Integrating Gel Auth's OAuth provider​

import http from "node:http";
import { URL } from "node:url";
import crypto from "node:crypto";

/**
 * You can get this value by running `gel instance credentials`.
 * Value should be:
 * `${protocol}://${host}:${port}/branch/${branch}/ext/auth/
 */
const GEL_AUTH_BASE_URL = process.env.GEL_AUTH_BASE_URL;
const SERVER_PORT = 3000;

/**
 * Generate a random Base64 url-encoded string, and derive a "challenge"
 * string from that string to use as proof that the request for a token
 * later is made from the same user agent that made the original request
 *
 * @returns {Object} The verifier and challenge strings
 */
const generatePKCE = () => {
  const verifier = crypto.randomBytes(32).toString("base64url");

  const challenge = crypto
    .createHash("sha256")
    .update(verifier)
    .digest("base64url");

  return { verifier, challenge };
};

const server = http.createServer(async (req, res) => {
  const requestUrl = getRequestUrl(req);

  switch (requestUrl.pathname) {
    case "/auth/authorize": {
      await handleAuthorize(req, res);
      break;
    }

    case "/auth/callback": {
      await handleCallback(req, res);
      break;
    }

    default: {
      res.writeHead(404);
      res.end("Not found");
      break;
    }
  }
});

/**
 * Redirects OAuth requests to Gel Auth OAuth authorize redirect
 * with the PKCE challenge, and saves PKCE verifier in an HttpOnly
 * cookie for later retrieval.
 *
 * @param {Request} req
 * @param {Response} res
 */
const handleAuthorize = async (req, res) => {
  const requestUrl = getRequestUrl(req);
  const provider = requestUrl.searchParams.get("provider");

  if (!provider) {
    res.status = 400;
    res.end("Must provider a 'provider' value in search parameters");
    return;
  }

  const pkce = generatePKCE();
  const redirectUrl = new URL("authorize", GEL_AUTH_BASE_URL);
  redirectUrl.searchParams.set("provider", provider);
  redirectUrl.searchParams.set("challenge", pkce.challenge);
  redirectUrl.searchParams.set(
    "redirect_to",
    `http://localhost:${SERVER_PORT}/auth/callback`
  );
  redirectUrl.searchParams.set(
    "redirect_to_on_signup",
    `http://localhost:${SERVER_PORT}/auth/callback?isSignUp=true`
  );

  res.writeHead(302, {
    "Set-Cookie": `gel-pkce-verifier=${pkce.verifier}; HttpOnly; Path=/; Secure; SameSite=Strict`,
    Location: redirectUrl.href,
  });
  res.end();
};

/**
 * Handles the PKCE callback and exchanges the `code` and `verifier
 * for an auth_token, setting the auth_token as an HttpOnly cookie.
 *
 * @param {Request} req
 * @param {Response} res
 */
const handleCallback = async (req, res) => {
  const requestUrl = getRequestUrl(req);

  const code = requestUrl.searchParams.get("code");
  if (!code) {
    const error = requestUrl.searchParams.get("error");
    res.status = 400;
    res.end(
      `OAuth callback is missing 'code'. OAuth provider responded with error: ${error}`
    );
    return;
  }

  const cookies = req.headers.cookie?.split("; ");
  const verifier = cookies
    ?.find((cookie) => cookie.startsWith("gel-pkce-verifier="))
    ?.split("=")[1];
  if (!verifier) {
    res.status = 400;
    res.end(
      `Could not find 'verifier' in the cookie store. Is this the same user agent/browser that started the authorization flow?`
    );
    return;
  }

  const codeExchangeUrl = new URL("token", GEL_AUTH_BASE_URL);
  codeExchangeUrl.searchParams.set("code", code);
  codeExchangeUrl.searchParams.set("verifier", verifier);
  const codeExchangeResponse = await fetch(codeExchangeUrl.href, {
    method: "GET",
  });

  if (!codeExchangeResponse.ok) {
    const text = await codeExchangeResponse.text();
    res.status = 400;
    res.end(`Error from the auth server: ${text}`);
    return;
  }

  const { auth_token } = await codeExchangeResponse.json();
  res.writeHead(204, {
    "Set-Cookie": `gel-auth-token=${auth_token}; HttpOnly; Path=/; Secure; SameSite=Strict`,
  });
  res.end();
};

/**
 * Get the user's profile information from Google
 */
async function getUserProfile(providerToken) {
  const response = await fetch(
    "https://accounts.google.com/.well-known/openid-configuration"
  );
  const discoveryDocument = await response.json();
  const response = await fetch(discoveryDocument.userinfo_endpoint, {
    headers: {
      Authorization: `Bearer ${providerToken}`,
      Accept: "application/json",
    },
  });
  return await response.json();
}