I would like to publish an app in google play, but I want to restrict the downloading under password or something like that. Is it possible? Is there any alternative?
Many thanks in advance,
Short Answer:
No.
Slightly Long Answer:
Applications available on Google Play cannot be downloaded only after a user authenticates. Any such feature will have to be implemented withing your application. At best, if this feature is needed for monetary reasons, publish a paid application.
Since the OP isn't descriptive beyond what you are looking for as a feature, it will be difficult to suggest an option / alternative that might suit your requirement. However, if you have a server setup that can communicate with the app, you can implement a feature which requires users, upon installation and running your app, to Sign Up for a new account and/or Login if already registered.
Again, I will circle back to the original point. Any such feature will have to implemented within the application itself. Google Play does not have such a feature.