I have a mobile app created in phongap (=html+javascript) and I need to encrypt e.g. a zip file using an asymmetric cypher like RSA, so once the file is encrypted it is practically impossible for anyone to get the contents without using private key that is only known to the server that the file will be sent to.
Is something like that possible in javascript? Assuming the app would run only on Android and iOS with HTML5 compatible browsers.
Javascript cryptography in web browsers is a flawed approach. But maybe you can use Cryptico for your task.