Last update: 22nd June 2021

We propose a new DID method based on ERC-1056 and is intended to use Ethereum addresses as fully self-managed Decentralized Identifiers (DIDs), it allows you to easily create and manage keys for these identities.

LACChain DID provide a scalable identity method for Ethereum addresses that gives any Ethereum address the ability to collect on-chain and off-chain data. Because this DID method allows any Ethereum key pair to become an identity, it is more scalable and privacy-preserving than smart contract based identity methods.

• Allow multiple controllers
• Allow to define a controller for each Verification Method
• Allow automatic key rotation
• Allow controller key recovery

The namestring that shall identify this DID method is: lac

A DID that uses this method MUST begin with the following prefix: did:lac. Per the DID specification, this string MUST be in lowercase. The remainder of the DID, after the prefix, is specified below.

The method specific identifier is composed by an optional Ethereum network identifier with a : separator, followed by a Hex-encoded Ethereum address (without a 0x prefix).

lac-did = "did:lac:" lac-specific-idstring
lac-specific-idstring = [lac-network  ":" ] lac-address
lac-network  = "main"
lac-address  = "0x" 40*HEXDIG

The lac-address is case-insensitive.

This specification currently only supports any Ethereum-based network, but in the future will support EOSIO networks.

Example lac DIDs:

• did:lac:0x2F2B37C890824242Cb9B0FE5614fA2221B79901E
• did:lac:main:0x2F2B37C890824242Cb9B0FE5614fA2221B79901E

{
	"@context": "https://www.w3.org/ns/did/v1",
	"id": "did:lac:main:0xadf1702b76419f428014d1386af487b2d8145f83",
	"controller": ["did:lac:main:0xadf1702b76419f428014d1386af487b2d8145f83"],
	"verificationMethod": [
		{
			"id": "did:lac:main:0xadf1702b76419f428014d1386af487b2d8145f83#vm-0",
			"type": "Secp256k1VerificationKey2018",
			"controller": "did:lac:main:0xadf1702b76419f428014d1386af487b2d8145f83",
			"publicKeyHex": "0xadf1702b76419f428014d1386af487b2d8145f83"
		}
	],
	"authentication": [
		"did:lac:main:0xadf1702b76419f428014d1386af487b2d8145f83#vm-0"
	],
	"capabilityInvocation": [
		"did:lac:main:0xadf1702b76419f428014d1386af487b2d8145f83#vm-0"
	],
	"capabilityDelegation": [
		"did:lac:main:0xadf1702b76419f428014d1386af487b2d8145f83#vm-0"
	],
	"assertionMethod": [
		"did:lac:main:0xadf1702b76419f428014d1386af487b2d8145f83#vm-0"
	],
	"keyAgreement": [
		"did:lac:main:0xadf1702b76419f428014d1386af487b2d8145f83#vm-0"
	],
	"service": [
		{
			"id":"did:lac:main:0xadf1702b76419f428014d1386af487b2d8145f83#mailbox",
			"type": "DIDComm",
			"serviceEndpoint": "https://mailbox.lacchain.net"
		}
	]
}

In order to create a lac DID, an Ethereum address, i.e., key pair, needs to be generated. At this point, no interaction with the target Ethereum network is required. The registration is implicit as it is impossible to brute force an Ethereum address, i.e., guessing the private key for a given public key on the Koblitz Curve (secp256k1). The holder of the private key is the entity identified by the DID.

To construct a valid DID document, first find all changes in history for an identity:

1. eth_call changed(address identity) on the DID Registry smart contract to get the latest block where a change occurred.
2. If result is null return.
3. Filter for events for all the above types with the contracts address on the specified block.
4. If event has a previous change then go to 3

After building the history of events for an address, interpret each event to build the DID document like so:

Verification Methods, Service Endpoints, etc. can be added using attributes. Attributes only exist on the blockchain as contract events of type DIDAttributeChanged and can thus not be queried from within solidity code.

event DIDAttributeChanged(
  address indexed identity,
  bytes name,
  bytes value,
  uint validTo,
  uint previousChange
);

While any attribute can be stored, for the DID document we currently support adding to each of these sections of the DID document:

• Verification Methods
• Service Endpoints

The name of the attribute added should follow this format: {type}/{controller}/{algotithm}/{enconding}

Where {type} can be:

• vm: for a generic Verification Method
• auth: for an Authentication Method
• asse: for an Assertion Purpose
• keya: for a Key Agreement
• dele: for a Delegation Key
• invo: for an Invocation Capability Key
• svc: for a Service

The {controller} represents the Verification Method controller, and can be any string DID or DID fragment.

The {algorithm} can be one of the following in compliant with the W3C specification https://w3c.github.io/did-spec-registries/:

• jwk: JsonWebKey2020,
• esecp256k1vk: EcdsaSecp256k1VerificationKey2019,
• esecp256k1rm: EcdsaSecp256k1RecoveryMethod2020,
• edd25519vk: Ed25519VerificationKey2018,
• gpgvk: GpgVerificationKey2020,
• rsavk: RsaVerificationKey2018,
• x25519ka: X25519KeyAgreementKey2019,
• ssecp256k1vk: SchnorrSecp256k1VerificationKey2019

And the {enconding} is the Public Key enconding type, the possible values are:

• hex: Hexadecimal -> Produces publicKeyHex field in the DID Document
• base64: Base64 -> Produces publicKeyBase64 field in the DID Document
• base58: Base58 -> Produces publicKeyBase58 field in the DID Document
• pem: PEM X.509 -> Produces publicKeyPem field in the DID Document

Note: The {encoding} only refers to the key encoding in the resolved DID document. Attribute values sent to the smart contract registry should always be hex encodings of the raw public key data.

The name of the attribute should follow this format:

svc//{type}/hex

Where: {type} is the service type field in the DID Document.

And, the serviceEndpoint must be in the value field of setAttribute function.

Each identity has a single address which maintains ultimate control over it, which is called the controller of the DID. By default, each identity is controlled by itself. As ongoing technological and security improvements occur, an owner can replace themselves with any other Ethereum address.

In this new version of smart contract it is possible to have multiple controllers associated to the DID, which allows capabilities of automatic key rotation and on-chain key recovery.

Before changing to a new controller it is necessary to register it:

addController(address identity, address controller);

After doing that, it is possible now to change controller by calling the following function:

changeController(address identity, address controller);

Also, if you want to delete one controller, just call the next function:

deleteController(address identity, address controller);

In this last case, there are some rules to comply before deleting one controller:

1. You cannot delete the current controller, for that case it is necessary to change to other controller
2. You cannot delete the all controllers, you must have at least one controller associated to the DID

The DID Document may be updated by invoking the relevant smart contract functions as defined by the DID Registry smart contract. This includes changes to the identity controller and adding additional attributes.

These functions will trigger the respective Ethereum events which are used to build the DID Document for a given identity as described in Enumerating Contract Events to build the DID Document.

DID attributes of the DID Document will be revoked automatically when their validity period expires.

To revoke a DID, the controller of the DID needs to be set to 0x0. Although, 0x0 is a valid Ethereum address, this will indicate the identity has no controller which is a common approach for invalidation.

changeController(address identity, '0x0');

If there is any other changes to the DID document after such a change, all preexisting keys and services will be considered revoked.

There are some features that this DID method implements related to security and privacy.

Automatic key rotation allows automatically and individually per DID account to rotate the main DID controller. The purpose of this mechanism is to increase the security of the full control of the DID document.

By default, is disabled for any DID the automatic key rotation. To enable this functionality for a specific account you need to execute the following smart contract method:

enableKeyRotation(address identity, uint keyRotationTime);

The keyRotationTime must be greater or equal than the minKeyRotationTime defined in the constructor of the smart contract.

To disable the automatic key rotation, just execute the next function:

disableKeyRotation(address identity);

The on-chain key recovery is one of the main features of this DID method, however it is optional and therefore we separate this functionality in a different smart contract. The main idea of this mechanism is to be able to regain full control of the DID by testing control of other controllers registered in the DID. The DIDRegistryRecoverable is a subclass of DIDRegistry, and it inherits all the functionality of the base DID Registry such as the Automatic Key Rotation.

The basic operation of key recovery function consist in to prove the ownership of n/2+1 controllers by executing the recover smart contract function signed by each of that controllers.

To prove the ownership of each n/2+1 controller different from the current controller of the DID, you must execute the following steps:

1. Off-chain sign the following function

   recover(address identity, address controller);

   The signature should be generated using the keccak256 hash of the following tightly packed parameters: byte(0x19), byte(0), address of did registry, 0, identity, "recover", controller
2. Extract the R, S, V parameters of the previous signature
3. Send a transaction invoking the smart contract function

   recover(address identity, v, r, s, address controller);

   Where, identity is always the original DID address and controller is the current controller that is trying to prove the ownership.

When it is successfully proven the ownership of n/2+1 controllers, the current controller of the DID will be changed to the last controller proved.