One Minute Mac Tip: Securely erase files from the command line

Security provisions are one of those “things” that Mac users have been snooty about—for good reason—for decades. However, I’d dare say that, even though the UNIX architecture of the underpinnings of Mac OS X is much more secure than most other popular operating systems (cough, Windows, cough), much of the security benefits that Mac users have enjoyed are really security-by-obscurity, which is not very secure at all. With the added popularity of Mac OS X, lots of responsibility suddenly shifts from the vendor (Apple, Inc.) to the individual users (this means you) to keep your data secure.

Apple has been on point, however, providing good security utilities built right into the operating system and easily available to end users. Of most common use is probably “Secure Empty Trash” which securely deletes files that you put into the trash. The counterpart to this function available in the Finder is, too few Mac users know, the srm or secure remove command-line utility.

srm can be thought of as simply a version of rm that overwrites file data before unlinking it from the file system. It comes with a few more options than rm comes with all geared towards tweaking just how it overwrites files. My favorite is -m, which the manual page says:

overwrite the file with 7 US DoD compliant passes (0xF6, 0x00, 0xFF, random, 0x00, 0xFF, random)

I had the perfect occasion to use srm today: I was transporting my SSH private key from one laptop to another via a temporary drive. I wanted to securely remove all traces of the private key file from the temporary drive after installing it in the new computer. (See this SSH public key tutorial if you don’t know why this might be important.)

After copying the private key file over, removing it securely looks like this:

srm -m private_key_file

It’s that easy.

To be confident that your file is truly overwritten with garbage, you can use the -n option. This is one way to retain a file, but completely corrupt it. Observe:

Meitar:~ meitar$ cat testfile 
Hello world.
Meitar:~ meitar$ srm -mn testfile 
Meitar:~ meitar$ cat testfile 
?
 ?)c?I
      P?Meitar:~ meitar$

That garbage you see after the second invocation of cat shows that the file really was trashed, that is, overwritten with garbage data. Now, a simple rm testfile can do the rest of the work.

As always, man srm will give you all the other juicy details.