wipe - Online in the Cloud

PROGRAM:

NAME

wipe - securely erase files from magnetic media

SYNOPSIS

wipe [options] path1 path2 ... pathn

CURRENT-VERSION

This manual page describes version 0.22 of wipe , released November 2010.

DESCRIPTION

Recovery of supposedly erased data from magnetic media is easier than what many people
would like to believe. A technique called Magnetic Force Microscopy (MFM) allows any
moderately funded opponent to recover the last two or three layers of data written to
disk; wipe repeatedly overwrites special patterns to the files to be destroyed, using the
fsync() call and/or the O_SYNC bit to force disk access. In normal mode, 34 patterns are
used (of which 8 are random). These patterns were recommended in an article from Peter
Gutmann (pgut001@cs.auckland.ac.nz) entitled "Secure Deletion of Data from Magnetic and
Solid-State Memory". A quick mode allows you to use only 4 passes with random patterns,
which is of course much less secure.

NOTE ABOUT JOURNALING FILESYSTEMS AND SOME RECOMMENDATIONS (JUNE 2004)

Journaling filesystems (such as Ext3 or ReiserFS) are now being used by default by most
Linux distributions. No secure deletion program that does filesystem-level calls can
sanitize files on such filesystems, because sensitive data and metadata can be written to
the journal, which cannot be readily accessed. Per-file secure deletion is better
implemented in the operating system.

Encrypting a whole partition with cryptoloop, for example, does not help very much either,
since there is a single key for all the partition.

Therefore wipe is best used to sanitize a harddisk before giving it to untrusted parties
(i.e. sending your laptop for repair, or selling your disk). Wiping size issues have been
hopefully fixed (I apologize for the long delay).

Be aware that harddisks are quite intelligent beasts those days. They transparently remap
defective blocks. This means that the disk can keep an albeit corrupted (maybe slightly)
but inaccessible and unerasable copy of some of your data. Modern disks are said to have
about 100% transparent remapping capacity. You can have a look at recent discussions on
Slashdot.

I hereby speculate that harddisks can use the spare remapping area to secretly make copies
of your data. Rising totalitarianism makes this almost a certitude. It is quite
straightforward to implement some simple filtering schemes that would copy potentially
interesting data. Better, a harddisk can probably detect that a given file is being
wiped, and silently make a copy of it, while wiping the original as instructed.

Recovering such data is probably easily done with secret IDE/SCSI commands. My guess is
that there are agreements between harddisk manufacturers and government agencies. Well-
funded mafia hackers should then be able to find those secret commands too.

Don't trust your harddisk. Encrypt all your data.

Of course this shifts the trust to the computing system, the CPU, and so on. I guess
there are also "traps" in the CPU and, in fact, in every sufficiently advanced mass-
marketed chip. Wealthy nations can find those. Therefore these are mainly used for
criminal investigation and "control of public dissent".

People should better think of their computing devices as facilities lended by the DHS.

IMPORTANT WARNING -- READ CAREFULLY

The author, the maintainers or the contributors of this package can NOT be held
responsible in any way if wipe destroys something you didn't want it to destroy. Let's
make this very clear. I want you to assume that this is a nasty program that will wipe out
parts of your files that you didn't want it to wipe. So whatever happens after you launch
wipe is your entire responsiblity. In particular, no one guarantees that wipe will
conform to the specifications given in this manual page.

Similarly, we cannot guarantee that wipe will actually erase data, or that wiped data is
not recoverable by advanced means. So if nasties get your secrets because you sold a
wiped harddisk to someone you don't know, well, too bad for you.

The best way to sanitize a storage medium is to subject it to temperatures exceeding
1500K. As a cheap alternative, you might use wipe at your own risk. Be aware that it is
very difficult to assess whether running wipe on a given file will actually wipe it -- it
depends on an awful lot of factors, such as : the type of file system the file resides on
(in particular, whether the file system is a journaling one or not), the type of storage
medium used, and the least significant bit of the phase of the moon.

Wiping over NFS or over a journalling filesystem (ReiserFS etc.) will most probably not
work.

Therefore I strongly recommend to call wipe directly on the corresponding block device
with the appropriate options. However THIS IS AN EXTREMELY DANGEROUS THING TO DO. Be sure
to be sober. Give the right options. In particular : don't wipe a whole harddisk (eg. wipe
-kD /dev/hda is bad) since this will destroy your master boot record. Bad idea. Prefer
wiping partitions (eg. wipe -kD /dev/hda2) is good, provided, of course, that you have
backed up all necessary data.

COMMAND-LINE OPTIONS

-f (force; disable confirmation query)
By default wipe will ask for confirmation, indicating the number of regular and
special files and directories specified on the command line. You must type "yes" for
confirmation, "no" for rejection. You can disable the confirmation query with the -f
(force) option.

-r (recurse into subdirectories)
Will allow the removal of the entire directory tree. Symbolic links are not followed.

-c (chmod if necessary)
If a file or directory to be wiped has no write permissions set, will do a chmod to
set the permission.

-i (informational, verbose mode)
This enables reporting to stdout. By default all data is written to stderr.

-s (silent mode)
All messages, except the confirmation prompt and error messages, are suppressed.

-q (quick wipe)
If this option is used, wipe will only make (by default) 4 passes on each file,
writing random data. See option -Q

-Q <number-of-passes>
Sets the number of passes for quick wiping. Default is 4.

-a (abort on error)
The program will exit with EXIT_FAILURE if a non-fatal error is encountered.

-R (set random device OR random seed command)

With this option which requires an argument you can specify an alternate /dev/random
device, or a command who's standard output will be hashed using MD5-hashed. The
distinction can be made using the -S option.

-S (random seed method)

This option takes a single-character argument, which specifies how the random
device/random seed argument is to be used. The default random device is /dev/random.
It can be set using the -R option.

The possible single-character arguments are:
r If you want the argument to be treated like a regular file/character device. This
will work with /dev/random, and might also work with FIFOs and the like.
c If you want the argument to be executed as a command. The output from the command
will be hashed using MD5 to provide the required seed. See the WIPE_SEEDPIPE
environment variable for more info.
p If you want wipe to get its seed by hashing environment variables, the current date
and time, its process id. etc. (the random device argument will not be used). This is
of course the least secure setting.

-M (select pseudo-random number generator algorythm)

During the random passes, wipe overwrites the target files with a stream of binary data,
created by the following choice of algorythms:
l will use (depending on your system) your libc's random() or rand() pseudorandom
generator. Note that on most systems, rand() is a linear congruential generator,
which is awfully weak. The choice is made at compile-time with the HAVE_RANDOM define
(see the Makefile).
a will use the Arcfour stream cipher as a PRNG. Arcfour happens to be compatible with
the well-known RC4 cipher. This means that under the same key, Arcfour generates
exactly the same stream as RC4...
r will use the fresh RC6 algorythm as a PRNG; RC6 is keyed with the 128-bit seed, and
then a null block is repeatedly encrypted to get the pseudo-random stream. I guess
this sould be quite secure. Of course RC6 with 20 rounds is slower than random(); the
compile-time option WEAK_RC6 allows you to use a 4-round version of RC6, which is
faster. In order to be able to use RC6, wipe must be compiled with ENABLE_RC6
defined; see the Makefile for warnings about patent issues.

In all cases the PRNG is seeded with the data gathered from the random device (see -R
and -S options).

-l <length>
As there can be some problems in determining the actual size of a block device (as
some devices do not even have fixed sizes, such as floppy disks or tapes), you might
need to specify the size of the device by hand; <length> is the device capacity
expressed as a number of bytes. You can use K (Kilo) to specify multiplication by
1024, M (Mega) to specify multiplication by 1048576, G (Giga) to specify
multiplication by 1073741824 and b (block) to specify multiplication by 512. Thus

1024 = 2b = 1K

20K33 = 20480+33 = 20513

114M32K = 114*1024*1024+32*1024.

-o <offset>
This allows you to specify an offset inside the file or device to be wiped. The
syntax of <offset> is the same as for the -l option.

-e Use exact file size: do not round up file size to wipe possible remaining junk on the
last block.

-Z Don't try to wipe file sizes by repeatedly halving the file size. Note that this is
only attempted on regular files so there is no use if you use wipe for cleaning a
block or special device.

-F Don't try to wipe file names. Normally, wipe tries to cover file names by renaming
them; this does NOT guarantee that the physical location holding the old file name
gets overwritten. Furthermore, after renaming a file, the only way to make sure that
the name change is physically carried out is to call sync (), which flushes out ALL
the disk caches of the system, whereas for ading and writing one can use the O_SYNC
bit to get synchronous I/O for one file. As sync () is very slow, calling sync ()
after every rename () makes filename wiping extremely slow.

-k Keep files: do not unlink the files after they have been overwritten. Useful if you
want to wipe a device, while keeping the device special file. This implies -F.

-D Dereference symlinks: by default, wipe will never follow symlinks. If you specify -D
however, wipe will consent to, well, wipe the targets of any symlinks you might
happen to name on the command line. You can't specify both -D and -r (recursive)
options, first because of possible cycles in the symlink-enhanced directory graph,
I'd have to keep track of visited files to guarantee termination, which, you'll
easily admit, is a pain in C, and, second, for fear of having a (surprise!!) block
device buried somewhere unexpected.

-v Show version information and quit.

-h Display help.

EXAMPLES

wipe -rcf /home/berke/plaintext/
Wipe every file and every directory (option -r) listed under /home/berke/plaintext/,
including /home/berke/plaintext/.

Regular files will be wiped with 34 passes and their sizes will then be halved a
random number of times. Special files (character and block devices, FIFOs...) will
not. All directory entries (files, special files and directories) will be renamed 10
times and then unlinked. Things with inappropriate permissions will be chmod()'ed
(option -c). All of this will happen without user confirmation (option -f).

wipe -kq /dev/hda3
Assuming /dev/hda3 is the block device corresponding to the third partition of the
master drive on the primary IDE interface, it will be wiped in quick mode (option -q)
i.e. with four random passes. The inode won't be renamed or unlinked (option -k).
Before starting, it will ask you to type ``yes''.

wipe -kqD /dev/floppy
Since wipe never follows symlinks unless explicitly told to do so, if you want to
wipe /dev/floppy which happens to be a symlink to /dev/fd0u1440 you will have to
specify the -D option. Before starting, it will ask you to type ``yes''.

wipe -rfi >wipe.log /var/log/*
Here, wipe will recursively (option -r) destroy everything under /var/log, excepting
/var/log. It will not attempt to chmod() things. It will however be verbose (option
-i). It won't ask you to type ``yes'' because of the -f option.

wipe -Kq -l 1440k /dev/fd0
Due to various idiosyncracies of the operating system, it's not always easy to obtain
the number of bytes a given device might contain (in fact, that quantity can be
variable). This is why you sometimes need to tell wipe the amount of bytes to
destroy. That's what the -l option is for. Plus, you can use b,K,M and G as
multipliers, respectively for 2^9 (512), 2^10 (1024 or a Kilo), 2^20 (a Mega) and
2^30 (a Giga) bytes. You can even combine more than one multiplier !! So that 1M416K
= 1474560 bytes.

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