Securely wipe disk
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Wiping a disk is done by simply writing new data over every single bit.
As mentioned in the Disk Encryption-Article there might be different scenarios for why you want to wipe a disk.
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 i.e. File Recovery-Software.
Preparation for block device encryption. You want to prepare your drive to securely set up Block device encryption inside the wiped area afterwards.
- 1 Introduction
- 2 Select a target
- 3 Overwrite the disk
- 4 Data remanence
- 5 Resources
Wipe all data left on the device
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.
However you might consider prefering the RNG-Method due to Security concerns. This is covered up in the Section about #Preparations for block device encryption.
Also read the section on the possibility of #Data remanence if you want to take wiping serious.
Preparations for block device encryption
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 if your at your desktop running a first person shooter can speed up things a lot.
You can always compare
/proc/sys/kernel/random/poolsize to keep an eye on your entropy pool.
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)
If you want to wipe sensitive data you can use anything matching your needs.
If you want to setup block device encryption you should always wipe at least with Pseudorandom data.
As a matter of course the best wiping practice is to never write unencrypted data.
Select a target
Use fdisk to locate all read/write devices. This will include USB drives if the user can access them. List the partition tables:
# fdisk -l
Check the output for lines that start with devices such as
/dev/sda. For example:
Disk /dev/sdc: 4063 MB, 4063232000 bytes 125 heads, 62 sectors/track, 1024 cylinders Units = cylinders of 7750 * 512 = 3968000 bytes Disk identifier: 0x00000000
In the preceding example the USB thumb drive is listed as
Overwrite the disk
If you have a modern 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=4M). For more information read the Block size section on Wikipedia.
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.
Zero-fill the disk by writing a zero byte to every addressable location on the disk using the /dev/zero stream.
# dcfldd if=/dev/zero of=/dev/sdX bs=4M
or the /dev/random stream:
# dcfldd if=/dev/urandom of=/dev/sdX bs=4M
The process is finished when dcfldd reports,
No space left on device. For example:
18944 blocks (75776Mb) written.dcfldd:: No space left on device
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. Simply 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
Data remanence after wiping is possible for a bunch of reasons.
Wiped HDD and other magnetic storage can get disassembled in a cleanroom and then analyzed with HighTech Equipment like a magnetic force microscope to guess on the overwritten data by computing around the measured residual magnetics.
To seriously make sure you wiped it out for ever you nevertheless might want to use random data and if you think you will really be able to sleep better feel free to give it more than one iteration.
older magnetic storage
Secure wiping of any "older" magnetic storage (especially Floppys, 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 ever copied there. Most resources even advise physical destruction in addition if you want to be really sure!
But not only old Media can be hard to wipe. Flash-Storage behaves very unforgiving as well due to wear leveling, transparent compression, ...
Filesystem, OS, Programms
The OS, executed Programs or a (journaling) filesystem also can do a lot to copy and spread your unencrypted data over the block device, but because you are writing to plain disks this should only be relevant in conjunction with one of the above.