add a license key mechanism to an existing software using C# visual studio 2010

I have a C# Visual studio S2010 project that was completed successfully by a vworker. here is the project: <[login to view URL]> I need you to enhance it by adding a licensing mechanism. You may need to edit some parts of the code but they should not have major impact on the core functionalities. basically: add a feature that allows for a license key to be pasted for the software to work. the license has a validity date. this validity date is encrypted in the license key itself. once validity date expires, software will not work. ## Deliverables I have a C# Visual studio S2010 project that was completed successfully by a vworker. here is the project: <[login to view URL]> I need you to enhance it by adding a licensing mechanism. You may need to edit some parts of the code but they should not have major impact on the core functionalities. basically: add a feature that allows for a license key to be pasted for the software to work. the license has a validity date. this validity date is encrypted in the license key itself. once validity date expires, software will not work. the main issue will be: 1. i will generate the encrypted license key from PHP, so your VS2010 will decrypt this key and read the validity date. 2. users cannot share keys - this means that the license is tied to an account. The software has an xml config file that can be edited from the software. you will add a new numeric field that is tied up to the identity if the user/license. This means the encryption/decryption of the license key is dependent on the numeric value manually saved in the xml file via the software on first install. 2.1 - the encryption algorythm does not have to be complicated - i cannot stress this enough. don't wrack your brains on this since my users are few and not intelligent. I propose a simple encryption from ascii to hex, as php has this already: [login to view URL] - so just look for the VS2010 function that converts hex to ascii. taking into account that the numeric id of the user needs to be multiplied or divided to the hex value 3. the validity date must not be circumvented if the user changes his PC date. 4. when license is expired, the software will inform the user of such 5. the software will have a page for you to encode/paste the license 6. ** the software is actually a windows service, and a user interface box is provided in the taskbar. you encode license from the UI box 7. licenses are usually valid on a monthly, quarterly or yearly basis. 8. you are not allowed to sell/reuse this software