Write an Android application

I need you to use some existing open source file manager android app and add two features to it. 1) It should be able to upload/download the files and directories to the server for which I will provide you the rest API using which you can post the files to the server. 2) It should encrypt the file before uploading and decrypt the file after downloading. The encryption and decryption algorithm code will also be provided. Let me know which open source app will you be using and how you will try to approach this task. Please provide your best bids. Thanks.