How to encrypt a string using public key cryptography

Note. I have updated this answer. Please scroll down the list before how this should be implemented, because this first way to do it is not the correct way to encrypt RSA.

One way I can think it is (but may not be standards compliant) and also note that this is not a pad:

public byte[] Encrypt(string m, Encoding encoding)
{
    byte[] bytes = encoding.GetBytes(m);
    byte[] returnBytes = new byte[0];
    for (int i = 0; i < bytes.Length; i++)
    {
        byte[] result = BigInteger.ModPow(bytes[i], (BigInteger)e, n).ToByteArray();
        int preSize = returnBytes.Length;
        Array.Resize(ref returnBytes, returnBytes.Length + result.Length + 1);
        (new byte[] { (byte)(result.Length) }).CopyTo(returnBytes, preSize);
        result.CopyTo(returnBytes, preSize + 1);
    }
    return returnBytes;
}

public string Decrypt(byte[] c, Encoding encoding)
{
    byte[] returnBytes = new byte[0];
    for (int i = 0; i < c.Length; i++)
    {
        int dataLength = (int)c[i];
        byte[] result = new byte[dataLength];
        for (int j = 0; j < dataLength; j++)
        {
            i++;
            result[j] = c[i];
        }
        BigInteger integer = new BigInteger(result);
        byte[] integerResult = BigInteger.ModPow(integer, d, n).ToByteArray();
        int preSize = returnBytes.Length;
        Array.Resize(ref returnBytes, returnBytes.Length + integerResult.Length);
        integerResult.CopyTo(returnBytes, preSize);
    }
    string decryptedString = encoding.GetString(returnBytes);
    return decryptedString;
}

      

        
        
        
      

    

It can be cross-platform because you have the ability to use a different datatype to represent e or n and pass it to a C # helper service like this one. Here's a test:

string stringToEncrypt = "Mary had a little lamb.";
Console.WriteLine("Encrypting the string: {0}", stringToEncrypt);
byte[] encryptedBytes = engine.Encrypt(stringToEncrypt, Encoding.UTF8);
Console.WriteLine("Encrypted text: {0}", Encoding.UTF8.GetString(encryptedBytes));
Console.WriteLine("Decrypted text: {0}", engine.Decrypt(encryptedBytes, Encoding.UTF8));

      

        
        
        
      

    

Output:

Encrypting the string: Mary had a little lamb.
Encrypted text: ☻6☻1♦☻j☻☻&♀☻g♦☻t☻☻1♦☻?  ☻g♦☻1♦☻g♦☻?♥☻?☻☻7☺☻7☺☻?♥☻?♂☻g♦☻?♥☻1♦☻$☺☻
c       ☻?☻
Decrypted text: Mary had a little lamb.

      

        
        
        
      

    

Update . Everything I said earlier is completely wrong in the RSA implementation. Wrong, wrong, wrong! This is the correct way to encrypt RSA:

• Convert the string to BigInteger data type.
• Make sure your integer is less than the n value you calculated for your algorithm, otherwise you won't be able to decrypt it.
• Encrypt an integer. RSA only works for integer encryption. This is clear.
• Decrypt it from an encrypted whole.
• I can't help but wonder that the BigInteger class was primarily created for cryptography.

As an example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;

namespace BytePadder
{
    class Program
    {
        const int p = 61;
        const int q = 53;
        const int n = 3233;
        const int totient = 3120;
        const int e = 991;
        const int d = 1231;

        static void Main(string[] args)
        {
            // ---------------------- RSA Example I ----------------------
            // Shows how an integer gets encrypted and decrypted.
            BigInteger integer = 1000;
            BigInteger encryptedInteger = Encrypt(integer);
            Console.WriteLine("Encrypted Integer: {0}", encryptedInteger);
            BigInteger decryptedInteger = Decrypt(encryptedInteger);
            Console.WriteLine("Decrypted Integer: {0}", decryptedInteger);
            // --------------------- RSA Example II ----------------------
            // Shows how a string gets encrypted and decrypted.
            string unencryptedString = "A";
            BigInteger integer2 = new BigInteger(Encoding.UTF8.GetBytes(unencryptedString));
            Console.WriteLine("String as Integer: {0}", integer2);
            BigInteger encryptedInteger2 = Encrypt(integer2);
            Console.WriteLine("String as Encrypted Integer: {0}", encryptedInteger2);
            BigInteger decryptedInteger2 = Decrypt(encryptedInteger2);
            Console.WriteLine("String as Decrypted Integer: {0}", decryptedInteger2);
            string decryptedIntegerAsString = Encoding.UTF8.GetString(decryptedInteger2.ToByteArray());
            Console.WriteLine("Decrypted Integer as String: {0}", decryptedIntegerAsString);
            Console.ReadLine();
        }

        static BigInteger Encrypt(BigInteger integer)
        {
            if (integer < n)
            {
                return BigInteger.ModPow(integer, e, n);
            }
            throw new Exception("The integer must be less than the value of n in order to be decypherable!");
        }

        static BigInteger Decrypt(BigInteger integer)
        {
            return BigInteger.ModPow(integer, d, n);
        }
    }
}

      

        
        
        
      

    

Output example:

Encrypted Integer: 1989
Decrypted Integer: 1000
String as Integer: 65
String as Encrypted Integer: 1834
String as Decrypted Integer: 65
Decrypted Integer as String: A