Function MontgomeryReduce

extra/yassl/taocrypt/src/integer.cpp:3735–3745

// U[N] --- multiplicative inverse of M mod 2**(WORD_BITS*N)

void MontgomeryReduce(word *R, word *T, const word *X, const word *M,
                      const word *U, unsigned int N)
{
    MultiplyBottom(R, T, X, U, N);
    MultiplyTop(T, T+N, X, R, M, N);
    word borrow = Subtract(T, X+N, T, N);
    // defend against timing attack by doing this Add even when not needed
    word carry = Add(T+N, T, M, N);
    (void)carry;            // shut up compiler
    CopyWords(R, T + (borrow ? N : 0), N);
}

// R[N] ----- result = A inverse mod 2**(WORD_BITS*N)
// T[3*N/2] - temporary work space