MAMEWorld Forums - News - Re: Vas WIP: decrypting Nihon System Omega

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ShouTime dumped the incredibly rare game Omega (Nihon System). It’s a ball-and-paddle game running on similar hardware to Sega’s Gigas. These games use an NEC MC-8123 CPU module containing a Z80 core, decryption circuitry, and an 8 KiB encryption key in battery-backed RAM.

This encryption scheme could have been brutal, requiring extensive analysis of a working CPU module to crack, if it weren’t for a fatal flaw: Sega used a simple linear congruential generator algorithm to create 8 KiB keys from 24-bit seeds. That means there are less than seventeen million encryption keys to test. Seventeen million might sound like a lot, but it’s far less than the total possible number of keys, and definitely small enough to apply a known plaintext attack in a reasonable amount of time.
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Amazing that the pcb is still running at this time. Lucked out in getting another pcb that uses the evil mc-8123 saved thanks to the earlier findings and work from years earlier by both Nicola and Dave.

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src/mame/machine/mc8123.cpp

Additionally, Dave Widel has dumped some tables from a NEC MC-8123B and Nicola has made progress decrypting the first one. So we will require all NEC MC-8123B's also as well as the 1 remaining custom Z80 (to be sent to Dave Widel)
Update: Nicola has cracked all of the custom Z80's and MC-8123 CPUs so we don't need them now.
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Counter Run is only game remaining from the src/mame/drivers/freekick.cpp file in which original pcb needs to be found (better if still in working condition). Fortunately the bootleg version of Counter Run is already supported in MAME, but it would be ideal if original version can be rescued/preserved.

Edited by gregf (02/20/18 10:35 PM)