.crin File Format

Version: 1.0  |  Status: Stable

Structure

Offset   Size   Field
0        8B     Magic: "CRYPTIN1" (ASCII)
8        1B     Version: 0x01
9        1B     Algorithm ID: 0x01 (AES-256-GCM + HKDF-SHA256)
10       16B    KDF Parameters (HKDF info string length + content)
26       32B    Dongle public key (K_identity pubkey) — AAD
58       16B    Salt (random, per-file)
74       12B    Nonce (random, AES-GCM)
86       N B    Ciphertext
86+N     16B    GCM Authentication Tag

Authenticated Data (AAD)

The header from offset 0 to 57 (inclusive) is authenticated as AEAD associated data. Any modification to the header invalidates the GCM tag.

Key Derivation

K_session = DERIVE_SESSION_KEY(dongle, nonce_session)
K_file    = HKDF-SHA256(ikm=K_session, salt=salt, info="cryptin-file-v1")

Decryption

1. Read and verify magic "CRYPTIN1"
2. Extract dongle pubkey from header
3. Verify dongle pubkey matches connected dongle (GET_PUBKEY)
4. Derive K_file using salt from header
5. Decrypt with AES-256-GCM using nonce from header
6. Verify GCM tag — if invalid: “Wrong dongle or corrupted file”