I ran across a problem this week where I needed to get the filename where an RSA encryption key was stored. These files are stored (for machine-scope keys) in C:\ProgramData\Microsoft\Crypto\RSA\MachineKeys, and have a filename that looks like a hash value followed by a SID. This is easy to find if you have access to the key:

var csp = new CspParameters
{
    Flags = CspProviderFlags.NoPrompt | 
            CspProviderFlags.UseMachineKeyStore | 
            CspProviderFlags.UseExistingKey,
    KeyContainerName = "dev.dev.domo.com"
};

var crypto = new RSACryptoServiceProvider(csp);

Console.WriteLine(csp.KeyContainerName);
Console.WriteLine(crypto.CspKeyContainerInfo.UniqueKeyContainerName);

But in my case I didn’t have access to the keyfile, as it had been created by another user and ACLed. The algorithm for deriving these filenames is not too difficult… It turns out you can take the container name, convert it to lowercase, add an extra null byte, compute the MD5 hash, and then convert the MD5 hash to a string in DWORD-sized chunks. Then you append the machine guid, which can be found in the registry.

public static class RsaCryptoServiceProviderExtensions
{
    public static string GetUniqueKeyContainerName(string containerName)
    {
        using (var rk = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Microsoft\Cryptography"))
        {
            if (rk == null)
            {
                throw new Exception("Unable to open registry key");
            }

            var machineGuid = (string)rk.GetValue("MachineGuid");

            using (var md5 = MD5.Create())
            {
                var containerNameArray = Encoding.ASCII.GetBytes(containerName.ToLower());
                var originalLength = containerNameArray.Length;
                Array.Resize(ref containerNameArray, originalLength + 1);

                var hash = md5.ComputeHash(containerNameArray);
                var stringBuilder = new StringBuilder(32);
                var binaryReader = new BinaryReader(new MemoryStream(hash));
                for (var i = 1; i <= 4; i++)
                {
                    stringBuilder.Append(binaryReader.ReadInt32().ToString("x8"));
                }

                stringBuilder.Append("_" + machineGuid);

                return stringBuilder.ToString();
            }
        }
    }
}

I hit a pretty frustrating problem the other day when I renewed my digital certificate for code signing and my drivers stopped working. When I renewed the certificate through VeriSign (Symantec), I was given the option of choosing a SHA-1 or a SHA-2 (with 256-bit digest) hashing algorithm for the certificate. After reading through the background information on these options, and understanding that the new and improved SHA-2 was supported on Vista and above, I determined that SHA-2 would meet my requirements and clicked the button.

I got the new cert, put it in my cert store, and reconfigured all my projects in Visual Studio to sign with the new cert. Everything was working perfectly. It installed and run fine on my test machine, but then one of my co-workers complained that the driver wouldn’t load on his machine. After a short while I realized that I had the same problem on my machine, I just wasn’t seeing it because I had a kernel debugger attached. (Note that there are some registry options that can enable mandatory kernel mode code signing even when the debugger is attached. Read more at MSDN.)

So now that I could reproduce the problem, I started troubleshooting the signing itself. Using “signtool.exe verify /kp”, I verified that the driver was signed, all certificates were valid, and the cross-certificate from Microsoft was also in the signing chain. Signtool claimed that everything was just fine, but the darn thing still wouldn’t load. I was getting the error ERROR_INVALID_IMAGE_HASH (0x80070241): “Windows cannot verify the digital signature for this file. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source.” After searching the event logs for a bit, I found the Microsoft-Windows-CodeIntegrity log which contained an Event 3004 with basically the same message as the error code above.

After exhausting my Google-Fu skills with no results, we started brainstorming about what was different between the build yesterday that worked and the one today. Somebody mentioned encryption algorithms, and the light bulbs went on in my head. Adding the specific case of SHA-2 to my searching yielded a couple of pages: Practical Windows Code and Driver Signing and PiXCL: Signing Windows 8 Drivers.

Some of my own testing showed that I couldn’t get a driver built with Visual Studio and a SHA-2 certificate to load on both Windows 7 and Windows 8. Theoretically you could sign it twice with each algorithm (using the /fd switch of signtool), but Visual Studio with the WDK is not really set up to do that, I didn’t want to go monkeying with MSBuild stuff too much.

After calling Symantec support and describing the problem we were basically told, “oh yeah, the support for SHA-2 is not really there for a lot of systems.” You think? Maybe you could have mentioned this when we were renewing the certificate?

The moral of the story, dear reader, is stick with the SHA-1 hash algorithm for signing your kernel mode drivers, at least for the time being.