Шифрование с открытым ключом с помощью RSACryptoServiceProvider

вот вам пример:

    public static void rsaPlayground()
    {
        byte[] data = new byte[] { 1, 2, 3, 4, 5 };
        RSACryptoServiceProvider csp = new RSACryptoServiceProvider();//make a new csp with a new keypair
        var pub_key = csp.ExportParameters(false); // export public key
        var priv_key = csp.ExportParameters(true); // export private key

        var encData = csp.Encrypt(data, false); // encrypt with PKCS#1_V1.5 Padding
        var decBytes = MyRSAImpl.plainDecryptPriv(encData, priv_key); //decrypt with own BigInteger based implementation
        var decData = decBytes.SkipWhile(x => x != 0).Skip(1).ToArray();//strip PKCS#1_V1.5 padding

    }

    public class MyRSAImpl 
    {

        private static byte[] rsaOperation(byte[] data, BigInteger exp, BigInteger mod)
        {
            BigInteger bData = new BigInteger(
                data    //our data block
                .Reverse()  //BigInteger has another byte order
                .Concat(new byte[] { 0 }) // append 0 so we are allways handling positive numbers
                .ToArray() // constructor wants an array
            );
            return 
                BigInteger.ModPow(bData, exp, mod) // the RSA operation itself
                .ToByteArray() //make bytes from BigInteger
                .Reverse() // back to "normal" byte order
                .ToArray(); // return as byte array

            /*
             * 
             * A few words on Padding:
             * 
             * you will want to strip padding after decryption or apply before encryption 
             * 
             */
        }

        public static byte[] plainEncryptPriv(byte[] data, RSAParameters key) 
        {
            MyRSAParams myKey = MyRSAParams.fromRSAParameters(key);
            return rsaOperation(data, myKey.privExponent, myKey.Modulus);
        }
        public static byte[] plainEncryptPub(byte[] data, RSAParameters key)
        {
            MyRSAParams myKey = MyRSAParams.fromRSAParameters(key);
            return rsaOperation(data, myKey.pubExponent, myKey.Modulus);
        }
        public static byte[] plainDecryptPriv(byte[] data, RSAParameters key)
        {
            MyRSAParams myKey = MyRSAParams.fromRSAParameters(key);
            return rsaOperation(data, myKey.privExponent, myKey.Modulus);
        }
        public static byte[] plainDecryptPub(byte[] data, RSAParameters key)
        {
            MyRSAParams myKey = MyRSAParams.fromRSAParameters(key);
            return rsaOperation(data, myKey.pubExponent, myKey.Modulus);
        }

    }

    public class MyRSAParams
    {
        public static MyRSAParams fromRSAParameters(RSAParameters key)
        {
            var ret = new MyRSAParams();
            ret.Modulus = new BigInteger(key.Modulus.Reverse().Concat(new byte[] { 0 }).ToArray());
            ret.privExponent = new BigInteger(key.D.Reverse().Concat(new byte[] { 0 }).ToArray());
            ret.pubExponent = new BigInteger(key.Exponent.Reverse().Concat(new byte[] { 0 }).ToArray());

            return ret;
        }
        public BigInteger Modulus;
        public BigInteger privExponent;
        public BigInteger pubExponent;
    }

Возьмите этот пример кодирования / декодирования

        byte[] toEncryptData = Encoding.ASCII.GetBytes("hello world");

        //Generate keys
        RSACryptoServiceProvider rsaGenKeys = new RSACryptoServiceProvider();
        string privateXml = rsaGenKeys.ToXmlString(true);
        string publicXml = rsaGenKeys.ToXmlString(false);

        //Encode with public key
        RSACryptoServiceProvider rsaPublic = new RSACryptoServiceProvider();
        rsaPublic.FromXmlString(publicXml);
        byte[] encryptedRSA = rsaPublic.Encrypt(toEncryptData, false);
        string EncryptedResult = Encoding.Default.GetString(encryptedRSA);


        //Decode with private key
        var rsaPrivate = new RSACryptoServiceProvider();
        rsaPrivate.FromXmlString(privateXml);
        byte[] decryptedRSA = rsaPrivate.Decrypt(encryptedRSA, false);
        string originalResult = Encoding.Default.GetString(decryptedRSA);