Securely wipe disk

From ArchWiki
Revision as of 15:03, 27 September 2012 by Nonix (talk | contribs) (dd and /dev/urandom: delete duplicates)
Jump to navigation Jump to search

Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary end

Wiping a disk is done by writing new data over every single bit.

Note: References to "disks" in this article also apply to loopback devices.

Common use cases

Wipe all data left on the device

There may be (possibly unencrypted) data left on the device and you want to protect against simple Forensic Investigation that would be possible with for example File Recovery Software.

If you are not going to set up block device encryption but just want to roughly wipe everything from the disk you could consider using /dev/zero or simple patterns instead of a cryptographically strong random number generator. (Referred to as RNG in this article from now on.) This allows to wipe big disks with maximum performance.

It is meant to provide a level of data erasure not allowing recovery with normal system functions and hardware interfaces like standard ATA/SCSI commands. Any File Recovery Software mentioned above then would need to be specialized on proprietary storage-hardware features.

Without at least undocumented drive commands or fiddling about the device’s controller or firmware to make them read out for example reallocated sectors (bad blocks that S.M.A.R.T. retired from use) in case of a HDD no data can get recreated.

Read the section on the possibility of #Data remanence if you want to take wiping serious. This is exceedingly important for all Flash storage devices.

Preparations for block device encryption

If you want to prepare your drive to securely set up Block device encryption inside the wiped area afterwards you really should use random data.

Warning: If Block device encryption is mapped on a partition that contains anything else than random/encrypted data, disclosure of usage patterns on the encrypted drive is possible and weakens the encryption the kind of it does for filesystem-level-encryption. Do never use /dev/zero, simple patterns (badblocks, eg.) or other unrandom data before setting up Block device encryption if you are serious about it!

Select a data source for overwriting

As just said If you want to wipe sensitive data you can use anything matching your needs.

If you want to setup block device encryption afterwards you should always wipe at least with Pseudorandom data.

For Data that is not truely random your disk's writing speed should be the only limiting factor. If you need random data performance may extremely depend on what you choose as source of entropy.

Note: Everything regarding Benchmarking disk wipes should get merged there.

Unrandom data

Warning: A drive that is abnormally fast in writing patterns or zeroing could be doing transparent compression. It is obviously presumable not all blocks get wiped this way. Some #Flash memory devices do "feature" that.

Pattern write test

Note: The badblocks command overwrites the drive at a much faster rate by generating data that is not truly random.

See also #Badblocks.

Random data

Kernel built-in RNG

Template:Moveto The Kernel built-in RNG /dev/random provides you the same quality random data you would use for keygeneration, but can be nearly impractical to use at least for wiping current HDD capacitys. What makes disk wiping take so long with is to wait for it to gather enough true entropy. In an entropy starved situation (e.g. remote server) this might never end while doing search operations on large directories or moving the mouse in X can slowly refill the entropy pool.

You can always compare /proc/sys/kernel/random/entropy_avail against /proc/sys/kernel/random/poolsize to keep an eye on your entropy pool.

Note: Data that is hard to compress (random data) will get written slower if the drive logic mentioned in the [#Unrandom data] warning tries in compressing it. This should not lead to #Data remanence though. As maximum write-speed is not the performance-bottleneck it can get completely neglected while wiping disks with random data.

Pseudorandom Data

A Good Compromise between Performance and Security might be the use of a pseudorandom number generator (like Frandom) or a cryptographically secure pseudorandom number generator like Yarrow (FreeBSD/OS-X) or Fortuna (the intended successor of Yarrow)

Merge-arrows-2.pngThis article or section is a candidate for merging with #Select_a_program.Merge-arrows-2.png

Notes: What refers to applications involved should get moved to the corresponding section. A short general introduction to unrandom (/dev/zero, write-test, ...) and random sources should complement from the section above. (Discuss in Talk:Securely wipe disk#)

There are three sources of random data commonly used for securely overwriting hard disk partitions; /dev/urandom, badblocks, and shred.

dd and /dev/urandom

#dd if=/dev/urandom of=/dev/<drive> bs=4096

Where /dev/<drive> is the drive to be encrypted.

Note: Using /dev/urandom will take a long time to completely overwrite a drive with "random" data. In the strictest sense, /dev/urandom is less random than /dev/random; however, /dev/random uses the kernel entropy pool and will halt overwriting until more input entropy once this pool has been exhausted. This makes the use of /dev/random for overwriting hard disks impractical.
Tip: A good wiping practice might be to not write unnecessary unencrypted data and to wipe cryptoheaders.

Select a program

/dev/<drive> is the drive to be encrypted.


Merge-arrows-2.pngThis article or section is a candidate for merging with Core_Utilities.Merge-arrows-2.png

Notes: Basic file operations are not specific to disk wiping! Unrelated stuff in this section should get merged and then deleted and replaced with a link to Core Utilities. Did you ever want to write an article about dd and Co? Then just go ahead. (Discuss in Talk:Securely wipe disk#)

Official documentation for dd and shred is linked to under #See also.



Note: cp does the same as dd without any operands but is not designed for more versatile disk wiping procedures.
Checking progress of dd while running

By default, there is no output of dd until the task has finished. With kill and the "USR1"-Signal you can force status output without actually killing the program. Open up a 2nd root terminal and issue the following command:

# killall -USR1 dd
Note: This will affect all other running dd-processes as well.


# kill -USR1 <PID_OF_dd_COMMAND>

For example:

# kill -USR1 $(pidof dd)

This causes the terminal in which dd is running to output the progress at the time the command was run. For example:

605+0 records in
605+0 records out
634388480 bytes (634 MB) copied, 8.17097 s, 77.6 MB/s
Dd spin-offs

Other dd alike programs feature periodical status output like i.e. a simple progress bar.


dcfldd is an enhanced version of dd with features useful for forensics and security. It accepts most of dd's parameters and includes status output. The last stable version of dcfldd was released on December 19, 2006.[1]


GNU ddrescue is a data recovery tool. It's capable of ignoring read errors what is a useless feature for disk wiping in almost any case. GNU ddrescue Manual


From Wikipedia: Shred is a Unix command that can be used to securely delete files and devices so that they can be recovered only with great difficulty with specialised hardware, if at all.

Shred uses three passes, writing pseudo-random data to the harddrive each pass. This can be reduced or increased.

# shred -v /dev/<drive>

This invokes shred with default settings, displaying the progress to stdout.

# shred --verbose --random-source=/dev/urandom -n1 /dev/<drive>

Invokes shred telling it to only do one pass, with entropy from /dev/urandom.


Badblocks is in e2fsprogs

For letting badblocks perform a disk wipe a destructive read-write test has to be done.

# badblocks -c 10240 -wsv /dev/<drive>


Badblocks can be made to write "random patterns" with the -t option.

Warning: A "random pattern" is a contradiction in itself. As badblocks does not reuse entropy like urandom but simply repeats one "random pattern" it should not be used where random data would be needed, e.g. for block device encryption.
# badblocks -wsvt random /dev/<drive>

Badblocks can run a settable number of consecutive passes with the -p option. Default is one pass.

# badblocks -wsvp <number> /dev/<drive>

This makes it more likely to find all weak blocks. As a side effect this could help in limiting #Data_remanence to very rare cases for most storage devices.

Note: S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology) is featured in almost every HDD still in use nowadays.

Select a target

Note: Fdisk will not work on GPT formatted devices. Use gdisk instead.

Use fdisk to locate all read/write devices the user has read acess to.

Check the output for lines that start with devices such as /dev/sdX.

This is an example for a HDD formatted to boot a linux system:

# fdisk -l
Disk /dev/sda: 250.1 GB, 250059350016 bytes, 488397168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00ff784a

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      206847      102400   83  Linux
/dev/sda2          206848   488397167   244095160   83  Linux

Or the Arch Install Medium written to a 4GB USB thumb drive:

# fdisk -l
Disk /dev/sdb: 4075 MB, 4075290624 bytes, 7959552 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x526e236e

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           0      802815      401408   17  Hidden HPFS/NTFS

Block size

Template:Wikipedia If you have a Advanced Format hard drive it is recommended that you specify a block size larger than the default 512 bytes. To speed up the overwriting process choose a block size matching your drive's physical geometry by appending the block size option to the dd command (i.e. bs=4096).

To quickly find the block size of the device issue the following command:

# dumpe2fs -h /dev/sdX | grep 'Block size:'

For more information read How to Find the Block Size on The Linux Information Project and the Block size section in the core GNU utilites manual.

Overwrite the disk

Warning: There is no confirmation regarding the sanity of this command so repeatedly check that the correct drive or partition has been targeted. Make certain that the of=... option points to the target drive and not to a system disk.

Zero-fill the disk by writing a zero byte to every addressable location on the disk using the /dev/zero stream.

# dd if=/dev/zero of=/dev/sdX bs=4096

or the /dev/random stream:

# dd if=/dev/urandom of=/dev/sdX bs=4096

The process is finished when dd reports, No space left on device:

dd: writing to ‘/dev/sdb’: No space left on device
7959553+0 records in
7959552+0 records out
4075290624 bytes (4.1 GB) copied, 1247.7 s, 3.3 MB/s

Data remanence

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: This section is too dependent on links to Wikipedia. Links to diverse and high quality resources should be added. (Discuss in Talk:Securely wipe disk#)

Template:Wikipedia The residual representation of data may remain even after attempts have been made to remove or erase the data.

Residual data may be removed by writing random data to the disk or with more than one iteration. However, more than one iteration may not significantly decrease the ability to reconstruct the data of hard disk drives. For more information see Secure deletion: a single overwrite will do it or Overwriting Hard Drive Data: The Great Wiping Controversy.

If the data can be located on the disk and you can confirm that it has never been copied anywhere else, a random number generator provides a quick and thorough alternative.

Residual magnetism

Wiped hard disk drives and other magnetic storage can get disassembled in a cleanroom and then analyzed with equipment like a magnetic force microscope. This may allow the overwritten data to be reconstructed by analyzing the measured residual magnetics.

This method of data recovery for current HDD's is largely theoretical and would require substantial financial resources. Nevertheless degaussing is still practiced.

Old magnetic storage

Securely wiping old magnetic storage (e.g. floppy disks, magnetic tape) is much harder due to much lower memory storage density. Many iterations with random data might be needed to wipe any sensitive data. To ensure that data has been completely erased most resources advise physical destruction.

Flash memory

Like older magnetic storage, flash memory can be difficult to wipe because of wear leveling and transparent compression. For more information see Reliably Erasing Data From Flash-Based Solid State Drives.

Filesystem, operating system, programs

The operating system, executed programs or journaling file systems may copy your unencrypted data throughout the block device. However, this should only be relevant in conjunction with one of the above, because you are writing to plain disks.

See also