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🤝 Verification

In this section, you'll play the role of a Verifier:

  • You'll take an Presentation object given to you by a Claimer;
  • You'll verify that its data is correct;
  • You'll verify that the attestation is valid, i.e., its hash exists on-chain and the attestation has not been revoked.
  • You'll verify that the Credential is owned by the Claimer sending it
Presentation

A Credential is used to create the Presentation object by the claimer. Unlike the credential, a presentation can hide some attributes that are not required by the verifiers and can contain a claimer-signed challenge. A presentation also contains a proof that the claimer owns the credential.

Create Presentation​

It's not enough to just send our credential as a Claimer as we also need to prove ownership of it. This is done by creating a presentation and signing the Verifier's challenge.

claimer/createPresentation.ts
import * as Kilt from '@kiltprotocol/sdk-js'

export async function createPresentation(
  credential: Kilt.ICredential,
  signCallback: Kilt.SignCallback,
  challenge?: string
): Promise<Kilt.ICredentialPresentation> {
  // Create the presentation from credential, DID and challenge.
  return Kilt.Credential.createPresentation({
    credential,
    signCallback,
    challenge
  })
}

Verify​

Let's write our verification script. Here we'll expose getChallenge which returns a random and unique challenge for the Claimer to sign; this is used to prove ownership. We'll also expose verifyPresentation which will do the actual verification. Copy the code below, this completes the Verifier code!

verify.ts
import type { ApiPromise } from '@polkadot/api'

import { config as envConfig } from 'dotenv'

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

import { createPresentation } from './claimer/createPresentation'
import { generateKeypairs } from './claimer/generateKeypairs'
import { generateLightDid } from './claimer/generateLightDid'

function getChallenge(): string {
  return Kilt.Utils.UUID.generate()
}

// Verifies validity, ownership & attestation.
async function verifyPresentation(
  api: ApiPromise,
  presentation: Kilt.ICredentialPresentation,
  challenge: string,
  trustedAttesterUris: Kilt.DidUri[]
): Promise<boolean> {
  try {
    await Kilt.Credential.verifyPresentation(presentation, { challenge })

    const attestationInfo = Kilt.Attestation.fromChain(
      await api.query.attestation.attestations(presentation.rootHash),
      presentation.rootHash
    )
    if (attestationInfo.revoked) {
      return false
    }
    // Returns true if no trusted attester URI is provided or, if it is, if it matches the one that issued the presented credential.
    return trustedAttesterUris.includes(attestationInfo.owner)
  } catch {
    return false
  }
}

export async function verificationFlow(
  credential: Kilt.ICredential,
  signCallback: Kilt.SignCallback,
  trustedAttesterUris: Kilt.DidUri[] = []
) {
  const api = Kilt.ConfigService.get('api')

  // Verifier sends a unique challenge to the claimer 🕊
  const challenge = getChallenge()

  // Create a presentation and send it to the verifier 🕊
  const presentation = await createPresentation(
    credential,
    signCallback,
    challenge
  )

  // The verifier checks the presentation.
  const isValid = await verifyPresentation(
    api,
    presentation,
    challenge,
    trustedAttesterUris
  )

  if (isValid) {
    console.log('Verification successful! You are allowed to enter the club 🎉')
  } else {
    console.log('Verification failed! 🚫')
  }
}

// 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 claimerDidMnemonic = process.env.CLAIMER_DID_MNEMONIC as string
      const { authentication } = generateKeypairs(claimerDidMnemonic)
      const claimerDid = generateLightDid(claimerDidMnemonic)
      const attesterDid = process.env.ATTESTER_DID_URI as Kilt.DidUri
      // Load credential and claimer DID
      const credential = JSON.parse(process.env.CLAIMER_CREDENTIAL as string)
      await verificationFlow(
        credential,
        async ({ data }) => ({
          signature: authentication.sign(data),
          keyType: authentication.type,
          keyUri: `${claimerDid.uri}${claimerDid.authentication[0].id}`
        }),
        [attesterDid]
      )
    } catch (e) {
      console.log('Error in the verification flow')
      throw e
    }
  })()
}

Run​

Run the verification flow on command line:

yarn ts-node verify.ts

That's it! All done :-)