ModularExponentiation.cs

using System;

namespace Algorithms.Numeric
{
    /// <summary>
    ///     Modular exponentiation is a type of exponentiation performed over a modulus
    ///     Modular exponentiation c is: c = b^e mod m where b is base, e is exponent, m is modulus
    ///     (Wiki: https://en.wikipedia.org/wiki/Modular_exponentiation).
    /// </summary>
    public class ModularExponentiation
    {
        /// <summary>
        ///     Performs Modular Exponentiation on b, e, m.
        /// </summary>
        /// <param name="b">Base.</param>
        /// <param name="e">Exponent.</param>
        /// <param name="m">Modulus.</param>
        /// <returns>Modular Exponential.</returns>
        public int ModularPow(int b, int e, int m)
        {
            // initialize result in variable res
            int res = 1;
            if (m == 1)
            {
                // 1 divides every number
                return 0;
            }

            if (m <= 0)
            {
                // exponential not defined in this case
                throw new ArgumentException(string.Format("{0} is not a positive integer", m));
            }

            for (int i = 0; i < e; i++)
            {
                res = (res * b) % m;
            }

            return res;
        }
    }
}