W3C Candidate Recommendation Draft
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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/.
The Working Group is actively seeking implementation feedback for this specification. In order to exit the Candidate Recommendation phase, the Working Group has set the requirement of at least two independent implementations for each mandatory feature in the specification. For details on the conformance testing process, see the test suites listed in the implementation report.
This document was published by the Verifiable Credentials Working Group as a Candidate Recommendation Draft using the Recommendation track.
Publication as a Candidate Recommendation does not imply endorsement by W3C and its Members. A Candidate Recommendation Draft integrates changes from the previous Candidate Recommendation that the Working Group intends to include in a subsequent Candidate Recommendation Snapshot.
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 03 November 2023 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. Examples of 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). Examples of cryptographic hash functions include SHA-3 and BLAKE-3.
Proof Generation is a process described by a proof serialization 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. Examples of proof serialization 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, REQUIRED, 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.
A conforming secured document is any concrete expression of the data model that follows the relevant normative requirements in Sections 2.1 Proofs, 2.2 Proof Purposes, 2.9 Contexts and Vocabularies, and 3.1 DataIntegrityProof.
A conforming controller document is any concrete expression of the data model that follows the relevant normative requirements in Sections 2.3 Controller Documents and 2.9 Contexts and Vocabularies.
A conforming verification method is any concrete expression of the data model that follows the relevant normative requirements in Sections 2.3.1 Verification Methods and 2.9 Contexts and Vocabularies.
A conforming document is either a conforming secured document, a conforming controller document, or a conforming verification method.
A conforming cryptographic suite specification is any document that follows the relevant normative requirements in Section 3. Cryptographic Suites.
A conforming processor is any algorithm realized as software and/or hardware that generates and/or consumes a conforming document according to the relevant normative statements in Section 4. Algorithms. Conforming processors MUST produce errors when non-conforming documents are consumed.
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 authentication.
This section specifies the data model that is used for expressing data integrity proofs, controller documents, and verification methods.
All of the data model properties and types in this specification map to URLs.
The vocabulary where these URLs are defined is the [SECURITY-VOCABULARY]. The
explicit mechanism that is used to perform this mapping in a secured document is
the @context
property.
The mapping mechanism is defined by JSON-LD [JSON-LD11]. To ensure a document
can be interoperably consumed without the use of a JSON-LD library, document authors
are advised to ensure that domain experts have 1) specified the expected order for
all values associated with a @context
property, 2) published cryptographic hashes
for each @context
file, and 3) deemed that the contents of each @context
file
are appropriate for the intended use case.
When a document is processed by a non-JSON-LD processor and there is a
requirement to use the same semantics as those used in a JSON-LD environment,
implementers are advised to 1) enforce the expected order and values in the
@context
property, and 2) ensure that each @context
file matches the known
cryptographic hashes for each @context
file.
Using static, versioned @context
files with published cryptographic hashes in
conjunction with JSON Schema is one acceptable approach to implementing the
mechanisms described above, which ensures proper term identification, typing,
and order, when a non-JSON-LD processor is used.
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].
When expressing a data integrity proof on an object, a
proof
property MUST be used. If present, its
value MUST be either a single object, or an unordered set of objects, expressed
using the properties below:
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.
dateTimeStamp
string.
expires
property is OPTIONAL. If present, it MUST be an [XMLSCHEMA11-2]
dateTimeStamp
string specifying when the proof expires.
domain
property is OPTIONAL. It conveys one or more security domains in which the
proof is meant to be used. If specified, the associated value MUST be either a
string, or an unordered set of strings. A verifier SHOULD use the value to
ensure that the proof was intended to be used in the security domain in which
the verifier is operating. The specification of the 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. Example domain values
include: domain.example
(DNS domain), https://domain.example:8443
(Web origin), mycorp-intranet
(bespoke 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 expressed with a header and encoding as described in Section
2.4 Multibase. The contents of this value are determined by a
specific cryptosuite and set to the proof value generated by the
Add Proof Algorithm for that cryptosuite. Alternative
properties with different encodings specified by the cryptosuite MAY be used,
instead of this property, to encode the data necessary to verify the digital
proof.
A proof can be added to a JSON document like the following:
{ "myWebsite": "https://hello.world.example/" };
The following proof secures the document above using the jcs-eddsa-2022
cryptography suite [DI-EDDSA], which produces a verifiable digital proof by
transforming the input data using the JSON Canonicalization Scheme (JCS)
[RFC8785] and then digitally signing it using an Edwards Digital Signature
Algorithm (EdDSA).
{ "myWebsite": "https://hello.world.example/", "proof": { "type": "DataIntegrityProof", "cryptosuite": "jcs-eddsa-2022", "created": "2023-03-05T19:23:24Z", "verificationMethod": "https://di.example/issuer#z6MkjLrk3gKS2nnkeWcmcxiZPGskmesDpuwRBorgHxUXfxnG", "proofPurpose": "assertionMethod", "proofValue": "zQeVbY4oey5q2M3XKaxup3tmzN4DRFTLVqpLMweBrSxMY2xHX5XTYV 8nQApmEcqaqA3Q1gVHMrXFkXJeV6doDwLWx" } }
Similarly, a proof can be added to a JSON-LD data document like the following:
{ "@context": {"myWebsite": "https://vocabulary.example/myWebsite"}, "myWebsite": "https://hello.world.example/" };
The following proof secures the document above by using the ecdsa-2019
cryptography suite [DI-ECDSA], which produces a verifiable digital proof by
transforming the input data using the RDF Dataset Canonicalization Scheme
[RDF-CANON] and then digitally signing it using the Elliptic Curve Digital
Signature Algorithm (ECDSA).
{ "@context": [ {"myWebsite": "https://vocabulary.example/myWebsite"}, "https://w3id.org/security/data-integrity/v2" ], "myWebsite": "https://hello.world.example/", "proof": { "type": "DataIntegrityProof", "cryptosuite": "ecdsa-2019", "created": "2020-06-11T19:14:04Z", "verificationMethod": "https://ldi.example/issuer#zDnaepBuvsQ8cpsWrVKw8fbpGpvPeNSjVPTWoq6cRqaYzBKVP", "proofPurpose": "assertionMethod", "proofValue": "zXb23ZkdakfJNUhiTEdwyE598X7RLrkjnXEADLQZ7vZyUGXX8cyJZR BkNw813SGsJHWrcpo4Y8hRJ7adYn35Eetq" } }
This specification enables the expression of dates and times, such as through
the created
and expires
properties. This information might be indirectly
exposed to an individual if a proof is processed and is detected to be outside
an allowable time range. When displaying date and time values related to the
validity of cryptographic proofs, implementers are advised to respect the
locale
and local calendar preferences of the individual [LTLI]. Conversion of
timestamps to local time values are expected to consider the time zone
expectations of the individual. See
Verifiable Credentials Data Model v2.0 for more details about
representing time values to individuals.
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": [ {"myWebsite": "https://vocabulary.example/myWebsite"}, "https://w3id.org/security/data-integrity/v2" ], "myWebsite": "https://hello.world.example/", "proof": [{ "type": "DataIntegrityProof", "cryptosuite": "eddsa-2022", "created": "2020-11-05T19:23:24Z", "verificationMethod": "https://ldi.example/issuer/1#z6MkjLrk3gKS2nnkeWcmcxiZPGskmesDpuwRBorgHxUXfxnG", "proofPurpose": "assertionMethod", "proofValue": "z4oey5q2M3XKaxup3tmzN4DRFTLVqpLMweBrSxMY2xHX5XTYVQeVbY8 nQAVHMrXFkXJpmEcqdoDwLWxaqA3Q1geV6" }, { "type": "DataIntegrityProof", "cryptosuite": "eddsa-2022", "created": "2020-11-05T13:08:49Z", "verificationMethod": "https://pfps.example/issuer/2#z6MkGskxnGjLrk3gK S2mesDpuwRBokeWcmrgHxUXfnncxiZP", "proofPurpose": "assertionMethod", "proofValue": "z5QLBrp19KiWXerb8ByPnAZ9wujVFN8PDsxxXeMoyvDqhZ6Qnzr5CG9 876zNht8BpStWi8H2Mi7XCY3inbLrZrm95" }] }
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": [ {"myWebsite": "https://vocabulary.example/myWebsite"}, "https://w3id.org/security/data-integrity/v2" ], "myWebsite": "https://hello.world.example/", "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#z6MkjLrk3gKS2nnkeWcmcxiZPGskmesDpuwRBorgHxUXfxnG", "proofPurpose": "assertionMethod", "proofValue": "zVbY8nQAVHMrXFkXJpmEcqdoDwLWxaqA3Q1geV64oey5q2M3XKaxup 3tmzN4DRFTLVqpLMweBrSxMY2xHX5XTYVQe" }, { "type": "DataIntegrityProof", "cryptosuite": "eddsa-2022", "created": "2020-11-05T21:28:14Z", "verificationMethod": "https://pfps.example/issuer/2#z6MkGskxnGjLrk3g KS2mesDpuwRBokeWcmrgHxUXfnncxiZP", "proofPurpose": "assertionMethod", "proofValue": "z6Qnzr5CG9876zNht8BpStWi8H2Mi7XCY3inbLrZrm955QLBrp19Ki WXerb8ByPnAZ9wujVFN8PDsxxXeMoyvDqhZ", "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.
There are many commonalities between this section on Controller Documents and similar sections in other securing mechanisms such as [VC-JOSE-COSE], as well as sections on similar concepts in specifications such as [DID-CORE]. The Working Group is currently discussing the possibility of moving this section to an independent Controller Document specification that can be referenced normatively. If this migration occurs, it is expected that there will be little to no impact on implementations, as the normative statements that exist in this section will remain in this or the new document as an additive set of requirements on top of the base Controller Document specification.
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 verificationMethod
property is REQUIRED for proofs, unlike controller
documents, for which it is optional. See section 2.1 Proofs.
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.
expires
property is OPTIONAL. It is set, in advance, by the
controller of a verification method to signal when that method
can no longer be used for verification purposes. If provided, it MUST be an
[XMLSCHEMA11-2] dateTimeStamp
string specifying when the
verification method SHOULD cease to be used. Once the value is set, it is
not expected to be updated, and systems depending on the value are expected to
not verify any proofs associated with the verification method at or after
the time of expiration.
revoked
property is OPTIONAL. It is set by the controller of a
verification method to signal when that method is to no longer to be used
for verification purposes, such as after a security compromise of the
verification method. If provided, it MUST be an [XMLSCHEMA11-2]
dateTimeStamp
string specifying when the verification method
SHOULD cease to be used. Once the value is set, it is not expected to be
updated, and systems depending on the value are expected to not verify any
proofs associated with the verification method at or after the time of
revocation.
{ "@context": [ "https://www.w3.org/ns/did/v1", "https://w3id.org/security/data-integrity/v2" ] "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 methods include JsonWebKey
and Multikey
.
A cryptographic suite specification is responsible for specifying the
verification method type
and its associated verification material
format. For examples, see
the Data Integrity ECDSA
Cryptosuites and
the Data Integrity EdDSA Cryptosuites. For a list of verification
method types, please see the [SECURITY-VOCABULARY].
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 easing implementation and providing support for formats that have historically had broad deployment.
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/jwk/v1", "https://w3id.org/security/multikey/v1" ] "id": "did:example:123456789abcdefghi", ... "verificationMethod": [{ "id": "did:example:123#_Qq0UL2Fq651Q0Fjd6TvnYE-faHiOpRlPVQcY_-tA4A", "type": "JsonWebKey", // 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 encodes key types into a single binary stream that is then encoded as a Multibase value as described in Section 2.4 Multibase.
When specifing a Multikey
, the object takes the following form:
type
property MUST contain the string Multikey
.
publicKeyMultibase
property is OPTIONAL. If present, its value MUST be a
Multibase encoded value as described in Section 2.4 Multibase.
secretKeyMultibase
property is OPTIONAL. If present, its value MUST be a
Multibase encoded value as described in Section 2.4 Multibase.
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 base-58-btc-encoded using the Bitcoin base-encoding alphabet. The
decoded binary data header is 0xed01
, 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, whose subtypes include symmetric keys and 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 base-58-btc-encoded using the Bitcoin base-encoding alphabet. The
decoded binary data header is 0x8026
, which specifies that the
remaining data is a 32-byte raw Ed25519 private key.
The JSON Web Key (JWK) data model is a specific type of verification method that uses the JWK specification [RFC7517] to encode key types into a set of parameters.
When specifing a JsonWebKey
, the object takes the following form:
type
property MUST contain the string JsonWebKey
.
publicKeyJwk
property is OPTIONAL. If present, its value MUST
be a map representing a JSON Web Key that
conforms to [RFC7517]. The map MUST NOT
include any members of the private information class, such as d
, as described
in the JWK
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 8 for an example of a
public key with a compound key identifier.
secretKeyJwk
property is OPTIONAL. If present, its value MUST be a map representing a JSON Web Key that conforms
to [RFC7517].
An example of an object that conforms to this data model is provided below:
{ "@context": ["https://www.w3.org/ns/security/jwk/v1"], "id": "did:example:123456789abcdefghi#key-1", "type": "JsonWebKey", "controller": "did:example:123456789abcdefghi", "publicKeyJwk": { "kty": "OKP", "alg": "EdDSA" "crv": "Ed25519", "kid": "key-1", "x": "_1EiHquO2aUx9JARSu0P8jdYT_OVneYxYOnOMAmUcFI", } }
In the example above, the publicKeyJwk
value contains the JSON Web Key.
The kty
property encodes the key type of "OKP", which means
"Octet string key pairs". The alg
property identifies the algorithm intended
for use with the public key. The crv
property identifies the particular
curve type of the public key. The kid
property specifies how the public key
might be referenced in software systems; if present, the kid
value SHOULD match the
id
property of the encapsulating JsonWebKey
object. Finally, the x
property specifies the point on the Ed25519 curve that is associated with the
public key.
The publicKeyJwk
property MUST NOT contain any property marked as
"Private" in any registry contained in the JOSE Registries [JOSE-REGISTRIES].
The JSON Web Key data model is also capable of encoding secret keys, sometimes referred to as private keys.
{ "@context": ["https://www.w3.org/ns/security/jwk/v1"], "id": "did:example:123456789abcdefghi#key-1", "type": "JsonWebKey", "controller": "did:example:123456789abcdefghi", "secretKeyJwk": { "kty": "OKP", "alg": "EdDSA" "crv": "Ed25519", "kid": "key-1", "d": "Q6JwjCUdThSnoxfXHSFt5C1nVFycY_ZpW7qVzK644_g", "x": "_1EiHquO2aUx9JARSu0P8jdYT_OVneYxYOnOMAmUcFI", } }
The private key example above is almost identical to the previous example of the
public key, except that the information is stored in the secretKeyJwk
property
(rather than the publicKeyJwk
), and the private key value is encoded in the d
property thereof (alongside the x
property, which still specifies the point on
the Ed25519 curve that is associated with the public 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-2.0].
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 [MULTIBASE] specification has been dispatched at IETF and may be standardized there. There is active discussion on this initiative in the Multiformats mailing list at IETF. If the Multibase draft is stabilized before this specification goes to the Proposed Recommendation phase, the table below will be replaced with normative references to the Multibase specification at IETF. It is the intention of the Working Group to ensure alignment between the Multibase values used in this specification and the Multibase values defined by the current Multibase community and any potential future IETF Multiformats Working Group.
A Multibase string includes a single character header which identifies the base and encoding alphabet used to encode a binary value, followed by the encoded binary value (using that base and alphabet). The common Multibase header values and their associated base encoding alphabets as provided below are normative:
Multibase Header | Description |
---|---|
u |
The base-64-url-no-pad alphabet is used to encode the bytes. The base-alphabet
consists of the following characters, in order:
ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_
|
z |
The base-58-btc alphabet is used to encode the bytes. The base-alphabet consists
of the following characters, in order:
123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz
|
Other Multibase encoding values MAY be used, but interoperability is not guaranteed between implementations using such values.
To base-encode a binary value into a Multibase string, an implementation MUST apply the algorithm in Section 4.1 Base Encode to the binary value, with the desired base encoding and alphabet from the table above, ensuring to prepend the associated Multibase header from the table above to the result. Any algorithm with equivalent output MAY be used.
To base-decode a Multibase string, an implementation MUST apply the algorithm in Section 4.2 Base Decode to the string following the first character (Multibase header), with the alphabet associated with the Multibase header. Any algorithm with equivalent output MAY be used.
The [MULTIHASH] specification has been dispatched at IETF and may be standardized. There is active discussion on this initiative in the Multiformats mailing list at IETF. If the IETF draft is stabilized before this specification goes to the Proposed Recommendation phase, the table below will be replaced with normative references to the Multihash specification. It is the intention of the Working Group to ensure alignment between the Multihash values used in this specification and the Multihash values defined by the current Multihash community and any potential future IETF Multiformats Working Group.
A Multihash value starts with a binary header, which identifies the specific cryptographic hash algorithm and parameters used to generate the digest, followed by the cryptographic digest value. The normative Multihash header values defined by this specification, and their associated output sizes and associated specifications, are provided below:
Multihash Identifier | Multihash Header | Description |
---|---|---|
sha2-256 |
0x12 |
SHA-2 with 256 bits (32 bytes) of output, as defined by [RFC6234]. |
sha2-384 |
0x20 |
SHA-2 with 384 bits (48 bytes) of output, as defined by [RFC6234]. |
sha3-256 |
0x16 |
SHA-3 with 256 bits (32 bytes) of output, as defined by [SHA3]. |
sha3-384 |
0x15 |
SHA-3 with 384 bits (48 bytes) of output, as defined by [SHA3]. |
Other Multihash encoding values MAY be used, but interoperability is not guaranteed between implementations.
To encode to a Multihash value, an implementation MUST prepend the associated Multihash header value to the cryptographic hash value.
To decode a Multihash value, an implementation MUST remove the prepended Multihash header value, which identifies the type of cryptographic hashing algorithm as well as its output length, leaving the raw cryptographic hash value which MUST match the output length associated with the Multihash header.
The Working Group is currently attempting to determine whether cryptographic hash
expression formats can be unified across all of the VCWG core specifications.
Candidates for this mechanism include digestSRI
and digestMultibase
. There
are arguments for and against unification that the WG is currently debating.
When a link to an external resource is included in a conforming secured document, it is desirable to know whether the resource that is identified has changed since the proof was created. This applies to cases where there is an external resource that is remotely retrieved as well as to cases where the verifier might have a locally cached copy of the resource.
To enable confirmation that a resource referenced by a conforming secured document
has not changed since the document was secured, an implementer MAY include a
property named digestMultibase
in any object
that includes an id
property. If present, the digestMultibase
value MUST be
a single string value, or an array of string values that are
Multibase-encoded Multihash
values.
An example of a resource integrity protected object is shown below:
{ ... "image": { "id": "https://university.example.org/images/58473", "digestMultibase": "zQmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n" }, ... }
Implementers are urged to consult appropriate sources, such as the FIPS 180-4 Secure Hash Standard and the Commercial National Security Algorithm Suite 2.0 to ensure that they are choosing a hash algorithm that is appropriate for their use case.
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.
Cryptographic suites that implement this specification can be used to secure verifiable credentials and verifiable presentations. Implementers that are addressing those use cases are cautioned that additional checks might be appropriate when processing those types of documents.
There are some use cases where it is important to ensure that the
verification method used in a proof is associated with the
issuer
in a
verifiable credential, or the
holder
in a
verifiable presentation, during the process of
validation. One
way to check for such an association is to ensure that the value of the
controller
property of a proof's verification method
matches the URL value used to identify the
issuer
or
holder
, respectively.
This particular association indicates that the
issuer
or
holder
, respectively,
is the controller of the verification method used to verify
the proof.
Document authors and implementers are advised to understand the difference
between the validity period of a proof, which is expressed
using the created
and expires
properties, and the validity period of
a credential,
which is expressed using the
validFrom
and
validUntil
properties.
While these properties might sometimes express the same validity periods, at
other times they might not be aligned. When verifying a
proof, it is important to ensure that the time of interest
(which might be the current time or any other time) is within the
validity period for the proof (that is, between
created
and
expires
).
When validating a
verifiable credential, it is important to ensure that the time of
interest is within the validity period for the
credential (that is,
betweeen
validFrom
and
validUntil
). Note that a
failure to validate either the validity period for the proof, or the validity period for the
credential, might result
in accepting data that ought to have been rejected.
Finally, implementers are also urged to understand that there is a difference
between the revocation time and
expiration time for a verification method,
and the revocation information associated with a verifiable credential.
The revocation time and
expiration time for a verification method
are expressed using the revocation
and expires
properties, respectively, and
are related to events such as a private key being compromised or expiring and
can provide timing information which might reveal details about a controller
such as their security practices or when they might have been compromised. The
revocation information for a verifiable credential is expressed using
the credentialStatus
property and is related to events such as an individual
losing the privilege that is granted by the verifiable credential and
does not provide timing information, which enhances privacy.
This section lists cryptographic hash values that might change during the Candidate Recommendation phase based on implementer feedback that requires the referenced files to be modified.
Implementations that perform JSON-LD processing MUST treat the following JSON-LD context URLs as already resolved, where the resolved document matches the corresponding hash values below:
URL and Media Type | Content |
---|---|
https://w3id.org/security/data-integrity/v2 application/ld+json |
sha256: v/POI0jhSjPansxhJAP1fwepCBZ2HK77fRZfCCyBDs0= sha3-512: Sg1PLFxKyEYQns9Zr0BoYXtFeDNfrHUDNMkyq4QEWv |
https://w3id.org/security/multikey/v1 application/ld+json |
sha256: uiwYLeLZL35HGEvMqPzwvq7m05hsUnv2ZMGVu8fFhZc= sha3-512: En0TOOp/cC10XtW/aQDtqKrEQZ2lRjGB/KAsJ+BQhRB |
https://w3id.org/security/jwk/v1 application/ld+json |
sha256: 9h/GLRVuGCl0rn/ye6lzf219aN7Sgzq9yBFqUoOI54k= sha3-512: VDH85TsaX6kH2nwmII0WXKzAi2MRNsJd+rJfYL5cw0b |
The security vocabulary terms that the JSON-LD contexts listed above resolve
to are in the https://w3id.org/security#
namespace. That is, all security terms in this vocabulary are of the form
https://w3id.org/security#TERM
, where TERM
is the name of a term.
Implementations that perform RDF processing MUST treat the following JSON-LD vocabulary URL as already resolved, where the resolved document matches the corresponding hash values below.
When dereferencing the https://w3id.org/security# URL, the data returned depends on HTTP content negotiation. These are as follows:
Media Type | Description and Cryptographic Hashes |
---|---|
application/ld+json |
The vocabulary in JSON-LD format [JSON-LD11]. sha256: LEaoTyf796eTaSlYWjfPe3Yb+poCW9TjWYTbFDmC0tc= sha3-512: f4DhJ3xhT8nT+GZ8UUZi4QC+HT//wXE2fRTgUP4UNw |
text/turtle |
The vocabulary in Turtle format [TURTLE]. sha256: McnhLyt7+/A/0iLb3CUXD0itNw+7bwwjtzOww/zwoyI= sha3-512: jZtZsqgPPPo+jphAcN8/St4VdRLLAmN3nEQhzs0twE |
text/html |
The vocabulary in HTML+RDFa Format [HTML-RDFA]. sha256: eUHP1xiSC157iTPDydZmxg/hvmX3g/nnCn+FO25d4dc= sha3-512: z53j8ryjVeX16Z/dby//ujhw37degwi09+LAZCTUB8 |
It is possible to confirm the digests listed above by running the following
command from a modern Unix command interface line:
curl -sL -H "Accept: <MEDIA_TYPE>" <DOCUMENT_URL> | openssl dgst -<DIGEST_ALGORITHM> -binary | openssl base64 -nopad -a
.
Authors of application-specific vocabularies and specifications SHOULD ensure that their JSON-LD context and vocabulary files are permanently cacheable using the approaches to caching described above or a functionally equivalent mechanism.
Implementations MAY load application-specific JSON-LD context files from the network during development, but SHOULD permanently cache JSON-LD context files used in conforming documents in production settings to increase their security and privacy characteristics. Caching goals MAY be achieved through approaches such as those described above or functionally equivalent mechanisms.
Some applications, such as digital wallets, that are capable of holding arbitrary verifiable credentials or other data-integrity-protected documents, from any issuer and using any contexts, might need to be able to load externally linked resources, such as JSON-LD context files, in production settings. This is expected to increase user choice, scalability, and decentralized upgrades in the ecosystem over time. Authors of such applications are advised to read the security and privacy sections of this document for further considerations.
For further information regarding processing of JSON-LD contexts and vocabularies, see Verifiable Credentials v2.0: Base Context and Verifiable Credentials v2.0: Vocabularies.
The @context
property is used to ensure that implementations are using the
same semantics when terms in this specification are processed. For example, this
can be important when properties like type
are processed and its value, such
as DataIntegrityProof
, are used.
If an @context
property is not provided in a document that is being secured or
verified, or the Data Integrity terms used in the document are not mapped by
existing values in the @context
property, implementations MUST inject or add
an @context
property with a value of
https://w3id.org/security/data-integrity/v2
.
Context injection is expected to be unnecessary sometimes, such as when the Verifiable
Credential Data Model v2.0 context (https://www.w3.org/ns/credentials/v2
)
exists as a value in the @context
property, as that context maps all of the
necessary Data Integrity terms that were previously mapped by
https://w3id.org/security/data-integrity/v2
.
HTML processors are designed to continue processing if recoverable errors are detected. JSON-LD processors operate in a similar manner. This design philosophy was meant to ensure that developers could use only the parts of the JSON-LD language that they find useful, without causing the processor to throw errors on things that might not be important to the developer. Among other effects, this philosophy led to JSON-LD processors being designed to not throw errors, but rather warn developers, when encountering things such as undefined terms.
When converting from JSON-LD to an RDF Dataset, such as when canonicalizing a document [RDF-CANON], undefined terms and relative URLs can be dropped silently. When values are dropped, they are not protected by a digital proof. This creates a mismatch of expectations, where a developer, who is unaware of how a JSON-LD processor works, might think that certain data was being secured, and then be surprised to find that it was not, when no error was thrown. This specification requires that any recoverable loss of data when performing JSON-LD transformations result in an error, to avoid a mismatch in the security expectations of developers.
Implementations that use JSON-LD processing, such as RDF Dataset
Canonicalization [RDF-CANON], MUST throw an error, which SHOULD be
DATA_LOSS_DETECTION_ERROR
, when data is dropped by a JSON-LD processor,
such as when an undefined term is detected in an input document.
Similarly, since conforming secured documents can be transferred from one security domain to another, conforming processors that process the conforming secured document cannot assume any particular base URL for the document. When deserializing to RDF, implementations MUST ensure that the base URL is set to null.
This section defines datatypes that are used by this specification.
This specification encodes cryptographic suite identifiers as enumerable
strings, which is useful in processes that need to efficiently encode such
strings, such as compression algorithms. In environments that support data types
for string values, such as RDF [RDF-CONCEPTS], cryptographic identifier
content is indicated using a literal value whose datatype is set to
https://w3id.org/security#cryptosuiteString
.
The cryptosuiteString
datatype is defined as follows:
https://w3id.org/security#cryptosuiteString
cryptosuite
property, as defined in Section 3.1 DataIntegrityProof.
Multibase-encoded strings are used to encode binary
data into ASCII-only formats, which are useful in environments that cannot
directly represent binary values. This specification makes use of this encoding.
In environments that support data types for string values, such as RDF
[RDF-CONCEPTS], Multibase-encoded content is
indicated using a literal value whose datatype is set to
https://w3id.org/security#multibase
.
The multibase
datatype is defined as follows:
https://w3id.org/security#multibase
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.
@protected
keyword.
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 be a string that identifies the
cryptographic suite. If the processing environment supports subtypes
of string
, the type of the cryptosuite
value MUST be the
https://w3id.org/security#cryptosuiteString
subtype of string
.
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.
One of the design patterns seen in Data Integrity cryptosuites from
2012 to 2020 was use of the type
property to establish a specific type for a
cryptographic suite. For example, the
Ed25519Signature2020 cryptographic suite was one such specification. This
led to a greater burden on cryptographic suite implementations, where every new
cryptographic suite required a new JSON-LD Context to be specified, resulting
in a sub-optimal developer experience. A streamlined version of this design
pattern emerged in 2020, such that a developer would only need to include a
single JSON-LD Context to support all modern cryptographic suites. This
encouraged more modern cryptosuites — such as the EdDSA Cryptosuites
[DI-EDDSA] and the ECDSA Cryptosuites [DI-ECDSA] — to be built
based on the streamlined pattern described in this section.
To improve the developer experience, authors creating new Data Integrity
cryptographic suite specifications SHOULD use the modern pattern — where
the type
is set to DataIntegrityProof
; the cryptosuite
property carries
the identifier for the cryptosuite; and any cryptosuite-specific cryptographic
data is encapsulated (i.e., not directly exposed as application layer data)
within proofValue
. A list of cryptographic suite specifications that are
known to follow this pattern is provided in the
Proof types section of
the Verifiable Credentials Specifications Directory.
The algorithms defined below are generalized in that they require a specific transformation algorithm, hashing algorithm, proof serialization 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 algorithm that generates the verification hash will eventually be defined in this specification.
The following algorithm specifies how to encode an array of bytes, where each byte represents a base-256 value, to a different base representation that uses a particular base alphabet, such as base-64-url-no-pad or base-58-btc. The required inputs are the bytes, targetBase, and baseAlphabet. The output is a string that contains the base-encoded value. All mathematical operations MUST be performed using integer arithmetic. Alternatives to the algorithm provided below MAY be used as long as the outputs of the alternative algorithm remain the same.
0
, length to
0
, begin to 0
, and end to the length of
bytes.
0
byte
values in bytes.
1
to the
value of size.
0
. Set basePosition to
size minus 1
. Perform the following loop as long as carry
does not equal 0
or i is less than length, and
basePosition does not equal -1
.
256
and add
it to carry.
1
and increment i by 1
.
1
.
0
, increment
baseEncodingPosition. This step skips the leading zeros in the
base-encoded result.
function baseEncode(bytes, targetBase, baseAlphabet) {
let zeroes = 0;
let length = 0;
let begin = 0;
let end = bytes.length;
// count the number of leading bytes that are zero
while(begin !== end && bytes[begin] === 0) {
begin++;
zeroes++;
}
// allocate enough space to store the target base value
const baseExpansionFactor = Math.log(256) / Math.log(targetBase);
let size = Math.floor((end - begin) * baseExpansionFactor + 1);
let baseValue = new Uint8Array(size);
// process the entire input byte array
while(begin !== end) {
let carry = bytes[begin];
// for each byte in the array, perform base-expansion
let i = 0;
for(let basePosition = size - 1;
(carry !== 0 || i < length) && (basePosition !== -1);
basePosition--, i++) {
carry += Math.floor(256 * baseValue[basePosition]);
baseValue[basePosition] = Math.floor(carry % targetBase);
carry = Math.floor(carry / targetBase);
}
length = i;
begin++;
}
// skip leading zeroes in base-encoded result
let baseEncodingPosition = size - length;
while(baseEncodingPosition !== size &&
baseValue[baseEncodingPosition] === 0) {
baseEncodingPosition++;
}
// convert the base value to the base encoding
let baseEncoding = baseAlphabet.charAt(0).repeat(zeroes)
for(; baseEncodingPosition < size; ++baseEncodingPosition) {
baseEncoding += baseAlphabet.charAt(baseValue[baseEncodingPosition])
}
return baseEncoding;
}
The following algorithm specifies how to decode an array of bytes, where each byte represents a base-encoded value, to a different base representation that uses a particular base alphabet, such as base-64-url-no-pad or base-58-btc. The required inputs are the sourceEncoding, sourceBase, and baseAlphabet. The output is an array of bytes that contains the base-decoded value. All mathematical operations MUST be performed using integer arithmetic. Alternatives to the algorithm provided below MAY be used as long as the outputs of the alternative algorithm remain the same.
0
,
zeroes to 0
, and decodedLength to 0
.
256
) and then multiplying by the
length of sourceEncoding minus the leading zeroes. Add 1 to the value
of size.
0
. Set byteOffset
to decodedSize minus 1
. Perform the following loop as long as,
carry does not equal 0
or i is less than
decodedLength, and byteOffset does not equal -1
:
256
.
256
, ensuring that integer
division is used to perform the division.
1
and increment i by 1
.
1
.
0
, increment
decodedOffset by 1
. This step skips the leading zeros in the
final base-decoded byte array.
0
.
1
, copy all bytes in decodedBytes, up
to decodedSize, starting at offset decodedOffset to
finalBytes.
function baseDecode(sourceEncoding, sourceBase, baseAlphabet) {
// build the base-alphabet to integer value map
baseMap = {};
for(let i = 0; i < baseAlphabet.length; i++) {
baseMap[baseAlphabet[i]] = i;
}
// skip and count zero-byte values in the sourceEncoding
let sourceOffset = 0;
let zeroes = 0;
let decodedLength = 0;
while(sourceEncoding[sourceOffset] === baseAlphabet[0]) {
zeroes++;
sourceOffset++;
}
// allocate the decoded byte array
const baseContractionFactor = Math.log(sourceBase) / Math.log(256);
let decodedSize = Math.floor((
(sourceEncoding.length - sourceOffset) * baseContractionFactor) + 1);
let decodedBytes = new Uint8Array(decodedSize);
// perform base-conversion on the source encoding
while(sourceEncoding[sourceOffset]) {
// process each base-encoded number
let carry = baseMap[sourceEncoding[sourceOffset]];
// convert the base-encoded number by performing base-expansion
let i = 0
for(let byteOffset = decodedSize - 1;
(carry !== 0 || i < decodedLength) && (byteOffset !== -1);
byteOffset--, i++) {
carry += Math.floor(sourceBase * decodedBytes[byteOffset]);
decodedBytes[byteOffset] = Math.floor(carry % 256);
carry = Math.floor(carry / 256);
}
decodedLength = i;
sourceOffset++;
}
// skip leading zeros in the decoded byte array
let decodedOffset = decodedSize - decodedLength;
while(decodedOffset !== decodedSize && decodedBytes[decodedOffset] === 0) {
decodedOffset++;
}
// create the final byte array that has been base-decoded
let finalBytes = new Uint8Array(zeroes + (decodedSize - decodedOffset));
let j = zeroes;
while(decodedOffset !== decodedSize) {
finalBytes[j++] = decodedBytes[decodedOffset++];
}
return finalBytes;
}
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; an [XMLSCHEMA11-2] dateTimeStamp
string (created) containing the current date and time,
accurate to at least one second, in Universal Time Code format. A
security domain (domain) and/or a
receiver-supplied challenge (challenge) MAY 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.
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
serialization 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 incrementally add a proof to a proof set or proof chain starting with a secured document containing either a proof or proof set/chain. Required inputs are a secured data document (securedDocument) and proof options (options). The proof options (options) must satisfy the criteria in section 4.3 Add Proof. A new secured data document is produced as output. Whenever this algorithm encodes strings, it MUST use UTF-8 encoding.
previousProof
attribute and that
attribute is a string, check whether
an element of the allProofs has a matching id
attribute.
If not, an error MUST be raised and SHOULD convey an error type of
PROOF_GENERATION_ERROR. If the
previousProof
attribute is an array, check that each element of that
array is a string which matches the id
attribute of an element
of the allProofs. If not, an error MUST be raised and SHOULD convey
an error type of PROOF_GENERATION_ERROR.
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). The proof options MAY contain the following values that help protect against relay and replay attacks: expectedProofPurpose, domain, and challenge. The expectedProofPurpose value is used to ensure that the proof was generated by the proof creator for the expected reason by the verifier, such as "authentication". The domain value is used by the proof creator to lock a proof to a particular security domain, and used by the verifier to ensure that a proof is not being used across different security domains. The challenge value is used by the verifier to ensure that an attacker is not replaying previously created proofs. Additional options are expected to be provided by libraries to ensure that digital proof time stamps do not deviate more than an acceptable time frame for a given use case. A verification result is produced as output.
proof
value removed.
In a proof set or proof chain, a secured data document has a
proof
attribute which contains a list of proofs
(allProofs).
The following algorithm specifies how to check the authenticity and
integrity of a secured data document by verifying each
proof in the allProofs. Required inputs are a
secured data document (securedDocument). A list of
verification results is produced as output.
previousProof
attribute, modify the associated
isProofVerified by iteratively checking that the proof identified by
the id
value in previousProof, along with every
previousProof
up the chain, is valid. If previousProof
is an
array, then all previous proofs identified by the array elements
MUST be valid for the current proof to be considered valid. Cycle detection,
such as keeping a list of previously visited proofs, MUST be used to guard
against proof cycles. If a cycle is detected in a proof chain,
then a PROOF_CHAIN_CYCLE_ERROR
MUST be raised.
The following algorithm specifies how to safely retrieve a verification method, such as a cryptographic public key, by using a verification method identifier contained in a data integrity proof. Required inputs are a data integrity proof (proof) and a set of dereferencing options (options). A verification method is produced as output.
The following example provides a minimum conformant controller document containing a minimum conformant verification method as required by the algorithm in this section:
{
"id": "https://controller.example/123",
"verificationMethod": [{
"id": "https://controller.example/123#key-456",
"type": "ExampleVerificationMethodType",
"controller": "https://controller.example/123",
// public cryptographic material goes here
}],
"authentication": ["#key-456"]
}
The algorithms described in this specification, as well as in various cryptographic suite specifications, throw specific types of errors. Implementers might find it useful to convey these errors to other libraries or software systems. This section provides specific URLs, descriptions, and error codes for the errors, such that an ecosystem implementing technologies described by this specification might interoperate more effectively when errors occur.
When exposing these errors through an HTTP interface, implementers SHOULD use [RFC9457] to encode the error data structure. If [RFC9457] is used:
type
value of the error object MUST be a URL that starts with the value
https://w3id.org/security#
and ends with the value in the section listed
below.
code
value MUST be the integer code described in the table below
(in parentheses, beside the type name).
title
value SHOULD provide a short but specific human-readable string for
the error.
detail
value SHOULD provide a longer human-readable string for the error.
proofPurpose
value in a proof did not match the expected value. See
Section 4.5 Verify Proof.
domain
value in a proof did not match the expected value. See Section
4.5 Verify Proof.
challenge
value in a proof did not match the expected value. See Section
4.5 Verify Proof.
verificationMethod
value in a proof was malformed. See Section
4.7 Retrieve Verification Method.
id
value in a controller document was malformed. See Section
4.7 Retrieve Verification Method.
proofPurpose
property in the proof. See Section
4.7 Retrieve Verification Method.
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-SPECS] 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.
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.
Section 5.1 Versioning Cryptography Suites emphasized the importance of providing relatively easy to understand information concerning the timeliness of particular cryptographic suite, while section 5.2 Protecting Application Developers further emphasized minimizing the number of options to be specified. Indeed, section 3. Cryptographic Suites lists requirements for cryptographic suites which include detailed specification of algorithm, transformation, hashing, and serialization. Hence, the name of the cryptographic suite does not need to include all this detail, which implies the parametric versioning mentioned in section 5.1 Versioning Cryptography Suites is neither necessary nor desirable.
The recommended naming convention for cryptographic suites is a string composed of a signature algorithm identifier, separated by a hyphen from an option identifier (if the cryptosuite supports incompatible implementation options), followed by a hyphen and designation of the approximate year that the suite was proposed.
For example, the [DI-EDDSA] is based on EdDSA digital signatures, supports
two incompatible options based on canonicalization approaches, and was proposed
in roughly the year 2022, so it would have two different cryptosuite names:
eddsa-rdfc-2022
and eddsa-jcs-2022
.
Although the [DI-ECDSA] is based on ECDSA digital signatures, supports the
same two incompatible canonicalization approaches as [DI-EDDSA], and supports
two different levels of security (128 bit and 192 bit) via two alternative sets
of elliptic curves and hashes, it has only two cryptosuite names:
ecdsa-rdfc-2019
and ecdsa-jcs-2019
. The security level and corresponding
curves and hashes are determined from the multi-key format of the public key
used in validation.
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-CANON] 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.
Some transformations, such as RDF Dataset Canonicalization [RDF-CANON], have mitigations for input data sets that can be used by attackers to consume excessive processing cycles. This class of attack is called dataset poisoning, and all modern RDF Dataset canonicalizers are required to detect these sorts of bad inputs and halt processing. The test suites for RDF Dataset Canonicalization includes such poisoned datasets to ensure that such mitigations exist in all conforming implementations. Generally speaking, cryptographic suite specifications that use transformations are required to mitigate these sorts of attacks, and implementers are urged to ensure that the software libraries that they use enforce these mitigations. These attacks are in the same general category as any resource starvation attack, such as HTTP clients that deliberately slow connections, thus starving connections on the server. Implementers are advised to consider these sorts of attacks when implementing defensive security strategies.
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-CANON], 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 4.7 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.
The way in which a transformation, such as canonicalization, is performed can affect the security characteristics of a system. Selecting the best canonicalization mechanisms depends on the use case. Often, the simplest mechanism that satisfies the desired security requirements is the best choice. This section attempts to provide simple guidance to help implementers choose between the two main canonicalization mechanisms referred to in this specification, namely JSON Canonicalization Scheme [RFC8785] and RDF Dataset Canonicalization [RDF-CANON].
If an application only uses JSON and does not depend on any form of RDF semantics, then using a cryptography suite that uses JSON Canonicalization Scheme [RFC8785] is an attractive approach.
If an application uses JSON-LD and needs to secure the semantics of the document, then using a cryptography suite that uses RDF Dataset Canonicalization [RDF-CANON] is an attractive approach.
Implementers are also advised that other mechanisms that perform no transformations are available, that secure the data by wrapping it in a cryptographic envelope instead of embedding the proof in the data, such as JWTs [RFC7519] and CWTs [RFC8392]. These approaches have simplicity advantages in some use cases, at the expense of some of the benefits provided by the approach detailed in this specification.
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.
This specification is designed in such a way that no network requests are required when verifying a proof on a conforming secured document. Readers might note, however, that JSON-LD contexts and verification methods can contain URLs that might be retrieved over a network connection. This concern exists for any URL that might be loaded from the network during or after verification.
To the extent possible, implementers are urged to permanently or aggressively
cache such information to reduce the attack surface on an
implementation that might need to fetch such URLs over the network. For
example, caching techniques for JSON-LD contexts are
described in Section 2.9 Contexts and Vocabularies, and some
verification methods, such as did:key
[DID-KEY], do not need to be
fetched from the network at all.
When it is not possible to use cached information, such as when a specific HTTP URL-based instance of a verification method is encountered for the first time, implementers are cautioned to use defensive measures to mitigate denial-of-service attacks during any process that might fetch a resource from the network.
Since the technology to secure documents described by this specification is generalized in nature, the security implications of its use might not be immediately apparent to readers. To understand the sort of security concerns one might need to consider in a complete software system, implementers are urged to read about how this technology is used in the verifiable credentials ecosystem [VC-DATA-MODEL-2.0]; see the section on Verifiable Credential Security Considerations for more information.
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.
When using the previousProof
feature defined in
2.1.2 Proof Chains, implementations are required to digitally sign
over one or more previous proofs, so as to include them in the
secured payload. This inevitably exposes information related to each
entity that added a previous proof.
At minimum, the verification method for the previous proof, such as a public key, is seen by the creator of the next proof in a proof chain. This can be a privacy concern if the creator of the previous proof did not intend to be included in a proof chain, but is an inevitable outcome when adding a non-repudiable digital signature to a document of any kind.
It is possible to use more advanced cryptographic mechanisms, such as a group signature, to hide the identity of the signer of a message, and it is also possible for a Data Integrity cryptographic suite to mitigate this privacy concern.
Fingerprinting concerns exist for any URL that might be loaded from the network during or after proof verification. This specification is designed in such a way that no network requests are necessary when verifying a proof on a conforming secured document. Readers might note, however, that JSON-LD contexts and verification methods can contain resource URLs that might be retrieved over a network connection leading to fingerprinting concerns.
For example, creators of conforming documents might craft unique per-document URLs for JSON-LD contexts and verification methods. When verifying such a document, a verifier fetching that information from the network would reveal their interest in the conforming document to the creator of the document, which might lead to a mismatch in privacy expectations for any entity that is not the creator of the document.
Implementers are urged to follow the guidance in Section 5.12 Network Requests on URL caching and implementing defensively when fetching URLs from the network. Usage of techniques such as Oblivious HTTP to retrieve resources from the network, without revealing the client that is making the request, are encouraged. Additionally, heuristics might be used to determine whether creators of conforming documents are using fingerprinting URLs in a way that might violate privacy expectations. These heuristics could be used to display warnings to entities that might process documents containing suspected fingerprinting URLs.
The way in which a transformation, namely canonicalization, is performed can affect the privacy characteristics of a system. Selecting the best canonicalization mechanism depends on the use case. This section attempts to provide simple guidance to help implementers pick between the two main canonicalization mechanisms referred to in this specification, namely JSON Canonicalization Scheme [RFC8785] and RDF Dataset Canonicalization [RDF-CANON], from a privacy perspective.
If an application does not require performing a selective disclosure of information in a secured document, nor does it utilize JSON-LD, then JSON Canonicalization Scheme [RFC8785] is an attractive approach.
If an application uses JSON-LD and might require selective disclosure of information in a secured document, then using a cryptography suite that uses RDF Dataset Canonicalization [RDF-CANON] is an attractive approach.
Implementers are also advised that other selective disclosure mechanisms that perform no transformations are available, that secure the data by wrapping it in a cryptographic envelope instead of embedding the proof in the data, such as SD-JWTs [SD-JWT]. This approach has simplicity advantages in some use cases, at the expense of some of the benefits provided by the approach detailed in this specification.
Since the technology to secure documents described by this specification is generalized in nature, the privacy implications of its use might not be immediately apparent to readers. To understand the sort of privacy concerns one might need to consider in a complete software system, implementers are urged to read about how this technology is used in the verifiable credentials ecosystem [VC-DATA-MODEL-2.0]; see the section on Verifiable Credential Privacy Considerations for more information.
The following section describes accessibility considerations that developers implementing this specification are urged to consider in order to ensure that their software is usable by people with different cognitive, motor, and visual needs. As a general rule, this specification is used by system software and does not directly expose individuals to information subject to accessibility considerations. However, there are instances where individuals might be indirectly exposed to information expressed by this specification and thus the guidance below is provided for those situations.
This specification enables the expression of dates and times related to the validity period of cryptographic proofs. This information might be indirectly exposed to an individual if a proof is processed and is detected to be outside an allowable time range. When exposing these dates and times to an individual, implementers are urged to take into account cultural normas and locales when representing dates and times in display software. In addition to these considerations, presenting time values in a way that eases the cognitive burden on the individual receiving the information is a suggested best practice.
For example, when conveying the expiration date for a particular set of digitally signed information, implementers are urged to present the time of expiration using language that is easier to understand rather than language that optimizes for accuracy. Presenting the expiration time as "This ticket expired three days ago." is preferred over a phrase such as "This ticket expired on July 25th 2023 at 3:43 PM." The former provides a relative time that is easier to comprehend than the latter time, which requires the individual to do the calculation in their head and presumes that they are capable of doing such a calculation.
This section is non-normative.
This section contains the substantive changes that have been made to this specification over time.
Changes since the First Public Working Draft:
JsonWebKey
and Multikey
definitions and context files.
Ed25519Signature2020
and moved into separate specification.
nonce
and expires
to proofs.
revoked
and expires
to verification methods.
domain
property to allow for an array of values.
cryptosuiteString
type to proofs to enable proof compression.
digestMultibase
property and multibase
data type for securing remote
content.
dateTimeStamp
.
This section is non-normative.
The specification authors would like to thank the base-x software library contributors and the Bitcoin Core developers who wrote the original code, shared under an MIT License, found in Section 4.1 Base Encode and Section 4.2 Base Decode.
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