Application lic Key Generator

hello we have a small application which is developed in VB.net, we need below urgently key generator software should be having below functions. 1. When we open the software first time it will open registration form. That form will be with public key(25 char long). 2. We will enter this public key in our Private key generator software. 3. This Private key generator software will allow us to generate key for 15 Days, 30 Days, 90Days Specific date range and for full key. 4. When will insert this private key in our software registration form it will allow users to run software for particular period only. regards Arvind