Disk Format

Header

The cryptio header starts at the beginning of the encrypted file, and contains two fields: a 12-byte initialization vector, and a 16-byte GCM authentication tag:

struct {
    uint8 iv[12];
    uint8 tag[16];
} CryptIOHeader;

The total size of this header is 28 bytes. Immediately following the header is the ciphertext data.

Overhead

Other than the 28-byte header, the AES-GCM mode introduces no additional overhead. Writing a 1024-byte plaintext with cryptio will result in a 1052-byte file.