Rockbox mail archiveSubject: Descrambler for multimedia
From: Björn Stenberg (bjorn_at_haxx.se)
My little christmas project has paid off. I just committed an update to 'descrambler' which supports descrambling Multimedia firmware files. I'll commit a new 'scrambler' tomorrow.
The Multimedia firmware files are compressed using a custom token packing algorithm, and then XORed with a 32-byte string. The descrambler tool finds this string automagically, simply by assuming that 0 is the most common byte in the plaintext. A simple statistical analysis then produces the string.
The compression was trickier, but thanks to it not being very efficient (gzip compresses the firmware to about 60% of the .ajz size) there was plenty of plaintext to look at in the compressed data.
Page was last modified "Jan 10 2012" The Rockbox Crew