When your user wants to open an encrypted file (encrypted with password protected mode), the encrypted file will pop up a dialog box to request your user to enter a Open Password. This dialog box will display the machine ID of your user's computer. your user can send his/her own machine ID to you, and you can create a Open Password based on your user's machine ID. After that your user can use this Open Password to open your file on his/her computer. Since the Open Password is created based on this user's machine ID, your user will not be able to share the Open Password with other users. Since the Open Password is bond to your user's computer, your user will not be able to re-distribute your files.
Use yourself encryption key to encrypt your files. The encrypted files can only be opened via a Open Password, which can be created using the 'Create Open Password' function in the software, Only the person knows the encryption key can create Open Password, and each Open Password is computed using the Machine ID of your user's computer and the encryption key. Open Password is unique for each computer.