Securely wipe disk
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Wiping a disk is done by writing new data over every single bit.
- 1 Common use cases
- 2 Select a data source for overwriting
- 3 Select a program
- 4 Select a target
- 5 Overwrite the disk
- 6 Data remanence
- 7 See also
Common use cases
Wipe all data left on the device
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, Floppys, 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.)
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.
/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.
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 RAM.)
As the pattern is written to every accesible block this effectively wipes the device.
Kernel built-in RNG
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/poolsize to keep an eye on your entropy pool.
The Kernels poolsize is
4096 bit. (512 Byte)
While Linux kernel 2.4 did have writable
/proc-entries for controlling the entropy-poolsize in newer kernels only
write_wakeup_threshold are writable.
The pool size is now hardcoded in kernel line 275 of
/* * Configuration information */ #define INPUT_POOL_WORDS 128 #define OUTPUT_POOL_WORDS 32 #define SEC_XFER_SIZE 512 #[...]
where poolsize is
4096 = INPUT * OUTPUT
/dev/random uses the kernel entropy pool and will halt overwriting until more input entropy once this pool has been exhausted. This can make it impractical for overwriting large hard disks.
/dev/urandom in contrast will reuse entropy when low on it so you won't get stuck. Nevertheless it might still take a long time to bottle-feed the neverending surge of large drives with data.
The output may contain less entropy than the corresponding read from /dev/random. However it is still intended as a pseudorandom number generator suitable for most cryptographic purposes,
There are also cryptographically secure pseudorandom number generators like Yarrow (FreeBSD/OS-X) or Fortuna (the intended successor of Yarrow).
Select a program
/dev/<drive> is the drive to be encrypted.
Official documentation for dd and shred is linked to under #See also.
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
# kill -USR1 <PID_OF_dd_COMMAND>
# 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
Other dd alike programs feature periodical status output like i.e. a simple progress bar.
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.
For letting badblocks perform a disk wipe a destructive read-write test has to be done.
# badblocks -c 10240 -wsv /dev/<drive>
Altough S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology) is Hardware-featured in almost every HDD still in use nowadays and it can automatically retire defect HDD Sectors in some cases SMART only passively wait for errors while 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 RAM.) As the pattern is written to every accesible block this effectively wipes the device.
Badblocks is in
badblocks [ -svwnfBX ] [ -b block-size ] [ -c blocks_at_once ] [ -e max_bad_blocks ] [ -d read_delay_factor ] [ -i input_file ] [ -o output_file ] [ -p num_passes ] [ -t test_pattern ] device [ last-block ] [ first-block ]
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.
Badblocks can be made to repeatedly write a single "random pattern" with the
# badblocks -wsvt random /dev/<drive>
Select a target
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
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
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
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
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.
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
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.
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
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.
- GNU Coreutils Manpage on Basic operations. Official documentation for dd and shred.
- Learn the DD command. - linuxquestions.org