Securely Erasing a Disk
Here are some tools and methods to wipe a disk securely.
Tools[edit | edit source]
dd[edit | edit source]
Write zeros or pseudorandom data:
# dd if=/dev/zero of=/dev/sda bs=4K status=progress
# dd if=/dev/urandom of=/dev/sda bs=4K status=progress
dcfldd[edit | edit source]
dcfldd
is based on an old version of dd
and has some extra features. It hasn't been updated since 2006. If you only want to write a bit pattern, also look at badblocks
.
You can specify a bit pattern:
# dcfldd pattern="00" of=/dev/sda
# dcfldd pattern="FF" of=/dev/sda
shred[edit | edit source]
Shred comes as part of coreutils
. It will overwrite a file with random data.
# shred /dev/sda
badblocks[edit | edit source]
Badblocks can be used to write to and verify a storage device.
# badblocks -v -w -s -b 4096 -t 0xff /dev/sda
Options are:
-v
: verbose-w
: write-s
: show progress-b
: block size-t
: test pattern
ATA Secure Erase[edit | edit source]
Modern ATA devices support a security feature which allows for the firmware to initiate a security erase. Depending on the manufacturer and type of media, an ATA secure erase may wipe the entire device or wipe only the encryption key rendering the data unusable as it can no longer be decrypted. For SSDs, both security erase and enhanced security erase will overwrite the encryption key and is the ideal way to wipe a SSD without inducing unnecessary wear.
The following output from a hdparm -I /dev/sdX
command on a SSD shows a both a security erase and enhanced secure erase will take 2 minutes.
Security:
Master password revision code = 65534
supported
enabled
not locked
not frozen
not expired: security count
supported: enhanced erase
Security level high
2min for SECURITY ERASE UNIT. 2min for ENHANCED SECURITY ERASE UNIT.
An enhanced secure erase will overwrite the entire disk several times with a distinct bit pattern including reallocated sectors that have previously triggered an I/O error. However, I have a Hitachi 3TB disk which reports a 508+ minute security erase but only a 2 minute enhanced security erase which suggests an enhanced security erase doesn't overwrite the entire drive for this model.
more than 508min for SECURITY ERASE UNIT. 2min for ENHANCED SECURITY ERASE UNIT.
Doing the erasure[edit | edit source]
An ATA secure erase can be accomplished with hdparm
.
- Ensure the device is
not frozen
. If it is frozen, either restart the computer or power cycle the disk (via hibernation or something)# hdparm -I /dev/sda
- Enable security using
hdparm
.# hdparm --user-master u --security-set-pass password /dev/sda
- Verify security is enabled.
# hdparm -I /dev/sda
- Issue the ATA secure erase command. Use
--security-erase-enhanced
if supported and desired.# hdparm --user-master u --security-erase password /dev/sda
- Once erased, ensure security is disabled.
# hdparm -I /dev/sda
DBAN[edit | edit source]
Darik's Boot and Nuke. A live disk which will automatically wipe any connected storage devices. This doesn't appear to be updated anymore and newer hardware might not be supported.
DBAN supports the following wipe methods:
- Quick Erase
- Canadian RCMP TSSIT OPS-II Standard Wipe
- American DoD 5220-22.M Standard Wipe
- Gutmann Wipe
- PRNG Stream Wipe
Methods[edit | edit source]
Here are some common wipe methods explained.
DoD 5220.22-M (C)[edit | edit source]
The DoD 5220.22-M (C) requires overwriting an entire device with a pseudo-random bit pattern once.
DoD 5220.22-M (E)[edit | edit source]
The DoD 5220.22-M (E) standard, also referred to as the DoD 3-pass method, requires the following:
- Overwrite with binary 0's
- Overwrite with binary 1's
- Overwrite with a random bit pattern
DoD 5220.22-M ECE[edit | edit source]
DoD 5220.22-M ECE combines DoD 5220.22-M E, C, and E to form a 7-pass variant.
Canadian RCMP TSSIT OPS-II[edit | edit source]
The Canadian RCMP TSSIT OPS-II wipe method is a 7-pass process:
- Overwrite with binary 0's
- Overwrite with binary 1's
- Overwrite with binary 0's
- Overwrite with binary 1's
- Overwrite with binary 0's
- Overwrite with binary 1's
- Overwrite with a random bit pattern and verify the write
Gutmann[edit | edit source]
For the overly paranoid, the Gutmann method uses 35-passes using pseudo-random and bit patterns to wipe a device. This is done with the assumption that intelligence agencies can somehow detect the previous bit values using magnetic force microscopes and image analysis.
See: https://en.wikipedia.org/wiki/Gutmann_method
Schneier[edit | edit source]
A 7-pass method by Bruce Schneier.
- Overwrite with binary 1's
- Overwrite with binary 0's
- Overwrite with a random bit pattern
- Overwrite with a random bit pattern
- Overwrite with a random bit pattern
- Overwrite with a random bit pattern
- Overwrite with a random bit pattern