I have a FBA application developed in sharepoint 2013. Now to log in to the application i have developed a custom login page which is a simple application page developed in C#. This login page has 3 controls (2 text box for user id and password and one button). Now i want to add a captcha to the page where i can also validate the captcha along with the user id and password. How should i achieve this? Also the condition that i have is that the captcha should be created using sever side code because of security purpose.