This specification describes a mechanism for ensuring the authenticity and integrity of Linked Data documents using mathematical proofs.

This is an experimental specification and is undergoing regular revisions. It is not fit for production deployment.

Introduction

Cryptographic proofs enable functionality that is extremely useful to implementors of distributed systems. For example, proofs can be used for purposes such as:

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 linked data documents through the use of mathematical proofs without sacrificing Linked Data features such as extensibility and composability.

Design Goals and Rationale

The Linked Data Proofs specification achieves the following design goals:

Simple for Developers
The proof format is designed to be easy to use for developers that don't have significant cryptography training. For example, cryptographic suite identifiers are used instead of specific cryptographic parameters to ensure that it is difficult to accidentally produce a weak digital proof.
Agile
Since digital proof mechanisms might be compromised without warning due to technological advancements, it is important that proof types can be easily and quickly replaced. This specification provides algorithm agility while still keeping the digital proof format easy for developers to understand.
Extensible
Creating and deploying new proof types is a fairly trivial undertaking to ensure that the proof format increases the rate of innovation in the digital proof space.
Graph Syntax Agnostic
Cryptographic proofs over graph data structures is a difficult problem, since graphs can be serialized in many different but equivalent ways. For advanced use cases, these proof mechanisms can be used across a variety of RDF-based graph serialization syntaxes such as JSON-LD, N-Quads, and TURTLE, without the need to regenerate the proof.

Terminology

The following terms are used to describe concepts involved in the generation and verification of Linked Data digital proofs.

linked data document
A document comprised of Linked Data.
linked data proof
A set of attributes that represent a Linked Data digital proof and the parameters required to verify it.
linked data signature
A type of Linked Data Proof that involves cryptographic signatures -- see the Linked Data Signatures specification [[LD-SIGNATURES]] for examples of signature algorithms.
signed linked data document
A linked data document that has been digitally signed.
public key
A cryptographic key that can be used to verify digital proofs created with a corresponding private key.
private key
A cryptographic key that can be used to generate digital proofs.
proof type
A specified set of cryptographic primitives bundled together into a cryptographic suite for the purposes of safety and convenience, by cryptographers for developers. A proof type typically consists of a canonicalization algorithm, a message digest algorithm, and a specific corresponding proof algorithm (see section ).
proof options
A set of options that is included in the proof data. These options might include controller, challenge, domain, or other data that is specific to the proof format.
proof purpose
The specific intent for the proof, the reason why an entity created it. Acts as a safeguard to prevent the proof from being misused for a purpose other than the one it was intended for.
verification method
A set of parameters required to independently verify the proof, such as an identifier for a public/private key pair that would be used in the proof.
controller
A link to a machine-readable object, such as a DID Document [[DID-CORE]], that contains authorization relations that explicitly permit the use of certain verification methods for specific purposes. For example, a controller object could contain statements that restrict a public key to being used only for signing Verifiable Credentials [[VC-DATA-MODEL]] and no other kinds of documents.
challenge
A random or pseudo-random value used by some authentication protocols to mitigate replay attacks.
domain
A string value that specifies the operational domain of a digital proof. This could be an Internet domain name like example.com, an ad-hoc value such as mycorp-level3-access, or a very specific transaction value like 8zF6T$mqP. A signer could include a domain in its digital proof to restrict its use to particular target, identified by the specified domain.

Linked Data Proof Overview

A linked data proof is comprised of information about the proof, parameters required to verify it, and the proof value itself. All of this information is provided using Linked Data vocabularies such as [[SECURITY-VOCABULARY]].

A linked data proof typically includes at least the following attributes:

type
Required. The specific proof type used. For example, an Ed25519Signature2018 type indicates that the proof includes a digital signature produced by an ed25519 cryptographic key.
proofPurpose
Required. The specific intent for the proof, the reason why an entity created it. Acts as a safeguard to prevent the proof from being misused for a purpose other than the one it was intended for. For example, a proof can be used for purposes of authentication, for asserting control of a Verifiable Credential (assertionMethod), and several others.
verificationMethod
Required. A set of parameters required to independently verify the proof, such as an identifier for a public/private key pair that would be used in the proof.
created
Required. The string value of an [[ISO8601]] combined date and time string generated by the Proof Algorithm.
domain
Optional. A string value specifying the restricted domain of the proof.
proof value
Required. One of any number of valid representations of proof value generated by the Proof Algorithm. Example: jws for detached JSON Web Signatures.

Since this specification is based on Linked Data, the terms type, created, and domain above map to URLs. The vocabulary where these terms are defined is the [[SECURITY-VOCABULARY]].

A proof can be added to a Linked Data document like the following:

{
  "@context": "https://w3id.org/identity/v1",
  "title": "Hello World!"
}
      

by adding the parameters outlined in this section:

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://www.w3.org/2018/credentials/examples/v1"
  ],
  "title": "Hello World!",
  "proof": {
    "type": "Ed25519Signature2018",
    "proofPurpose": "assertionMethod",
    "created": "2017-09-23T20:21:34Z",
    "verificationMethod": "did:example:123456#key1",
    "challenge": "2bbgh3dgjg2302d-d2b3gi423d42",
    "domain": "example.org",
    "jws": "eyJ0eXAiOiJK...gFWFOEjXk"
  }
}
      

The proof example above uses the Ed25519Signature2018 proof type to produce a verifiable digital proof.

Create a separate section detailing an optional mechanism for authenticating public key ownership via bi-directional links. How to establish trust in key owner entities is out of scope but examples can be given.
Specify algorithm agility mechanisms (additional attributes from the security vocab can be used to indicate other signing and hash algorithms). Rewrite algorithms to be parameterized on this basis and move `RsaSignature2018` definition to a single supported mechanism; specify its identifier as a URL. In order to make it easy to specify a variety of combinations of algorithms, introduce a core type `LinkedDataProof` that allows for easy filtering/discover of proof nodes, but that type on its own doesn't specify any default proof or hash algorithms, those need to be given via other properties in the nodes.
Add an explicit check on key type to prevent an attacker from selecting an algorithm that could abuse how the key is used/interpreted.
Add a note indicating that selective disclosure proof mechanisms can be compatible with Linked Data Proofs; for example, an algorithm could produce a merkle tree from a canonicalized set of N-Quads and then sign the root hash. Disclosure would involve including the merkle paths for each N-Quad that is to be revealed. This mechanism would merely consume the normalized output differently (this, and the proof mechanism would be modifications to this core spec). It might also be necessary to generate proof parameters such as a private key/seed that can be used along with an algorithm to deterministically generate nonces that are concatenated with each N-Quad to prevent rainbow table or similar attacks.

Verification Attributes

Each proof contains a set of attributes that are used by verifiers to ensure that the verification material, such as cryptographic keys, have been authorized by their controllers specifically for a purpose that is appropriate to a given situation. These attributes include: verificationMethod, which describes the cryptographic keys or other methods used in the proof, proofPurpose, which identifies the purpose for which the method is to be used, and the link to a controller, which provides external confirmation that a given method is indeed intended to be used for the stated purpose.

Verification Method

Add section explaining what a Verification Method parameter is, and its relationship with the Controller document.

Proof Purpose

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 similar goal but allows for finer-grained restrictions.

The following is a list of commonly used proof purpose values.

authentication
Indicates that a given proof is only to be used for the purposes of an authentication protocol.
assertionMethod
Indicates that a proof can only be used for making assertions, for example signing a Verifiable Credential.
keyAgreement
Indicates that a proof is used for for key agreement protocols, such as Elliptic Curve Diffie Hellman key agreement used by popular encryption libraries.
contractAgreement
Indicates that a proof is used for proofs that an entity agrees to a contract.

Note: The Authorization Capabilities [[ZCAP]] specification defines additional proof purposes for that use case, such as capabilityInvocation and capabilityDelegation.

Controller

Add section explaining what a Controller document is, and how it is used to ensure that a given method is authorized to be used for a purpose.
Add examples of common Controller documents, such as DID Documents published on a ledger-based registry, or on a mutable medium in combination with an integrity protection mechanism such as Hashlinks.

Multiple Proofs

The Linked Data Proofs 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).

Proof Sets

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": [
    "https://www.w3.org/2018/credentials/v1",
    "https://www.w3.org/2018/credentials/examples/v1"
  ],
  "title": "Hello World!",
  "proof": [{
    "type": "Ed25519Signature2018",
    "proofPurpose": "assertionMethod",
    "created": "2019-08-23T20:21:34Z",
    "verificationMethod": "did:example:123456#key1",
    "challenge": "2bbgh3dgjg2302d-d2b3gi423d42",
    "domain": "example.org",
    "jws": "eyJ0eXAiOiJK...gFWFOEjXk"
  },
  {
    "type": "RsaSignature2018",
    "proofPurpose": "assertionMethod",
    "created": "2017-09-23T20:21:34Z",
    "verificationMethod": "https://example.com/i/pat/keys/5",
    "challenge": "2bbgh3dgjg2302d-d2b3gi423d42",
    "domain": "example.org",
    "jws": "eyJ0eXAiOiJK...gFWFOEjXk"
  }]
}
        

Proof Chains

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 order needs to be preserved, is represented by associating an ordered list of proofs with the proofChain key in a document.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://www.w3.org/2018/credentials/examples/v1"
  ],
  "title": "Hello World!",
  "proofChain": [{
    "type": "Ed25519Signature2018",
    "proofPurpose": "assertionMethod",
    "created": "2019-08-23T20:21:34Z",
    "verificationMethod": "did:example:123456#key1",
    "domain": "example.org",
    "jws": "eyJ0eXAiOiJK...gFWFOEjXk"
  },
  {
    "type": "RsaSignature2018",
    "proofPurpose": "assertionMethod",
    "created": "2017-09-23T20:21:34Z",
    "verificationMethod": "https://example.com/i/pat/keys/5",
    "domain": "example.org",
    "jws": "eyJ0eXAiOiJK...gFWFOEjXk"
  }]
}
        

Proof Types

Linked Data Signatures

A linked data signature is a type of [[!LD-PROOF]], and is comprised of information about the signature, parameters required to verify it, and the signature value itself. All of this information is provided using Linked Data vocabularies such as the [[!SECURITY-VOCABULARY]].

A linked data signature typically includes at least the following attributes:

type (required)
A URI that identifies the digital signature suite that was used to create the signature. For example: Ed25519Signature2018.
created (required)
The string value of an [[!ISO8601]] combined date and time string generated by the Signature Algorithm.
domain (optional)
A string value specifying the restricted domain of the signature.
nonce (optional, but strongly recommended)
A string value that is included in the digital signature and MUST only be used once for a particular domain and window of time. This value is used to mitigate replay attacks.
signature value (required)
One of any number of valid representations of signature value generated by the Signature Algorithm. Example: jws for detached JSON Web Signatures.

Since this specification is based on Linked Data, the terms type, created, domain, nonce, and jws above map to URLs. The vocabulary where these terms are defined is the [[SECURITY-VOCABULARY]].

A signature can be added to a Linked Data document like the following:

  {
    "@context": "https://www.w3.org/2018/credentials/examples/v1",
    "title": "Hello World!"
  }
        

by adding the parameters outlined in this section:

  {
    "@context": "https://www.w3.org/2018/credentials/examples/v1",
    "title": "Hello World!",
    "proof": {
      "type": "Ed25519Signature2018",
      "proofPurpose": "assertionMethod",
      "created": "2019-08-23T20:21:34Z",
      "verificationMethod": "did:example:123456#key1",
      "domain": "example.org",
      "jws": "eyJ0eXAiOiJK...gFWFOEjXk"
    }
  }
        

The signature example above uses the Ed25519Signature2018 signature suite to produce a verifiable digital signature.

Create a separate section detailing an optional mechanism for authenticating public key control via bi-directional links. How to establish trust in key controller entities is out of scope but examples can be given.
Specify algorithm agility mechanisms (additional attributes from the security vocab can be used to indicate other signing and hash algorithms). Rewrite algorithms to be parameterized on this basis and move `RsaSignature2018` definition to a single supported mechanism; specify its identifier as a URL. In order to make it easy to specify a variety of combinations of algorithms, introduce a core type `LinkedDataSignature` that allows for easy filtering/discover of signature nodes, but that type on its own doesn't specify any default signature or hash algorithms, those must be given via other properties in the nodes.
Add a note indicating that this specification should not be construed to indicate that public key owners should be restricted to a single public key or that systems that use this spec and involve real people should identify each person as only ever being a single entity rather than perhaps N entities with M keys. There are no such restrictions and in many cases those kinds of restrictions are ill-advised due to privacy considerations.
Add an explicit check on key type to prevent an attacker from selecting an algorithm that may abuse how the key is used/interpreted.
Add a note indicating that selective disclosure signature mechanisms can be compatible with Linked Data Signatures; for example, an algorithm could produce a merkle tree from a canonicalized set of N-Quads and then sign the root hash. Disclosure would involve including the merkle paths for each N-Quad that is to be revealed. This mechanism would merely consume the normalized output differently (this, and the proof mechanism would be modifications to this core spec). It may also be necessary to generate signature parameters such as a private key/seed that can be used along with an algorithm to deterministically generate nonces that are concatenated with each N-Quad to prevent rainbow table or similar attacks.

Other Proof Types

TODO: Add links and examples to proof types that are not Linked Data Signatures, such as biometrics-based proofs and ZKP proofs.

Advanced Terminology

These terms are relevant only to implementors of new cryptographic suites.

canonicalization algorithm
An algorithm that takes an input document that has more than one possible representation and always transforms it into a deterministic representation. For example, alphabetically sorting a list of items is a type canonicalization. This process is sometimes also called normalization.
message digest algorithm
An algorithm that takes an input message and produces a cryptographic output message that is often many orders of magnitude smaller than the input message. These algorithms are often 1) very fast, 2) non-reversible, 3) cause the output to change significantly when even one bit of the input message changes, and 4) make it infeasible to find two different inputs for the same output.
proof algorithm
An algorithm that takes an input message and produces an output value where the receiver of the message can mathematically verify that the message has not been modified in transit and came from someone possessing a particular secret.

Creating New Proof Types

A Linked Data 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. Ed25519Signature2018) 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 details the cryptographic primitives that are available to proof type developers.

At a minimum, a proof type is expected have the following attributes:

id
A URL that identifies the cryptographic suite. For example: https://w3id.org/security#Ed25519Signature2018.
type
The value ProofSuite.
canonicalizationAlgorithm
A URL that identifies the canonicalization algorithm to use on the document. For example: https://w3id.org/security#URDNA2015.
digestAlgorithm
A URL that identifies the message digest algorithm to use on the canonicalized document. For example: https://www.ietf.org/assignments/jwa-parameters#SHA256
proofAlgorithm
A URL that identifies the proof algorithm to use on the data to be signed. For example: https://w3id.org/security#ed25519

A complete example of a proof type is shown in the next example:

{
  "id": "https://w3id.org/security/v1#Ed25519Signature2018",
  "type": "Ed25519VerificationKey2018",
  "canonicalizationAlgorithm": "https://w3id.org/security#URDNA2015",
  "digestAlgorithm": "https://www.ietf.org/assignments/jwa-parameters#SHA256",
  "signatureAlgorithm": "https://w3id.org/security#ed25519"
}
      

Algorithms

The algorithms defined below are generalized in that they require a specific canonicalization algorithm, message digest algorithm, and proof algorithm to be used to achieve the algorithm's intended outcome.

Proof Algorithm

The proof parameters should be included as headers and values in the data to be signed.

The following algorithm specifies how to create a digital proof that can be later used to verify the authenticity and integrity of a linked data document. A linked data document, document, proof options, options, and a private key, privateKey, are required inputs. The proof options MUST contain an identifier for the public/private key pair, and an [[!ISO8601]] combined date and time string, created, containing the current date and time, accurate to at least one second, in Universal Time Code format. A domain might also be specified in the options. A signed linked data document is produced as output. Whenever this algorithm encodes strings, it MUST use UTF-8 encoding.

  1. Create a copy of document, hereafter referred to as output.
  2. Generate a canonicalized document by canonicalizing document according to a canonicalization algorithm (e.g. the URDNA2015 [[!RDF-DATASET-NORMALIZATION]] algorithm).
  3. Create a value tbs that represents the data to be signed, and set it to the result of running the Create Verify Hash Algorithm, passing the information in options.
  4. Digitally sign tbs using the privateKey and the the digital proof algorithm (e.g. JSON Web Proof using RSASSA-PKCS1-v1_5 algorithm). The resulting string is the proof value.
  5. Add a proof node to output containing a linked data proof using the appropriate type and proof value values as well as all of the data in the proof options (e.g. created, and if given, any additional proof options such as domain).
  6. Return output as the signed linked data document.

Proof Verification Algorithm

This algorithm is highly specific to digital signatures and needs to be generalized to other proof mechanisms such as Equihash.

The following algorithm specifies how to check the authenticity and integrity of a signed linked data document by verifying its digital proof. This algorithm takes a signed linked data document, signed document and outputs a true or false value based on whether or not the digital proof on signed document was verified. Whenever this algorithm encodes strings, it MUST use UTF-8 encoding.

Specify how the public key can be obtained, through some out-of-band process and passed in or it can be retrieved by derefencing its URL identifier, etc.
  1. Get the public key by dereferencing its URL identifier in the proof node of the default graph of signed document. Confirm that the linked data document that describes the public key specifies its owner and that its owner's URL identifier can be dereferenced to reveal a bi-directional link back to the key. Ensure that the key's owner is a trusted entity before proceeding to the next step.
  2. Let document be a copy of signed document.
  3. Remove any proof nodes from the default graph in document and save it as proof.
  4. Generate a canonicalized document by canonicalizing document according to the canonicalization algorithm (e.g. the URDNA2015 [[!RDF-DATASET-NORMALIZATION]] algorithm).
  5. Create a value tbv that represents the data to be verified, and set it to the result of running the Create Verify Hash Algorithm, passing the information in proof.
  6. Pass the proof value, tbv, and the public key to the proof algorithm (e.g. JSON Web Proof using RSASSA-PKCS1-v1_5 algorithm). Return the resulting boolean value.

Create Verify Hash Algorithm

This algorithm is too specific to digital signatures and needs to be generalized for algorithms such as Equihash.

The following algorithm specifies how to create the data that is used to generate or verify a digital proof. It takes a canonicalized linked data document, canonicalized document, canonicalization algorithm, a message digest algorithm, and proof options, input options (by reference). The proof options MUST contain an identifier for the public/private key pair, and an [[!ISO8601]] combined date and time string, created, containing the current date and time, accurate to at least one second, in Universal Time Code format. A domain might also be specified in the options. Its output is a data that can be used to generate or verify a digital proof (it is usually further hashed as part of the verification or signing process).

  1. Let options be a copy of input options.
  2. If the proof value parameter, such as jws, exists in options, remove the entry.
  3. If created does not exist in options, add an entry with a value that is an [[!ISO8601]] combined date and time string containing the current date and time accurate to at least one second, in Universal Time Code format. For example: 2017-11-13T20:21:34Z.
  4. Generate output by:
    1. Creating a canonicalized options document by canonicalizing options according to the canonicalization algorithm (e.g. the URDNA2015 [[!RDF-DATASET-NORMALIZATION]] algorithm).
    2. Hash canonicalized options document using the message digest algorithm (e.g. SHA-256) and set output to the result.
    3. Hash canonicalized document using the message digest algorithm (e.g. SHA-256) and append it to output.
  5. This last step needs further clarification. Signing implementations usually automatically perform their own integrated hashing of an input message, i.e. signing algorithms are a combination of a raw signing mechanism and a hashing mechanism such as RS256 (RSA + SHA-256). Current implementations of RSA-based Linked Data Proof suites therefore do not perform this last step before passing the data to a signing algorithm as it will be performed internally. The Ed25519Proof2018 algorithm also does not perform this last step -- and, in fact, uses SHA-512 internally. In short, this last step should better communicate that the 64 bytes produced from concatenating the SHA-256 of the canonicalized options with the SHA-256 of the canonicalized document are passed into the signing algorithm with a presumption that the signing algorithm will include hashing of its own.
    Note: It is presumed that the 64-byte output will be used in a signing algorithm that includes its own hashing algorithm, such as RS256 (RSA + SHA-256) or EdDsa (Ed25519 which uses SHA-512).
  6. Return output.

Security Considerations

The following section describes security considerations that developers implementing this specification should be aware of in order to create secure software.

TODO: We need to add a complete list of security considerations.