Implementing AES Encryption for Data at Rest

Overview

AES (Advanced Encryption Standard) is a symmetric block cipher standardized by NIST (FIPS 197) used to protect classified and sensitive data. This skill covers implementing AES-256 encryption in GCM mode for encrypting files and data stores at rest, including proper key derivation, IV/nonce management, and authenticated encryption.

Objectives

• Implement AES-256-GCM encryption and decryption for files
• Derive encryption keys from passwords using PBKDF2 and Argon2
• Manage initialization vectors (IVs) and nonces securely
• Encrypt and decrypt entire directory trees
• Implement authenticated encryption to detect tampering
• Handle large files with streaming encryption

Key Concepts

AES Modes of Operation

Key Derivation

Never use raw passwords as encryption keys. Always derive keys using:

• PBKDF2: NIST-approved, widely supported (minimum 600,000 iterations as of 2024)
• Argon2id: Winner of Password Hashing Competition, memory-hard
• scrypt: Memory-hard, good alternative to Argon2

Nonce/IV Management

• GCM requires a 96-bit (12-byte) nonce that must NEVER be reused with the same key
• Generate nonces using os.urandom() (CSPRNG)
• Store nonce alongside ciphertext (it is not secret)

Implementation Steps

1. Install the cryptography library: pip install cryptography
2. Generate or derive an encryption key
3. Create a random nonce for each encryption operation
4. Encrypt data using AES-256-GCM with the key and nonce
5. Store nonce + ciphertext + authentication tag together
6. For decryption, extract nonce, verify tag, and decrypt

Encrypted File Format

[salt: 16 bytes][nonce: 12 bytes][ciphertext: variable][tag: 16 bytes]

Security Considerations

• Always use authenticated encryption (GCM, CCM) to prevent tampering
• Never reuse a nonce with the same key (catastrophic in GCM)
• Use at least 256-bit keys for long-term data protection
• Securely wipe keys from memory after use when possible
• Rotate encryption keys periodically per organizational policy
• For disk-level encryption, consider XTS mode (AES-XTS)

Validation Criteria

• [ ] AES-256-GCM encryption produces valid ciphertext
• [ ] Decryption recovers original plaintext exactly
• [ ] Authentication tag detects any ciphertext modification
• [ ] Key derivation uses sufficient iterations/parameters
• [ ] Nonces are never reused for the same key
• [ ] Large files (>1GB) can be processed via streaming
• [ ] Encrypted file format includes all necessary metadata