Task 2.1

using System;
using System.Text;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Crypto.Signers;
using Org.BouncyCastle.Security;
using Org.BouncyCastle.Asn1.Sec;
using Org.BouncyCastle.Math;

namespace ECDSA
{
    class Program
    {
        static void Main(string[] args)
        {
            string message = "Confidential Message";
            Console.WriteLine("Сообщение: " + message);

            var keyPair = GenerateKeyPair();
            var privateKey = (ECPrivateKeyParameters)keyPair.Private;
            var publicKey = (ECPublicKeyParameters)keyPair.Public;

            byte[] signature = SignMessage(message, privateKey);
            Console.WriteLine("Подпись (Base64): " + Convert.ToBase64String(signature));

            bool isValid = VerifySignature(message, signature, publicKey);
            Console.WriteLine("Подпись действительна? " + (isValid ? "Да" : "Нет"));
        }

        static AsymmetricCipherKeyPair GenerateKeyPair()
        {
            var curve = SecNamedCurves.GetByName("secp256k1");
            var domainParams = new ECDomainParameters(curve.Curve, curve.G, curve.N, curve.H);

            var keyGenParams = new ECKeyGenerationParameters(domainParams, new SecureRandom());
            var keyGen = new Org.BouncyCastle.Crypto.Generators.ECKeyPairGenerator();
            keyGen.Init(keyGenParams);

            return keyGen.GenerateKeyPair();
        }

        static byte[] SignMessage(string message, ECPrivateKeyParameters privateKey)
        {
            var signer = new ECDsaSigner();
            signer.Init(true, privateKey);

            byte[] msgHash = DigestMessage(message);

            BigInteger[] signatureComponents = signer.GenerateSignature(msgHash);
            BigInteger r = signatureComponents[0];
            BigInteger s = signatureComponents[1];

            var seq = new Org.BouncyCastle.Asn1.DerSequence(
                new Org.BouncyCastle.Asn1.DerInteger(r),
                new Org.BouncyCastle.Asn1.DerInteger(s));

            return seq.GetEncoded();
        }

        static bool VerifySignature(string message, byte[] signatureBytes, ECPublicKeyParameters publicKey)
        {
            var signer = new ECDsaSigner();
            signer.Init(false, publicKey);

            byte[] msgHash = DigestMessage(message);

            try
            {
                var asn1 = Org.BouncyCastle.Asn1.Asn1Object.FromByteArray(signatureBytes);
                var seq = (Org.BouncyCastle.Asn1.Asn1Sequence)asn1;
                var r = ((Org.BouncyCastle.Asn1.DerInteger)seq[0]).Value;
                var s = ((Org.BouncyCastle.Asn1.DerInteger)seq[1]).Value;

                return signer.VerifySignature(msgHash, r, s);
            }
            catch
            {
                return false;
            }
        }

        static byte[] DigestMessage(string message)
        {
            var sha256 = System.Security.Cryptography.SHA256.Create();
            return sha256.ComputeHash(Encoding.UTF8.GetBytes(message));
        }
    }
}