Securely wipe disk

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

The most common usecase for completely and irrevocably wiping a device will be when the device it going to be given away or also maybe sold. There may be (unencrypted) data left on the device and you want to protect against simple forensic investigation that is mere child's play with for example File Recovery software.

If you want to quickly wipe everything from the disk /dev/zero or simple patterns allow maximum performance while adequate randomness can be advantageous in some cases that should be covered up in #Data remanence.

Every overwritten bit means to provide a level of data erasure not allowing recovery with normal system functions (like standard ATA/SCSI commands) and hardware interfaces. Any file recovery software mentioned above then would need to be specialized on proprietary storage-hardware features.

In case of a HDD data recreation will not be possible 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).

There are different wiping issues with different physical storage technologys, most notably all Flash memory based devices and older magnetic storage (old HDD's, floppy disks, tape).

Preparations for block device encryption

If you want to prepare your drive to securely set up Disk Encryption#Block device encryption inside the wiped area afterwards you really should use #Random data generated by a trusted cryptographically strong random number generator (referred to as RNG in this article from now on).

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 being comparable with 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 truly random your disk's writing speed should be the only limiting factor. If you need random data, the required system performance to generate it may extremely depend on what you choose as source of entropy.

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

Unrandom data

Overwriting with /dev/zero or simple patterns is considered secure in most resources. In the case of current HDD's it should be sufficient for fast disk wipes.

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

#Badblocks can write simple patterns to every block of a device and then read and check them searching for damaged areas (just like memtest86* does with memory).

As the pattern is written to every accesible block this effectively wipes the device.

Random data

Note: Data that is hard to compress (random data) will get written slower, if the drive logic mentioned in the #Unrandom data warning tries 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.

Kernel built-in RNG

The kernel build in RNG's /dev/(u)random are highly recommended for producing reliable random data providing the same security level that is used for the creation of cryptographic keys. The random number generator gathers environmental noise from device drivers and other sources into an entropy pool.

/dev/random

uses an entropy pool of 4096 bits (512 Bytes) to generate random data and stops when the pool is exhausted until it get's (slowly) refilled. /dev/random is absolutely not designed for wiping entire HDD's, but rather to generate cryptographic keys (e.g. SSL/SSH).

/dev/urandom

reuses existing entropy pool data while the pool is replenished and although not suited for the most crucial cryptographic purposes, for example the generation of longterm keys, its quality should be sufficient for a paranoid disk wipe, preparing for block device encryption, wiping LUKS keyslots, wiping single files and many other purposes.

One can compare the actual available entropy /proc/sys/kernel/random/entropy_avail against /proc/sys/kernel/random/poolsize to keep an eye on the entropy pool. More information on the kernel RNG is available via man 4 random.

Apart from the applications discussed above there are others that depend on enough entropy being available, e.g. using the system as a wifi access point with encryption.

For much better performance to refill the entropy pool, one can consider using a pseudorandom number generator instead of the kernel, e.g. Frandom.

Select a program

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

Coreutils

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

Dd

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.

Or:

# 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

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]

ddrescue

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

shred

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.[2] Shred uses three passes, writing pseudo-random data to the device during each pass. This can be reduced or increased.

The following command invokes shred with its default settings and displays the progress.

# shred -v /dev/<drive>

Alternatively, shred can be instructed to do only one pass with entropy from, e.g. /dev/urandom.

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

Badblocks

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

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

Select a target

Note: Fdisk will not work on GPT formatted devices. Use gdisk (gptfdisk) 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 for 4KB).

Fdisk prints physical and logical sector size for every disk.

Alternatively sysfs does expose information:

/sys/block/sdX/queue/physical_block_size
/sys/block/sdX/queue/logical_block_size
/sys/block/sdX/alignment_offset

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. iflag and oflag as below will try to disable buffering, which is senseless for a constant stream.

# dd if=/dev/zero of=/dev/sdX iflag=nocache oflag=direct bs=4096

Or the /dev/urandom 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

Template:Wikipedia

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

Residual data may get wiped by writing (random) data to the disk with a single or even more than one iteration. However, more than one iteration may not significantly decrease the possibility to reconstruct the data of hard disk drives. For more information see Secure deletion: a single overwrite will do it - The H Security.

Random data

If the data can get exactly located on the disk and was never copied anywhere else, wiping with random data can be thoroughgoing and impressively quick as long there is enough entropy in the pool.

A good example is cryptsetup using /dev/urandom for wiping the LUKS keyslots.

Hardware specific issues

Flash memory

Wikipedia:Write amplification and other characteristics make Flash memory a stubborn target for reliable wiping. As there is a lot of transparent abstraction in between data as seen by a device's controller chip and the operating system sight data is never overwritten in place and wiping particular blocks or files is not reliable.

Other "features" like transparent compression (all SandForce SSD's) can compress your /dev/zero or pattern stream so if wiping is fast beyond belief this might be the case.

Disassembling Flash memory devices, unsoldering the chips and analyzing data content without the controller in between is feasible without difficulty using simple hardware. Data recovery companys do it for cheap money.

For more information see: Reliably Erasing Data From Flash-Based Solid State Drives.

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 a practiced countermeasure.

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.

Operating system, programs and filesystem

Note: This is not specific to any hardware obviously.

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

See also

• GNU Coreutils Manpage on Basic operations. Official documentation for dd and shred.

• Learn the DD command. - linuxquestions.org