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Express

Integrate your custom backend

This guide shows how to build a custom backend that generates JWT tokens for FastAuth transaction signing using Express.js.

Prerequisites​

  • Node.js and npm/pnpm installed
  • RSA key pair for JWT signing

Step 1: Generate RSA Key Pair​

# Generate private key
openssl genrsa -out jwtRS256.key 2048

# Generate public key
openssl rsa -in jwtRS256.key -pubout -out jwtRS256.pub

Step 2: Install Dependencies​

npm install express jsonwebtoken
npm install -D @types/express @types/jsonwebtoken @types/node typescript

Step 3: Create the Express Server​

import { Request, Response } from "express";
import express from "express";
import jwt from "jsonwebtoken";
import * as fs from "node:fs";
import { createPublicKey } from "crypto";

const app = express();
const PORT = 3000;
const JWT_ISSUER = "https://fa-custom-backend.com";
const JWT_SUBJECT = "user+1@fa-custom-backend.com";

Step 4: Create JWT Generation Endpoint​

app.get("/jwt", (_: Request, response: Response) => {
    // Your transaction payload (hex-encoded)
    const SIGNING_PAYLOAD = Buffer.from("Hello world").toString("hex");

    const token = jwt.sign(
        { fatxn: SIGNING_PAYLOAD }, // FastAuth transaction claim
        { key: fs.readFileSync("./jwtRS256.key") },
        {
            expiresIn: "1h",
            algorithm: "RS256",
            issuer: JWT_ISSUER,
            subject: JWT_SUBJECT,
        },
    );

    response.status(200).send(token);
});

Step 5: Start Server and Extract Public Key​

app.listen(PORT, () => {
    console.log("Server running at PORT: ", PORT);
    console.log("Jwt issuer: ", JWT_ISSUER);

    // Extract public key components for contract configuration
    const publicKey = createPublicKey(fs.readFileSync("./jwtRS256.pub"));
    const jwk = publicKey.export({ format: "jwk" });

    console.log("Jwt public key [n]: ", jwk.n);
    console.log("Jwt public key [e]: ", jwk.e);
}).on("error", (error: Error) => {
    throw new Error(error.message);
});

Step 6: Run the Server​

# Compile TypeScript
npx tsc index.ts

# Run the server
node index.js

Step 7: Test JWT Generation​

# Get JWT token
curl http://localhost:3000/jwt

Next Steps​

  1. Customize the SIGNING_PAYLOAD with your actual transaction data
  2. Implement user authentication before JWT generation
  3. Deploy your JwtRS256Guard contract with your custom jwt verification logic
  4. Register your guard in the JwtGuardRouter contract
  5. Test your integration with the FastAuth contract

Code​

You can find the complete code in the examples folder.