When a user wants to open an encrypted file (encrypted with password protected mode), the encrypted file will pop up a dialog box to request the user to enter a Open Password. This dialog box will display the machine ID of the user's computer. The user can send his/her own machine ID to you, and you can create a Open Password based on the user's machine ID. After that the user can use this Open Password to open your file on his/her computer. Since the Open Password is created based on the user's machine ID, the user will not be able to share the Open Password with other users. Since the Open Password is bond to the user's computer, the 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 the user's computer and the encryption key. The Open Password is unique for each computer.