Suppose I wanted to remove sensitive data from a USB drive. Deleting it isn’t quite enough, since there are undelete tools readily available. But can I do something simple without buying software? I am not trying to defeat the efforts of a state-operated spy agency who will spare no expense to find my old bits, just want to make it impossible for the average guy who might pick this up if I lose it.
I was thinking of creating a binary file full of zeroes or random data, copying it a zillion times until the drive is full, then deleting them again.
I have a software development background so I could write a Java program or DOS script or whatever to do this kind of stuff. But I’m not that hardware savvy so I don’t know what measures are adequate.
For what it’s worth no overwrriting scheme is completely foolproof and with the right equipment and patience data can in principle be recovered no matter how many times it has been overwritten. On the other hand, presumably it’s not the CIA you’re concerned about undeleting your files (hopefully), so I’d go with any of the many freeware or shareware software available designed to overwrite a few times and be done with it.
With magnetic hard drives, it is often said that you should overwrite it with junk data seven times, because it takes that long for the original magnetic patterns on the disk to be erased.
It is easier with flash drives - once you’ve erased your data once, it’s gone. Just format the disk and fill it up once with random or unimportant data, and you’re good to go.
How sensitive?
Thumb drives are cheap, when I take one out of commision, I crack the case and mutulate the innards with a wire cutters or wrench of something to break the chip into smaller pieces.
Wouldn’t a low-level format be good enough to keep the average schmoe out? I know that with my USB thumbdrive, at least, if I just do a normal format, my recovery utilities can get at the data. If I do a low-level format, none of my recovery programs can find anything.
There are plenty of free disk erasers out there. I use DBAN on CD for the hard drives of our old workstations and Disk Eraser for things like USB drives (since it runs from within Windows). Here’s a link:
Very simple to use. Install it. Delete contents of the USB drive, right-click and choose Secure Erase or Erase Free Space (or whatever the context choice is, I haven’t installed it on my new machine). Double-check that the settings are on 3x overwrite, it will do more but that will take a very long time.
I use it to move data from one secured computer to another. I copy a file to it, copy it to the other computer, then delete the file. I carry it around with me. Occasionally the data is considered “sensitive” but not government classified (or I wouldn’t be doing this with it at all).
So I don’t want to destroy it. Just occasionally scrub it down.
In which case a common sense approach would be to treat the USB drive in the same manner in that you would the secured computers.
Leave it in either secured area after each transfer and treat it with the same level of security as you would the computers.
At my company we have taken on a military contract and prohibited USB drives from the build site.
Anything going in must be on a CD-R.
In the BIOS, USB capability switched off and BIOS passwords enabled.
This is all part of the requirements expected of us to protect the data that our customer will provide.
I would assume your Company would like to maintain at least a certain level of security that “sensitive” data could not walk out the back door.
Get them to provide a dedicated USB drive for this exact purpose.
Be pro-active in this.
As an interim you could use basic Winzip Password Encryption for the files prior to putting them on the USB drive, provided of course the secured computers have Winzip available to them.
A Company will expect you to cut corners but it’s up to you to cover your own butt.
The most thorough thing you can use will probably be the *nix application shred. You can use it on almost anything, but be careful: It could damage flash memory. Also remember that you can’t ensure secure deletion on journaling file systems.
This is just not true (movies and TV notwithstanding). Hard-drives may allow recovery after some measure of overwriting, but eventually the old data is unrecoverable. It’s like annual layers in ice cores - the first few hundred years, you can see the layers, then (as they get more compressed) you can’t, so you need to use sampling to isolate them. Eventually you cannot even isolate the layers, and you use averaging and statistical techniques. On a hard drive the leaked magnetic domains get smaller and less distinct the more you overwrite the data. Eventually the noise floor means that you cannot extract the old data from the newer data. About 7 times does the job - 14 to be sure. But this does take a fair bit of time.
It certainly is not the case for Flash based storage - once a bit has been written to there is no way of recovering previous data. A data cell is a data cell containing a 1 or a 0. No technology exists to determine that the cell had a 1, then a 1 then a 0, and then the current 1. But… Flash storage uses “wear levelling.” This ensures that new data is written to the least used cells, not over top of the previous data. Thus, if you have never written more than 1Gb total data to your 1Gb Flash stick, it is actually possible that all the data you have ever written to the drive is still recoverable - using specialized tools. Even if you have written more than 1Gb, the last 1Gb is still possibly recoverable. This is how the police can recover deleted messages from cellphones - for quite some time back.
Flash overwriting requires special tools, but you do only need to do it once. Formatting is not good enough, due to the way file systems on flash work, and copying large amounts of data may not do the job either, due to the particular wear-levelling algorithm.
I was under the impression that a low-level format (which takes about 2-3 minutes) completely overwrites all Flash card sectors with either 0s or 1s. This is opposed to the normal format which just seems to clear the directory and nothing else. Is my understanding incorrect?
Congratulations on having, basically, reinvented the wheel.
The “Disk Manager Programs,” that came and maybe still do. with a new HD, wrote & rewrote 1’s and 0’s repeatedly over every sector and track for several times.