From 16ed08caf78951bf6bb92945c3e729bb06111f6f Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher
Date: Tue, 3 Jun 2025 16:04:11 +0200
Subject: Documentation: make file-format.md more structured

Also mention that empty files are stored empty.
---
 Documentation/file-format.md | 53 ++++++++++++++++++++++++++++++++++----------
 1 file changed, 41 insertions(+), 12 deletions(-)

diff --git a/Documentation/file-format.md b/Documentation/file-format.md
index 7cce72c..7c2e2c8 100644
--- a/Documentation/file-format.md
+++ b/Documentation/file-format.md
@@ -1,43 +1,67 @@
 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
 
-Data block, AES-SIV mode (used in reverse mode, or when explicitly enabled with `-init -aessiv`)
+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
 
-Data block, XChaCha20-Poly1305 (enabled via `-init -xchacha`)
+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
 
-Full block overhead (AES-GCM and AES-SIV mode) = 32/4096 = 1/128 = 0.78125 %
+Overhead = (24+16)/4096 = 0.98 %
 
-Full block overhead (XChaCha20-Poly1305 mode) = 40/4096 = \~1 %
+Examples
+========
 
-Example: 1-byte file, AES-GCM and AES-SIV mode
-----------------------------------------------
+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
 
-Example: 5000-byte file, , AES-GCM and AES-SIV mode
----------------------------------------------------
+5000-byte file, , AES-GCM and AES-SIV mode
+------------------------------------------
 
 	Header       18 bytes
 	Data block 4128 bytes
@@ -45,19 +69,24 @@ Example: 5000-byte file, , AES-GCM and AES-SIV mode
 
 Total: 5082 bytes
 
-Example: 1-byte file, XChaCha20-Poly1305 mode
-----------------------------------------------
+1-byte file, XChaCha20-Poly1305 mode
+------------------------------------
 
 	Header     18 bytes
 	Data block 41 bytes
 
 Total: 59 bytes
 
-Example: 5000-byte file, XChaCha20-Poly1305 mode
-----------------------------------------------
+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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