Cryptonite is an Android app that brings the FUSE based cryptographic filesystem EncFS and TrueCrypt to Android, you can link it to your Dropbox account with a single tap, after that you will be able to read and write on Dropbox EncFS volumes, exporting, viewing or uploading new files. Dropbox claims to keep data already encrypted in their servers but if anyone finds out your password account they will be able to read the files, encrypting them with Cryptonite you are placing a second security layer on top and block Dropbox built-in backdoor to your data.
To access your files offline sync them to a local folder with an app providing online storage synchronization, e.g. FolderSync. EncFS has a front end interface but Truecrypt is only available as a command line version, rooted phones that support the FUSE kernel, e.g. CyanogenMod, can mount an EncFS or Truecrypt volume, there is a Truecrypt work around to avoid having to use a rooted file browser, by typing “truecrypt –fs-options=”uid=1000,gid=1000,umask=0002″ volume.tc /sdcard/tc“. EncFS will use the encryption ciphers found in the system encryption libraries, Cryptonite allows you to select the encryption method, from a “Quick” Blowfish 128bit up to a “Paranoia” AES256bit with filename block encoding, other preferences include saving temporary files on an external SD card, setting up the mount storage point, clearing the cache and the “Chuck Norris mode” for experienced users that do not want to receive any security warning from the app.
You can browse, export and open encrypted EncFS directories and files on your Dropbox and to your phone, when you open a file from a decrypted EncFS volume Cryptonite will produce a temporary copy in “/data/data/csh.cryptonite/app_open/path_to_your_file”, anyone with access to your phone could recover those files, the app includes a text viewer that works in memory and does not save any temporary copy, there are plans to add an image viewer in the future but right now there isn’t one and if you open an image a temporary copy could be made on the phone outside the encrypted container.
Note: App still in development and intended for advanced users.