How to verify data against signature with public key that uses sha1ecdsa?

Question

Knowing little about cryptography I have great problems with what seems to be a simple task.

I have .pem certificate, bytes of data, and signature of that data. I want to check if someone changed the data by matching it against signature.

My try:

private bool VerifySignature(byte[] data, byte[] signature)
{
  try
  {
    X509Certificate certificate = new X509Certificate("cert_filename.pem");
    if (certificate == null)
      return false;

    DSACryptoServiceProvider dsa = (DSACryptoServiceProvider)certificate.PublicKey.Key;

    return dsa.VerifyData(data, signatureData);
  }
  catch
  {
    return false;
  }
}

But it gives me an error

'Algorithm of certificates key is not supported' (System.NotSupportedException).

Looking into loaded certificate it says that the signature algorithm is 'sha1ecdsa'.

I am trying only to verify data against signature. What am I missing here? I would like to do it without any external solutions as it seems to be really trivial task.

Update: I am trying to achieve same functionality as in below Java code:

private boolean verify(byte[] data, byte[] signature)
{
  boolean isLicenseCorrect = false;

  Signature sig = Signature.getInstance("SHA1WithECDSA");
  sig.initVerify(certificate.getPublicKey());
  sig.update(data);

  return sig.verify(signature);
}

as per ur update if you want to convert the java code to C# [link](http://dotnetslackers.com/articles/security/Hashing_MACs_and_Digital_Signatures_in_NET.aspx) this might help have a look — too_cool, May 20 '15 at 06:40

score 6 · Answer 1 · answered May 09 '15 at 21:41

6

Although DSA and ECDSA are related, they are not the same. Why not try ECDsaCryptoServiceProvider? Note that the ECDSA support for Elliptic Curves only includes NIST named curves.

answered May 09 '15 at 21:41

Maarten Bodewes

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Unfortunately I cannot access this class in my Windows Forms code. I have only ECDsa and ECDsaCng, none of which I am able to use. – kasper May 11 '15 at 14:48
OK, so *why* are you not able to use those classes? They should provide a better target as `DSACryptoServiceProvider` is simply never going to work. Note that I don't have your setup (I'm not using Windows Forms myself, I just know things about crypto and what I see passing by here on SO). – Maarten Bodewes May 11 '15 at 15:00
As I have mentioned before, I don't know much about crypto, and can't figure out by myself how to solve my task. There are many articles on the net that uses RSACryptoServiceProvider or DSACryptoServiceProvider, but I couldn't make it work with them. – kasper May 13 '15 at 07:08
I've added Java code I'd like to port to C# as a reference. – kasper May 19 '15 at 10:10

score 0 · Answer 2 · answered Jul 01 '16 at 01:06

.NET 4.6.1 added improved support for ECDSA. While I'm not a fan of your catch-everything-and-return-false, I'll keep it here for comparison:

private bool VerifySignature(byte[] data, byte[] signature)
{
    try
    {
        // new cannot return null, so no point in a null check. It would have thrown.

        using (X509Certificate certificate = new X509Certificate("cert_filename.pem"))
        using (ECDsa ecdsa = certificate.GetECDsaPublicKey())
        using (RSA rsa = certificate.GetRSAPublicKey())
        // Improved DSA is 4.6.2
        {
            // You said the cert was ECDSA-SHA1, but that doesn't mean the original data was.
            // I assumed it was.
            if (ecdsa != null)
                return ecdsa.VerifyData(data, signature, HashAlgorithmName.SHA1);

            if (rsa != null)
                return rsa.VerifyData(data, signature, HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);

            return false;
        }
    }
    catch
    {
        return false;
    }
}

Note that in .NET 4.6 the RSA base class has the Sign/Verify methods defined, and in 4.6.1 the ECDsa base class got a similar treatment. New code shouldn't talk about the *CryptoServiceProvider types unless loading pre-existing named keys.

It's also worth noting that the Get[Algorithm]PublicKey methods return null when the public key isn't of that algorithm type, so a null-check is warranted.

How to verify data against signature with public key that uses sha1ecdsa?

2 Answers2