How to wipe free disk space in Linux?

Question

When a file is deleted, its contents may still be left in the filesystem, unless explicitly overwritten with something else. "wipe" can securely erase files, but does not seem to allow erasing free disk space not used by any files.

What should I use to achieve this?

Answer 1

You can use a suite of tools called secure-delete.

sudo apt-get install secure-delete This is a similar question to How to wipe free disk space in Linux?

Here is the answer I gave then;

You can use a suite of tools called secure-delete.

sudo apt-get install secure-delete

This has four tools:

srm - securely delete an existing file
smem - securely delete traces of a file from ram
sfill - wipe all the space marked as empty on your hard drive
sswap - wipe all the data from you swap space.

From the man page of srm

srm is designed to delete data on mediums in a secure manner which can not be recovered by thiefs, law enforce‐ment or other threats. The wipe algorythm is based on the paper "Secure Deletion of Data from Magnetic and Solid-State Memory" presented at the 6th Usenix Security Symposium by Peter Gutmann, one of the leading civilian cryptographers.

The secure data deletion process of srm goes like this:

• 1 pass with 0xff
• 5 random passes. /dev/urandom is used for a secure RNG if available.
• 27 passes with special values defined by Peter Gutmann.
• 5 random passes. /dev/urandom is used for a secure RNG if available.
• Rename the file to a random value
• Truncate the file

As an additional measure of security, the file is opened in O_SYNC mode and after each pass an fsync() call is done. srm writes 32k blocks for the purpose of speed, filling buffers of disk caches to force them to flush and overwriting old data which belonged to the file.

Answer 2

The quickest way, if you only need a single pass and just want to replace everything with zeros, is:

cat /dev/zero > zero.file
rm zero.file

(run from a directory on the filesystem you want to wipe)

There will be a time during this operation when there will be no free space at all on the filesystem, which can be tens of seconds if the resulting file is large and fragmented so takes a while to delete. To reduce the time when freespace is completely zero:

dd if=/dev/zero of=zero.small.file bs=1024 count=102400
cat /dev/zero > zero.file
rm zero.small.file
rm zero.file

This should be enough to stop someone reading the old file contents without an expensive forensic operation. For a slightly more secure, but slower, variant replace /dev/zero with /dev/urandom. For more paranoia run multiple steps with /dev/urandom, though if you need that much effort the shred utility from the coreutils package is the way to go:

dd if=/dev/zero of=zero.small.file bs=1024 count=102400
shred -z zero.small.file
cat /dev/zero > zero.file
rm zero.small.file
shred -z zero.file
rm zero.file

Note that in the above the small file is shredded before creating the larger, so it can be removed as soon as the larger is complete instead of having to wait for it to be shredded leaving the filesystem with zero free space for the time that takes. The shred process with take a long time over a large file and unless you are trying to hide something from the NSA isn't really necessary IMO.

All of the above should work on any filesystem.

Answer 3

WARNING

I was shocked by how many files photorec could retrieve from my disk, even after wiping.

Whether there is more security in filling the "free space" only 1 time with 0x00 or 38 times with different cabalistic standards is more of an academic discussion. The author of the seminal 1996 paper on shredding wrote himself an epilogue saying that this is obsolete and unecessary for modern hardware. There is no documented case of data being physically replaced zeroes and recovered afterwards.

The true fragile link in this procedure is the filesystem. Some filesystems reserve space for special use, and it is not made available as "free space". But your data may be there. That includes photos, personal plain-text emails, whatever. I have just googled reserved+space+ext4 and learned that 5% of my home partition was reserved. I guess this is where photorec found so much of my stuff. Conclusion: the shredding method is not the most important, even the multi-pass method still leaves data in place.

You can try # tune2fs -m 0 /dev/sdn0 before mounting it. (If this will be the root partition after rebooting, make sure run -m 5 or -m 1 after unmounting it).

But still, one way or another, there may be some space left.

The only truly safe way is to wipe the whole partition, create a filesystem again, and then restore your files from a backup.

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Fast way (recommended)

Run from a directory on the filesystem you want to wipe:

dd if=/dev/zero of=zero.small.file bs=1024 count=102400
dd if=/dev/zero of=zero.file bs=1024
sync ; sleep 60 ; sync
rm zero.small.file
rm zero.file

Notes: the purpose of the small file is to reduce the time when free space is completely zero; the purpose of sync is to make sure the data is actually written.

This should be good enough for most people.

Slow way (paranoid)

There is no documented case of data being recovered after the above cleaning. It would be expensive and resource demanding, if possible at all.

Yet, if you have a reason to think that secret agencies would spend a lot of resources to recover your files, this should be enough:

dd if=/dev/urandom of=random.small.file bs=1024 count=102400
dd if=/dev/urandom of=random.file bs=1024
sync ; sleep 60 ; sync
rm random.small.file
rm random.file

It takes much longer time.

Warning. If you have chosen the paranoid way, after this you would still want to do the fast wipe, and that's not paranoia. The presence of purely random data is easy and cheap to detect, and raises the suspicion that it is actually encrypted data. You may die under torture for not revealing the decryption key.

Very slow way (crazy paranoid)

Even the author of the seminal 1996 paper on shredding wrote an epilogue saying that this is obsolete and unecessary for modern hardware.

But if yet you have a lot of free time and you don't mind wasting your disk with a lot of overwritting, there it goes:

dd if=/dev/zero of=zero.small.file bs=1024 count=102400
sync ; sleep 60 ; sync
shred -z zero.small.file
dd if=/dev/zero of=zero.file bs=1024
sync ; sleep 60 ; sync
rm zero.small.file
shred -z zero.file
sync ; sleep 60 ; sync
rm zero.file

Note: this is essentially equivalent to using the secure-delete tool.

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Before the edit, this post was a rewrite of David Spillett's. The "cat" command produces an error message, but I can't write comments on other people's posts.

Answer 4

I use dd to allocate one or more big files big file to fill up the free space, then use a secure deletion utility.

To allocate files with dd try

dd if=/dev/zero of=delete_me bs=1024 count=102400

to generate a file named delete_me that is 100 MB in size. (Here bs is the "block size" set to 1k, and count is the number of blocks to allocate.)

Then use your favorite secure deletion utility (I've been using shred) on the files so created.

But NOTE THIS: buffering means even if you do the whole disk, you may not get absolutely everything!

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This link recommends scrub for free space wiping. Haven't tried it.

Answer 5

You probably already have the GNU coreutils package installed on your system. It provides the command shred.

Answer 6

Here's how to do it with a GUI.

1. Install BleachBit
2. Run as root by clicking Applications - System Tools - BleachBit as Administrator.
3. In the preferences, tell it which paths you want. Generally it guesses them well. You want to include one writeable path for each partition. Generally that is /home/username and /tmp, unless they are the same partition, in which case just pick one.
4. Check the box System - Wipe Free Disk Space.
5. Click Delete.

The advance of BleachBit over dd (which otherwise is very nice) is when the disk is finally full, BleachBit creates small files to wipe the inodes (which contains metadata like filenames, etc).

Answer 7

Easier: scrub -X dump

This will create a dump folder in the current location and create file until the disk is full. You can choose a pattern with the -p option (nnsa|dod|bsi|old|fastold|gutmann)

It's not easy to get scrub installed (see the Ubuntu Forums on this), but once the installation is done, you've a really SIMPLE and efficient tool in your hand.

Answer 8

Once the file is gone off the file system's record, the data that is left on the hard disk is meaningless sequence of 1's and 0's. If you are looking to replace that meaningless sequence with another meaningless sequence, I can advice some commercial products for safely erasing drives, like arconis.