2009 Jun 27 - Sat
Securely Erasing Files
On a Linux system, there are a number of tools available for over-writing a file
with random data and then deleting the file and hiding the name of the name of the file as well.
Of course, there are certain caveats that go along with this. If you focus only on
securely deleting files, you will miss file content that may have been written to bad sectors,
file journals, sectors released when files have been relocated from one area to another
(as in when you edit or shorten files),
and various other disk dead areas.
On popular tool is a utility called shred, and is found natively on most distributions. In
the most basic form:
shred --remove filename
If you use the -v (verbose) option, you can see how many times it over-writes a file, and with
what patterns it uses. It also uses a descending 0 write in order to obliterate
a file name.
If you need to recurse sub-directories:
find * -depth -type f | xargs shred --remove
If you have created then moved or erased files and want to ensure that the released content
is overwritten, then you need to over-write drive free space and then release it. There are some
poeople who suggest using dd to fill the free space and then use shred to overwrite and delete the single
large file.
An alternative is to use
scrub, a tool built by the
Lawrence Livermore National Library folks. It uses various national standards for selecting suitable patterns
and over-writing strategies. Source can be found at
Sourceforge.
A quick way to apply all 0's to the free space of a drive:
dd if=/dev/zero of=zerofile bs=1M
sync
rm zerofile
If you can't get scrub to work, then the above command with the shred might be a good combination.
To ensure you have all the data, not just what was located in files or drive free space, one needs to
apply scrub/shred to whole partitions and/or drives. The Gentoo Wiki talks about ways of
securely deleting drives and partitions.
For near-absolute protection of data, I've known companies to specify that once a drive is no longer
useful, that it be crushed and sent to landfill.
|
