using System;

using Org.BouncyCastle.Asn1.X9;

using Org.BouncyCastle.Security;

​

public class EcSignatureString {

    public static void Main() {

        // running with .NET 4.7.2 and BouncyCastle 1.89

​

        Console.WriteLine("EC signature string (ECDSA with SHA256) DER-encoding");

        var dataToSignString = "The quick brown fox jumps over the lazy dog";

        var dataToSign = System.Text.Encoding.UTF8.GetBytes(dataToSignString);

        Console.WriteLine("dataToSign: " + dataToSignString);

        try {

            Console.WriteLine("\n* * * sign the plaintext with the EC private key * * *");

            string ecPrivateKeyPem = loadEcPrivateKeyPem();

            string signatureBase64 = ecSignDerToBase64(ecPrivateKeyPem, dataToSign);

            Console.WriteLine("used private key:");

            Console.WriteLine(ecPrivateKeyPem);

            Console.WriteLine("\nsignature (Base64): " + signatureBase64);  

            // verify

            Console.WriteLine("\n* * * verify the signature against hash of plaintext with the EC public key * * *");

            string signatureReceivedBase64 = signatureBase64;

            string ecPublicKeyPem = loadEcPublicKeyPem();

            Console.WriteLine("used public key:");

            Console.WriteLine(ecPublicKeyPem);

            //bool signatureVerified = false;

            bool signatureVerified = ecVerifySignatureDerFromBase64(ecPublicKeyPem, dataToSign, signatureReceivedBase64);

            Console.WriteLine("\nsignature verified: " + signatureVerified);

        }

        catch(ArgumentNullException) {

            Console.WriteLine("The data was not signed or verified");

        }

    }

    private static string ecSignDerToBase64(string ecPrivateKeyPem, byte[] messageByte) {

        var privateKey = PrivateKeyFactory.CreateKey(Convert.FromBase64String(getEcPrivateKeyFromPemStripped(ecPrivateKeyPem)));

        var signer = SignerUtilities.GetSigner(X9ObjectIdentifiers.ECDsaWithSha256.Id);

        signer.Init(true, privateKey);

        signer.BlockUpdate(messageByte, 0, messageByte.Length);

        var signature = signer.GenerateSignature();

        return Base64Encoding(signature);

    }   

    private static bool ecVerifySignatureDerFromBase64(string ecPublicKeyPem, byte[] messageByte, String signatureBase64) {

        var publicKey = PublicKeyFactory.CreateKey(Convert.FromBase64String(getEcPublicKeyFromPemStripped(ecPublicKeyPem)));

        var verifier = SignerUtilities.GetSigner(X9ObjectIdentifiers.ECDsaWithSha256.Id);

        verifier.Init(false, publicKey);

        verifier.BlockUpdate(messageByte, 0, messageByte.Length);

        return verifier.VerifySignature(Convert.FromBase64String(signatureBase64));

    }

    private static string getEcPrivateKeyFromPemStripped(string ecPrivateKeyPem) {

        string ecPrivateKeyHeaderPem = "-----BEGIN EC PRIVATE KEY-----\n";

        string ecPrivateKeyFooterPem = "-----END EC PRIVATE KEY-----";

        string ecPrivateKeyDataPem = ecPrivateKeyPem.Replace(ecPrivateKeyHeaderPem, "").Replace(ecPrivateKeyFooterPem, "").Replace("\n", "");

        return ecPrivateKeyDataPem;

    }

    private static string getEcPublicKeyFromPemStripped(string ecPublicKeyPem) {

        string ecPublicKeyHeaderPem = "-----BEGIN PUBLIC KEY-----\n";

        string ecPublicKeyFooterPem = "-----END PUBLIC KEY-----";

        string ecPublicKeyDataPem = ecPublicKeyPem.Replace(ecPublicKeyHeaderPem, "").Replace(ecPublicKeyFooterPem, "").Replace("\n", "");

        return ecPublicKeyDataPem;

    }

    private static byte[] getEcPrivateKeyEncodedFromPem(string ecPrivateKeyPem) {

        string ecPrivateKeyHeaderPem = "-----BEGIN EC PRIVATE KEY-----\n";

        string ecPrivateKeyFooterPem = "-----END EC PRIVATE KEY-----";

        string ecPrivateKeyDataPem = ecPrivateKeyPem.Replace(ecPrivateKeyHeaderPem, "").Replace(ecPrivateKeyFooterPem, "").Replace("\n", "");

        return Base64Decoding(ecPrivateKeyDataPem);

    }

​

    private static byte[] getEcPublicKeyFromPem(string ecPublicKeyPem) {

        string ecPublicKeyHeaderPem = "-----BEGIN PUBLIC KEY-----\n";

        string ecPublicKeyFooterPem = "-----END PUBLIC KEY-----";

        string ecPublicKeyDataPem = ecPublicKeyPem.Replace(ecPublicKeyHeaderPem, "").Replace(ecPublicKeyFooterPem, "").Replace("\n", "");

        return Base64Decoding(ecPublicKeyDataPem);

    }

​

    private static string loadEcPrivateKeyPem() {

        // this is a sample key - don't worry !

        return "-----BEGIN EC PRIVATE KEY-----\n"

            + "MEECAQAwEwYHKoZIzj0CAQYIKoZIzj0DAQcEJzAlAgEBBCAU2f8tzo99Z1HoxJlY\n"

            + "96yXUhFY5vppVjw1iPKRfk1wHA==\n"

            + "-----END EC PRIVATE KEY-----";

    }

​

    private static string loadEcPublicKeyPem() {

        // this is a sample key - don't worry !

        return "-----BEGIN PUBLIC KEY-----\n"

            + "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEzb7yAFWup6iDqJiEq764rAumsV2M\n"

            + "rspZxaP3WGpwHaC4Uff3N4UbJZF7Zac1c6W7KJl0eeCP0205Q3UEpwxndQ==\n"

            + "-----END PUBLIC KEY-----";

    }

​

    static string Base64Encoding(byte[] input) {

        return Convert.ToBase64String(input);

    }

​

    static byte[] Base64Decoding(String input) {

        return Convert.FromBase64String(input);

    }

}