| name | implementing-aes-encryption-for-data-at-rest |
| description | 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 m |
| domain | cybersecurity |
| subdomain | cryptography |
| tags | ["cryptography","encryption","aes","data-at-rest","symmetric-encryption"] |
| version | 1.0 |
| author | mahipal |
| license | Apache-2.0 |
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
| Mode | Authentication | Parallelizable | Use Case |
|---|
| GCM | Yes (AEAD) | Yes | Network data, file encryption |
| CBC | No | Decrypt only | Legacy systems, disk encryption |
| CTR | No | Yes | Streaming encryption |
| CCM | Yes (AEAD) | No | IoT, constrained environments |
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
- Install the
cryptography library: pip install cryptography
- Generate or derive an encryption key
- Create a random nonce for each encryption operation
- Encrypt data using AES-256-GCM with the key and nonce
- Store nonce + ciphertext + authentication tag together
- 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
