Modify Emdeded String in C# compiled exe

Question

I have an issue where I need to be able to have a compiled exe ( .net 3.5 c# ) that I will make copies of to distribute that will need to change a key for example before the exe is sent out.

I cannot compile each time a new exe is needed. This is a thin client that will be used as part of a registration process.

Is it possible to add a entry to a resource file with a blank value then when a request comes in have another application grab the blank default thin client, copy it, populate the blank value with the data needed.

If yes how? If no do you have any ideas? I have been scratching my head for a few days now and the limitation as due to the boundaries I am required to work in.

The other idea I has was to inject the value into a method, which I have no idea how I would even attempt that.

Thanks.

Answer 1

The accepted answer is GARBAGE! I HAVE DONE THIS SUCCESSFULLY. MUCH EASIER

Just put your base application (.net) that needs the key somewhere with a string resource FILLED WITH "XXXXXXXXXXXXXXX" (more than you'll need)

.Net resources are usually kept at the top of the code so you will find them fast skipping the first 100,000 bytes in my case.

Then you just read it in and look for those XXXXXX's. When you find them you replace them with the real API key and replace the rest of the X's with spaces you just trim off in code. This is the answer. It works and it works well.

        ApiToken at = new ApiToken(UserId, SelectedCID);
        at.MakeToken();

        byte[] app = System.IO.File.ReadAllBytes(Path.Combine(AppDomain.CurrentDomain.GetData("DataDirectory").ToString(), "notkeyedapp.exe"));

        for (int i = 100000; i < app.Length; i++)
        {
            if (app[i] == 0x58 && app[i + 1] == 0x58 && app[i + 2] == 0x58)
            {
                for (int j = 0; j < 128; j++)
                {
                    if (at.Token.Length >= j + 1)
                        app[i + j] = System.Text.Encoding.ASCII.GetBytes(at.Token[j].ToString())[0];
                    else
                        app[i + j] = 0x20;

                }
                break;
            }
        }
        string filename = "SoftwareProduct for - " + BaseModel.CompanyName.Replace(".", "") + ".exe";
        return File(app, System.Net.Mime.MediaTypeNames.Application.Octet, filename);

Answer 2

Instead of embedding the key in the assembly, put it in the app.config file (or another file delivered with the application) and prevent your application from running if the key is not present and valid. To protect it against modification by users, also add an RSA signature the config file.

This code could be used to generate XML containing your key.

public static void Main()
{
   Console.WriteLine(GenerateKey());
}

public static Byte[] Transform(Byte[] bytes, ICryptoTransform xform)
{
   using (System.IO.MemoryStream stream = new System.IO.MemoryStream())
   {
      using (CryptoStream cstream = new CryptoStream(stream, xform, CryptoStreamMode.Write))
      {
         cstream.Write(bytes, 0, bytes.Length);
         cstream.Close();
         stream.Close();
         return stream.ToArray();
      }
   }
}

public static string GenerateKey()
{
   RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
   // This is the private key and should never be shared.
   // Generate your own with RSA.Create().ToXmlString(true).
   String rsaPrivateKey = "<RSAKeyValue><Modulus>uPCow37yEzlKQXgbqO9E3enSOXY1MCQB4TMbOZyk9eXmc7kuiCMhJRbrwild0LGO8KE3zci9ETBWVVSJEqUqwtZyfUjvWOLHrf5EmzribtSU2e2hlsNoB2Mu11M0SaGd3qZfYcs2gnEnljfvkDAbCyJhUlxmHeI+35w/nqSCjCk=</Modulus><Exponent>AQAB</Exponent><P>4SMSdNcOP0qAIoT2qzODgyl5yu9RubpIU3sSqky+85ZqJHXLUDjlgqAZvT71ROexJ4tMfMOgSWezHQwKWpz3sw==</P><Q>0krr7cmorhWgwCDG8jmzLMo2jafAy6tQout+1hU0bBKAQaPTGGogPB3hTnFIr84kHcRalCksI6jk4Xx/hiw+sw==</Q><DP>DtR9mb60zIx+xkdV7E8XYaNwx2JeUsqniwA3aYpmpasJ0N8FhoJI9ALRzzp/c4uDiuRNJIbKXyt6i/ZIFFH0qw==</DP><DQ>mGCxlBwLnhkN4ind/qbQriPYY8yqZuo8A9Ggln/G/IhrZyTOUWKU+Pqtx6lOghVdFjSxbapn0W8QalNMFGz7AQ==</DQ><InverseQ>WDYfqefukDvMhPHqS8EBFJFpls/pB1gKsEmTwbJu9fBxN4fZfUFPuTnCIJsrEsnyRfeNTAUFYl3hhlRYZo5GiQ==</InverseQ><D>qB8WvAmWFMW67EM8mdlReI7L7jK4bVf+YXOtJzVwfJ2PXtoUI+wTgH0Su0IRp9sR/0v/x9HZlluj0BR2O33snQCxYI8LIo5NoWhfhkVSv0QFQiDcG5Wnbizz7w2U6pcxEC2xfcoKG4yxFkAmHCIkgs/B9T86PUPSW4ZTXcwDmqU=</D></RSAKeyValue>";

   rsa.FromXmlString(rsaPrivateKey);
   String signedData = "<SignedData><Key>Insert your key here</Key></SignedData>";
   Byte[] licenseData = System.Text.Encoding.UTF8.GetBytes(signedData);
   Byte[] sigBytes = rsa.SignData(licenseData, new SHA1CryptoServiceProvider());
   String sigText = System.Text.Encoding.UTF8.GetString(Transform(sigBytes, new ToBase64Transform()));
   System.Text.StringBuilder sb = new StringBuilder();
   using (System.Xml.XmlWriter xw = System.Xml.XmlTextWriter.Create(sb))
   {
      xw.WriteStartElement("License");
      xw.WriteRaw(signedData);
      xw.WriteElementString("Signature", sigText);
      xw.WriteEndElement();
   }
   return sb.ToString();
}

Example output from this code:

<?xml version="1.0" encoding="utf-16"?>
<License>
  <SignedData>
    <Key>Insert your key here</Key>
  </SignedData>
  <Signature>cgpmyqaDlHFetCZbm/zo14NEcBFZWaQpyHXViuDa3d99AQ5Dw5Ya8C9WCHbTiGfRvaP4nVGyI+ezAAKj287dhHi7l5fQAggUmh9xTfDZ0slRtvYD/wISCcHfYkEhofXUFQKFNItkM9PnOTExZvo75pYPORkvKBF2UpOIIFvEIU=</Signature>
</License>

Then you can use code like this to verify it. You never have to distribute the private key:

public static Boolean CheckLicenseSignature(String licXml)
{
   try
   {
      System.Xml.XmlDocument xd = new System.Xml.XmlDocument();
      xd.LoadXml(licXml);
      String licSig = xd.SelectSingleNode("/License/Signature").InnerText;
      RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
      String rsaPublicKey = "<RSAKeyValue><Modulus>uPCow37yEzlKQXgbqO9E3enSOXY1MCQB4TMbOZyk9eXmc7kuiCMhJRbrwild0LGO8KE3zci9ETBWVVSJEqUqwtZyfUjvWOLHrf5EmzribtSU2e2hlsNoB2Mu11M0SaGd3qZfYcs2gnEnljfvkDAbCyJhUlxmHeI+35w/nqSCjCk=</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>";
      rsa.FromXmlString(rsaPublicKey);
      Byte[] licenseData = System.Text.Encoding.UTF8.GetBytes(xd.SelectSingleNode("/License/SignedData").OuterXml);
      return rsa.VerifyData(licenseData, new SHA1CryptoServiceProvider(), Transform(System.Text.Encoding.UTF8.GetBytes(licSig), new FromBase64Transform()));
   }
   catch (System.Xml.XmlException ex)
   {
      return false;
   }
   catch (InvalidOperationException ex)
   {
      return false;
   }
}