doc: file-format.md: describe XChaCha20-Poly1305

Different nonce size.

Documentation/file-format.md

@@ -18,22 +18,46 @@ Data block, AES-SIV mode (used in reverse mode, or when explicitly enabled with
 	16 bytes SIV
 	1-4096 bytes encrypted data
 
-Full block overhead = 32/4096 = 1/128 = 0.78125 %
+Data block, XChaCha20-Poly1305 (enabled via `-init -xchacha`)
 
-Example: 1-byte file
---------------------
+	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 %
+
+Full block overhead (XChaCha20-Poly1305 mode) = 40/4096 = \~1 %
+
+Example: 1-byte file, AES-GCM and AES-SIV mode
+----------------------------------------------
 
 	Header     18 bytes
 	Data block 33 bytes
 
 Total: 51 bytes
 
-
-Example: 5000-byte file
------------------------
+Example: 5000-byte file, , AES-GCM and AES-SIV mode
+---------------------------------------------------
 
 	Header       18 bytes
 	Data block 4128 bytes
 	Data block  936 bytes
 
 Total: 5082 bytes
+
+Example: 1-byte file, XChaCha20-Poly1305 mode
+----------------------------------------------
+
+	Header     18 bytes
+	Data block 41 bytes
+
+Total: 59 bytes
+
+Example: 5000-byte file, XChaCha20-Poly1305 mode
+----------------------------------------------
+
+	Header       18 bytes
+	Data block 4136 bytes
+	Data block  944 bytes
+
+Total: 5098 bytes