Handling C#, MVC entity validation errors in a meaningful way

MVC's entity framework is a convenient tool for abstracting databases. However, when something goes wrong the debug messages are not very meaningful. Especially when an entity validation exception occurs.

The following try, catch block catches an entity validation exception and concatenates all validation error messages into a single string. Then this string can be displayed in a debug message.

try
{
   entities.SaveChanges();
}
catch (System.Data.Entity.Validation.DbEntityValidationException ex)
{
   var errorMessages = ex.EntityValidationErrors.SelectMany(x => x.ValidationErrors).Select(x => x.ErrorMessage);
   //Join the list to a single string. 
   var fullErrorMessage = string.Join("; ", errorMessages);
   throw new Exception(fullErrorMessage);
}

The unfortunate cookies

Cookies sent over plain HTTP to Google websites can reveal information about a user

Disclaimer

The following has been reported to Google and is considered not an issue

Recently while visiting Google scholar I noticed that on the top right corner my Google username was displayed.

This appeared to me very strange, since I was not accessing this service using HTTPs. I fired up Wireshark and I revisited scholar once again. From the captured traffic it was obvious that my browser was sending a bunch of cookies over plain HTTP. I stored these cookies to a file, I imported them to a Firefox private browsing window and I visited Google scholar once again. To my surprise my username was still there. Moreover I was able to see my citations and my updates just like if I was signed in. By observing the cookies I noticed that most of them were for the domain *.google.gr, so as next step I visited http://www.google.gr/ig  in the same private session: all gadgets that do not require authentication (like weather) were there!

But the surprises continued. I edited the cookies file and I replaced the domain *.google.gr with *.youtube.com, I loaded the new file in a new Firefox private browsing window and I visited http://www.youtube.com. As it can be observed from the screenshot, my username, my subscriptions, as well as posts of my friends in google+, all were there!

It is astonishing how much information about a user can be gained simple by monitoring a mere HTTP session. 

Edit 1:

Even if the user logs out, the captured cookies continue to reveal the same information

Convert video files and embed subtitles using VLC

VLC player, by VideoLAN, is a handy media player with many features. VLC, among other things, enables the conversion of video files, from one format to another, enabling the same time the incorporation of subtitles.

Suppose that  we want to convert an h.254 video file to DivX with embedded subtitles. Suppose also that subtitles are stored in a separate (.srt) file with the same name as the video file.  Here are the steps that should be followed:

Run VLC and from the Media menu, select Convert/Save (Ctrl + R).

Select Convert/Save

In the file selection area, press Add, and choose the video file to be converted. Moreover, on the button-left menu press the arrow and select  Convert.

Select the Convert optionn

In the Destination area, press Browse, and select where your file should be saved (Note that you have to add the filename as well the extension). In the Settings area, select the Convertion profile and press the Edit selected profile button.

Press the button marked with the black square

In the new window select the Subtitles tab, check the Subtitles check box, select DVB subtitle on the listbox on the left, and check the Overlay subtitles on the video check box. Then press Save.

Subtitle options

Now by pressing Start, your video will be converted to desired format and the subtitles will be embedded in the output video file. 

Send a facebook message using php

Facebook API does not provide any method for sending a message to the inbox of a user. Fortunately, messages can also be sent using XMPP, which is used by facebook chat, but can also be used for sending a message to a user that is offline (i.e., the message will appear in the user's inbox).

In this link you can find a php class that utilizes facebook's XMPP functionality and sends a message to a facebook user.

In order to use this class you need to obtain an API key, by registering your application here. In order to use this class you should provide, your API key, your user id, the current authorization token, and the user id of the user to which you wish to send a message.

In order to get your user id, you can user the facebook php sdk and use the getUser() method of the Facebook class. In order to get the authorization token you can invoke the getAccessToken() of the Facebook class

A proxy re-encryption implementation

Proxy re-encryption is scheme that allows a proxy to re-encrypt a ciphertext, encrypted with the public key of a user A, into a ciphertext that can be decrypted with a private key of a user B, without having access to the private key of A or B, as well as to the plaintext.

Green and Ateniese describe an Identity-based proxy re-encryption scheme in their paper and prove its security. An implementation of their solution can be found in my github repository. This is a python implementation using the Charm Crypto tool

Export excel diagrams to pdf and use them in latex

If you are creating your diagrams using Excel 2007, there is an easy way to export them in .pdf and then use them as figures in your latex documents.

 Open your excel document, select your diagram and then press the office logo and select save as->pdf or exps. Choose a file name and select save as file type tou be PDF (*.pdf). This will result in a pdf file with the diagram and a lot of blank space. In order to remove the blank space use pdfcrop. This a utility is included both in MikTex (latex for windows) as well as in texlive-extra-utils ubuntu package. Alternatevily you can download it from here

Running an IIS7 site from a network drive

Running a web site, located in a network share, in IIS7 can be really tricky, as it usually ends up with IIS7 complaining about permission problem. This usually happens because IIS processes run as a different user, who is not allowed to access network shares. In this blog post it is shown how a web site can be run in the context of a user that is eligible to access a network share.

 But before we start an important note: Running a web site in the context of a privileged user may possibly entail security risks.

In this blog post the following setup is considered: a network drive with IP 192.168.2.6 and share called fotiou, which is password-protected. The share has been mapped, by user User_NAME, to a network drive (Z:), windows have been configured to connect to that drive on start up, and our web site is located in Z:\wordpress.

 From the IIS7 manager console add a new web site. In our case the site is named wordpress. In the Physical path textbox insert the full URI to the website and not the mapped drive (in our case this would be \\192.168.2.6\fotiou\wordpress), and press OK.

Now navigate in the Application Pools located above the Sites option (see the picture below)

From the Application Pools list select the newly created site (named wordpress in our example, as depicted below)

Right click and select Advanced Settings. The in the Process Model tab, edit the Identity option, by selecting the Custom account option, and by setting it to the current user, as in the picture below

Now navigate back to the Sites tree, select your web site and double click the Authentication option

Double click on the Anonymous Authentication option and select Application pool identity

Now your web site can be run without permission problems.