How to sign a message with existing private key by using ECDsa on dotnet core 3.1 MacOS?

Question

I generated a key by running following command:

openssl ecparam -genkey -name secp256k1 -out private.key

Worth mentioning that I had to use secp256k1 curve.

Here is private.key:

Then I tried to sign some random plain-text message:

var privateKey = CngKey.Import(privateKeyBytes, CngKeyBlobFormat.EccPrivateBlob);
var signer = new ECDsaCng(privateKey);
var signed = signer.SignData(messageBytes, HashAlgorithmName.SHA256);

But encountered a problem when called CngKey.Import: Windows Cryptography Next Generation (CNG) is not supported on this platform. (MacOS)

Is there any way to accomplish it cross-platform?

You could try the Bouncy Castle libs. I'm pretty sure that the normal .NET also doesn't support `secp256k1` curve, only the `secp256r1` curve. .NET's crypto libs are very dated and not very portable. — Maarten Bodewes, Feb 08 '20 at 22:41

score 1 · Answer 1 · edited Sep 03 '20 at 19:40

This library uses pure C# to safely generate keys, sign and verify ECDSA-secp256k1 signatures. Runs on .NET Standard 1.3+. Here is an example:

using System;
using EllipticCurve;

PrivateKey privateKey = PrivateKey.fromPem(privateKeyPem);

Signature signature = Ecdsa.sign(message, privateKey);
Console.WriteLine(signature.toBase64());

PublicKey publicKey = privateKey.publicKey();
Console.WriteLine(Ecdsa.verify(message, signature, publicKey));

score 0 · Accepted Answer · answered Feb 09 '20 at 13:37

Thanks to Maarten Bodewes I found BouncyCastle.NetCore package.

Here is a solution:

public void Main()
{
    var asymmetricCipherKeyPair = ReadAsymmetricCipherKeyPairFromPem("./key.pem");
    var signature = GetSignature(asymmetricCipherKeyPair.Private, "Some message");
}

private string GetSignature(AsymmetricKeyParameter privateKeyParameter, string message)
{
    var signer = SignerUtilities.GetSigner("SHA-256withECDSA");

    signer.Init(true, privateKeyParameter);

    signer.BlockUpdate(Encoding.ASCII.GetBytes(message), 0, Encoding.ASCII.GetBytes(message).Length);

    var signature = signer.GenerateSignature();

    return Convert.ToBase64String(signature);
}

private AsymmetricCipherKeyPair ReadAsymmetricCipherKeyPairFromPem(string pathToPem)
{
    using var reader = File.OpenText(pathToPem); 
    var keyPair = (AsymmetricCipherKeyPair)new PemReader(reader).ReadObject();

    return keyPair;
}

How to sign a message with existing private key by using ECDsa on dotnet core 3.1 MacOS?

2 Answers2