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ENCRYPTION_MODES.md

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git-cas Encryption Modes

git-cas supports three distinct envelope encryption schemes, each optimized for different security and performance profiles.

Mode Integrity Privacy Deduplication Use Case
Whole Full Object High None Small sensitive files (keys, configs).
Framed Per-Frame High None Standard large assets. Prevents blob swapping.
Convergent Block-Level Medium High Shared libraries or public binaries where storage cost is priority.

1. Whole-Object Encryption (whole)

The entire file is encrypted as a single ciphertext.

  • Mechanism: AES-256-GCM with a random 96-bit nonce.
  • Deduplication: None (each store creates a unique ciphertext even for identical content).
  • Integrity: The GCM auth tag covers the entire object.

2. Framed Encryption (framed)

The default for v6.0.0. The file is split into chunks, and each chunk is encrypted as an independent AES-GCM "frame".

  • Mechanism: AES-256-GCM. The frame index is bound to the AAD (Additional Authenticated Data).
  • Integrity Guarantee: Prevents "Replay Attacks" or blob-swapping between different manifests, as each frame's metadata is cryptographically bound to its position in the asset.
  • Deduplication: None.

3. Convergent Encryption (convergent)

Allows deduplication across different files or versions while still providing encryption at rest.

  • Mechanism: The encryption key is derived from the SHA-256 hash of the content itself.
  • Trade-off: Vulnerable to "Confirmation Attacks" (an attacker who guesses the content can verify its presence in the CAS).
  • Deduplication: Global. Identical chunks across any file share the same ciphertext and OID.

Key Management

All modes use Envelope Encryption:

  1. A Data Encryption Key (DEK) is generated per asset (or per chunk).
  2. The DEK is encrypted for one or more Key Encryption Keys (KEKs) (e.g., passphrases or OS keys).
  3. The encrypted DEK is stored in the manifest or vault metadata.