PublicKey.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using Mono.Math;

namespace YamRsa
{
  class PublicKey
  {
    private const int HEX = 16;

    public readonly BigInteger Module;
    public readonly BigInteger Exponent;

    public PublicKey(BigInteger m, BigInteger e)
    {
      Module = m;
      Exponent = e;
    }
    /// 
    /// Requires 0 <= plain < Module
    /// 
 /// 
    public BigInteger Encrypt(BigInteger plain)
    {
      return plain.ModPow(Exponent, Module);
    }

    public static PublicKey FromString(string me)
    {
      var pair = me.Split(new[] {'#'}, 2);
      if (pair.Length != 2)
        throw new ArgumentException("Wrong format", "me");
      return new PublicKey(ParseHex(pair[0]), ParseHex(pair[1]));
    }

    private static BigInteger ParseHex(IEnumerable hexString)
    {
      return hexString.Aggregate(new BigInteger(),
        (integer, @char) => integer*HEX + int.Parse(new string(new[] {@char}), NumberStyles.AllowHexSpecifier));
    }

    public override string ToString()
    {
      return string.Format("{0}#{1}", Module.ToString(HEX), Exponent.ToString(HEX));
    }
  }
}


CryptoProviderRSA.cs

using System;
using System.Linq;
using Mono.Math;

namespace YamRsa
{
  public class CryptoProviderRSA
  {
    private PublicKey _publicKey;

    public void ImportPublicKey(string key)
    {
      _publicKey = PublicKey.FromString(key);
    }

    private const int MAX_CRYPT_BITS = 1024;

    public byte[] Encrypt(byte[] data)
    {
        // must ensure that any data block would be < key's modulus
        // hence -1
        int portionLen = (_publicKey.Module.BitCount() - 1) / 8;
        var prevCrypted = new byte[portionLen];

      return Enumerable
        .Range(0, ((data.Length - 1) / portionLen) + 1)
        .Select(i => data.Skip(i * portionLen).Take(portionLen))
        .SelectMany(portion =>
       {
         var portbuf = portion.Select((b, i) => (byte)(b ^ prevCrypted[i])).ToArray();
         var cp = _publicKey.Encrypt(new BigInteger(portbuf)).GetBytes();
         var length = Math.Min(portbuf.Length, cp.Length);
         Array.Copy(cp, prevCrypted, length);
         Array.Clear(prevCrypted, length, prevCrypted.Length - length);
         return BitConverter.GetBytes((ushort)portbuf.Length)
           .Concat(BitConverter.GetBytes((ushort)cp.Length))
           .Concat(cp)
           .ToArray();
       })
       .ToArray();
    }
  }
}


Program.cs

using System;
using System.Linq;
using System.Text;

namespace YamRsa
{
  class Program
  {
    static void Main(string[] args)
    {
      var rsa = new CryptoProviderRSA();
      rsa.ImportPublicKey(args[0]);
      string data = @"";
      var encoded = rsa.Encrypt(Encoding.ASCII.GetBytes(data));
      Console.WriteLine(Convert.ToBase64String(encoded));
      Console.ReadLine();
    }
  }
}