JWT暗号工学ラボ：クレーム改ざん・署名検証・アルゴリズム混乱攻撃の検証

// Production Node.js JWT Verification with jsonwebtoken
import jwt from 'jsonwebtoken';

const JWT_SECRET = process.env.JWT_SECRET_KEY; // High-entropy secret (>= 256 bits)

export function verifyUserToken(authHeader: string | undefined) {
  if (!authHeader || !authHeader.startsWith('Bearer ')) {
    throw new Error('Missing or malformed Authorization header');
  }

  const token = authHeader.slice(7);

  // SECURE VERIFICATION:
  // 1. Explicitly whitelist allowed algorithms (rejects "none" or RS256 confusion)
  // 2. Enforces expiration (exp) and expected issuer (iss)
  const decoded = jwt.verify(token, JWT_SECRET, {
    algorithms: ['HS256'], // NEVER let the header dictate the algorithm!
    issuer: 'https://auth.curious-techie.com',
    audience: 'https://api.curious-techie.com',
    clockTolerance: 5 // 5 seconds tolerance for clock skew
  });

  return decoded;
}