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This specification describes a Data Integrity Cryptosuite for use when generating a digital signature using the Elliptic Curve Digital Signature Algorithm (ECDSA).
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This is an experimental specification and is undergoing regular revisions. It is not fit for production deployment.
This document was published by the Verifiable Credentials Working Group as a Working Draft using the Recommendation track.
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This document is governed by the 2 November 2021 W3C Process Document.
This specification defines a cryptographic suite for the purpose of creating, and verifying proofs for ECDSA signatures in conformance with the Data Integrity [VC-DATA-INTEGRITY] specification. ECDSA signatures are specified in [FIPS-186-5] with elliptic curves P-256 and P-384 specified in [NIST-SP-800-186]. [FIPS-186-5] includes the deterministic ECDSA algorithm which is also specified in [RFC6979].
This specification uses either the RDF Dataset Canonicalization Algorithm [RDF-CANON] or the JSON Canonicalization Scheme [RFC8785] to transform the input document into its canonical form. It uses one of two mechanisms to digest and sign: SHA-256 [RFC6234] as the message digest algorithm and ECDSA with Curve P-256 as the signature algorithm, or SHA-384 [RFC6234] as the message digest algorithm and ECDSA with Curve P-384 as the signature algorithm.
The elliptic curves P-256 and P-384 of [NIST-SP-800-186] are referred to as secp256r1 and secp384r1 respectively in [SECG2]. In addition, this notation is sometimes used in ECDSA software libraries.
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."
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, and MUST NOT 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 proof is any concrete expression of the data model that complies with the normative statements in this specification. Specifically, all relevant normative statements in Sections 2. Data Model and 3. Algorithms of this document MUST be enforced.
A conforming processor is any algorithm realized as software and/or hardware that generates or consumes a conforming proof. Conforming processors MUST produce errors when non-conforming documents are consumed.
This document also contains examples that contain JSON and JSON-LD content. Some
of these examples contain characters that are invalid JSON, such as inline
comments (//
) and the use of ellipsis (...
) to denote
information that adds little value to the example. Implementers are cautioned to
remove this content if they desire to use the information as valid JSON or
JSON-LD.
The following sections outline the data model that is used by this specification for verification methods and data integrity proof formats.
The cryptographic material used to verify a data integrity proof is called the verification method. This suite relies on public key material represented using [MULTIBASE] and [MULTICODEC]. This suite supports public key use for both digital signature generation and verification, according to [FIPS-186-5].
This suite MAY be used to verify Data Integrity Proofs [VC-DATA-INTEGRITY] produced by ECDSA public key material encoded as a Multikey. Loss-less key transformation processes that result in equivalent cryptographic material MAY be utilized.
This definition should go in the Data Integrity specification and referenced from there.
The type
of the verification method MUST be Multikey
.
The controller
of the verification method MUST be a URL.
The publicKeyMultibase
property of the verification method MUST be
a public key encoded according to [MULTICODEC] and formatted according to
[MULTIBASE]. The multicodec encoding of a P-256 public key is the
two-byte prefix 0x1200
followed by the 33-byte compressed public key data.
The 35 byte value is then encoded using base58-btc (z
) as the prefix.
The multicodec encoding of a P-384 public key is the
two-byte prefix 0x1201
followed by the 49-byte compressed public key data.
The 51 byte value is then encoded using base58-btc (z
) as the prefix.
Any other encodings MUST NOT be allowed.
Developers are advised to not accidentally publish a representation of a private
key. Implementations of this specification will raise errors in the event of a
[MULTICODEC] value other than 0x1200
or 0x1201
being used in a
publicKeyMultibase
value.
{
"id": "https://example.com/issuer/123#key-0",
"type": "Multikey",
"controller": "https://example.com/issuer/123",
"publicKeyMultibase": "zDnaerx9CtbPJ1q36T5Ln5wYt3MQYeGRG5ehnPAmxcf5mDZpv"
}
{
"id": "https://example.com/issuer/123#key-0",
"type": "Multikey",
"controller": "https://example.com/issuer/123",
"publicKeyMultibase": "z82LkvCwHNreneWpsgPEbV3gu1C6NFJEBg4srfJ5gdxEsMGRJ
Uz2sG9FE42shbn2xkZJh54"
}
{
"@context": [
"https://www.w3.org/ns/did/v1",
"https://w3id.org/security/data-integrity/v1"
],
"id": "did:example:123",
"verificationMethod": [{
"id": "https://example.com/issuer/123#key-1",
"type": "Multikey",
"controller": "https://example.com/issuer/123",
"publicKeyMultibase": "zDnaerx9CtbPJ1q36T5Ln5wYt3MQYeGRG5ehnPAmxcf5mDZpv"
}, {
"id": "https://example.com/issuer/123#key-2",
"type": "Multikey",
"controller": "https://example.com/issuer/123",
"publicKeyMultibase": "z82LkvCwHNreneWpsgPEbV3gu1C6NFJEBg4srfJ5gdxEsMGRJ
Uz2sG9FE42shbn2xkZJh54"
}],
"authentication": [
"did:example:123#key-1"
],
"assertionMethod": [
"did:example:123#key-2"
],
"capabilityDelegation": [
"did:example:123#key-2"
],
"capabilityInvocation": [
"did:example:123#key-2"
]
}
This specification should not specify multikey formats. It should, instead,
point to a multikey registry and/or specification. Examples of these
sorts of documents include the DID Specification Registries for
Verification Method Types, the key types in the
Multikey2021 JSON-LD Context, and key definitions in the Security Vocabulary.
Ideally, the specification that this one points to would define all possible
multikeys listed in the Multicodec Registry
and define how to encode them as multibase values in fields such as
publicKeyMultibase
and secretKeyMultibase
. The referenced specification
should also include an extensibility mechanism and registry for new values as
they are added to the Multicodec Registry.
This suite relies on detached digital signatures represented using [MULTIBASE] and [MULTICODEC].
The verificationMethod
property of the proof MUST be a URL.
Dereferencing the verificationMethod
MUST result in an object
containing a type
property with the value set to
Multikey
.
The type
property of the proof MUST be DataIntegrityProof
.
The cryptosuite
property of the proof MUST be ecdsa-2019
.
The created
property of the proof MUST be an [XMLSCHEMA11-2]
formatted date string.
The proofPurpose
property of the proof MUST be a string, and MUST
match the verification relationship expressed by the verification method
controller
.
The proofValue
property of the proof MUST be a detached ECDSA
produced according to [FIPS-186-5], encoded according to [MULTIBASE] using
the base58-btc base encoding.
{
"@context": [
{"title": "https://schema.org/title"},
"https://w3id.org/security/data-integrity/v1"
],
"title": "Hello world!",
"proof": {
"type": "DataIntegrityProof",
"cryptosuite": "ecdsa-2019",
"created": "2020-11-05T19:23:24Z",
"verificationMethod": "https://example.com/issuer/123#key-2",
"proofPurpose": "assertionMethod",
"proofValue": "z4oey5q2M3XKaxup3tmzN4DRFTLVqpLMweBrSxMY2xHX5XTYVQeVbY8nQA
VHMrXFkXJpmEcqdoDwLWxaqA3Q1geV6"
}
}
The following section describes multiple Data Integrity cryptographic suites that utilize the Elliptic Curve Digital Signature Algorithm (ECDSA) [FIPS-186-5].
The ecdsa-2019
cryptographic suite takes an input document, canonicalizes
the document using the Universal RDF Dataset Canonicalization Algorithm
[RDF-CANON], and then cryptographically hashes and signs the output
resulting in the production of a data integrity proof. The algorithms in this
section also include the verification of such a data integrity proof.
To generate a proof, the algorithm in Section 4.1: Add Proof in the Data Integrity [VC-DATA-INTEGRITY] specification MUST be executed. For that algorithm, the cryptographic suite specific transformation algorithm is defined in Section 3.1.3 Transformation (ecdsa-2019), the hashing algorithm is defined in Section 3.1.4 Hashing (ecdsa-2019), and the proof serialization algorithm is defined in Section 3.1.6 Proof Serialization (ecdsa-2019).
To verify a proof, the algorithm in Section 4.2: Verify Proof in the Data Integrity [VC-DATA-INTEGRITY] specification MUST be executed. For that algorithm, the cryptographic suite specific transformation algorithm is defined in Section 3.1.3 Transformation (ecdsa-2019), the hashing algorithm is defined in Section 3.1.4 Hashing (ecdsa-2019), and the proof verification algorithm is defined in Section 3.1.7 Proof Verification (ecdsa-2019).
The following algorithm specifies how to transform an unsecured input document into a transformed document that is ready to be provided as input to the hashing algorithm in Section 3.1.4 Hashing (ecdsa-2019).
Required inputs to this algorithm are an unsecured data document (unsecuredDocument) and transformation options (options). The transformation options MUST contain a type identifier for the cryptographic suite (type) and a cryptosuite identifier (cryptosuite). A transformed data document is produced as output. Whenever this algorithm encodes strings, it MUST use UTF-8 encoding.
DataIntegrityProof
and options.cryptosuite is not
set to the string ecdsa-2019
then a PROOF_TRANSFORMATION_ERROR
MUST be
raised.
The following algorithm specifies how to cryptographically hash a transformed data document and proof configuration into cryptographic hash data that is ready to be provided as input to the algorithms in Section 3.1.6 Proof Serialization (ecdsa-2019) or Section 3.1.7 Proof Verification (ecdsa-2019).
The required inputs to this algorithm are a transformed data document (transformedDocument) and canonical proof configuration (canonicalProofConfig). A single hash data value represented as series of bytes is produced as output.
The following algorithm specifies how to generate a proof configuration from a set of proof options that is used as input to the proof hashing algorithm.
The required inputs to this algorithm are proof options (options). The proof options MUST contain a type identifier for the cryptographic suite (type) and MUST contain a cryptosuite identifier (cryptosuite). A proof configuration object is produced as output.
DataIntegrityProof
and
proofConfig.cryptosuite is not set to ecdsa-2019
, an
INVALID_PROOF_CONFIGURATION
error MUST be raised.
INVALID_PROOF_DATETIME
error MUST be raised.
The following algorithm specifies how to serialize a digital signature from a set of cryptographic hash data. This algorithm is designed to be used in conjunction with the algorithms defined in the Data Integrity [VC-DATA-INTEGRITY] specification, Section 4: Algorithms. Required inputs are cryptographic hash data (hashData) and proof options (options). The proof options MUST contain a type identifier for the cryptographic suite (type) and MAY contain a cryptosuite identifier (cryptosuite). A single digital proof value represented as series of bytes is produced as output.
The following algorithm specifies how to verify a digital signature from a set of cryptographic hash data. This algorithm is designed to be used in conjunction with the algorithms defined in the Data Integrity [VC-DATA-INTEGRITY] specification, Section 4: Algorithms. Required inputs are cryptographic hash data (hashData), a digital signature (proofBytes) and proof options (options). A verification result represented as a boolean value is produced as output.
The following section describes security considerations that developers implementing this specification should be aware of in order to create secure software.
Ensuring that cryptographic suites are versioned and tightly scoped to a very small set of possible key types and signature schemes (ideally one key type and size and one signature output type) is a design goal for most Data Integrity cryptographic suites. Historically, this has been done by defining both the key type and the cryptographic suite that uses the key type in the same specification. The downside of doing so, however, is that there might be a proliferation of different key types in multikey that result in different cryptosuites defining the same key material differently. For example, one cryptosuite might use compressed Curve P-256 keys while another uses uncompressed values. If that occurs, it will harm interoperability. It will be important in the coming months to years to ensure that this does not happen by fully defining the multikey format in a separate specification so cryptosuite specifications, such as this one, can refer to the multikey specification, thus reducing the chances of multikey type proliferation and improving the chances of maximum interoperability for the multikey format.
The following section describes privacy considerations that developers implementing this specification should be aware of in order to avoid violating privacy assumptions.
This cryptography suite does not provide for selective disclosure or unlinkability. If signatures are re-used, they can be used as correlatable data.