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back to article Crypto attack unveils hidden backups

A German cryptographer has discovered a technique that discloses the presence of a hidden encrypted volume in a disc backup. The sophisticated comparison-based technique doesn't allow the encrypted backup volumes to be read, but it does allow police or other interested parties to determine that it is there. The approach, …

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Anonymous Coward

Useless?

"By comparing an encrypted backup image file with an older image, made using the same key"

So to know a hidden volume exists, whoever's looking needs to image it, wait until the target changes it and then image it again. Not much use to the police is it? "Oh, here, have your drive back and if you have a hidden volume, make sure you save something else to it ready for when we seize again in a few weeks".

...or did I misread?

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Silver badge

Hmm...

So a company that makes a security product "cracks" the protection of a different security product (well, only in the sense that you need an older image of the disk, which is surely a bit unlikely) and says "use our product instead"...?

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Stop

I'm beginning to spot the flaw in this plan ....

(apologies to Blackadder)

"By comparing an encrypted backup image file with an older image, made using the same key ...."

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Easy workaround?

Unless I misread, if you switch cyphers between your two encrypted volumes, you're safe. Seems like this would be good practice anyway and trivial to implement - well, at least on linux anyway. Windows users may be limited by whatever features their chosen software provides I guess.

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Anonymous Coward

@Useless?

Depends how often you cross that border, or the border of a cooperating government that imaged your disk last time, don't it?

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Not just detecting hidden volumes

First, all present hidden volumes can be detected if you have access to a previous snapshot of the visible volume (assuming the contents of the hidden volume have changed) - put simply, something will have changed in a place where it shouldn't have changed.

This can be overcome, but no-one does it - it's difficult and expensive. Note that if volumes are backed up it is quite easy to get a previous snapshot of a filing system.

[Markus Kuhn suggested that the original StegFS could withstand a two-snapshot attack - but it doesn't pass my "will it convince a Jury?" test., and the code of the original version is moribund anyway.]

However this attack is not just about detecting hidden volumes, it's about getting some of the plaintext content too - which is far more important, as people frequently don't need or use hidden volumes.

This type of attack is nothing new. I won't go into OTFE modes as it's verra complicated Captain, and none of the usual modes are completely secure anyway. Rekeying solves some of the problems, but introduces others.

Does TurboCrypt do any better? I don't know - they have probably plugged a small known hole, but there's more to an OTFE solution.

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Coat

Encrypted backups are asking for trouble....

Too many files that are known and predictable compared to the target data.

There shouldn't be that much data that's secret so it would be better to code it and bury it in something like a MP3 or video file.

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Paris Hilton

Back up boner

"There shouldn't be that much data that's secret so it would be better to code it and bury it in something like a MP3 or video file."

There isn't anything left that IS secret these days is there?

It's hard to imagine a sector of the secret and not so secret service that hasn't had all its files dumped as CDs on the last train to Smallville.

Or do I mean MP's three.

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Bronze badge

encrypt it all

until you put the file through hex to ascii. and try and find anything that resembles anything that looks like a password string and identifier

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and....?

"We got you, some bits of your hard drive have changed"

"yeah. and?"

"did you write files to that bit of your hard drive?"

"sure, that's what hard drives are for"

"so why aren't they there now?"

"because I wiped them with heidi erasor"

"why would you do a thing like that?"

"because I was going though US customs and its now our company policy to do so in case they (for example) decide to make a "backup copy" of all our valuable customer data"

"we wouldn't do that"

"so where did you get THAT backup from then?"

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