index.js

const Koa = require("koa");
const Router = require("@koa/router");
const { isReady, PrivateKey, Field, Signature } = require("snarkyjs");

const PORT = process.env.PORT || 3000;

const app = new Koa();
const router = new Router();

async function getSignedCreditScore(userId) {
  // We need to wait for SnarkyJS to finish loading before we can do anything
  await isReady;

  // The private key of our account. When running locally the hardcoded key will
  // be used. In production the key will be loaded from a Vercel environment
  // variable.
  const privateKey = PrivateKey.fromBase58(
    process.env.PRIVATE_KEY ??
      "EKF65JKw9Q1XWLDZyZNGysBbYG21QbJf3a4xnEoZPZ28LKYGMw53"
  );

  // We get the users credit score. In this case it's 787 for user 1, and 536
  // for anybody else :)
  const knownCreditScore = (userId) => (userId === "1" ? 787 : 536);

  // We compute the public key associated with our private key
  const publicKey = privateKey.toPublicKey();

  // Define a Field with the value of the users id
  const id = Field(userId);

  // Define a Field with the users credit score
  const creditScore = Field(knownCreditScore(userId));

  // Use our private key to sign an array of Fields containing the users id and
  // credit score
  const signature = Signature.create(privateKey, [id, creditScore]);

  return {
    data: { id: id, creditScore: creditScore },
    signature: signature,
    publicKey: publicKey,
  };
}

router.get("/user/:id", async (ctx) => {
  ctx.body = await getSignedCreditScore(ctx.params.id);
});

app.use(router.routes()).use(router.allowedMethods());

app.listen(PORT);