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:

-----BEGIN EC PARAMETERS-----
BgUrgQQACg==
-----END EC PARAMETERS-----
-----BEGIN EC PRIVATE KEY-----
MHQCAQEEIMaqRFP3zkco2B2xZaIDQ0/JGcgOqzLPcTTJPqvpu+UooAcGBSuBBAAK
oUQDQgAEp0mktO9BMlsVw9lG8EIxr9wZizBHSlUv3VIbf3sTBmCxE4myJvZcgF8b
qDp0T/o9TqnsBw15LU3wdnqV4tJp6w==
-----END EC 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?

Answer 1

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;
}