searching into over exposed photos (page 7 of the “
Sensor Noise Camera
Identification: Countering Counter-Forensics” research).
7.9 Data Shredding
One day, you may need a given file no more – whether it’s encrypted or not.
That day, don’t just throw it in the bin: the file will stay there or, at least, leave
some traces of its presence, possibly making it recoverable. In this chapter, we
will cover all the known methods to completely destroy every evidence within
your computer, especially the ROM memories storing your data.
7.9.1 How to perform Data Shredding
When the possibility to recover deleted files
from a PC was discovered,
dozens of free and commercial software sprang out, allowing to solve this
problem. At the moment, we can break down the tools into three broad
categories:
-
Disk Cleaners
-
File Wipers
-
Physical drive destruction
7.9.1.1 Disk Cleaners
Such category includes software using different methods to
sanitize a given
Hard Disk. Essentially, they clear the drive sectors still containing
phantom data
information (a sort of in-memory reminiscence), which will be used by the
operating system only when no free space is left on the Hard Disk.
However, the reliability of Disk Cleaners has been
placed under discussion
by many industry experts, since the techniques used are too weak and only
focused on winning the “speed benchmark competitions”; furthermore, many of
the software performing this task use to leave traces within the operating system
proprietary logs.
BleachBit
BleachBit is an opensource program that cleans the disk space, optimizes the
computer performance and ensures a better privacy for the user. Available for
Windows,
macOS and GNU/Linux, Bleachbit maintains the promise providing
tools designed to remove cache, cookies, history and logs from the main
browser, also integrating a convenient feature to
verify and rewrite the
unallocated disk space (we are going to cover this topic in the “File Shredding”
chapter).
Naturally, installing the program on
Debian is a no-brainer:
$ su
$ apt-get install bleachbit
However, if for any reason it’s not available among the Operating System
repositories, you can download
[118]
and install the package directly from the
official site. Since the program is extremely easy to use (it actually requires just
two clicks) we won’t explore it further.
Other Disk Cleaning software
We can find many other tools designed for Disk Cleaning online. The
following list includes those I feel to recommend:
-
CCleaner
[119]
for Windows, macOS and Android
-
Glary Utilities
[120]
for
Windows and Android
-
Clean Master
[121]
7.9.1.2 File Shredding
The
File Shredding practice deals with this situation
with a more straight-
forward approach, overwriting the memory position which the pre-existing file
was allocated to with random bytes. The more that position will be overwritten,
the more will be the chances of erasing the original file data for good. There are
different opinions about the number of reiterations required for a proper File
Shredding elimination: for example, the NSA recommends 3, the Department of
Defense 7, while Peter Gutmann (who invented the Gutmann method) used even
35 reiterations for his most famous pattern. Everybody will make their own
assessment, but 35 may indeed be a disproportionate number, even if the
explanation makes sense (of course it does!); actually, however, 5-6 reiterations
may be enough for the randomization to allow an
infinite number of file
rebuilding hypotheses. For the sake of clarity, today, the Gutmann method is not
effective anymore, because his studies were based on old patterns used in the
IDE Hard Disks of the late '90s. Furthermore, we have to consider that, from
2001 on, many storage media manufacturers focused on Data Shredding, to the
point to standardize their products with a technology known as ATA Secure
Eras
[122]
e; according to a research conducted in 2011, however, only half of the
worldwide manufacturers adopted this feature.
How to perform File Shredding
File Shredding is not particularly difficult; you
can find many tools for any
Operating System. Unfortunately, however, no one (except BleachBit) is cross-
platform, therefore we must summarize one for each OS:
-
CCleaner
[123]
includes the Drive Wiping feature to clear the space occupied
by deleted files. Only available for
Windows.
-
Once again, a huge number of File Shredding tools is also available for
Windows: Eraser
[124]
, Securely File Shredder
[125]
, Freeraser
[126]
, WipeFile
[127]
,
Secure Eraser
[128]
and many more.
-
On
Mac OS/OSX, Permanent Eraser
[129]
is seemingly the most reliable one
-
The
most recommended tool for GNU/Linux is shred
[130]
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