OAuth Implementation

Implement OAuth 2.0 and OpenID Connect for secure authentication.

OAuth 2.0 Flows

Flow	Use Case
Authorization Code	Web apps (most secure)
Authorization Code + PKCE	SPAs, mobile apps
Client Credentials	Service-to-service
Refresh Token	Session renewal

Authorization Code Flow (Express)

const express = require('express');
const jwt = require('jsonwebtoken');

// Step 1: Redirect to authorization
app.get('/auth/login', (req, res) => {
  const state = crypto.randomBytes(16).toString('hex');
  req.session.oauthState = state;

  const params = new URLSearchParams({
    client_id: process.env.CLIENT_ID,
    redirect_uri: process.env.REDIRECT_URI,
    response_type: 'code',
    scope: 'openid profile email',
    state
  });

  res.redirect(`${PROVIDER_URL}/authorize?${params}`);
});

// Step 2: Handle callback
app.get('/auth/callback', async (req, res) => {
  if (req.query.state !== req.session.oauthState) {
    return res.status(400).json({ error: 'Invalid state' });
  }

  const tokenResponse = await fetch(`${PROVIDER_URL}/token`, {
    method: 'POST',
    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
    body: new URLSearchParams({
      grant_type: 'authorization_code',
      code: req.query.code,
      redirect_uri: process.env.REDIRECT_URI,
      client_id: process.env.CLIENT_ID,
      client_secret: process.env.CLIENT_SECRET
    })
  });

  const tokens = await tokenResponse.json();
  // Store tokens securely and create session
});

PKCE for Public Clients

function generatePKCE() {
  const verifier = crypto.randomBytes(32).toString('base64url');
  const challenge = crypto
    .createHash('sha256')
    .update(verifier)
    .digest('base64url');
  return { verifier, challenge };
}

Security Requirements

• Always use HTTPS
• Validate redirect URIs strictly
• Use PKCE for public clients
• Store tokens in HttpOnly cookies
• Implement token rotation
• Use short-lived access tokens (15 min)

Additional Implementations

See references/python-java.md for:

• Python Flask with Authlib OIDC provider
• OpenID Connect discovery and JWKS endpoints
• Java Spring Security OAuth2 server
• Token introspection and revocation

Never Do

• Store tokens in localStorage
• Use implicit flow
• Skip state parameter validation
• Expose client secrets in frontend
• Use long-lived access tokens