Securing a license key with RSA key.

Question

Hello, it's late, I'm tired, and probably being quite dense....

I have written an application that I need to secure so it will only run on machines that I generate a key for. What I am doing for now is getting the BIOS serial number and generating a hash from that, I then am encrypting it using a XML RSA private key. I then sign the XML to ensure that it is not tampered with. I am trying to package the public key to decrypt and verify the signature with, but every time I try to execute the code as a different user than the one that generated the signature I get a failure on the signature.

Most of my code is modified from sample code I have found since I am not as familiar with RSA encryption as I would like to be. Below is the code I was using and the code I thought I needed to use to get this working right...

Any feedback would be greatly appreciated as I am quite lost at this point the original code I was working with was this, this code works fine as long as the user launching the program is the same one that signed the document originally...

 CspParameters cspParams = new CspParameters();
            cspParams.KeyContainerName = "XML_DSIG_RSA_KEY";
            cspParams.Flags = CspProviderFlags.UseMachineKeyStore;

            // Create a new RSA signing key and save it in the container. 
            RSACryptoServiceProvider rsaKey = new RSACryptoServiceProvider(cspParams)
            {
                PersistKeyInCsp = true,
            };

This code is what I believe I should be doing but it's failing to verify the signature no matter what I do, regardless if it's the same user or a different one...

RSACryptoServiceProvider rsaKey = new RSACryptoServiceProvider();
            //Load the private key from xml file
            XmlDocument xmlPrivateKey = new XmlDocument();
            xmlPrivateKey.Load("KeyPriv.xml");
            rsaKey.FromXmlString(xmlPrivateKey.InnerXml);

I believe this to have something to do with the key container name (Being a real dumbass here please excuse me) I am quite certain that this is the line that is both causing it to work in the first case and preventing it from working in the second case....

cspParams.KeyContainerName = "XML_DSIG_RSA_KEY";

Is there a way for me to sign/encrypt the XML with a private key when the application license is generated and then drop the public key in the app directory and use that to verify/decrypt the code? I can drop the encryption part if I can get the signature part working right. I was using it as a backup to obfuscate the origin of the license code I am keying from.

Does any of this make sense? Am I a total dunce?

Thanks for any help anyone can give me in this..

Answer 1

I used this method to sign xml documents using a private key stored in an xml file that I then embedded into the application .dll as a resource. I think you may be struggling with permissions accessing the keystore, and this would also create hassles transferring the code to other servers etc.

Here is the code to get the private key as an embedded resource and sign the document: (Sign is the name of the class this method is located in, Licensing.Private.Private.xml is a combination of the default namespace + folder + filename of the resource)

public static void SignDocument(XmlDocument xmldoc)
{
    //Get the XML content from the embedded XML privatekey.
    Stream s = null;
    string xmlkey = string.Empty;
    try
    {
        s = typeof(Sign).Assembly.GetManifestResourceStream("Licensing.Private.Private.xml");

        // Read-in the XML content.
        StreamReader reader = new StreamReader(s);
        xmlkey = reader.ReadToEnd();
        reader.Close();
    }
    catch (Exception e)
    {
        throw new Exception("Error: could not import key:",e);
    }

    // Create an RSA crypto service provider from the embedded
    // XML document resource (the private key).
    RSACryptoServiceProvider csp = new RSACryptoServiceProvider();
    csp.FromXmlString(xmlkey);
    //Creating the XML signing object.
    SignedXml sxml = new SignedXml(xmldoc);
    sxml.SigningKey = csp;

    //Set the canonicalization method for the document.
    sxml.SignedInfo.CanonicalizationMethod = SignedXml.XmlDsigCanonicalizationUrl; // No comments.

    //Create an empty reference (not enveloped) for the XPath transformation.
    Reference r = new Reference("");

    //Create the XPath transform and add it to the reference list.
    r.AddTransform(new XmlDsigEnvelopedSignatureTransform(false));

    //Add the reference to the SignedXml object.
    sxml.AddReference(r);

    //Compute the signature.
    sxml.ComputeSignature();

    // Get the signature XML and add it to the document element.
    XmlElement sig = sxml.GetXml();
    xmldoc.DocumentElement.AppendChild(sig);
}

Use the following code the generate the private.xml and public.xml keys. Keep the private.xml file secure, obviously.

RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
File.WriteAllText(@"C:\privateKey.xml", rsa.ToXmlString(true));  // Private Key
File.WriteAllText(@"C:\publicKey.xml", rsa.ToXmlString(false));  // Public Key

Answer 2

Guess, the problem is that different users don't have access to the key that is stored for the 1st user (Please note: I am not a cryptography expert).