Deleting a file on Android removes the reference to it rather than the contents, which remain on the storage medium until something else happens to occupy the same space. iShredder overwrites instead.
It implements more than twenty published erasure algorithms, including the widely cited defence and standards body specifications, which differ mainly in how many overwriting passes they perform and what pattern each pass writes. Files, folders, photographs, messages, call logs, and contacts can be targeted individually, and free space can be overwritten so that data deleted before the application was installed is also addressed. The practical caveat is significant and worth stating plainly, because flash storage does not behave like the magnetic media those standards were written for. A controller distributes writes across cells to spread wear, so an overwrite is not guaranteed to land on the physical location holding the original data, which means multiple pass algorithms provide less additional assurance than their reputation suggests and full disk encryption combined with destroying the key is the more reliable approach on modern devices.
It is most useful for satisfying a documented process that requires a named standard, and for devices where encryption was never enabled. Where encryption is in place, discarding the key achieves more in an instant than any number of passes.
Link: iShredder on Google Play