.gksend format specification.
The .gksend format is an encrypted envelope that carries one or more documents — typically .gkp files — to a recipient identified by a public key.
Section 1 — Overall structure
A .gksend file has three blocks:
- Fixed 32-byte header starting with the magic bytes
GKSE(4 bytes:0x47 0x4B 0x53 0x45). - Send manifest (SendManifest) encoded in canonical CBOR.
- Encrypted payload with AES-256-GCM.
Section 2 — Fixed header (32 bytes)
| Offset | Size | Field |
|---|---|---|
| 0 | 4 | Magic GKSE |
| 4 | 2 | Major version |
| 6 | 2 | Minor version |
| 8 | 8 | Manifest size (uint64 little-endian) |
| 16 | 8 | Encrypted payload size (uint64 little-endian) |
| 24 | 8 | Emission timestamp (uint64 little-endian, Unix seconds) |
Section 3 — Send manifest
The SendManifest carries:
from: identifier of the sender and Ed25519 public signing key.to: identifier of the recipient and X25519 encryption public key.alg: envelope algorithm (default"X25519+HKDF+AES-256-GCM").nonce: AES-GCM nonce (12 bytes).items: list of items carried, each with: name, MIME type, cleartext size, BLAKE3 fingerprint.sig: Ed25519 signature of the sender over the manifest.
Section 4 — Key derivation
The encryption key of the payload is derived by HKDF-SHA-256 from the X25519 shared secret between sender and recipient, with a context including the envelope identifier and the operation type.
Section 5 — Payload
The encrypted payload is an archive concatenating the items declared in items. After decryption, the recipient can split the items and verify their individual BLAKE3 fingerprints.
Section 6 — Revocation
A .gksend envelope does not support revocation at the format level. Revocation is carried by the .gkp documents transported inside, each according to its own revocation model.
Section 7 — Compatibility
A conformant receiving client MUST:
- reject a file whose magic bytes are not
GKSE; - verify the Ed25519 signature of the manifest before any decryption attempt;
- verify the BLAKE3 fingerprint of each item after decryption;
- silently reject items whose fingerprint does not match.