The license is computed by webframework server. The Apps is communicating with the back end through secure webservice. Cryptographic asymetric key is used to produce a License.
Licensing Apps Layout
Communication to the backend with Webservice
Cryptographic assymetric key
The registration of the Apps need to communicate to the server only during registration. Later validation of the Apps is done offline.
Tested on Hardware and emulator : Ice Cream Sandwich 4.0.3 - 4.0.4 API level 15
Tested on Emulator :Marshmallow 6.0 API level 23
Paypal need TLS1.2 which is only available on API lever 16+.