KMS Envelope Encryption Specification

The Cryptographic Architecture

Envelope encryption uses a two-tier key hierarchy: 1. Key Encryption Key (KEK) / Customer Master Key (CMK): - Managed inside the Hardware Security Module (HSM) of AWS KMS, Azure Key Vault, or GCP Cloud KMS. - Never leaves the HSM unencrypted. 2. Data Encryption Key (DEK): - A 256-bit symmetric key (AES-256) generated on-demand by KMS. - KMS returns both the Plaintext DEK (used immediately in client RAM) and the Ciphertext DEK (encrypted under the CMK).

AES-GCM Payload Structure

When serializing encrypted payloads to storage:

┌─────────────────┬──────────────┬──────────────────┬──────────────┐
│ Ciphertext DEK  │ Nonce / IV   │ Ciphertext Data  │ Auth Tag     │
│ (e.g. 512 bytes)│ (12 bytes)   │ (Variable size)  │ (16 bytes)   │
└─────────────────┴──────────────┴──────────────────┴──────────────┘

- AES-GCM (Galois/Counter Mode) provides both confidentiality and authenticated integrity. If the ciphertext or nonce is modified in transit, decryption will fail immediately.