> Can it really be done with just a multiplication?

Yes, because it takes advantage of the fact that on a computer A*B 
actually calculates (A*B)mod(2^N), when N is the number of bits in the 
destination register.

For example, with 8-bit numbers, the inverse of multiply-by-3 is 
multiply-by-171. Note that it only works if the multiplicand is greater 
than 2 and prime. So that narrows down the possible values of SECRET2 in 
the OP quite a lot.

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