restrict web application users to specific number of terminals using certificates

Hi I am developing a cloud based application. It is a niche application, and not a public general consumer facing application. For specific need wrt the nature of the application and also to keep traffic costs low-as this will be a free application, I need to restrict the access for each user to certain number of terminal. IE: each user can use only 1 computer to access the system at any time. But he need not use the same computer every time. He can always log off and login using another computer. Now there are many ways I am considering to implement this. One of them is to use some sort of certificate to identify along with the username.

Questions: 1. Is this possible using any off the shelf technology? 2. Using the existing technologies, how much effort does the user need to go thru? eg:does he have to install anything every time he wants to use another system etc. 3. Any links that provide information about using such a system in a web application?

Must have Requirement: Should be OS/browser independent

Please share your general experience if you have faced such scenario during development of web application.

PS: Please note that I am not restricting people using the application, but the terminals using the application. IE several people might use the application using same credentials. But at any time, only one terminal can access the application with a given credential. Another scenario might be where an account specifically require to use 2 terminals. In this case 2 different terminal can use the application at any given point of time but not开发者_如何学Python more.