Secure delete with rm on OS X

http://paksoy.net/post/1385176457/easy-secure-delete-with-rm-on-os-x

by mustpax • 16 years ago

24 26 16 years ago

santry• 16 years ago

Or use srm.

There is no guarantee this goes on the same sectors on the disk. I think that would only be guaranteed by a "secure delete call" to the drive's firmware.

The only way I know to securely erase the empty space on the drive is a call like "cat /dev/urandom > file; rm file". This will error once the drive is full, then be deleted.

KC8ZKF• 16 years ago

Disk Utility can overwrite empty space on a device with zeros. It also has the dubious 7 and 35 pass overwrite options.

atomical• 16 years ago

Why dubious?

mustpax• 16 years ago

Interesting. But that would only be true for journaling file systems right? Or are there consumer hard disk drives that transparently journal writes a la ssd's?

pmorici• 16 years ago

Newer generations of SSD's have a command called "TRIM". It is meant as a way to speed up SSD drives by alerting the drive to blocks that are no longer in use by the file system and can be pre erased so to speak which makes future writes to that location faster. Windows 7 and newer versions of the Linux kernel support it.

tzs• 16 years ago

It's not just journaling that is a problem. Some filesystems and/or operating systems will move files or parts of files to improve performance. For example OS X does some automatic defragmentation of files. Some systems move files that are frequently accessed together to be near each other on the disk.

When the system does such an automatic move of a file or part of a file, does it clear out the source block on the disk, or just mark it free? If the latter, then a secure erase that just overwrites the current blocks of the file will leave those other blocks untouched in the free space.

If you have a file that you must truly be able to get rid of without destroying your whole hard disk, that file should be kept encrypted on the disk, and the plaintext version should never be stored on disk. Getting rid of the file is then a simple matter of forgetting the encryption key.

Someone2• 16 years ago

"cat /dev/urandom > file" may fail before the disk is full. I have seen a system do that because it wants to have room on the disk for e.g. log files or in-file-system swap.

Also, this will not work with disk quota, and you will have to make sure that you write to the same disk as where the original files lived (think mount points)

Full-disk encryption will not work in all countries, as law enforcement could imprison you if you do not give them the keys (recently happened in the UK). It also would not enable you to point at your drive and say "take a look. That file isn't there"

I think the only way to do this would be with support from the file system. It would have to support two things: overwriting file data (including that in on-disk snapshots), and removing evidence that a file or directory existed with a given name.

Edit: I just realize that the disk must support this, too. It could have written your data, verified the write, seen it to be unreliable, and rewritten it somewhere else on the disk. I doubt disks would keep track of links to these unreliable copies. So, in short, I doubt there is any 100% reliable way to erase data from a disk. Copying what you want to a different drive and destroying the drive probably is your best option, but that may be a bit time-consuming and expensive if you want to safely erase a single file frequently.

jrockway4• 16 years ago

Secure delete is not reliable with modern filesystems. Just use full-disk encryption -- it's free.

wmf• 16 years ago

Good thing HFS+ is not a modern filesystem. Also, I'm not sure what FDE has to do with recovering unlinked files.

jrockway4• 16 years ago

With secure delete, someone with your disk can't read deleted files. With an encrypted filesystem, they can't read the non-deleted files, either.

newman314• 16 years ago

SSDs with FDE...

http://www.computerworld.com/s/article/9131684/Full_disk_enc...

coin• 16 years ago

I haven't been able to find a free bootable full-disk encryption solution for the Mac. TrueCrypt is free but doesn't support bootable whole-disk encryption for the Mac. The only solution I found was PGP Whole Disk Encryption which isn't free.

sp4rki3• 16 years ago

I've generally seen full disk encryption on a OS volume as a moot point. Why is there a need to have full disk encryption on a bootable volume, when the only thing you should be protecting from prying eyes is your content? Full disk encryption makes sense in a data volume, not in a bootable volume where just having a ~ directory and it's children encrypted covers mostly everyone's need for security.

jrockway4• 16 years ago

So you know for a fact that every application you ever use doesn't write data outside of ~?

You may feel comfortable about that, but I don't. /tmp, for example, can contain plenty of sensitive data.

Groxx2• 16 years ago

Know what's easier?

Right click the trash bin, and hold Command. You'll see "Secure Empty Trash". You can also do the rm -P from the trash bin, which I wish they allowed you to do under normal circumstances. (dterm makes this easy.)

MikeCapone• 16 years ago

Thanks. I knew you could hold "alt" when left-clicking on the volume control to select inputs, but I didn't know about that one. Very useful, much better than going in the Finder's preferences...

thought_alarm• 16 years ago

For what it's worth, holding down Option/Alt will empty the Trash without confirmation.

Also, from the menu you can select:

    Finder > Secure Empty Trash

You can even give it a keyboard shortcut, if you like. Or, as you mentioned, there's an option to always securely empty the Trash in the Finder Preferences.

Groxx2• 16 years ago

Holding alt / control / command to modify a right-click menu is one of the most useful and worst UI decisions in OSX (IMO). Easily 95% of people I've encountered on Macs don't even know it exists, and when they do, they don't know how many places it exists.

I really wish there were some kind of hinting to help people discover this.

Niten• 16 years ago

This isn't an OS X specific feature, it's a feature from the FreeBSD userland that Apple borrowed into OS X.

Secure delete with rm on OS X

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