dbbackup

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Renz 607d2e50e9 feat: Phase 4 Tasks 1-2 - Implement AES-256-GCM encryption library

Implemented complete encryption library:

internal/encryption/encryption.go (426 lines):
- AES-256-GCM authenticated encryption
- PBKDF2 key derivation (100,000 iterations, SHA-256)
- EncryptionWriter: streaming encryption with 64KB chunks
- DecryptionReader: streaming decryption
- EncryptionHeader: magic marker, version, algorithm, salt, nonce
- Key management: passphrase or direct key
- Nonce increment for multi-chunk encryption
- Authenticated encryption (prevents tampering)

internal/encryption/encryption_test.go (234 lines):
- TestEncryptDecrypt: passphrase, direct key, wrong password
- TestLargeData: 1MB file encryption (0.04% overhead)
- TestKeyGeneration: cryptographically secure random keys
- TestKeyDerivation: PBKDF2 deterministic derivation

Features:
✅ AES-256-GCM (strongest symmetric encryption)
✅ PBKDF2 with 100k iterations (OWASP recommended)
✅ 12-byte nonces (GCM standard)
✅ 32-byte salts (security best practice)
✅ Streaming encryption (low memory usage)
✅ Chunked processing (64KB chunks)
✅ Authentication tags (integrity verification)
✅ Wrong password detection (GCM auth failure)
✅ File format versioning (future compatibility)

Security Properties:
- Confidentiality: AES-256 (military grade)
- Integrity: GCM authentication tag
- Key derivation: PBKDF2 (resistant to brute force)
- Nonce uniqueness: incremental counter
- Salt randomness: crypto/rand

Test Results: ALL PASS (0.809s)
- Encryption/decryption: ✅
- Large data (1MB): ✅
- Key generation: ✅
- Key derivation: ✅
- Wrong password rejection: ✅

Status: READY FOR INTEGRATION
Next: Add --encrypt flag to backup commands