[ixtab]

You're right... but actually, it's a bit different. I had assumed so far that only the PID (which is in turn derived from the serial number) constitutes the key, in which case it would indeed have been even worse than what I wrote above (the PID is really only 7 characters + "*" + 2-character checksum), and those 7 characters don't even span the entire possible range (O and 0 are excluded).

However, providing the correct PID, but a wrong serial number, also fails to decrypt the file. Thus, the key space is more likely to be something like 36^13 again, which leads back to the first calculation, multiplied by another factor of ~ 50000 (yes, the serials are 16 characters, but there are only few valid "prefixes")