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DID SPEC: Decentralized Identifiers

shortname:      DID
name:           Decentralized Identifiers
type:           Standard
status:         Valid
version:        0.1
editor:         Aitor Argomaniz <aitor@nevermined.io>
contributors:   

This specification is based on the W3C DID specification, which was at version 0.11 as of August 2018.

Motivation

The main motivations of this SPEC are:

  • Design a solution to extend the current architecture to use Decentralized Identifiers (DIDs) and DID Documents (DDOs)
  • Understand how to register information on-chain with off-chain integrity associated
  • Understand how to resolve DIDs into DDOs
  • Design a solution facilitating alignment of on-chain and off-chain information
  • Establishing the mechanism to know if the DDO associated with a DID was modified
  • Defining the common mechanisms, interfaces and APIs to implemented the designed solution
  • Define how Nevermined assets can be modeled with a DID/DDO data model
  • Understand how DID hubs are formed, and how they integrate a business and storage layer

Specification

Requirements are:

  • The DID resolving capabilities MUST be exposed in the client libraries, enabling to resolve a DDO directly in a totally transparent way
  • ASSETS are DATA objects describing RESOURCES under control of a PUBLISHER
  • DLT stores on-chain only the essential information about ASSETS
  • PROVIDERS store the ASSET metadata off-chain
  • DLT doesn't store any ASSET metadata
  • Nevermined doesn't store ASSET contents (e.g. files)
  • An ASSET is modeled in Nevermined as on-chain information stored in the DLT and metadata stored in METADATA API
  • ASSETS on-chain information only can be modified by OWNERS or DELEGATED USERS
  • ASSETS can be resolved using a Decentralized ID (DID) included on-chain and off-chain
  • A DID Document (DDO) should include the ASSET metadata
  • Any kind of object registered in Nevermined MUST have a DID allowing one to uniquely identify that object in the system
  • ASSET DDO (and the metadata included as part of the DDO) is associated to the ASSET information stored on-chain using a common DID
  • A DID can be resolved to get access to a DDO
  • ASSET DDOs can be updated without updating the on-chain information
  • ASSET information stored in the DLT will include a checksum attribute
  • The ASSET on-chain checksum attribute SHOULD include a one-way HASH calculated using the DDO content
  • After the DDO resolving, the DDO HASH can be calculated off-chain to validate if the on-chain and off-chain information is aligned
  • A HASH not matching with the checksum on-chain means the DDO was modified without the on-chain update
  • The function to calculate the hash MUST BE standard

Proposed Solution

Decentralized IDs (DIDs)

A DID is a unique identifier that can be resolved or de-referenced to a standard resource describing the entity (a DID Document or DDO). If we apply this to Nevermined, the DID would be the unique identifier of an object represented in Nevermined (i.e. the Asset ID of an ASSET or the Actor ID of a USER). The DDO SHOULD include the METADATA information associated with this object. The DDO is stored off-chain in Nevermined.

In Nevermined, a DID is a string that looks like:

did:nv:0ebed8226ada17fde24b6bf2b95d27f8f05fcce09139ff5cec31f6d81a7cd2ea

which follows the generic DID scheme. Details about how to compute the DID are given below.

Publishing and Consumption Flow

Here you have the complete flow using as example a new ASSET:

DID Resolver

Steps:

  1. A Data Publisher, using the Smart Contracts, register the new Resource (ie. ASSET) providing the DID and the DID of the Provider acting as Public service returning the DDO of the Resource (ASSET)
  2. The DLT register the Resource using the Service Agreement Smart Contract and after of that register the identity using the DidRegistry Smart Contract. In this point, the attribute is raised as a new event
  3. The Data Publisher publishes the DDO in the Metadata api
  4. A Data Consumer (it could be a frontend application or a backend software), having a DID and using a client library (Python, Javascript, Java, etc) get the service-ddo attribute associated to the DID directly from the Smart Contracts
  5. The Data Consumer, using the provider public url, query directly to the provider passing the DID to obtain the DDO

DID Documents (DDOs)

If a DID is the index key in a key-value pair, then the DID Document is the value to which the index key points. The combination of a DID and its associated DID Document forms the root record for a decentralized identifier.

A DDO document is composed of standard DDO attributes:

  • @context
  • id
  • created
  • updated
  • publicKey
  • authentication
  • proof
  • verifiableCredential
  • service

Asset metadata can be included as one of the objects inside the "service" array, with type "metadata".

DDO Services

Each type of asset (dataset, algorithm, workflow, etc, ..) typically will have associated different kind of services. There are 2 type of services that are commonly added to all the assets: * metadata - describing the asset * provenance - describing the asset provenance

Each service is distinguished by the DDO.service.type attribute.

Each service has an attributes section where all the information related to the service is added. As mandatory content, the attributes section will have a main sub-section. This one is important because it must include all the mandatory information that a service has to provide and never change because it is used to calculate the integrity checksum of the service.

A part of the attributes.main sub-section, other optional sub-sections can be added (like: attributes.curation or attributes.extra) depending on the service type.

Example:

"service": [  
  {  
    "index": 0,
    "type": "metadata",
    "serviceEndpoint": "https://service/api/v1/metadata/assets/ddo/did:nv:0ebed8226ada17fde24b6bf2b95d27f8f05fcce09139ff5cec31f6d81a7cd2ea",
    "attributes": {  
      "main": {},
      "additionalInformation": {},
      "curation": {}
    }
  },
  {  
    "index": 1,
    "type": "provenance",
    "serviceEndpoint": "https://service/api/v1/provenance/assets/ddo/did:nv:0ebed8226ada17fde24b6bf2b95d27f8f05fcce09139ff5cec31f6d81a7cd2ea",
    "attributes": {  
      "main": {},
      "additionalInformation": {}
    }
  },
  {  
    "index": 2,
    "type": "access",
    "serviceEndpoint": "https://service/api/v1/access/assets/ddo/did:nv:0ebed8226ada17fde24b6bf2b95d27f8f05fcce09139ff5cec31f6d81a7cd2ea",
    "attributes": {  
      "main": {},
      "additionalInformation": {}
    }
  }
]

Integrity

The Integrity policy for identity and metadata is a sub-specification allowing to validate the integrity of the Metadata associated to an on-chain object (initially an ASSET).

How to compute the integrity checksum

An ASSET in the system is composed by on-chain information maintained by the DLT and off-chain Metadata information (DDO) stored by the PROVIDER.

Technically a user could update the DDO accessing directly to the off-chain database, modifying attributes (e.g. License information, description, etc.) relevant to a previous consumption agreement with an user.

The motivation of this is to facilitate a mechanism allowing to the CONSUMER of an object, to validate if the DDO was modified after a previous agreement.

This hash composing the integrity checksum is calculated in the following way:

  • The complete content of the service[index].attributes.main is serialized in a common string
  • The string generated is is Hashed using SHA3-256 algorithm (You might have to convert the string to bytes first.)
  • The hash generated as a result of this process is stored in the proof.checksum[index].checksum attribute
  • The previous 3 steps are repeated for every individual service include in the service array. The hash generated is always stored in the proof.checksum array using as key the index of the service computed
  • During the serialization process, the objects to serialize (service[index].attributes.main are prepared using the following process:
  • The object is sorted alphabetically independently of the existing nested levels
  • In the JSON generated, all the characters between entries are removed (\n, \t, \r, whitespaces, etc.)
  • As a result must be generated a string of only one line
  • After hashing, in the DDO, the checksums should be represented as a hex string beginning with 0x and ending with 64 hex characters (e.g. 0x52b5c93b82dd9e7ecc3d9fdf4755f7f69a54484941897dc517b4adfe3bbc3377)
  • After generating each individual checksum the complete proof.checksum entry is sorted, serialized and hashed as previously described in the other checksums
  • The final hash generated as a result of hashing the checksums (DID CHECKSUM or DID HASH) will be the ID part of the DID (the string after the prefix did:nv:)

Because this DID HASH will be stored on-chain and emitted as an event, a validator could use this information to check if something changed regarding the initial registration.

DID Document Proof

A proof on a DID Document is cryptographic proof of the integrity of the DID Document. In the DID Specification the proof attribute is optional. We enforce the usage of the proof attribute to demonstrate the Owner of an Asset is signing the proof of integrity of some Asset attributes. The information to sign by the owner is the integrity checksum defined in the above section.

const signature = Sign.signMessage(DID)

The DID Document (DDO) SHOULD include the following proof information:

  • type - Type of proof, in our case "DDOIntegritySignature"
  • created - Date and time when the proof was created
  • creator - Address of the user providing the proof
  • signatureValue - Result of the signature given by the creator
  • checksum - Checksums of the individual services included in the DDO

Here is an example proof section to add in the DDO:

"proof": {
    "type": "DDOIntegritySignature",
    "created": "2016-02-08T16:02:20Z",
    "creator": "0x00Bd138aBD70e2F00903268F3Db08f2D25677C9e",
    "signatureValue": "0xc9eeb2b8106e…6abfdc5d1192641b",
    "checksum": {
        "0": "0x52b5c93b82dd9e7ecc3d9fdf4755f7f69a54484941897dc517b4adfe3bbc3377",
        "1": "0x999999952b5c93b82dd9e7ecc3d9fdf4755f7f69a54484941897dc517b4adfe3"
    }    
}

Using the proof information, a third-party with access to the DDO could validate the creator signed a specific integrity checksum referring to an Asset.

Length of a DID

The length of a DID must be compliant with the underlying storage layer and function calls. Given that decentralized virtual machines make use of contract languages such as Solidity and WASM, it is advised to fit the DID in structures such as bytes32.

It would be nice to store the did:nv: prefix in those 32 bytes, but that means fewer than 32 bytes would be left for storing the rest (25 bytes since "did:nv:" takes 7 bytes if using UTF-8). If the rest is a secure hash, then we need a 25-byte secure hash, but secure hashes typically have 28, 32 or more bytes, so that won't work.

Only the hash value needs to be stored, not the did:nv: prefix, because it should be clear from context that the value is a Nevermined DID.

How to compute a DID

The DID (id) string begins with did:nv: and is followed by a string representation of a bytes32.

As is described previously, the DID is calculating doing the Hash (SHA3-256) of the DDO.proof.checksum entry

Registry

To register the different kind of objects can be stored in a simple register contract named DidRegistry. This DidRegistry can act as generic/flexible way to associate Resources (ie. Assets) to the public providers resolving the DDO (and Metadata included) of those resources.

The key of the Identity entity in Nevermined is the DID. Each entity will have a unique DID.

To resolve the DDO associated to a Resource (Asset), associated to this Resource DID we have the DID of the Provider giving access to this Resource. The Provider will have associated a mapping (key - value) of attributes. One of those can be used to related with the public service returning the DDO of a specific resource.

Applied to Assets, typically are part of a Service Agreement. The suggested approach to implement this is:

  • Associate the Resource (ie. Asset DID) to the Provider DID
  • Each Provider will have associated a Public URL where the provider is exposing the DDOs

Here is a draft DidRegistry implementation:

// This piece of code is for reference only!
// Doesn't include any validation, types could be reviewed, enums, etc

contract DidRegistry {

    event DIDAttributeRegistered(
        bytes32 indexed did,
        address indexed owner,
        bytes32 indexed checksum,
        string value,
        uint updatedAt
    );

    mapping(bytes32 => DIDRegister) private didRegister;
    function registerAttribute(bytes32 _did, bytes32 _checksum, string _value) public {
        // ....

        emit DIDAttributeRegistered(_did, msg.sender, _checksum, _value, block.number);
    }

}

To register the provider publicly resolving the DDO associated to a DID, we will register an attribute with the public hostname of that provider:

registerAttribute("21tDAKCERh95uGgKbJNHYp", "328aabb94534935864312", "https://myprovider.example.com/ddo")

Resolver

The resolving capabilities will be encapsulated in the Nevermined SDK libraries (JavaScript, Python, etc.), allowing to resolve a DDO directly speaking with the Smart Contracts.

No third-party requests or API need to be integrated. This allows to have a simple a seam-less integration from the consumer side.

This is a generic definition of what could be exposed in the client libraries from an API point of view:

function DDO resolve(String did)  {
  // black magic
  return ddo;
}

To resolve a DID to the associated DDO, some information is stored on-chain associated to the DID. In the approach recommended in the scope of this SPEC, this is stored as an attribute associated to the DidAttributeRegistered event.

Because the DID and key are indexed parameters of the event, a consumer in any supported web3 language could filter the DidAttributeRegistered events filtering by the DID and the key named "service-ddo".

A DDO pointing to a DID could be resolved hierarchically using the same mechanism.

This is an example in JavaScript using web3.js:

const event = contractInstance.DidAttributeRegistered(
    { did: '21tDAKCERh95uGgKbJNHYp' },
    { fromBlock: 0, toBlock: 'latest' }
);

Here in Python using web3.py:

event = mycontract.events.DidAttributeRegistered.createFilter(fromBlock='latest', argument_filters={'did': '21tDAKCERh95uGgKbJNHYp'})

This logic could be encapsulated in the client libraries (SDK) in different languages, allowing to the client applications to get the attributes enabling to resolve the DDO associated to the DID.

Using this information a consumer can query directly to the provider able to return the DDO.

References