simpleRsa.cs

public class RSA
    {
        private int _n;
        private int _e;
        private int _d;
        private int _phi;

        public RSA(int p, int q)
        {
            _n = p * q;
            _phi = (p - 1) * (q - 1);
            _e = FindE(_phi);

            if (_e > 0)
            {
                _d = FindD(_e, _phi);
                if (_d < 0) throw new Exception("input is not allowed");
            } else
                if (_d < 0) throw new Exception("input is not allowed");

        }

        public byte[] Encrypt(byte[] st)
        {
            byte[] m = new byte[st.Length];
            
            for (int i = 0; i < st.Length; i++)
            {   
                m[i] = (byte)PowerMod(st[i], _e, _n);
            }

            return m;
        }

        public byte[] Decrypt(byte[] st)
        {
            byte[] m = new byte[st.Length];

            for (int i = 0; i < st.Length; i++)
            {
                m[i] = (byte)PowerMod(st[i], _d, _n);
            }

            return m;
        }

        private int PowerMod(byte n, int exp, int m)
        {
            if (exp == 0) return 1;
            int r = 1;
            for (int i = 0; i < exp; i++)
			{
			    r = (r * n) % m;
			}

            return r;
        }

        private int FindE(int n)
        {
            for (int i = n - 1; i > 1; i--)
            {
                if (Gcd(n, i) == 1) return i;
            }

            return -1;
        }

        public int FindD(int e, int phi)
        {
            for (int i = 1; i < phi; i++)
            {
                if ((i * e) % phi == 1) return i;
            }
            return -1;
        }

        private int Gcd(int a, int b)
        {
            while (a != 0)
            {
                var tmp = a;
                a = a % b;
                b = tmp;
            }

            return b;
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            string st = "Attack at dawn";
            RSA rsa = new RSA(11, 13);
            var m = st.ToList().Select(c => (byte)c).ToArray();

            Console.WriteLine("Source message:");

            for (int i = 0; i < m.Length; i++)
            {
                Console.Write("{0:D3} ", m[i]);
            }

            Console.WriteLine();
            Console.WriteLine("Encrypted message:");

            m = rsa.Encrypt(m);

            for (int i = 0; i < m.Length; i++)
            {
                Console.Write("{0:D3} ", m[i]);
            }

            Console.WriteLine();
            Console.WriteLine("Decrypted message:");

            m = rsa.Decrypt(m);

            for (int i = 0; i < m.Length; i++)
            {
                Console.Write("{0:D3} ", m[i]);
            }

            Console.Read();
        }
    }