encryption in android apps

Hi,

In my android application I need to save several sensitive files and I
want to encrypt them.
But I have doubts the way to store the key on the device!
The application is protected with PIN code and the is also
communication with the back-end server. But such communication should
be as
less as possible. This implies that I can't store the secret key on
the server and get it whenever needed.
So does anybody has a practical solution?

Thanks



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