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+# Relicario Security Model
+
+## Cryptographic Protection
+
+Relicario uses two-factor vault decryption:
+1. **Passphrase** — user-memorized, zxcvbn score ≥3 required
+2. **Reference image** — JPEG carrying 256-bit secret via DCT steganography
+
+Key derivation: Argon2id (64 MiB memory, 3 iterations, 4 parallelism)
+Encryption: XChaCha20-Poly1305 (192-bit nonce, 256-bit key)
+
+## Manifest Integrity
+
+The manifest (`manifest.enc`) is encrypted with AEAD, which provides:
+
+- **Confidentiality**: Contents unreadable without master key
+- **Integrity**: Any modification detected and rejected on decrypt
+- **Authenticity**: Only master key holders can create valid ciphertexts
+
+### What AEAD Does NOT Protect
+
+- **Item deletion**: An attacker with write access can delete `.enc` files
+  or git-revert commits. The manifest decrypts successfully but won't
+  contain the deleted items.
+
+- **Rollback attacks**: An attacker can replace `manifest.enc` with an
+  older valid version. AEAD accepts any ciphertext created with the key.
+
+### Mitigation
+
+Item deletion and rollback are detectable via **git history**:
+
+```bash
+git log --oneline items/
+```
+
+For environments where git history could be rewritten (force-push):
+
+1. Enable device authentication (commit signing + pre-receive hook)
+2. Use a git server that rejects non-fast-forward pushes
+3. Regular backups with `relicario backup export`
+
+## Device Authentication
+
+When enabled, device authentication provides:
+
+- **Commit authorship**: All commits signed by registered device keys
+- **Push access control**: Deploy keys managed via Gitea API
+- **Instant revocation**: One command cuts off both signing and push
+
+See `docs/superpowers/specs/2026-05-02-device-authentication-design.md`.
+
+## Access Control
+
+Without device authentication, access control is transport-layer only:
+
+- **CLI**: SSH key authentication to git remote
+- **Extension**: Git credentials in browser storage
+
+Device registration was optional before v0.4.0. With device auth enabled,
+all commits must be signed by a registered device.