Using SSH Keys for License generation and validation in .NET Applications


This blog post is going to present how can you implement license functionality in your .NET application. Providing license in your .NET application is very challenging because there is no standard procedure for the implementation. You are free to use whatever you want. But be notice, there is no license which is 100% safe and cannot be cracked or bypassed.

For this purpose I have selected the CocoaFob library for registration code generation and verification in Objective-C applications. Mainly the library is for Objective -C based applications, like iQS mobile applications and other OSX based applications.  This is very interesting library but you cannot use it in .NET applications, there is no implementation for .NET Framework.

The library uses DSA to generate registration keys, which is very hard for hackers to produce key generators. The library is also specific because it generates license key in human readable form, when the bytes are converted in to Base32 string to avoid ambiguous characters. It also groups codes in sets of five characters separated by dashes. Also DSA has encryption algorithm generates the license which is different every time because of a random element introduced during the process.

So the License key is produced using a 512-bit DSA key looks like on the following sample:


More information about CocoaFob can be found at GitHub page:

The library is using BouncyCastle.Crypto Nuget package for DSA encryption and decryption.

The library CocoaFob for .NET contains two classes:

  1. LicenseData class which provide License properties which is used in license generation. It an be anything: Name, Product number, email, date of expiration etc.
  2. LicenseGenerator  class which is responsible for encrypting and validating the license.

For this blog post the License data class has the flowing implementation:

public class LicenseData

protected internal string productCode;
protected internal string name;
protected internal string email;

public virtual string toLicenseStringData()
 StringBuilder result = new StringBuilder();
 if (productCode != null)

 //name is mandatory property
 if (name == null)
 throw new System.Exception("name cannot be null");

 if (email != null)
 return result.ToString();

As can be seen from the code snippet above the License data contains username, product key and email address. Also, only name property is mandatory, which means you can generate license key based on the user name only.

Generating the License Key

One we have License data we can process of License key generation. License is generated using DSA encryption which uses SSH private key.  You can generate public and private SSH keys  using any of the available tools, eg. OpenSSH, GitHub bash, …. More information about private and public key generation you can find at this link. Once we have public and private keys we can generate license and validate it. One important thing to remember is that you have to care about your private key. It should always be secure and no one should have access to it.

The public key is used for license validation, and it is usualy packed with the application as a part of the deployment stuff. So the process of generating the license is show on the flowing code snippet:

public string makeLicense(LicenseData licenseData)
     if (!CanMakeLicenses)
       throw new System.InvalidOperationException("The LicenseGenerator cannot make licenses as it was not configured with a private key");
        var dsa = SignerUtilities.GetSigner("SHA1withDSA");
        dsa.Init(true, privateKey);
        string stringData = licenseData.toLicenseStringData();
        byte[] licBytes = Encoding.UTF8.GetBytes(stringData);
        dsa.BlockUpdate(licBytes, 0, licBytes.Length);
        byte[] signed = dsa.GenerateSignature();
        string license = ToLicenseKey(signed);
        return license;
    catch (Exception e)
        throw new LicenseGeneratorException(e);

First the DSA encryption is created based on the publicKey we have provided as an argument. Then licBytes is generated from the License data, and converted in to UTF8 formatted bytes. Then we have update DSA provider with licBytes. Now the DSA provider can generate signature in bytes. The signature is converted in to LicenseKey by calling ToLicenseKey method. The method is shown on the following code snippet:

private string ToLicenseKey(byte[] signature)
    /* base 32 encode the signature */
    var result = Base32.ToString(signature);

    /* replace O with 8 and I with 9 */
    result = result.Replace("O", "8").Replace("I", "9");

    /* remove padding if any. */
    result = result.Replace("=", "");

    /* chunk with dashes */
    result = split(result, 5);
    return result;

The magic happen in this method during the conversion of signature from bytes to human readable string. Conversion is done using Base32 string helper method.

Verify the License Key

The License verification process is defined in varifyLicense method. You have to provide SSH publicKey as well as

public virtual bool verifyLicense(LicenseData licenseData, string license)
	if (!CanVerifyLicenses)
		throw new System.InvalidOperationException("The LicenseGenerator cannot verify licenses as it was not configured with a public key");
        //Signature dsa = Signature.getInstance("SHA1withDSA", "SUN");
        var dsa = SignerUtilities.GetSigner("SHA1withDSA");
        dsa.Init(false, publicKey);

        string stringData = licenseData.toLicenseStringData();
        byte[] msgBytes = Encoding.UTF8.GetBytes(stringData);
        dsa.BlockUpdate(msgBytes, 0, msgBytes.Length);

        var dec = FromLicenseKey(license);
        var retVal = dsa.VerifySignature(dec);
        return retVal; 
    catch (Exception e)
        throw new LicenseGeneratorException(e);

As can be seen from the code above, the validation process is done by generating licenseData, converting the license Key in to signatere and the validation process return true is the license is valid, otherwize return false.

The whole project is published at git hub, an can be downloaded from


Testing the Library

The Library solution contains unit test project which you can see how to use this library in the real scenario in order to implement licensing in .NET app.

Happy programming!


C# Implementation of the Finite Element method

I have just published the source code for Finite element method at GitHub. You can find it at

The project is at very beginign stage, and contains very few samples and implementation.

Currently you can solve simple plane load problem, and calculate node displacement.

The more code will be added soon.

The project will be Cross OS and Cross Platform, and will be complatible with .NET Core.


How to Install Windows 10 on Microsoft Azure


It is great time Windows 10 is released and there are huge amount of hype everywhere on the internet. I am using Windows 10 from the beginning through the Insider program, and all I can say the Windows 10 is the greatest and the best OS ever made.
There are plenty of blog posts and articles about the Windows 10, and everybody who is using previous version of the Windows OS should upgrade to Windows 10. As soon as you upgrade you will see the difference and better experience in all aspects.
For this blog post I would show you how to install Windows 10 on Azure, and how to enable some of the great feature. So lets start.

1. First of all go to your Microsoft Azure Subscription Portal.


2. Create New Azure Virtual Machine


3. Start defining Virtual Machine Parameters  by defining OS version. Select Windows 10 Enterprise (x64)


4. Define Virtual Machine Name, username, and password.

Note: This User name should not be developer username, because you have some restriction for Administrator when developing Windows Application. For this reason I have defined bhrnjica_admin user name. My developer username will be bhrnjica, which will be defined later.


5. Define Cloud service name, DNS Name and Region. In this step it is also neccessery to open all port you will need. In my case I have opened the following port:

  • Remote Desktop – this is default without this port you are not able to connect to this machine.
  • PowerShell in case you are working on PowerShell.
  • MSSQL port if you are planning to install Microsoft SQL Server on this machine.
  • MySQL is you are planning to install MySQL server.


6. The last step is to enable some components to be installed on the virtual machine. You have option to include some Antimalware software as well as some Virtual Machine Agent.


7. After you click on the last OK button, wait some time (few minutes) and you should see the following status on you Microsoft Azure Portal.


8. To connect to your created Windows 10 machine, click Connect button to download RDP connection (See picture above). Double click to downloaded RDP file, enter your credentials.


9. Then you have your Windows 10 Azure Machine.



10. Enjoy on your Windows 10 Azure Machine

How to manually fix content error in Microsoft Word


Today while I was editing a document suddenly Word crashed and the document became unable to be opened. Every time I wanted to open it the same error was:

We’re sorry. We can’t open DocumentName.docx because we found a problem with its contents.
When I want to see Details the following message shows:
Unspecified error
Location: Part: /word/document.xml, Line 2, Column: 0.

Then I wanted to open auto-saved version of the document, the same error appeared. So I changed the DocumentName.docx  file in to, and extract the content, found document.xml file and opened it in Notepad++.

The picture below shows the content of the DocumentName.docx file, renamed in to zip and extracted:


If you didn’t know the word document with doc or docx extension is actually zip file, which you can extract.

I have opened the docuemnt.xml and try to find error in second line. This was mission impossible because all xml content is placed in this line. As you can see on the picture below:


To find which element cause the content problem we have to format XML content. I prefer using Notepad++ and XML Tool PlugIn. It can be installed through Plungins menu of Notepad++.


To format xml content choose option Pretty print (XML only – with line breaks) . After the content is formatted, back the Document.xml to zip, change the extension from zip to docx, and open the word document.

The same error appear, but when you choose for details you can read in which line is the error. Find the line in Notepad++ and delete whole tag element:


Copy document.xml back in to zip file, rename the extension and try to open. If you have another error, repeat the process again, otherwise your document is opened.


Oktobarski sastanak Bihac .NET UG

Termin sastanka: 31. 10. 2012. 17:00, Tehnički fakultet Bihać

Nakon ljetnje pauze i septembaskih obaveza zakazujemo naredni sastanak Bihac .NET UG. Naime, u srijedu 31. oktobra sastaćemo se na starom mjestu, na Tehničkom fakultetu u Bihaću i održati sastanak na temu razvoja Windows 8 aplikacija. Ovog puta moja malenkost će održati predavanje pod nazivom “Consuming data in Windows 8 Store apps” u narednom tekstu date su detaljne informacije o predavanju.

Tema: Consuming data in Windows 8 Store app / Konzumacija podataka u Windows 8 Store aplikacijama

Level: 300

Predavač: Bahrudin Hrnjica, Microsoft MVP

Opis: Windows 8 Store aplikacije izvršavaju se u izoliranom okruženju kojim upravlja Windows Run-Time. Ova okolina pruža mogućnosti razvoja aplikacija pod nazivom Windows Store app, odnosno novi tip aplikacija koje se vrte na Windows 8 OS. Jedna od karakteristika ovih aplikacija je što su prilagođene za pokretanje na raznim uređajima od notebooka, tableta do PCa, i što mogu da se distribuiraju na Windows Store globalnu prodavnicu Windows 8 aplikacija. Jednistvenost platforme daje i određena ograničenja, a u ovom predavanju ćemo vidjeti na koji način Windows Store aplikacijama obezbijediti podatke iz različitih izvora podataka, a posebno sa SQL Servera. Widnows Store aplikacija nisu u mogućnosti da direktno pristupe Entity Framework ORM maperima, pa smo prisiljeni da koristimo ili ASP.NET Web API ili WCF. Demo primjeri će pokazati kako koristiti HTTPClient klasu za pristup podacima preko pomenutih serverskih tehnolgija. S druge strane na predavanju će biti pokazano kako koristiti SQLite C++-cross-platform biblioteku u Windows Store applikacijama direktno bez korištenja pomenutih Rest i WCF tehnologije. Kroz demo primjere predavanje će dati kompletnu sliku kako Windows Store aplikacija mogu koristiti podatke iz vanjskih izvora bez kojih je teško zamisliti današnju modernu Windows 8 Store aplikaciju.

Na sastanku će biti i nekoliko nagrada na najsretnije i najaktivnije članove. Nagrade su: majica, usb stik, …..

Golden Ratio and GPdotNET v2 User Interface

First of all, I am not a designer. In fact I don’t think I can design anything beautiful. Mostly, if I want to design something first asked some of my friends or people who knows about design, before decided to do. So design implementation of GPdotNET v2 was very frustrated for me. Anything I have implemented, I thought it was not so good. I still think that the current design of GPdotNET v2 is not something which I can say very well. But something interesting happen while I have designed current UX. I knew if something needs to be designed well, it must be based on golden ratio. So I have decided to implement golden ration anywhere where is possible in the app.

This blog post is going to present one approach of implementation of golden ratio in Application, as a good recipe for design. If you don’t know what is golden ratio you can found on this link.

So let’s first see the Start Screen.

Start Screen of GPdotNET

The first implementation of golden Ratio was dimension of the main window. So when you open GPdotNET, the size of initial Window is size=(932:579) which means that the height=579, and width=579*1,61=932, which is depicted on picture below as well.

Then I concentrate on my main toolbar to see if possible to implement golden ratio there. So if you look for example Common group, you can see that the width and height are designed by golden ratio, as well as height = width=H/Φ of tool button.

I have also implement golden ration to parts of group bar. The number a, b, c are implemented based on golden ratio on the following way: c=(a+b)*Φ, b=a*Φ (see picture below).

Other group bars are also implemented with golden ratio by simple adding half width of Common group toolbar for each additional tool button.

Create new Model Dialog

Other full implementation of golden ration was in Create New Dialog
From picture you can see that the size of dialog is based on golden Ration, as well as parts of the dialog. One interesting thing is that the position of Cancel and OK buttons are also based on golden ratio. Length of line a, is the same as length b multiple by Φ.

Export Dialog

Export dialog is pretty simple so golden ratio is implemented in 3 elements height (a) of ListBox, height (b) and width (c) of dialog in relations b=a*Φ , c= b*Φ, which you can see from the picture below.

I don’t know is my design is better than previous but I know you can find some interesting proportions in form of golden ration. :)

GPdotNET 2.0 beta 1 is coming

GPdotNET v2.0
GPdotNET v2.0

In the few days I will release the first beta of the second version of GPdotNET open source project for modelling and optimization by using Genetic Programming and Genetic Alogorithm. Comparing with previous version, this version brings several new features as well as improvements of existing. The following is the main feature which this beta brings:

  1. GPdotNET v2 is runnign od Windows and Linux based OS.
  2. GPdotNET v2 now supports optimization modul which is integrated with existing GP modeling, as well as a separate module for finding optimum of analytic defined function, please see my previous post about it.
  3. New simplified GUI with only one big toolbar containing all avaliable options.
  4. New File format which you can open in Notepad and modify manually.
  5. New memory improvement, with using memory pooling.

For each of this main features I will write blog post as well as User Guide and Tutorials.

Here is some screen shoots how GPdotNET looks like in Fedora 17.

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