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Always love seeing applications of Feistel cipher. Used it with AES as the PRF for implementing FPE in legacy systems.

Just want to note that this approach (regardless of PRF) probably wouldn't have worked in 1991. Recomputing the cipher state at every pixel is probably ~10x slower than the single shift + xor in the iterative LFSR approach.

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Hello, yes the approach is slower in my implementation, but I've the feeling that a suitable F (much simpler) and a low number of rounds could do the trick. However the highlight in the original post was that the ports were not able to reproduce the effect. Given that the ports are AFAIK successive and use higher resolutions, I bet that the CPU was not an issue in that case.

EDIT: I just randomly checked that 4 rounds of F = ((r * 31) ^ (r >> 3)) & 0xff provide more or less the same result. Multiplying for 31 is the same as shifting 5 bits on the left and subtracting the number again, so it's just 4 rounds of bit shifting and xor.

You should time it :)

No doubt the original code is still faster :-)

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