SignES256.cs

public static string SignES256(string privateKey, string header, string payload)
		{
			CngKey key = CngKey.Import(
				Convert.FromBase64String(privateKey),
				CngKeyBlobFormat.Pkcs8PrivateBlob);

			using (ECDsaCng dsa = new ECDsaCng(key))
			{
				dsa.HashAlgorithm = CngAlgorithm.Sha256;
				var unsignedJwtData =
					Encode(Encoding.UTF8.GetBytes(header)) + "." + Encode(Encoding.UTF8.GetBytes(payload));
				var signature =
					dsa.SignData(Encoding.UTF8.GetBytes(unsignedJwtData));
				return unsignedJwtData + "." + Encode(signature);
			}
		}