implementing-aes-encryption-for-data-at-rest

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
nist_csf	["PR.DS-01","PR.DS-02","PR.DS-10"]

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.

When to Use

• When deploying or configuring implementing aes encryption for data at rest capabilities in your environment
• When establishing security controls aligned to compliance requirements
• When building or improving security architecture for this domain
• When conducting security assessments that require this implementation

Common Misconfigurations & Verification

• AES-ECB mode: identical plaintext blocks produce identical ciphertext blocks (the "ECB penguin"). Never use modes.ECB() for data at rest — use AES-256-GCM (AEAD). Detect by encrypting a buffer of repeated 16-byte blocks and checking the ciphertext for repeated blocks.
• Static or reused GCM nonce: reusing a 96-bit nonce with the same key is catastrophic — it leaks the XOR of plaintexts and allows authentication-key recovery. Generate the nonce with os.urandom(12) per message; never hardcode it. Verify nonces differ across two encryptions of the same plaintext.
• CBC/CTR without a MAC (no auth tag): unauthenticated ciphertext is malleable and enables padding-oracle attacks. Use GCM/CCM, or encrypt-then-HMAC.
• Dropping/ignoring the GCM tag on decrypt: decryption MUST fail if the tag is wrong.
• Raw password as key, or weak KDF: derive with PBKDF2 (≥600k iters) or Argon2id; use a random per-file salt.
• The critical test — tamper rejection: flip one byte of the ciphertext (or the tag) and confirm decryption raises InvalidTag and returns NO plaintext. A scheme that returns data after tampering is broken. Also confirm decrypt with the wrong key fails rather than yielding garbage.

Prerequisites

• Familiarity with cryptography concepts and tools
• Access to a test or lab environment for safe execution
• Python 3.8+ with required dependencies installed
• Appropriate authorization for any testing activities

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