I want to be able to build a good file encryption on android for a cloud app.
The files used are mainly pdfs, pngs, docs, audio, video, etc (all sorts of files).
My constrains are that it should be fast enough, so the users of the cloud app don't wait forever to upload/download. Also, the encryption must be secure, thus, if the device gets stolen nobody can get a hand on the data.
I know a bit about encryption, I read about AES (understood the math behind, not so complicated), but I am not sure how to properly do a good file encryption, from the usability point of view.
I do hope you get my problem, if not, please tell what is not clear.
Therefore, my question is:
Which steps should I follow (things to read, etc) in order to have a proper encryption which is also fast enough?
Example: I saw that Spotify is doing a sort of encryption of their files which seems to be fast enough.
Known fact: All I know so far, is that the storage of the key used for encryption is safe enough, so that is not a problem.