcdefghi", "secretKeyMultibase": "z3u2fprgdREFtGakrHr6zLyTeTEZtivDnYCPZmcSt16EYCER" }
In the example above, the secretKeyMultibase
value starts with the letter z
,
which is the [MULTIBASE] header that conveys that the binary data is
base58-encoded using the Bitcoin base-encoding alphabet. The decoded binary data
[MULTICODEC] header is 0x1300
, which specifies that the remaining data
is a 32-byte raw Ed25519 private key.
Verification methods can be embedded in or referenced from properties associated with various verification relationships as described in 2.3.2 Verification Relationships. Referencing verification methods allows them to be used by more than one verification relationship.
If the value of a verification method property is a map, the verification method has been
embedded and its properties can be accessed directly. However, if the value is a
URL string, the verification method has
been included by reference and its properties will need to be retrieved from
elsewhere in the controller document or from another controller document. This
is done by dereferencing the URL and searching the resulting resource for a
verification method map with an
id
property whose value matches the URL.
{ ... "authentication": [ // this key is referenced and might be used by // more than one verification relationship "did:example:123456789abcdefghi#keys-1", // this key is embedded and may *only* be used for authentication { "id": "did:example:123456789abcdefghi#keys-2", "type": "Multikey", // external (property value) "controller": "did:example:123456789abcdefghi", "publicKeyMultibase": "z6MkmM42vxfqZQsv4ehtTjFFxQ4sQKS2w6WR7emozFAn5cxu" } ], ... }
A verification relationship expresses the relationship between the controller and a verification method.
Different verification relationships enable the associated verification methods to be used for different purposes. It is up to a verifier to ascertain the validity of a verification attempt by checking that the verification method used is contained in the appropriate verification relationship property of the controller document.
The verification relationship between the controller and the verification method is explicit in the controller document. Verification methods that are not associated with a particular verification relationship cannot be used for that verification relationship. For example, a verification method in the value of the `authentication` property cannot be used to engage in key agreement protocols with the controller—the value of the `keyAgreement` property needs to be used for that.
The controller document does not express revoked keys using a verification relationship. If a referenced verification method is not in the latest controller document used to dereference it, then that verification method is considered invalid or revoked.
The following sections define several useful verification relationships. A controller document MAY include any of these, or other properties, to express a specific verification relationship. In order to maximize global interoperability, any such properties used SHOULD be registered in the Data Integrity Specification Registries [TBD: DIS-REGISTRIES].
The authentication
verification relationship is used to
specify how the controller is expected to be authenticated, for
purposes such as logging into a website or engaging in any sort of
challenge-response protocol.
authentication
property is OPTIONAL. If present, the associated
value MUST be a set of one or more
verification methods. Each verification method MAY be embedded or
referenced.
{ "@context": [ "https://www.w3.org/ns/did/v1", "https://w3id.org/security/multikey/v1" ], "id": "did:example:123456789abcdefghi", ... "authentication": [ // this method can be used to authenticate as did:...fghi "did:example:123456789abcdefghi#keys-1", // this method is *only* approved for authentication, it may not // be used for any other proof purpose, so its full description is // embedded here rather than using only a reference { "id": "did:example:123456789abcdefghi#keys-2", "type": "Multikey", "controller": "did:example:123456789abcdefghi", "publicKeyMultibase": "z6MkmM42vxfqZQsv4ehtTjFFxQ4sQKS2w6WR7emozFAn5cxu" } ], ... }
If authentication is established, it is up to the application to decide what to do with that information.
This is useful to any authentication verifier that needs to check to
see if an entity that is attempting to authenticate is, in fact,
presenting a valid proof of authentication. When a verifier receives
some data (in some protocol-specific format) that contains a proof that was made
for the purpose of "authentication", and that says that an entity is identified
by the id
, then that verifier checks to ensure that the proof can be
verified using a verification method (e.g., public key) listed under
`authentication` in the controller document.
Note that the verification method indicated by the
`authentication` property of a controller document can
only be used to authenticate the controller. To
authenticate a different controller, the entity associated with
the value of controller
needs to authenticate with its
own controller document and associated
`authentication` verification relationship.
The assertionMethod
verification relationship is used to
specify how the controller is expected to express claims, such as for
the purposes of issuing a Verifiable Credential [VC-DATA-MODEL-2].
assertionMethod
property is OPTIONAL. If present, the
associated value MUST be a set of
one or more verification methods. Each verification method MAY be
embedded or referenced.
This property is useful, for example, during the processing of a verifiable credential by a verifier. During verification, a verifier checks to see if a verifiable credential contains a proof created by the controller by checking that the verification method used to assert the proof is associated with the `assertionMethod` property in the corresponding controller document.
{ "@context": [ "https://www.w3.org/ns/did/v1", "https://w3id.org/security/multikey/v1" ], "id": "did:example:123456789abcdefghi", ... "assertionMethod": [ // this method can be used to assert statements as did:...fghi "did:example:123456789abcdefghi#keys-1", // this method is *only* approved for assertion of statements, it is not // used for any other verification relationship, so its full description is // embedded here rather than using a reference { "id": "did:example:123456789abcdefghi#keys-2", "type": "Multikey", // external (property value) "controller": "did:example:123456789abcdefghi", "publicKeyMultibase": "z6MkmM42vxfqZQsv4ehtTjFFxQ4sQKS2w6WR7emozFAn5cxu" } ], ... }
The keyAgreement
verification relationship is used to
specify how an entity can generate encryption material in order to transmit
confidential information intended for the controller, such as for
the purposes of establishing a secure communication channel with the recipient.
keyAgreement
property is OPTIONAL. If present, the associated
value MUST be a set of one or more
verification methods. Each verification method MAY be embedded or
referenced.
An example of when this property is useful is when encrypting a message intended for the controller. In this case, the counterparty uses the cryptographic public key information in the verification method to wrap a decryption key for the recipient.
{ "@context": "https://www.w3.org/ns/did/v1", "id": "did:example:123456789abcdefghi", ... "keyAgreement": [ // this method can be used to perform key agreement as did:...fghi "did:example:123456789abcdefghi#keys-1", // this method is *only* approved for key agreement usage, it will not // be used for any other verification relationship, so its full description is // embedded here rather than using only a reference { "id": "did:example:123#zC9ByQ8aJs8vrNXyDhPHHNNMSHPcaSgNpjjsBYpMMjsTdS", "type": "X25519KeyAgreementKey2019", // external (property value) "controller": "did:example:123", "publicKeyMultibase": "z6LSn6p3HRxx1ZZk1dT9VwcfTBCYgtNWdzdDMKPZjShLNWG7" } ], ... }
The capabilityInvocation
verification relationship is used
to specify a verification method that might be used by the
controller to invoke a cryptographic capability, such as the
authorization to update the controller document.
capabilityInvocation
property is OPTIONAL. If present, the
associated value MUST be a set of
one or more verification methods. Each verification method MAY be
embedded or referenced.
An example of when this property is useful is when a controller needs to access a protected HTTP API that requires authorization in order to use it. In order to authorize when using the HTTP API, the controller uses a capability that is associated with a particular URL that is exposed via the HTTP API. The invocation of the capability could be expressed in a number of ways, e.g., as a digitally signed message that is placed into the HTTP Headers.
The server providing the HTTP API is the verifier of the capability and it would need to verify that the verification method referred to by the invoked capability exists in the `capabilityInvocation` property of the controller document. The verifier would also check to make sure that the action being performed is valid and the capability is appropriate for the resource being accessed. If the verification is successful, the server has cryptographically determined that the invoker is authorized to access the protected resource.
{ "@context": [ "https://www.w3.org/ns/did/v1", "https://w3id.org/security/multikey/v1" ], "id": "did:example:123456789abcdefghi", ... "capabilityInvocation": [ // this method can be used to invoke capabilities as did:...fghi "did:example:123456789abcdefghi#keys-1", // this method is *only* approved for capability invocation usage, it will not // be used for any other verification relationship, so its full description is // embedded here rather than using only a reference { "id": "did:example:123456789abcdefghi#keys-2", "type": "Multikey", // external (property value) "controller": "did:example:123456789abcdefghi", "publicKeyMultibase": "z6MkmM42vxfqZQsv4ehtTjFFxQ4sQKS2w6WR7emozFAn5cxu" } ], ... }
The capabilityDelegation
verification relationship is used
to specify a mechanism that might be used by the controller to delegate
a cryptographic capability to another party, such as delegating the authority
to access a specific HTTP API to a subordinate.
capabilityDelegation
property is OPTIONAL. If present, the
associated value MUST be a set of
one or more verification methods. Each verification method MAY be
embedded or referenced.
An example of when this property is useful is when a controller chooses
to delegate their capability to access a protected HTTP API to a party other
than themselves. In order to delegate the capability, the controller
would use a verification method associated with the
capabilityDelegation
verification relationship to
cryptographically sign the capability over to another controller. The
delegate would then use the capability in a manner that is similar to the
example described in 2.3.2.4 Capability Invocation.
{ "@context": [ "https://www.w3.org/ns/did/v1", "https://w3id.org/security/multikey/v1" ], "id": "did:example:123456789abcdefghi", ... "capabilityDelegation": [ // this method can be used to perform capability delegation as did:...fghi "did:example:123456789abcdefghi#keys-1", // this method is *only* approved for granting capabilities; it will not // be used for any other verification relationship, so its full description is // embedded here rather than using only a reference { "id": "did:example:123456789abcdefghi#keys-2", "type": "Multikey", // external (property value) "controller": "did:example:123456789abcdefghi", "publicKeyMultibase": "z6MkmM42vxfqZQsv4ehtTjFFxQ4sQKS2w6WR7emozFAn5cxu" } ], ... }
The term Linked Data is used to describe a recommended best practice for exposing, sharing, and connecting information on the Web using standards, such as URLs, to identify things and their properties. When information is presented as Linked Data, other related information can be easily discovered and new information can be easily linked to it. Linked Data is extensible in a decentralized way, greatly reducing barriers to large scale integration.
With the increase in usage of Linked Data for a variety of applications, there is a need to be able to verify the authenticity and integrity of Linked Data documents. This specification adds authentication and integrity protection to data documents through the use of mathematical proofs without sacrificing Linked Data features such as extensibility and composability.
While this specification provides mechanisms to digitally sign Linked Data, the use of Linked Data is not necessary to gain some of the advantages provided by this specification.
A data integrity proof is designed to be easy to use by developers and therefore
strives to minimize the amount of information one has to remember to generate a
proof. Often, just the cryptographic suite name (e.g.
eddsa-2022
) is required from developers to initiate the creation of
a proof. These cryptographic suites are often created or reviewed by
people that have the requisite cryptographic training to ensure that safe
combinations of cryptographic primitives are used. This section specifies the
requirements for authoring cryptographic suite specifications.
The requirements for all data integrity cryptographic suite specifications are as follows:
type
and any
parameters that can be used with the suite.
The following language was deemed to be contentious: The specification MUST
provide a link to an interoperability test report to document which
implementations are conformant with the cryptographic suite specification.
The Working Group is seeking feedback on whether or not this is desired given
the important role that cryptographic suite specifications play in ensuring
data integrity.
A number of cryptographic suites follow the same basic pattern when expressing a
data integrity proof. This section specifies that general design pattern, a
cryptographic suite type called a DataIntegrityProof
, which reduces the burden
of writing and implementing cryptographic suites through the reuse of design
primitives and source code.
When specifing a cryptographic suite that utilizes this design pattern, the
proof
value takes the following form:
type
property MUST contain the string DataIntegrityProof
.
cryptosuite
property MUST contain a string specifying the name of the
cryptosuite.
proofValue
property MUST be used, as specified in 2.1 Proofs.
Cryptographic suite designers MUST use mandatory proof
value properties
defined in Section 2.1 Proofs, and MAY define other properties
specific to their cryptographic suite.
The algorithms defined below are generalized in that they require a specific transformation algorithm, hashing algorithm, proof generation algorithm, and proof verification algorithm to be specified by a particular cryptographic suite (see Section 3. Cryptographic Suites).
At present the creation of the verification hash is delegated to the cryptographic suite specification when generating and verifying a proof. It is expected that this algorithm is going to be common to most cryptographic suites. It is predicted that the verification hash generation algorithm will eventually be defined in this specification.
The following algorithm specifies how to add a digital proof to a document, which can be used to verify the authenticity and integrity of an unsecured data document. Required inputs are an unsecured data document (unsecuredDocument) and proof options (options). The proof options MUST contain a type identifier for the cryptographic suite (type) and any other properties needed by the cryptographic suite type; an identifier for the verification method (verificationMethod) that can be used to verify the authenticity of the proof; and an [XMLSCHEMA11-2] combined date and time string (created) containing the current date and time, accurate to at least one second, in Universal Time Code format. A security domain (domain) and receiver-supplied challenge (challenge) might also be specified in the options. A secured data document is produced as output. Whenever this algorithm encodes strings, it MUST use UTF-8 encoding.
PROOF_GENERATION_ERROR
MUST be raised.
PROOF_GENERATION_ERROR
MUST be raised.
PROOF_GENERATION_ERROR
MUST be raised.
PROOF_GENERATION_ERROR
MUST be
raised.
While the output of the hashing algorithm
can be a single value, such as a 32 byte SHA2-256 value, implementers are
advised that some cryptographic suite(s) might define hashData
to be
comprised of multiple values that might be processed independently in the proof
generation algorithm. For example, this approach is known to be taken in certain
cryptographic suites that allow selective disclosure or unlinkability via the
digital proof.
The following algorithm specifies how to check the authenticity and integrity of a secured data document by verifying its digital proof. Required inputs are a secured data document (securedDocument) and proof options (options). A verification result is produced as output.
MALFORMED_PROOF_ERROR
MUST be raised.
MALFORMED_PROOF_ERROR
MUST be raised.
MISMATCHED_PROOF_PURPOSE_ERROR
MUST be raised.
proof
value removed.
CREATED_TIME_DEVIATION_ERROR
MUST be raised.
INVALID_DOMAIN_ERROR
MUST be raised.
INVALID_CHALLENGE_ERROR
MUST be
raised.
Specify how proof.verificationMethod can be obtained,
e.g., through some out-of-band process, by
dereferencing its URL identifier, etc. It is expected that the public key
is retrieved by dereferencing its URL identifier in the proof
node
of the default graph of securedDocument. The algorithm needs to
confirm that the verification method that describes the cryptographic
material, such as the public key, specifies its controller, and
that its controllers's URL identifier can be dereferenced to reveal a
bi-directional link back to the key. Ensure that the key's controller is a
trusted entity before proceeding to the next step.
The following section describes security considerations that developers implementing this specification should be aware of in order to create secure software.
Cryptography secures information through the use of secrets. Knowledge of the necessary secret makes it computationally easy to access certain information. The same information can be accessed if a computationally-difficult, brute-force effort successfully guesses the secret. All modern cryptography requires the computationally difficult approach to remain difficult throughout time, which does not always hold due to breakthroughs in science and mathematics. That is to say that Cryptography has a shelf life.
This specification plans for the obsolescence of all cryptographic approaches by asserting that whatever cryptography is in use today is highly likely to be broken over time. Software systems have to be able to change the cryptography in use over time in order to continue to secure information. Such changes might involve increasing required secret sizes or modifications to the cryptographic primitives used. However, some combinations of cryptographic parameters might actually reduce security. Given these assumptions, systems need to be able to distinguish different combinations of safe cryptographic parameters, also known as cryptographic suites, from one another. When identifying or versioning cryptographic suites, there are several approaches that can be taken which include: parameters, numbers, and dates.
Parametric versioning specifies the particular cryptographic parameters that are
employed in a cryptographic suite. For example, one could use an identifier such
as RSASSA-PKCS1-v1_5-SHA1
. The benefit to this scheme is that a well-trained
cryptographer will be able to determine all of the parameters in play by the
identifier. The drawback to this scheme is that most of the population that
uses these sorts of identifiers are not well trained and thus will not understand
that the previously mentioned identifier is a cryptographic suite that is no
longer safe to use. Additionally, this lack of knowledge might lead software
developers to generalize the parsing of cryptographic suite identifiers
such that any combination of cryptographic primitives becomes acceptable,
resulting in reduced security. Ideally, cryptographic suites are implemented
in software as specific, acceptable profiles of cryptographic parameters instead.
Numbered versioning might specify a major and minor version number such as
1.0
or 2.1
. Numbered versioning conveys a specific order and suggests that
higher version numbers are more capable than lower version numbers. The benefit
of this approach is that it removes complex parameters that less expert
developers might not understand with a simpler model that conveys that an
upgrade might be appropriate. The drawback of this approach is that its not
clear if an upgrade is necessary, as software version number increases often
don't require an upgrade for the software to continue functioning. This can
lead to developers thinking their usage of a particular version is safe, when
it is not. Ideally, additional signals would be given to developers that use
cryptographic suites in their software that periodic reviews of those
suites for continued security are required.
Date-based versioning specifies a particular release date for a specific cryptographic suite. The benefit of a date, such as a year, is that it is immediately clear to a developer if the date is relatively old or new. Seeing an old date might prompt the developer to go searching for a newer cryptographic suite, where as a parametric or number-based versioning scheme might not. The downside of a date-based version is that some cryptographic suites might not expire for 5-10 years, prompting the developer to go searching for a newer cryptographic suite only to not find one that is newer. While this might be an inconvenience, it is one that results in safer ecosystem behavior.
The following text is currently under debate:
It is highly encouraged that cryptographic suite identifiers are versioned
using a year designation. For example, the cryptographic suite identifier
ecdsa-2022
implies that the suite is probably an acceptable of ECDSA in the
year 2025, but might not be a safe choice in the year 2042. A date-based
versioning mechanism, however, is not enough by itself. All cryptographic
suites that follow this specification are intended to be registered
[VC-EXTENSION-REGISTRY] in a way that clearly signal which cryptosuites
are deprecated, standardized, or experimental. Cryptosuite registration will
follow CFRG, IETF, NIST, FIPS, and safecurves guidance. Use of deprecated suites
are expected to throw errors in implementations unless a useUnsafeCryptosuites
option is used specifying exactly the unsafe cryptosuite to use.
Use of experimental suites are expected to throw errors in implementations
unless a useExperimentalCryptosuites
option is used specifying exactly
the experimental cryptosuite to use.
The following text is debated due to the governance rules of the VC
Extension Registry not being finalized:
Developers are urged to always refer to the [VC-EXTENSION-REGISTRY] for
the latest standardized cryptography suites as well as cross-checking their
usage against, at least, the
COSE Algorithms Registry, the
JOSE Algorithms Registry, and the latest
FIPS Digital Signature Standard
guidance. Depending on your jurisdiction you may also need to consider
additional guidance from
ETSI
or other regional regulatory and standards bodies.
Modern cryptographic algorithms provide a number of tunable parameters and options to ensure that the algorithms can meet the varied requirements of different use cases. For example, embedded systems have limited processing and memory environments and might not have the resources to generate the strongest digital signatures for a given algorithm. Other environments, like financial trading systems, might only need to protect data for a day while the trade is occurring, while other environments might need to protect data for multiple decades. To meet these needs, cryptographic algorithm designers often provide multiple ways to configure a cryptographic algorithm.
Cryptographic library implementers often take the specifications created by cryptographic algorithm designers and specification authors and implement them such that all options are available to the application developers that use their libraries. This can be due to not knowing which combination of features a particular application developer might need for a given cryptographic deployment. All options are often exposed to application developers.
Application developers that use cryptographic libraries often do not have the requisite cryptographic expertise and knowledge necessary to appropriately select cryptographic parameters and options for a given application. This lack of expertise can lead to an inappropriate selection of cryptographic parameters and options for a particular application.
This specification sets the priority of constituencies to protect application developers over cryptographic library implementers over cryptographic specification authors over cryptographic algorithm designers. Given these priorities, the following recommendations are made:
The guidance above is meant to ensure that useful cryptographic options and parameters are provided at the lower layers of the architecture while not exposing those options and parameters to application developers who may not fully understand the balancing benefits and drawbacks of each option.
The VCWG is seeking guidance on adding language to allow the use of experimental or deprecated cryptography. By default, those features will be disabled and will require the application developer to specifically allow use on a per-cryptographic suite basis. There will be requirements for all implementing libraries to throw errors or warnings when deprecated or experimental options are selected without the appropriate override flags.
Cryptographic agility is a practice by which one designs frequently connected information security systems to support switching between multiple cryptographic primitives and/or algorithms. The primary goal of cryptographic agility is to enable systems to rapidly adapt to new cryptographic primitives and algorithms without making disruptive changes to the systems' infrastructure. Thus, when a particular cryptographic primitive, such as the SHA-1 algorithm, is determined to be no longer safe to use, systems can be reconfigured to use a newer primitive via a simple configuration file change.
Cryptographic agility is most effective when the client and the server in the information security system are in regular contact. However, when the messages protected by a particular cryptographic algorithm are long-lived, as with Verifiable Credentials, and/or when the client (holder) might not be able to easily recontact the server (issuer), then cryptographic agility does not provide the desired protections.
Cryptographic layering is a practice where one designs rarely connected information security systems to employ multiple primitives and/or algorithms at the same time. The primary goal of cryptographic layering is to enable systems to survive the failure or one or more cryptographic algorithms or primitives without losing cryptographic protection on the payload. For example, digitally signing a single piece of information using RSA, ECDSA, and Falcon algorithms in parallel would provide a mechanism that could survive the failure of two of these three digital signature algorithms. When a particular cryptographic protection is compromised, such as an RSA digital signature using 768-bit keys, systems can still utilize the non-compromised cryptographic protections to continue to protect the information. Developers are urged to take advantage of this feature for all signed content that might need to be protected for a year or longer.
This specification provides for both forms of agility. It provides for cryptographic agility, which allows one to easily switch from one algorithm to another. It also provides for cryptographic layering, which allows one to simultaneously use multiple cryptographic algorithms, typically in parallel, such that any of those used to protect information can be used without reliance on or requirement of the others, while still keeping the digital proof format easy to use for developers.
At times, it is beneficial to transform the data being protected during the cryptographic protection process. Such "in-line" transformation can enable a particular type of cryptographic protection to be agnostic to the data format it is carried in. For example, some Data Integrity cryptographic suites utilize RDF Dataset Canonicalization [RDF-DATASET-C14N] which transforms the initial representation into a canonical form [N-QUADS] that is then serialized, hashed, and digitally signed. As long as any syntax expressing the protected data can be transformed into this canonical form, the digital signature can be verified. This enables the same digital signature over the information to be expressed in JSON, CBOR, YAML, and other compatible syntaxes without having to create a cryptographic proof for every syntax.
Being able to express the same digital signature across a variety of syntaxes is beneficial because systems often have native data formats with which they operate. For example, some systems are written against JSON data, while others are written against CBOR data. Without transformation, systems that process their data internally as CBOR are required to store the digitally signed data structures as JSON (or vice-versa). This leads to double-storing data and can lead to increased security attack surface if the unsigned representation stored in databases accidentally deviates from the signed representation. By using transformations, the digital proof can live in the native data format to help prevent otherwise undetectable database drift over time.
This specification is designed to avoid requiring the duplication of signed information by utilizing "in-line" data transformations. Application developers are urged to work with cryptographically protected data in the native data format for their application and not separate storage of cryptographic proofs from the data being protected. Developers are also urged to regularly confirm that the cryptographically protected data has not been tampered with as it is written to and read from application storage.
The VCWG is seeking feedback on normative language that cryptographic suite implementers need to follow to ensure that they do not utilize data transformation mechanisms that can map to the same output. That is, given different inputs for canonicalization scheme #1 and canonicalization scheme #2, they must not produce the same output value. As an analogy, this is the same requirement for cryptographic hashing mechanisms and is why those schemes are designed to be collision resistant. Cryptographic canonicalization mechanisms have the same requirement. At present, this isn't a problem because the three expected canonicalization schemes — the Universal RDF Dataset Canonicalization Algorithm 2015 [RDF-DATASET-C14N], JSON Canonicalization Scheme [RFC8785], and a theoretical future base-encoding canonicalization — have entirely different outputs.
The VCWG is seeking feedback on whether to explain why modern canonicalization schemes are simpler than the far more complex XML Canonicalization schemes of the early 2000s. Some readers seem to be under the impression that all canonicalization is difficult and has to be avoided at all costs (including costs to application developers). The WG would like to understand if it would be helpful to include a section explaining why some simpler data syntaxes (such as JSON) are easier to canonicalize than more complex data syntaxes (such as XML).
The inspectability of application data has effects on system efficiency and developer productivity. When cryptographically protected application data, such as base-encoded binary data, is not easily processed by application subsystems, such as databases, it increases the effort of working with the cryptographically protected information. For example, a cryptographically protected payload that can be natively stored and indexed by a database will result in a simpler system that:
Similarly, a cryptographically protected payload that can be processed by multiple upstream networked systems increases the ability to properly layer security architectures. For example, if upstream systems do not have to repeatedly decode the incoming payload, it increases the ability for a system to distribute processing load by specializing upstream subsystems to actively combat attacks. While a digital signature needs to always be checked before taking substantive action, other upstream checks can be performed on transparent payloads — such as identifier-based rate limiting, signature expiration checking, or nonce/challenge checking — to reject obviously bad requests.
Additionally, if a developer is not able to easily view data in a system, the ability to easily audit or debug system correctness is hampered. For example, requiring application developers to cut-and-paste base-encoded application data makes development more challenging and increases the chances that obvious bugs will be missed because every message needs to go through a manually operated base-decoding tool.
There are times, however, where the correct design decision is to make data opaque. Data that does not need to be processed by other application subsystems, as well as data that does not need to be modified or accessed by an application developer, can be serialized into opaque formats. Examples include digital signature values, cryptographic key parameters, and other data fields that only need to be accessed by a cryptographic library and need not be modified by the application developer. There are also examples where data opacity is appropriate when the underlying subsystem does not expose the application developer to the underlying complexity of the opaque data, such as databases that perform encryption at rest. In these cases, the application developer continues to develop against transparent application data formats while the database manages the complexity of encrypting and decrypting the application data to and from long-term storage.
This specification strives to provide an architecture where application data remains in its native format and is not made opaque, while other cryptographic data, such as digital signatures, are kept in their opaque binary encoded form. Cryptographic suite implementers are urged to consider appropriate use of data opacity when designing their suites, and to weigh the design trade-offs when making application data opaque versus providing access to cryptographic data at the application layer.
Implementers must ensure that a verification method is bound to a particular controller by going from the verification method to the controller document, and then ensuring that the controller document also contains the verification method.
When an implementation is verifying a proof, it is imperative that it verify not only that the verification method used to generate the proof is listed in the controller document, but also that it was intended to be used to generate the proof that is being verified. This process is known as "verification relationship validation".
The process for verification relationship validation is outlined in Section 4.3 Retrieve Verification Method.
This process is used to ensure that cryptographic material, such as a private cryptographic key, is not misused by application to an unintended purpose. An example of cryptographic material misuse would be if a private cryptographic key meant to be used to issue a Verifiable Credential was instead used to log into a website (that is, for authentication). Not checking a verification relationship is dangerous because the restriction and protection profile for some cryptographic material could be determined by its intended use. For example, some applications could be trusted to use cryptographic material for only one purpose, or some cryptographic material could be more protected, such as through storage in a hardware security module in a data center versus as an unencrypted file on a laptop.
When an implementation is verifying a proof, it is imperative that it verify that the proof purpose match the intended use.
This process is used to ensure that proofs are not misused by an application for an unintended purpose, as this is dangerous for the proof creator. An example of misuse would be if a proof that stated its purpose was for securing assertions in verifiable credentials was instead used for authentication to log into a website. In this case, the proof creator attached proofs to any number of verifiable credentials that they expected to be distributed to an unbounded number of other parties. Any one of these parties could log into a website as the proof creator if the website erroneously accepted such a proof as authentication instead of its intended purpose.
One of the algorithmic processes used by this specification is canonicalization, which is a type of transformation. Canonicalization is the process of taking information that might be expressed in a variety of semantically equivalent ways as input, and expressing all output in a single way, called a "canonical form".
The security of a resulting data integrity proof that utilizes canonicalization is highly dependent on the correctness of the algorithm. For example, if a canonicalization algorithm converts two inputs that have different meanings into the same output, then the author's intentions can be misrepresented to a verifier. This can be used as an attack vector by adversaries.
Additionally, if semantically relevant information in an input is not present in the output, then an attacker could insert such information into a message without causing proof verification to fail. This is similar to another transformation that is commonly used when cryptographically signing messages: cryptographic hashing. If an attacker is able to produce the same cryptographic hash from a different input, then the cryptographic hash algorithm is not considered secure.
Implementers are strongly urged to ensure proper vetting of any canonicalization algorithms to be used for transformation of input to a hashing process. Proper vetting includes, at a minimum, association with a peer reviewed mathematical proof of algorithm correctness; multiple implementations and vetting by experts in a standards setting organization is preferred. Implementers are strongly urged not to invent or use new mechanisms unless they have formal training in information canonicalization and/or access to experts in the field who are capable of producing a peer reviewed mathematical proof of algorithm correctness.
The following section describes privacy considerations that developers implementing this specification should be aware of in order to create privacy enhancing software.
When a digitally-signed payload contains data that is seen by multiple verifiers, it becomes a point of correlation. An example of such data is a shopping loyalty card number. Correlatable data can be used for tracking purposes by verifiers, which can sometimes violate privacy expectations. The fact that some data can be used for tracking might not be immediately apparent. Examples of such correlatable data include, but are not limited to, a static digital signature or a cryptographic hash of an image.
It is possible to create a digitally-signed payload that does not have any correlatable tracking data while also providing some level of assurance that the payload is trustworthy for a given interaction. This characteristic is called unlinkability which ensures that no correlatable data are used in a digitally-signed payload while still providing some level of trust, the sufficiency of which must be determined by each verifier.
It is important to understand that not all use cases require or even permit unlinkability. There are use cases where linkability and correlation are required due to regulatory or safety reasons, such as correlating organizations and individuals that are shipping and storing hazardous materials. Unlinkability is useful when there is an expectation of privacy for a particular interaction.
There are at least two mechanisms that can provide some level of unlinkability. The first method is to ensure that no data value used in the message is ever repeated in a future message. The second is to ensure that any repeated data value provides adequate herd privacy such that it becomes practically impossible to correlate the entity that expects some level of privacy in the interaction.
A variety of methods can be used to achieve unlinkability. These methods include ensuring that a message is a single use bearer token with no information that can be used for the purposes of correlation, using attributes that ensure an adequate level of herd privacy, and the use of cryptosuites that enable the entity presenting a message to regenerate new signatures while not compromising the trust in the message being presented.
Selective disclosure is a technique that enables the recipient of a previously-signed message (that is, a message signed by its creator) to reveal only parts of the message without disturbing the verifiability of those parts. For example, one might selectively disclose a digital driver's license for the purpose of renting a car. This could involve revealing only the issuing authority, license number, birthday, and authorized motor vehicle class from the license. Note that in this case, the license number is correlatable information, but some amount of privacy is preserved because the driver's full name and address are not shared.
Not all software or cryptosuites are capable of providing selective disclosure.
If the author of a message wishes it to be selectively disclosable by
its recipient, then they need to enable selective disclosure on the specific
message, and both need to use a capable cryptosuite. The author might
also make it mandatory
to disclose certain parts of the message. A recipient that wants to selectively
disclose partial content of the message needs to utilize software that is able to perform the
technique. An example of a cryptosuite that supports selective disclosure is
bbs-2022
.
It is possible to selectively disclose information in a way that does not preserve unlinkability. For example, one might want to disclose the inspection results related to a shipment, which include the shipment identifier or lot number, which might have to be correlatable due to regulatory requirements. However, disclosure of the entire inspection result might not be required as selectively disclosing just the pass/fail status could be deemed adequate. For more information on disclosing information while preserving privacy, see Section 6.1 Unlinkability.
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