modular_inverse

There is a need for the value of integer N' such that B^-1(B-1)-N^-1N'=1,

static comp modular_inverse(
　　bigint * bim
)

bim

There is a need for the value of integer N' such that B^-1(B-1)-N^-1N'=1,
where B^-1(B-1) mod N=1. Actually, only the least significant part of
N' is needed, hence the definition N0'=N' mod b. We reproduce below the
simple algorithm from an article by Dusse and Kaliski to efficiently
find N0' from N0 and b