notes.txt

Zip file format
===============

This is all copied from the Wikipedia page on the Zip file format and I'm
keeping it here for reference:
https://en.wikipedia.org/wiki/Zip_(file_format)

File headers
------------
Offset Bytes Description 
0      4     Local file header signature = 0x04034b50 (read as a little-endian number)
4      2     Version needed to extract (minimum)
6      2     General purpose bit flag
8      2     Compression method
10     2     File last modification time
12     2     File last modification date
14     4     CRC-32
18     4     Compressed size
22     4     Uncompressed size
26     2     File name length (n)
28     2     Extra field length (m)
30     n     File name
30+n   m     Extra field 

This is immediately followed by the compressed data.

Data descriptor
---------------
If the bit at offset 3 (0x08) of the general-purpose flags field is set, then
the CRC-32 and file sizes are not known when the header is written. The fields
in the local header are filled with zero, and the CRC-32 and size are appended
in a 12-byte structure (optionally preceded by a 4-byte signature) immediately
after the compressed data:

Offset Bytes   Description[24]
0      0/4     Optional data descriptor signature = 0x08074b50
0/4      4     CRC-32
4/8      4     Compressed size
8/12     4     Uncompressed size 


Central directory
-----------------
Offset Bytes Description[24]
0      4     Central directory file header signature = 0x02014b50
4      2     Version made by
6      2     Version needed to extract (minimum)
8      2     General purpose bit flag
10     2     Compression method
12     2     File last modification time
14     2     File last modification date
16     4     CRC-32
20     4     Compressed size
24     4     Uncompressed size
28     2     File name length (n)
30     2     Extra field length (m)
32     2     File comment length (k)
34     2     Disk number where file starts
36     2     Internal file attributes
38     4     External file attributes
42     4     Relative offset of local file header. This is the number of bytes between the start of the first disk on which the file occurs, and the start of the local file header. This allows software reading the central directory to locate the position of the file inside the ZIP file.
46     n     File name
46+n   m     Extra field
46+n+m k     File comment


End of central directory record (EOCD)
--------------------------------------
After all the central directory entries comes the end of central directory
(EOCD) record, which marks the end of the ZIP file:

Offset Bytes Description[24]
0      4     End of central directory signature = 0x06054b50
4      2     Number of this disk
6      2     Disk where central directory starts
8      2     Number of central directory records on this disk
10     2     Total number of central directory records
12     4     Size of central directory (bytes)
16     4     Offset of start of central directory, relative to start of archive
20     2     Comment length (n)
22     n     Comment