When to Use

• Encrypting files, database fields, or app storage
• Password hashing (bcrypt, argon2)
• Key management, rotation, derivation
• TLS/certificate configuration
• Auditing code for crypto mistakes
• Mobile secure storage (Keychain, Keystore)

Algorithm Selection

Critical Rules

1. Never reuse IVs/nonces — AES-GCM + repeated nonce = catastrophic
2. Use authenticated encryption (AEAD) — Plain CBC enables padding oracles
3. Hash passwords, don't encrypt — Hashing is one-way
4. No hardcoded keys — Use env vars, KMS, or Vault
5. No Math.random() for crypto — Use CSPRNG only
6. Constant-time comparisons — Prevent timing attacks on secrets
7. Separate keys by purpose — Encryption ≠ signing ≠ backup

File Encryption (CLI)

# age (modern, simple)
age -p -o file.age file.txt
age -d -o file.txt file.age

# GPG
gpg -c --cipher-algo AES256 file.txt

Platform-Specific

See patterns.md for code snippets:

• Password hashing (Node, Python, Go)
• Envelope encryption with KMS
• JWT with RS256 key rotation
• Secure token generation

See mobile.md for:

• iOS Keychain wrapper
• Android EncryptedSharedPreferences
• SQLCipher setup
• Biometric auth integration
• Certificate pinning

See infra.md for:

• TLS certificate auto-renewal
• HashiCorp Vault policies
• mTLS between services
• Backup encryption verification

Audit Checklist

• No plaintext passwords in DB/logs/env
• No secrets in git history
• No hardcoded keys in source
• No Math.random() for security
• No deprecated algorithms (MD5, SHA1, DES)
• No disabled cert validation
• IVs/nonces never reused
• PBKDF2 iterations ≥600k / bcrypt cost ≥12
• TLS 1.2+ enforced, old protocols disabled
• Key rotation procedure documented