Cryptography

Symmetric Encryption same key for encrypt/decrypt
• AES-256-GCM: standard for data at rest and in transit

Asymmetric (Public-Key) key pair
• RSA: encrypt with public key, decrypt with private key
• ECDH: key exchange
• RSA/ECDSA: digital signatures

Hash Functions one-way, fixed output | Algorithm | Status | Use | |---|---|---| | SHA-256 | Safe | Bitcoin, certificates, integrity | | SHA-3 | Safe | Modern alternative | | bcrypt | Safe | Password hashing (slow by design) | | Argon2id | Safe | Modern password hashing | | MD5 / SHA-1 | BROKEN | Never use for security |

Random Numbers: ``javascript // Cryptographically secure (NEVER use Math.random() for security!) const bytes = crypto.getRandomValues(new Uint8Array(32)); // browser const bytes = require('crypto').randomBytes(32); // Node.js ``

TLS Handshake: `` 1. Client Hello → supported TLS, random 2. Server Hello → chosen cipher, certificate 3. Client verifies certificate against trusted CA 4. ECDHE key exchange → shared session key (perfect forward secrecy) 5. Encrypted channel established ``

JWT Best Practices: ```javascript // DO: strong algorithm, short expiry const token = jwt.sign({ userId: 42 }, process.env.JWT_SECRET, { algorithm: 'HS256', expiresIn: '15m' });

// DO: verify algorithm explicitly jwt.verify(token, secret, { algorithms: ['HS256'] });

// DON'T: algorithm: 'none' (critical vulnerability) // DON'T: store sensitive data in payload (it's base64, not encrypted) ```

Key Management Rules:
• Never hardcode secrets in source code
• Use AWS Secrets Manager / HashiCorp Vault
• Rotate keys regularly
• Use HSM for critical private keys