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File Format
===========
Empty files are stored as empty files.
Non-empty files contain a *Header* and one or more *Data blocks*.
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
Overhead = (16+16)/4096 = 1/128 = 0.78125 %
Data block, AES-SIV mode
------------------------
AES-SIV is used in reverse mode, or when explicitly enabled with `-init -aessiv`.
16 bytes nonce
16 bytes SIV
1-4096 bytes encrypted data
Overhead = (16+16)/4096 = 1/128 = 0.78125 %
Data block, XChaCha20-Poly1305
------------------------------
Enabled via `-init -xchacha`
24 bytes nonce
1-4096 bytes encrypted data
16 bytes Poly1305 tag
Overhead = (24+16)/4096 = 0.98 %
Examples
========
0-byte file (all modes)
-----------------------
(empty)
Total: 0 bytes
1-byte file, AES-GCM and AES-SIV mode
-------------------------------------
Header 18 bytes
Data block 33 bytes
Total: 51 bytes
5000-byte file, , AES-GCM and AES-SIV mode
------------------------------------------
Header 18 bytes
Data block 4128 bytes
Data block 936 bytes
Total: 5082 bytes
1-byte file, XChaCha20-Poly1305 mode
------------------------------------
Header 18 bytes
Data block 41 bytes
Total: 59 bytes
5000-byte file, XChaCha20-Poly1305 mode
---------------------------------------
Header 18 bytes
Data block 4136 bytes
Data block 944 bytes
Total: 5098 bytes
See Also
========
https://nuetzlich.net/gocryptfs/forward_mode_crypto/ / https://github.com/rfjakob/gocryptfs-website/blob/master/docs/forward_mode_crypto.md
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