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gregf
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Re: Vas WIP: decrypting Nihon System Omega
02/13/17 03:14 AM




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ShouTime dumped the incredibly rare game Omega (Nihon System). Itís a ball-and-paddle game running on similar hardware to Segaís 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 werenít 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 itís 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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There are still 2 games from the src/mame/drivers/freekick.cpp file in which original pcbs need to be found (better if still in working condition). Counter Run and Gigas Mark II. I thought entries of these two would be on undumped Wiki, but not yet even though the bootleg versions are already supported in MAME.







Entire thread
Subject Posted by Posted on
* Vas WIP: decrypting Nihon System Omega Vas Crabb 02/13/17 01:38 AM
. * Re: Vas WIP: decrypting Nihon System Omega gregf  02/13/17 03:14 AM
. * Re: Vas WIP: decrypting Nihon System Omega Diet Go Go Fan  02/13/17 04:16 AM
. * Re: Vas WIP: decrypting Nihon System Omega Haze  02/13/17 03:44 PM

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