Decrypting and browsing the file system for iPhone OS 4.0 is very easy, and can be done on Windows, Mac OS X, and Linux.

First, you’ll need a copy of VFDECRYPT: Windows |  Mac OS X |  Linux

Next, you’ll need the decryption keys.

iPhone 3G

0da2d3316d5ee7cd1858e4035e451387cd8156e97535fb09028859e68e5b7b39a6649552

iPhone 3GS

62ea9bf9971e6c410231646f916f80330f9cbc1d1c585f0c03dab6b6f7158dc0a9c5efaf

iPod Touch 2G

d88fa434f6a8c50fd49cd0923879da5c3079c59c0e534cff521a9cecbcad48d84b4daef0

iPod Touch 3G

382dee11b9d80387b16ac2030ee1e903b78d9743a31a18bcafc922b7921eca85ab0aebf7

Next, you’ll need to run the following command if you’re on Mac OS X or Linux:

./vfdecrypt -i <dmg> -o decrypted_fs.dmg -k <key>

If you’re on Windows, run this:

vfdecrypt.exe -i <dmg> -o decrypted_fs.dmg -k <key>

Replace <dmg> with the big extracted DMG from the IPSW. Replace <key> with the appropriate decryption key.

Running that command will spit out the decrypted DMG, which you can then mount and browse.

A lot of people have been wondering how to re-encrypt/re-compile the DMG file to put it back inside of the IPSW file. Well, you can’t easily do it. There are a lot of checks that iTunes have to conduct beforehand. The only tools that are able to modify and recompile the DMG/IPSW is PwnageTool and sn0wbreeze. Unless you know how those 2 tools work, you won’t be able to manually do this yourself. Also, those tools don’t support 4.0 beta, so don’t even bother.