Permanently delete files Android

Question

I found an android app named Super Erase that deletes files and folder permanently from android device so that the file deleted cant be recovered anymore..here is the applicatio

Answer 1

When you delete file with standard methods like file.delete() or runtime.exec("rm -f my_file") the only job that kernel does is removing info about file from auxiliary filesystem structures. But storage sectors that contain actual data remain untouched. And because of this recovering is possible.

This gives an idea about how we can try to remove file entirely - we should erase all sectors somehow. Easiest approach is to rewrite all file content with random data few times. After each pass we must flush file buffers to ensure that new content is written to storage. All existing methods of secure file removal spin around above principle. For example this one. Note that there is no universal method that works well across all storage types and filesystems. I guess you should experiment by yourself and try to implement some of the existing approaches or design your own. E.g. you can start from next:

1. Overwrite and flush file 10 times with random data (use FileOutputStream methods). Note!!! Don't use zeros or another low entropy data. Some filesystems may optimize such sparse files and leave some sectors with original content. You can use /dev/urandom file as source of random data (this is a virtual file and it is endless). It gives better results and works faster then well-known Random class.
2. Rename and move file 10 times. Choose new file names randomly.
3. Then truncate file with FileChannel.truncate().
4. And finally remove file with File.delete().

Of course you can write all logic in native code, it may be even somewhat easier than in Java. Described algorithm is just an example. Try doing in that way.