Decrypting a CPS2 game.

[l_oliveira]

Masks are created by comparing sets. Matches are one state and mismatches are the other. To reduce risk of collisions from encryption (the digest is only 0-65535 and the chance of collisions is big enough to cause up to a dozen errors on a mask) we add a third set to make it stronger. It only work with sets that have the exact same code on several regional variations.

Games that are a single release or have no match between regional variations I had to be disassemble in IDA from MAME's digest to then discern what was code and what was data...

I manually created the masks in that particular cases. I went where Razoola wasn't willing to with those ones (he did it for some very popular games but he automated as much as he could of the process while I did mine all manually)

 

[Darksoft]

Absolutely amazing job. Hats off.

 

[creeping1]

Many thanks for your reply.

Masks are created by comparing sets. Matches are one state and mismatches are the other. To reduce risk of collisions from encryption (the digest is only 0-65535 and the chance of collisions is big enough to cause up to a dozen errors on a mask) we add a third set to make it stronger. It only work with sets that have the exact same code on several regional variations.Wow

Wow very smart!

Games that are a single release or have no match between regional variations I had to be disassemble in IDA from MAME's digest to then discern what was code and what was data...2T

Was SSF2XJ one of those games, or is it close enough to SSF2T clones to do set comparing? I'm very interested to see the differences for 940311 vs 940223 of SSF2XJ. Maybe I can just regenerate the mask from what has been generated in https://cps2.avalaunch.net/downloads/ssf2xjdi.zip and the original encrypted roms, and then use that for 940311.

 

[l_oliveira]

I have masks for the games I decrypted myself. But I can for example make masks out of existing sets from other decrypters after "unpatching" them and comparing/analyzing vs the original encrypted set and MAME's decrypted digest. It takes some work, though.