libgocryptfs/Documentation/file-format.md

File Format

Header

 2 bytes header version (big endian uint16, currently 2)
16 bytes file id

Data block, default AES-GCM mode

16 bytes GCM IV (nonce)
1-4096 bytes encrypted data
16 bytes GHASH

Data block, AES-SIV mode (used in reverse mode, or when explicitly enabled with -init -aessiv)

16 bytes nonce
16 bytes SIV
1-4096 bytes encrypted data

Full block overhead = 32/4096 = 1/128 = 0.78125 %

Example: 1-byte file

Header     18 bytes
Data block 33 bytes

Total: 51 bytes

Example: 5000-byte file

Header       18 bytes
Data block 4128 bytes
Data block  936 bytes

Total: 5082 bytes