Xml Signature for XmlElement fails to verify

Question

I apologize in advance for the rather lengthy block of code, but it's the smallest compilable example I could produce. I already omitted all error checking from the original code. I'm using Visual Studio 2012 and .NET 4.5, although this is nothing new to 4.5, it should work with any version.

I am trying to sign an XML documents' elements to protect them from tampering. I don't want to protect the whole document, but only certain elements. Maybe even different elements with different keys.

However, when I sign three example elements and try to verify them, the first one always verifies, the other two fail. To make it even worse, the first one even succeeds if I modify it after being signed. I have googled a lot, read a lot of tutorials and even asked a theoretical question here, but I don't have any clue what I'm doing wrong. Can anybody spot my mistake?

Note: I'd be more than happy to offer the same bounty that's on friday's question to anybody solving this.

The certificate was created by executing:

"C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A\Bin\makecert" -r -pe -n "CN=XMLDSIG_Test" -b 01/01/2013 -e 01/01/2014 -sky signing -ss my

The Test xml file is:

<?xml version="1.0" encoding="utf-8" ?>
<PackageRoot>
  <Package>
    <Changes >
     <Change/>
    </Changes>
  </Package>
  <Package>
    <Changes>
     <Change/>
     <Change/>
    </Changes>
  </Package>
  <Package>
    <Changes>
     <Change/>
     <Change/>
     <Change/>
    </Changes>
  </Package>
</PackageRoot>

The code to sign and verify:

namespace SOExample
{
  using System;
  using System.Security.Cryptography.X509Certificates;
  using System.Security.Cryptography.Xml;
  using System.Xml;

  public static class Program
  {
    public static void Sign(this XmlElement element, X509Certificate2 certificate)
    {
        var identifier = Guid.NewGuid().ToString();

        element.SetAttribute("Id", identifier);

        var signedXml = new SignedXml(element) { SigningKey = certificate.PrivateKey };

        var reference = new Reference("#" + identifier);

        reference.AddTransform(new XmlDsigEnvelopedSignatureTransform());

        signedXml.AddReference(reference);

        signedXml.ComputeSignature();

        var xmlDigitalSignature = signedXml.GetXml();

        element.AppendChild(element.OwnerDocument.ImportNode(xmlDigitalSignature, true));
    }

    public static bool VerifySignature(this XmlElement element, X509Certificate2 certificate)
    {
      var signedXml = new SignedXml(element);

      XmlNodeList nodeList = element.GetElementsByTagName("Signature");

      if (nodeList.Count != 1) return false;

      signedXml.LoadXml((XmlElement)nodeList[0]);

      return signedXml.CheckSignature(certificate, true);
    }

    public static void Main()
    {
        var xmlDoc = new XmlDocument { PreserveWhitespace = true };

        xmlDoc.Load("ExamplePackage.xml");

        var certificate = GetCertificateBySubject("CN=XMLDSIG_Test");

        foreach (XmlElement root in xmlDoc.GetElementsByTagName("PackageRoot"))
        {
          foreach (XmlElement package in root.GetElementsByTagName("Package"))
          {
            package.Sign(certificate);
          }
        }

        xmlDoc.Save("test_signed.xml");

        Console.WriteLine("XML file signed.");
        Console.WriteLine("Press any key to verify");
        Console.ReadLine();

        var signedDoc = new XmlDocument();

        signedDoc.Load("test_signed.xml");

        foreach (XmlElement root in xmlDoc.GetElementsByTagName("PackageRoot"))
        {
          foreach (XmlElement package in root.GetElementsByTagName("Package"))
          {
            Console.Write("Verifying Package " + package.GetAttribute("Id"));
            var success = package.VerifySignature(certificate);
            Console.WriteLine(success ? " successful!" : " failed!");
          }
        }

        Console.WriteLine("Done.");
        Console.ReadLine();
    }

    private static X509Certificate2 GetCertificateBySubject(string certificateSubject)
    {
      var store = new X509Store("My", StoreLocation.CurrentUser);

      store.Open(OpenFlags.ReadOnly | OpenFlags.OpenExistingOnly);

      foreach (X509Certificate2 c in store.Certificates)
      {
        if (c.Subject == certificateSubject)
        {
          store.Close();
          return c;
        }
      }

      store.Close();
      return null;
    }
  }
}

Answer 1

You have a bug in your test code. The second foreach loops again over xmlDoc instead of signedDoc. Fixing this will change the outcome to fail for all nodes.
Why they fail I don't yet know.

---

I couldn't find out why they fail with your code but I found a way to make it work. The difference: All signatures are direct childs of the root element:

public static void Main()
{

    // ...

    var signedDoc = new XmlDocument { PreserveWhitespace = true };

    signedDoc.Load("test_signed.xml");

    foreach (XmlElement root in signedDoc.GetElementsByTagName("PackageRoot"))
    {
        foreach (XmlElement signature in root.GetElementsByTagName("Signature"))
        {
            var success = signature.VerifySignature(certificate);
            Console.WriteLine(success ? " successful!" : " failed!");
        }
    }

    Console.WriteLine("Done.");
    Console.ReadLine();
}

public static void Sign(this XmlElement element, X509Certificate2 certificate)
{
    var identifier = Guid.NewGuid().ToString("N");
    element.SetAttribute("Id", identifier);

    var signedXml = new SignedXml(element) { SigningKey = certificate.PrivateKey };
    signedXml.AddReference(new Reference("#" + identifier));
    signedXml.ComputeSignature();

    var xmlDigitalSignature = signedXml.GetXml();

    element.OwnerDocument.DocumentElement.AppendChild(
        element.OwnerDocument.ImportNode(xmlDigitalSignature, true));
}

public static bool VerifySignature(this XmlElement element, X509Certificate2 certificate)
{
    var signedXml = new SignedXml(element.OwnerDocument);
    signedXml.LoadXml(element);

    return signedXml.CheckSignature(certificate, true);
}

One important detail to notice: PreserveWhitespace needs to be set to true for signedDoc, too.