Task: Montgomery reduction
C#: using System;
using System.Numerics;

namespace MontgomeryReduction {
    public static class Helper {
        public static int BitLength(this BigInteger v) {
            if (v < 0) {
                v *= -1;
            }

            int result = 0;
            while (v > 0) {
                v >>= 1;
                result++;
            }

            return result;
        }
    }

    struct Montgomery {
        public static readonly int BASE = 2;

        public BigInteger m;
        public BigInteger rrm;
        public int n;

        public Montgomery(BigInteger m) {
            if (m < 0 || m.IsEven) throw new ArgumentException();

            this.m = m;
            n = m.BitLength();
            rrm = (BigInteger.One << (n * 2)) % m;
        }

        public BigInteger Reduce(BigInteger t) {
            var a = t;

            for (int i = 0; i < n; i++) {
                if (!a.IsEven) a += m;
                a = a >> 1;
            }
            if (a >= m) a -= m;
            return a;
        }
    }

    class Program {
        static void Main(string[] args) {
            var m = BigInteger.Parse("750791094644726559640638407699");
            var x1 = BigInteger.Parse("540019781128412936473322405310");
            var x2 = BigInteger.Parse("515692107665463680305819378593");

            var mont = new Montgomery(m);
            var t1 = x1 * mont.rrm;
            var t2 = x2 * mont.rrm;

            var r1 = mont.Reduce(t1);
            var r2 = mont.Reduce(t2);
            var r = BigInteger.One << mont.n;

            Console.WriteLine("b :  {0}", Montgomery.BASE);
            Console.WriteLine("n :  {0}", mont.n);
            Console.WriteLine("r :  {0}", r);
            Console.WriteLine("m :  {0}", mont.m);
            Console.WriteLine("t1:  {0}", t1);
            Console.WriteLine("t2:  {0}", t2);
            Console.WriteLine("r1:  {0}", r1);
            Console.WriteLine("r2:  {0}", r2);
            Console.WriteLine();
            Console.WriteLine("Original x1       : {0}", x1);
            Console.WriteLine("Recovered from r1 : {0}", mont.Reduce(r1));
            Console.WriteLine("Original x2       : {0}", x2);
            Console.WriteLine("Recovered from r2 : {0}", mont.Reduce(r2));

            Console.WriteLine();
            Console.WriteLine("Montgomery computation of x1 ^ x2 mod m :");
            var prod = mont.Reduce(mont.rrm);
            var @base = mont.Reduce(x1 * mont.rrm);
            var exp = x2;
            while (exp.BitLength() > 0) {
                if (!exp.IsEven) prod = mont.Reduce(prod * @base);
                exp >>= 1;
                @base = mont.Reduce(@base * @base);
            }
            Console.WriteLine(mont.Reduce(prod));
            Console.WriteLine();
            Console.WriteLine("Alternate computation of x1 ^ x2 mod m :");
            Console.WriteLine(BigInteger.ModPow(x1, x2, m));
        }
    }
}

C++: #include<iostream>
#include<conio.h>
using namespace std;
typedef unsigned long ulong;

int ith_digit_finder(long long n, long b, long i){
 
    while(i>0){
        n/=b;
        i--;
    }
    return (n%b);
}

long eeuclid(long m, long b, long *inverse){        
    
    long A1 = 1, A2 = 0, A3 = m,
         B1 = 0, B2 = 1, B3 = b,
         T1, T2, T3, Q;

         cout<<endl<<"eeuclid() started"<<endl;

        while(1){
            if(B3 == 0){
                *inverse = 0;
                return A3;      
            }

            if(B3 == 1){
                *inverse = B2; 
                return B3;      
            }

            Q = A3/B3;

            T1 = A1 - Q*B1;
            T2 = A2 - Q*B2;
            T3 = A3 - Q*B3;

            A1 = B1; A2 = B2; A3 = B3;
            B1 = T1; B2 = T2; B3 = T3;

       }
    cout<<endl<<"ending eeuclid() "<<endl;
}

long long mon_red(long m, long m_dash, long T, int n, long b = 2){

    long long A,ui, temp, Ai;       
    if( m_dash < 0 ) m_dash = m_dash + b;
    A = T;
    for(int i = 0; i<n; i++){
    
        Ai = ith_digit_finder(A, b, i);
        ui = ( ( Ai % b) * m_dash ) % b;
        temp  = ui*m*power(b, i);
        A = A + temp;
    }
    A = A/power(b, n);
    if(A >= m) A = A - m;
    return A;
}

int main(){
    long a, b, c, d=0, e, inverse = 0;
    cout<<"m >> ";
    cin >> a;
    cout<<"T >> ";
    cin>>b;
    cout<<"Radix b >> ";
    cin>>c;
    eeuclid(c, a, &d);      
    e = mon_red(a, -d, b, length_finder(a, c), c);
    cout<<"Montgomery domain representation = "<<e;
    return 0;
}