SRM(1) General Commands Manual SRM(1)
NAME
srm - secure remove (secure_deletion toolkit)
SYNOPSIS
srm [-d] [-f] [-l] [-l] [-r] [-v] [-z] files
DESCRIPTION
srm is designed to delete data on mediums in a secure manner which can not be recovered by thiefs, law enforcement or other threats. The
wipe algorythm is based on the paper "Secure Deletion of Data from Magnetic and Solid-State Memory" presented at the 6th Usenix Security
Symposium by Peter Gutmann, one of the leading civilian cryptographers.
The secure data deletion process of srm goes like this:
* 1 pass with 0xff
* 5 random passes. /dev/urandom is used for a secure RNG if available.
* 27 passes with special values defined by Peter Gutmann.
* 5 random passes. /dev/urandom is used for a secure RNG if available.
* Rename the file to a random value
* Truncate the file
As an additional measure of security, the file is opened in O_SYNC mode and after each pass an fsync() call is done. srm writes 32k blocks
for the purpose of speed, filling buffers of disk caches to force them to flush and overwriting old data which belonged to the file.
COMMANDLINE OPTIONS
-d ignore the two special dot files . and .. on the commandline. (so you can execute it like "srm -d .* *")
-f fast (and insecure mode): no /dev/urandom, no synchronize mode.
-l lessens the security. Only two passes are written: one mode with 0xff and a final mode random values.
-l -l for a second time lessons the security even more: only one random pass is written.
-r recursive mode, deletes all subdirectories.
-v verbose mode
-z wipes the last write with zeros instead of random data
LIMITATIONS
NFS Beware of NFS. You can't ensure you really completely wiped your data from the remote disks.
Raid Raid Systems use stripped disks and have got large caches. It's hard to wipe them.
swap, /tmp, etc.
Some of your data might have a temporary (deleted) copy somewhere on the disk. You should use sfill which comes with the
secure_deletion package to ensure to wipe also the free diskspace. However, If already a small file aquired a block with your pre-
cious data, no tool known to me can help you here. For a secure deletion of the swap space sswap is available.
BUGS
No bugs. There was never a bug in the secure_deletion package (in contrast to my other tools, whew, good luck ;-) Send me any that you
find. Patches are nice too :)
AUTHOR
van Hauser / THC <vh@thc.org>
DISTRIBUTION
The newest version of the secure_deletion package can be obtained from http://www.thc.org
srm and the secure_deletion package is (C) 1997-2003 by van Hauser / THC (vh@thc.org)
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by
the Free Software Foundation; Version 2.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MER-
CHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
SEE ALSO
sfill (1), sswap (1), sdmem (1)
SRM(1)
Check Out this Related Man Page
WIPE(1) User Commands WIPE(1)
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 repeat-
edly 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.auck-
land.ac.nz) entitled "Secure Deletion of Data from Magnetic and Solid-State Memory". A quick mode allows you to use only 4 passes with ran-
dom 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 parti-
tion.
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 manu-
facturers 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 investiga-
tion 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 guar-
antees 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, pro-
vided, 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 lin-
ear 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 num-
ber of bytes. You can use K (Kilo) to specify multiplication by 1024, M (Mega) to specify multiplication by 1048576, G (Giga) to spec-
ify 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 writ-
ing 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.
BUGS
/LIMITATIONS
Wipe should work on harddisks and floppy disks; however the internal cache of some harddisks might prevent the necessary writes to be done
to the magnetic surface. It would be funny to use it over NFS. Under CFS (Cryptographic File System) the fsync() call has no effect; wipe
has not much use under it anyway - use wipe directly on the corresponding encrypted files. Also, under Linux, when using a device mounted
thru a loopback device, synchronous I/O does not get propagated cleanly.
For wiping floppy disks, at least under Linux, there is no way, besides obscure floppy-driver specific ioctl's to determine the block size
of the disk. In particular, the BLKGETSIZE ioctl is not implemented in the floppy driver. So, for wiping floppies, you must specify the
size of the floppy disk using the -l option, as in the last example. This option is normally not needed for other fixed block devices, like
IDE and SCSI devices.
File name wiping is implemented since version 0.12. I don't know how efficient it is. It first changes the name of the file to a random-
generated name of same length, calls sync (), then changes the name to a random-generated name of maximal length.
File size wiping is implemented by repeatedly truncating the file to half of its size, until it becomes empty; sync () is called between
such operations.
Note that it is still not possible to file creation date and permission bits portably. A wipe utility working at the block device level
could be written using the ext2fs library.
AUTHOR AND LICENCE
Wipe was written by Berke Durak (to find my email address, just type echo berke1ouvaton2org|tr 12 @. in a shell).
Wipe is released under the conditions of the GNU General Public License.
FILES
/dev/random is used by default to seed the pseudo-random number generators.
ENVIRONMENT VARIABLES
WIPE_SEEDPIPE If set, wipe will execute the command specified in it (using popen()), and will hash the command's output with the MD5 mes-
sage-digest algorythm to get a 128-bit seed for its PRNG. For example, on systems lacking a /dev/random device, this variable might be set
in /etc/profile to a shell script which contains various commands such as ls, ps, who, last, etc. and which are run asynchronously in order
to get an output as less predictable as possible.
SEE ALSO
open(2), fsync(2), sync(8), bdflush(2), update(8), random(3)
Linux Sun Nov 7 09:41:23 EST 2010 WIPE(1)