implementing-envelope-encryption-with-aws-kms

---
name: implementing-envelope-encryption-with-aws-kms
description: Implements envelope encryption with AWS KMS, encrypting data locally with a data encryption key (DEK) and protecting that DEK with a KMS-managed key (KEK), covering the encrypt/decrypt flow, KMS key types, and security validation criteria. Use when designing key management for encrypting large or numerous data objects on AWS, or when reducing direct KMS API call volume.
license: Apache-2.0
---

# Implementing Envelope Encryption with AWS KMS

## Overview

Envelope encryption is a strategy where data is encrypted with a data encryption key (DEK), and the DEK itself is encrypted with a master key (KEK) managed by AWS KMS. This approach allows encrypting large volumes of data locally while keeping the master key secure in a hardware security module (HSM) managed by AWS. This skill covers implementing envelope encryption using AWS KMS GenerateDataKey API.


## When to Use

- When deploying or configuring implementing envelope encryption with aws kms 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

## 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

- Understand the envelope encryption pattern and its advantages
- Generate data encryption keys using AWS KMS GenerateDataKey
- Encrypt/decrypt data locally using DEKs
- Store encrypted DEK alongside ciphertext
- Implement key caching to reduce KMS API calls
- Handle key rotation with automatic re-encryption
- Implement multi-region encryption for disaster recovery

## Key Concepts

### Envelope Encryption Flow

1. Call `kms:GenerateDataKey` to get plaintext DEK + encrypted DEK
2. Use plaintext DEK to encrypt data locally (AES-256-GCM)
3. Store encrypted DEK alongside ciphertext
4. Discard plaintext DEK from memory
5. For decryption: call `kms:Decrypt` on encrypted DEK, then decrypt data

### Advantages Over Direct KMS Encryption

| Aspect | Direct KMS | Envelope Encryption |
|--------|-----------|-------------------|
| Max data size | 4 KB | Unlimited |
| Latency | Network round-trip per operation | Local encryption |
| Cost | $0.03/10,000 requests | Fewer KMS requests |
| Offline | Not possible | Yes (with cached DEKs) |

### KMS Key Types

- **AWS Managed**: AWS creates and manages (`aws/s3`, `aws/ebs`)
- **Customer Managed**: You create and manage policies
- **Custom Key Store**: Backed by CloudHSM cluster

## Security Considerations

- Never store plaintext DEK; only keep encrypted DEK
- Use key policies to restrict who can call GenerateDataKey and Decrypt
- Enable AWS CloudTrail logging for all KMS API calls
- Implement key rotation (automatic annual rotation for CMKs)
- Use encryption context for authenticated encryption metadata
- Handle KMS throttling with exponential backoff

## Validation Criteria

- [ ] GenerateDataKey returns plaintext and encrypted DEK
- [ ] Data encrypts correctly with plaintext DEK using AES-256-GCM
- [ ] Encrypted DEK can be decrypted via KMS Decrypt API
- [ ] Decrypted DEK recovers the original data
- [ ] Plaintext DEK is wiped from memory after use
- [ ] Encryption context is validated during decryption
- [ ] Key rotation re-encrypts DEKs with new master key