Computing WC3 Map Key Value (crc of some kind)

[Strilanc]

When hosting wc3 games, there is a required value that I don't know how to compute yet.

Here is what I know about it:
- It depends only on files within the mpq (repackaging the mpq with no compression, etc, has no effect on it)
- It varies only slightly if the difference between the two mpqs is small. If you save a map twice in a row it seems to change very little
- It is not a standard crc32 of any of the individual files
- I believe it includes a crc of the war3map.j file, because the files changes only a little every save [it includes a save time]

What do I do now? Does blizzard have some standard non-standard crc32 they use? [They do use the standard one in other places]

I currently use a packet sniffer to retrieve the value when a game is hosted, but that means I can only run it on valid maps, which makes it difficult to see which files are being read.