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Double Submit Cookie

Double submit cookie is one of the prevention method for Cross-Site Request Forgery (CSRF). Same as the synchronizer token pattern, it is using a cryptographic random value to validate a request agent. However, the implementation is different since the way it is saved is dissimilar. Here, the generated token value is saved in the browser, not on the server. Therefore servers no need to save huge amounts of data. When a user makes a request, the server compared the token value in the form DOM element (in the body) and token in the cookie header of the request. 
(Click on the images to view clearly)
Source code of the implemented program can be downloaded from here.
A login page, index.html has been implemented to enroll users.



Once the user entered the credentials, the form data will be submitted to login_handler.php using POST method.


If the username and password are correct, the user session is started and the username will be assigned to the session variable 'uname'. By default, the session identifier is saved as a cookie in the browser when session_start() is activated. Then the 32-byte length random binary is created and it is converted to a hex value. The random value (CSRF token) is assigned to the variable ‘token’. Further, the token is set as a cookie in the browser by passing the parameter like, token name, token value, and life span (1 day). Then the user will be redirected to home.html. The failed logins are redirected to index.php. 


In home.html, using getCookie function, particular CSRF token value will be extracted from the cookie and the derived value is assigned to the DOM element which has id as dom_csrf.



When the user submits the home.html form, the form data will be sent to form_submit.php.


The form_submit.php checks whether the token value in cookies is available or not. At the same time, token value in the form DOM element is compared with token value in the cookie. If both the values are same, the program displays a success message. If not CSRF token not matched message will be shown when the token is mismatched or the token not exist.

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