libgocryptfs/Documentation/file-format.md
2019-01-05 12:27:55 +01:00

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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