Task1.5

using System;
using System.Collections.Generic;
using System.Numerics;

namespace RSA
{
    class Program
    {
        private static readonly int p = 61;
        private static readonly int q = 53;
        private static readonly int n = p * q;
        private static readonly int e = 17;
        private static readonly int d = 2753;

        static void Main(string[] args)
        {
            string message = "Привет";
            Console.WriteLine($"Оригинальное сообщение: {message}");

            List<BigInteger> encrypted = EncryptMessage(message);

            Console.Write("\nЗашифрованное сообщение: ");
            foreach (var val in encrypted)
            {
                Console.Write(val + " ");
            }

            string decrypted = DecryptMessage(encrypted);
            Console.WriteLine($"\n\nРасшифрованное сообщение: {decrypted}\n\n");
        }

        static BigInteger EncryptChar(char ch)
        {
            BigInteger m = ch;
            return BigInteger.ModPow(m, e, n);
        }

        static char DecryptChar(BigInteger cipher)
        {
            BigInteger m = BigInteger.ModPow(cipher, d, n);
            return (char)(int)m;
        }

        static List<BigInteger> EncryptMessage(string message)
        {
            List<BigInteger> result = new List<BigInteger>();

            foreach (char ch in message)
            {
                result.Add(EncryptChar(ch));
            }

            return result;
        }

        static string DecryptMessage(List<BigInteger> encrypted)
        {
            string result = "";

            foreach (BigInteger cipher in encrypted)
            {
                result += DecryptChar(cipher);
            }

            return result;
        }
    }
}