[Solved]: Emulating boolean circuits using addition and multiplication (mod 5)

Problem Detail: 

I'm trying to use gates that do addition and multiplication modulo 5 to emulate logic gates.

Assuming false and true are mapped to 0 and 1 respectively (with 2, 3, and 4 being invalid), I figured out I can map the operations like this:

a and b -> a*b (mod 5) a or b -> 2*(a+b)*(a+b+2) (mod 5) 

I was wondering if there was a simpler approach.

For the application I have in mind, a toy example of secure multi party computation using secret sharing, I haven't shown/discovered/figured-out yet if it's safe to re-use private values. If I have to recompute a, b, and a+b two times in order to do an or, costs would be exponential in the length of the circuit. (I'm only using tiny circuits so that's not a big deal, but it would be interesting to know if it was just a non-issue via a clever transformation.)

Asked By : Craig Gidney

Answered By : Yuval Filmus

There are many other solutions. For example, even keeping your True and False, you can have $a\lor b = 1 - (1-a)(1-b) = a+b-ab = a + (1-a)b$ and so on.

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Question Source : http://cs.stackexchange.com/questions/17908

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