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
scrub(1) scrub scrub(1)
NAME
scrub - write patterns on disk/file
SYNOPSIS
scrub [OPTIONS] special-file
scrub [OPTIONS] file
scrub -X [OPTIONS] directory
DESCRIPTION
Scrub iteratively writes patterns on files or disk devices to make retrieving the data more difficult. Scrub operates in one of three
modes:
1) The special file corresponding to an entire disk is scrubbed and all data on it is destroyed. This mode is selected if file is a char-
acter or block special file. This is the most effective method.
2) A regular file is scrubbed and only the data in the file (and optionally its name in the directory entry) is destroyed. The file size
is rounded up to fill out the last file system block. This mode is selected if file is a regular file. See CAVEATS below.
3) directory is created and filled with files until the file system is full, then the files are scrubbed as in 2). This mode is selected
with the -X option. See CAVEATS below.
OPTIONS
Scrub accepts the following options:
-v, --version
Print scrub version and exit.
-r, --remove
Remove the file after scrubbing.
-p, --pattern PATTERN
Select the patterns to write. See SCRUB METHODS below. The default, nnsa, is reasonable for sanitizing modern PRML/EPRML encoded
disk devices.
-b, --blocksize blocksize
Perform read(2) and write(2) calls using the specified blocksize (in bytes). K, M, or G may be appended to the number to change the
units to KiBytes, MiBytes, or GiBytes, respectively. Default: 4M.
-f, --force
Scrub even if target contains signature indicating it has already been scrubbed.
-S, --no-signature
Do not write scrub signature. Later, scrub will not be able to ascertain if the disk has already been scrubbed.
-X, --freespace
Create specified directory and fill it with files until write returns ENOSPC (file system full), then scrub the files as usual. The
size of each file can be set with -s, otherwise it will be the maximum file size creatable given the user's file size limit or 1g if
unlimited.
-D, --dirent newname
After scrubbing the file, scrub its name in the directory entry, then rename it to the new name. The scrub patterns used on the
directory entry are constrained by the operating system and thus are not compliant with cited standards.
-s, --device-size size
Override the device size (in bytes). Without this option, scrub determines media capacity using OS-specific ioctl(2) calls. K, M,
or G may be appended to the number to change the units to KiBytes, MiBytes, or GiBytes, respectively.
-L, --no-link
If file is a symbolic link, do not scrub the link target. Do remove it, however, if --remove is specified.
-R, --no-hwrand
Don't use a hardware random number generator even if one is available.
-t, --no-threads
Don't generate random data in parallel with I/O.
-h, --help
Print a summary of command line options on stderr.
SCRUB METHODS
nnsa 4-pass NNSA Policy Letter NAP-14.1-C (XVI-8) for sanitizing removable and non-removable hard disks, which requires overwriting all
locations with a pseudorandom pattern twice and then with a known pattern: random(x2), 0x00, verify.
dod 4-pass DoD 5220.22-M section 8-306 procedure (d) for sanitizing removable and non-removable rigid disks which requires overwriting
all addressable locations with a character, its complement, a random character, then verify. NOTE: scrub performs the random pass
first to make verification easier: random, 0x00, 0xff, verify.
bsi 9-pass method recommended by the German Center of Security in Information Technologies (http://www.bsi.bund.de): 0xff, 0xfe, 0xfd,
0xfb, 0xf7, 0xef, 0xdf, 0xbf, 0x7f.
gutmann
The canonical 35-pass sequence described in Gutmann's paper cited below.
schneier
7-pass method described by Bruce Schneier in "Applied Cryptography" (1996): 0x00, 0xff, random(x5)
pfitzner7
Roy Pfitzner's 7-random-pass method: random(x7).
pfitzner33
Roy Pfitzner's 33-random-pass method: random(x33).
usarmy US Army AR380-19 method: 0x00, 0xff, random. (Note: identical to DoD 522.22-M section 8-306 procedure (e) for sanitizing magnetic
core memory).
fillzero
1-pass pattern: 0x00.
fillff 1-pass pattern: 0xff.
random 1-pass pattern: random(x1).
random2
2-pass pattern: random(x2).
old 6-pass pre-version 1.7 scrub method: 0x00, 0xff, 0xaa, 0x00, 0x55, verify.
fastold
5-pass pattern: 0x00, 0xff, 0xaa, 0x55, verify.
custom=string
1-pass custom pattern. String may contain C-style numerical escapes:
nn (octal) or xnn (hex).
CAVEATS
Scrub may be insufficient to thwart heroic efforts to recover data in an appropriately equipped lab. If you need this level of protection,
physical destruction is your best bet.
The effectiveness of scrubbing regular files through a file system will be limited by the OS and file system. File systems that are known
to be problematic are journaled, log structured, copy-on-write, versioned, and network file systems. If in doubt, scrub the raw disk
device.
Scrubbing free blocks in a file system with the -X method is subject to the same caveats as scrubbing regular files, and in addition, is
only useful to the extent the file system allows you to reallocate the target blocks as data blocks in a new file. If in doubt, scrub the
raw disk device.
On MacOS X HFS file system, scrub attempts to overwrite a file's resource fork if it exists. Although MacOS X claims it will support addi-
tional named forks in the future, scrub is only aware of the traditional data and resource forks.
scrub cannot access disk blocks that have been spared out by the disk controller. For SATA/PATA drives, the ATA "security erase" command
built into the drive controller can do this. Similarly, the ATA "enhanced security erase" can erase data on track edges and between
tracks. The DOS utility HDDERASE from the UCSD Center for Magnetic Recording Research can issue these commands, as can modern versions of
Linux hdparm. Unfortunately, the analogous SCSI command is optional according to T-10, and not widely implemented.
EXAMPLES
To scrub a raw device /dev/sdf1 with default NNSA patterns:
# scrub /dev/sdf1
scrub: using NNSA NAP-14.1-C patterns
scrub: please verify that device size below is correct!
scrub: scrubbing /dev/sdf1 1995650048 bytes (~1GB)
scrub: random |................................................|
scrub: random |................................................|
scrub: 0x00 |................................................|
scrub: verify |................................................|
To scrub the file /tmp/scrubme with a sequence of 0xff 0xaa bytes:
# scrub -p custom="xffxaa" /tmp/scrubme
scrub: using Custom single-pass patterns
scrub: scrubbing /tmp/scrubme 78319616 bytes (~74MB)
scrub: 0xffaa |................................................|
AUTHOR
Jim Garlick <garlick@llnl.gov>
This work was produced at the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48 with the
DOE. Designated UCRL-CODE-2003-006, scrub is licensed under terms of the GNU General Public License.
SEE ALSO
DoD 5220.22-M, "National Industrial Security Program Operating Manual", Chapter 8, 01/1995.
NNSA Policy Letter: NAP-14.1-C, "Clearing, Sanitizing, and Destroying Information System Storage Media, Memory Devices, and other Related
Hardware", 05-02-08, page XVI-8.
"Secure Deletion of Data from Magnetic and Solid-State Memory", by Peter Gutmann, Sixth USENIX Security Symposium, San Jose, CA, July
22-25, 1996.
"Gutmann Method", Wikipedia, http://en.wikipedia.org/wiki/Gutmann_method.
Darik's boot and Nuke FAQ: http://dban.sourceforge.net/faq/index.html
"Tutorial on Disk Drive Data Sanitization", by Gordon Hugues and Tom Coughlin,
http://cmrr.ucsd.edu/people/Hughes/DataSanitizationTutorial.pdf.
"Guidelines for Media Sanitization", NIST special publication 800-88, Kissel et al, September, 2006.
shred(1), hdparm(8)
scrub-2.5.2 2012-06-20 scrub(1)