USB Lock/Encryption

Thanks Bruce for talking me through this. It had worried me for a while but is obvious when you really think about it. I wonder how they would monitor for changes to know they need to re-encrypt?

I think that is right, but zeroing all the free space on the drive would not do a lot for its longevity, let alone if one used the kind of multiple random overwrite specified for magnetic disk drives to be securely deleted.

For most applications, blanking the file (say, with the space character, or random data) then saving it, and only then deleting it, will provide "enough" security; an undelete program would restore only the most recent version of the file, the one with the bogus data. The real data might be there in unallocated sectors, but it would take more work to find the sectors and reassemble them than our data is worth, plus the aggregate effect is to be storing random data all over the drive, further overwriting older data.

On the other hand, if we are engaging in truly critical or life-threatening work, something more secure is definitely needed. The best answer I've seen is encrypted pseudo-filesystems like TrueCrypt, but to use them as a filesystem requires that the driver be installed by an admin, which we've seen can be problemmatic. (There is a TrueCrypt Explorer, but I think it takes us back to the same problem, the file must be copied from the encrypted container to the unencrypted file system to be used, and then we have to figure out how to securely delete it...)

As you say, filling the unallocated space with random characters would also work, while reducing the lifespan of the drive and taking so long that people might not want to do it regularly.

See http://www.microsoft.com/technet/sysinternals/Security/SDelete.mspx

I've done some research, and the results are both more and less troublesome than I had expected.

It turns out that there is a good chance any block of storage on the drive might get swapped out with blocks that are in reserve ... flash drives have blocks in reserve that can be put into service if another block fails, or if performance or other factors might benefit. The blocks that are most likely to be treated this way are directories, but that's not a hard and fast rule.

If you zero out every single block in the file system, there is no guarantee that you have cleared out any of those spare blocks that might hold data. On the other hand, in order to get at those blocks to see what they contain, you have to do some serious and potentially destructive work on the flash memory chips.

The actual filesystem itself behaves in a normal way; if you zero out the free space, it is zeroed out. It is also possible, though I haven't found incontrovertible evidence, that the data stored on the flash drive is more binary than on a hard drive (i.e. that it is either on or off, and doesn't reveal its previous state), so it might not be necessary to overwrite files multiple times in order to remove residual magnetic fields the way you might with a traditional hard drive.

If you restrict yourself to the actual filesystem (which is all most of us would be able to do), it is possible to see where a program has saved its file by writing a brand new version, then deleting the older one. This is the behavior I would maintain is most likely to be something that would put data at risk, beyond having everything in plain text and "losing" the flash drive into the hands of your adversary.

What I conclude is this: if you use something that only stores its data encrypted, whether in output files or temporary files, you should be fine. If you can get a virtual drive like TrueCrypt to work, and have the application store everything there, from temporary files to finished documents, you should be fine. Forensic techniques that analyze all of the storage areas on the flash drive will find only encrypted data, even on the sections that have been swapped into "reserve".

If you use software that makes temporary files then deletes them without blanking them first, those files are recoverable with easily available software (though only the most recent version). If you use encryption software that "encrypts in place", overwriting the file, the plain text file itself would likely be obscured by the encrypted version.

Data files that have been deleted, once the directory entry is overwritten, may not be recoverable with "undelete" software, but the contents may still be available on the file system. An inspection of the filesystem with a filesystem dump program would reveal such content that had not been overwritten. It might be a chore to put it all together and/or prove its integrity once you did it, but some confidential or damaging information might be revealed. If the free space on the drive were zeroed, that source of confidential information would be gone.

That said, there is no way to prove that the flash drive itself hasn't swapped out one or more sectors of data before it was encrypted or blanked, and that data might be recoverable by forensic tools. But we knew that.

So if you want to keep "normal" people away from your data, use regular encryption, and clear the free space.

If you have information that you just do not want to fall into other hands, use something that you are confident never stores even a temporary file on disk without it being encrypted ... and uses some techniques to avoid keyloggers and screen readers and so forth. (Cryptnote is a free program that meets many of those criteria, but I haven't been able to figure out if it is free enough to be usable in the PA suite.) An encrypted virtual drive like TrueCrypt can make other programs "safer", if you can use it on a machine where an Administrator has used it once (to set up the file system driver), and if you can be sure that the program isn't writing odd things to other places on the system, and if you are sure an adversary hasn't installed a keylogger or other surveillance.

Another option, suggested on the TrueCrypt site, is to use one of the LiveCD distributions, if the machine you are using allows booting from a CD or your USB drive (which might be a problem, if they have locked down the computer enough so the TrueCrypt drive can't be used by regular users -- I wouldn't want an adversary to boot my machine with a LiveCD whether or not I had some confidential information there). It would provide you with an environment over which you have a fair amount of control, and let you mount and use a TrueCrypt filesystem as Administrator. It would not protect you from hardware attacks, such as a hardware keylogger, if one was hidden on the machine.

At any rate, the question each of us should ask is how much risk can we accept. If encrypting individual files and wiping free space is likely to remove enough confidential data, there might not be a problem. If your life or freedom might be compromised if someone with a forensic laboratory analyzed your drive, don't put that data on your drive.