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🧾 Attestation

This section covers how the Attester receives and processes a Credential and how you can:

  • Attest or deny it
  • Store the attestation information on the chain

Attest a Credential​

attester/attestCredential.ts
import { config as envConfig } from 'dotenv'

import * as Kilt from '@kiltprotocol/sdk-js'

import { generateAccount } from './generateAccount'
import { generateCredential } from '../claimer/generateCredential'
import { generateKeypairs } from './generateKeypairs'
import { generateLightDid } from '../claimer/generateLightDid'

export async function attestCredential(
  attesterAccount: Kilt.KiltKeyringPair,
  attesterDid: Kilt.DidUri,
  credential: Kilt.ICredential
): Promise<void> {
  const api = Kilt.ConfigService.get('api')

  // Get CType and root hash from the provided credential.
  const { cTypeHash, claimHash } = Kilt.Attestation.fromCredentialAndDid(
    credential,
    attesterDid
  )

  // Create the tx and authorize it.
  const tx = api.tx.attestation.add(claimHash, cTypeHash, null)
  const extrinsic = api.tx.did.dispatchAs(attesterAccount.address, tx)

  // Submit the tx to write the attestation to the chain.
  console.log('Attester -> create attestation...')
  await Kilt.Blockchain.signAndSubmitTx(extrinsic, attesterAccount)
}

export async function attestingFlow(
  claimerDid: Kilt.DidUri,
  attesterAccount: Kilt.KiltKeyringPair,
  attesterDid: Kilt.DidUri,
  // eslint-disable-next-line @typescript-eslint/no-unused-vars
  signCallback: Kilt.SignExtrinsicCallback
): Promise<Kilt.ICredential> {
  // First the claimer.
  const credential = generateCredential(claimerDid, {
    age: 27,
    name: 'Mia Musterfrau'
  })

  // ... send the request to the attester

  // The attester checks the attributes and attests the provided credential.
  await attestCredential(attesterAccount, attesterDid, credential)

  // Return the generated credential.
  return credential
}

// Don't execute if this is imported by another file.
if (require.main === module) {
  ;(async () => {
    envConfig()

    try {
      await Kilt.connect(process.env.WSS_ADDRESS as string)

      const attesterAccountMnemonic = process.env
        .ATTESTER_ACCOUNT_MNEMONIC as string
      const { account: attesterAccount } = generateAccount(
        attesterAccountMnemonic
      )

      const attesterDidMnemonic = process.env.ATTESTER_DID_MNEMONIC as string
      const { authentication, assertionMethod } =
        generateKeypairs(attesterDidMnemonic)
      const attesterDidUri = Kilt.Did.getFullDidUriFromKey(authentication)

      const claimerDidMnemonic = process.env.CLAIMER_DID_MNEMONIC as string
      const claimerDid = await generateLightDid(claimerDidMnemonic)

      const credential = await attestingFlow(
        claimerDid.uri,
        attesterAccount,
        attesterDidUri,
        async ({ data }) => ({
          signature: assertionMethod.sign(data),
          keyType: assertionMethod.type
        })
      )
      console.log('The claimer build their credential and now has to store it.')
      console.log('Add the following to your .env file. ')
      console.log(`CLAIMER_CREDENTIAL='${JSON.stringify(credential)}'`)
    } catch (e) {
      console.log('Error while going throw attesting workflow')
      throw e
    }
  })()
}

The attestCredential function loads the account and DID of the Attester and issues an attestation for the credential received from the Claimer. The credential is valid from the time an Attester attests it on chain until the time it is revoked.

In the attestingFlow function, the Claimer generates the demo credential and sends it to the Attester. The Attester checks the attributes and either attests or denies the attestation if the attributes are invalid. Once the attestation is written on the chain, the Attester can share all or part of the attested credentials with verifiers.

Run​

Run the code from the command line:

yarn ts-node attester/attestCredential.ts

Summary​

Your job as an Attester is complete. You've attested a credential and written the attestation hash onto the chain.

Let's move on to set up the Verifier!