I'm looking at the Seibu SPI graphics encryption at the moment. For now, I'm fixing the small mistakes in the tilemap layers of Senkyu etc., after that I might look at the sprites as well.
Most of the work has already been done on the tilemap layer, but there are some mistakes. I have understood while they are happening and already fixed one of them. It's clear how the algorithm works, it's a bitswap + arithmetic calculations (additions instead of simple XORs - this is not common). Replacing the lookup table with formulas and fixing the remaining bugs is just a matter of time and patience at this point.