RSA - create signature and verify

RSA_SignAndVerify.cs

// ----------------------------------------------------
// Notice, this code is using BouncyCastle NuGet:
// https://www.nuget.org/packages/BouncyCastle/1.8.4/
// Created by:    Itsho
// Creation Date: 20 December 2018
// ----------------------------------------------------

using Org.BouncyCastle.Asn1.Pkcs;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Generators;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.OpenSsl;
using Org.BouncyCastle.Security;
using System;
using System.IO;
using System.Text;

namespace Test
{
	public class Program
	{
		[STAThread]
		private static void Main(string[] args)
		{
			// the 'usual' usage of keypair in "textbook" RSA (which is an asymmetric algorithm) is:
			// public key  = ENCRYPTING / VERIFYING
			// private key = DECRYPTING / SIGNING
			var rsav = new RsaSignAndVerify();
			var keyPair = rsav.GenerateRandomKeyPair();

			// print keys to console 
			rsav.PrintKeys(keyPair);

			// content we like to sign
			var textToSign = "Data to sign. can be a file or anything :-)";

			// display data we are about to sign
			Console.WriteLine($"Text to sign: {textToSign}");

			// server side - generates the signature by using the PRIVATE key
			var signature = rsav.ServerGenerateSignature(textToSign, (Org.BouncyCastle.Crypto.Parameters.RsaKeyParameters)keyPair.Private);

			// client side - validates the signature by using the PUBLIC key
			var isSignatureValid = rsav.ClientValidateSignature(textToSign, signature, (Org.BouncyCastle.Crypto.Parameters.RsaKeyParameters) keyPair.Public);
			Console.WriteLine($"Signature isValid: {isSignatureValid}");
		}
	}
  
	public class RsaSignAndVerify
	{
		public AsymmetricCipherKeyPair GenerateRandomKeyPair()
		{
			var rsaKeyPairGen = new RsaKeyPairGenerator();
			rsaKeyPairGen.Init(new KeyGenerationParameters(new SecureRandom(), 2048));
			return rsaKeyPairGen.GenerateKeyPair(); ;
		}

		public bool ClientValidateSignature(string sourceData, byte[] signature, RsaKeyParameters publicKey)
		{
			byte[] tmpSource = Encoding.ASCII.GetBytes(sourceData);

			ISigner signClientSide = SignerUtilities.GetSigner(PkcsObjectIdentifiers.Sha256WithRsaEncryption.Id);
			signClientSide.Init(false, publicKey);
			signClientSide.BlockUpdate(tmpSource, 0, tmpSource.Length);

			return  signClientSide.VerifySignature(signature);
		}

		public byte[] ServerGenerateSignature(string sourceData, RsaKeyParameters privateKey)
		{
			byte[] tmpSource = Encoding.ASCII.GetBytes(sourceData);

			ISigner sign = SignerUtilities.GetSigner(PkcsObjectIdentifiers.Sha256WithRsaEncryption.Id);
			sign.Init(true, privateKey);
			sign.BlockUpdate(tmpSource, 0, tmpSource.Length);
			return sign.GenerateSignature();
		}

		public void PrintKeys(AsymmetricCipherKeyPair keyPair)
		{
			using (TextWriter textWriter1 = new StringWriter())
			{
				var pemWriter1 = new PemWriter(textWriter1);
				pemWriter1.WriteObject(keyPair.Private);
				pemWriter1.Writer.Flush();

				string privateKey = textWriter1.ToString();
				Console.WriteLine(privateKey);
			}

			using (TextWriter textWriter2 = new StringWriter())
			{
				var pemWriter2 = new PemWriter(textWriter2);
				pemWriter2.WriteObject(keyPair.Public);
				pemWriter2.Writer.Flush();
				string publicKey = textWriter2.ToString();
				Console.WriteLine(publicKey);
			}
		}

		private byte[] ConvertHexString(string hexString)
		{
			byte[] data = new byte[hexString.Length / 2];
			for (int index = 0; index < data.Length; index++)
			{
				string byteValue = hexString.Substring(index * 2, 2);
				data[index] = byte.Parse(byteValue, System.Globalization.NumberStyles.HexNumber, System.Globalization.CultureInfo.InvariantCulture);
			}

			return data;
		}
	}
}