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 guide 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

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

Compliance Framework Mapping

This skill supports compliance evidence collection across multiple frameworks:

• SOC 2: CC6.7 (Restriction on Transmission), CC6.1 (Logical Access)
• ISO 27001: A.10.1 (Cryptographic Controls)
• NIST 800-53: SC-12 (Cryptographic Key Management), SC-13 (Cryptographic Protection), SC-8 (Transmission Confidentiality)
• NIST CSF: PR.DS (Data Security)

Claw GRC Tip: When this skill is executed by a registered agent, compliance evidence is automatically captured and mapped to the relevant controls in your active frameworks.

Deploying This Skill with Claw GRC

Agent Execution

Register this skill with your Claw GRC agent for automated execution:

# Install via CLI
npx claw-grc skills add implementing-aes-encryption-for-data-at-rest

# Or load dynamically via MCP
grc.load_skill("implementing-aes-encryption-for-data-at-rest")

Audit Trail Integration

When executed through Claw GRC, every step of this skill generates tamper-evident audit records:

• SHA-256 chain hashing ensures no step can be modified after execution
• Evidence artifacts (configs, scan results, logs) are automatically attached to relevant controls
• Trust score impact — successful execution increases your agent's trust score

Continuous Compliance

Schedule this skill for recurring execution to maintain continuous compliance posture. Claw GRC monitors for drift and alerts when re-execution is needed.