How to verify a signed license key using C#, Ed25519 and the NSEC crypto package. See: https://keygen.sh

_README.md

Setup dependencies

dotnet --version
# => 6.0.101

dotnet add package NSec.Cryptography

Run the example

dotnet run
# => Dataset: {"test":1}
#    Valid: True

ed25519.cs

using System;
using System.Text;
using NSec.Cryptography;

public class Program
{
  private const string licenseKey = "key/eyJ0ZXN0IjoxfQ==._f34UAAtNKM8TXGLlnblBGCLSy22Oa_gjp4jn0CpvEuwQ2gAcH0IntFKwtbyV5iWnH9_x8l0R144oHp2_GviAw==";
  private const string publicKeyHex = "e8601e48b69383ba520245fd07971e983d06d22c4257cfd82304601479cee788";

  public static void Main()
  {
    var algorithm = SignatureAlgorithm.Ed25519;

    // Read signature
    var licenseStringTokens = licenseKey.Split('.');
    var encodedSignature = convertBase64UrlString(licenseStringTokens[1]);
    var signatureBytes = Convert.FromBase64String(encodedSignature);

    // Read data
    var signingData = licenseStringTokens[0].Split('/');
    var signingDataBytes = Encoding.UTF8.GetBytes($"key/{signingData[1]}");

    // Read public key
    var publicKeyBytes = Convert.FromHexString(publicKeyHex);
    var publicKey = PublicKey.Import(algorithm, publicKeyBytes, KeyBlobFormat.RawPublicKey);

    // Verify
    var valid = algorithm.Verify(publicKey, signingDataBytes, signatureBytes);
    if (valid)
    {
      var encodedDataset = convertBase64UrlString(signingData[1]);
      var decodedDataset = Convert.FromBase64String(encodedDataset);
      var dataset = Encoding.UTF8.GetString(decodedDataset);

      Console.WriteLine($"Dataset: {dataset}");
    }

    Console.WriteLine($"Valid: {valid}");
  }

  private static string convertBase64UrlString(string s)
  {
    return s.Replace("-", "+").Replace("_", "/");
  }
}

Thank you!. This gist helped me solve a problem that I had been banging my head on for a day :)