A word on the dangers of TrueCrypt’s hidden volumes feature

Douglas Crawford

Douglas Crawford

September 19, 2013

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When it comes to protecting your data from prying eyes, there are few better programs than the free and open source (FOSS*) TrueCrypt, as Scotland Yard found out when it was unable to access David Miranda’s hard drive in the recent shocking incident where partner of Brazilian journalist Glenn Greenwald, a Guardian reporter involved in the release of documents leaked by whistleblowing here Edward Snowden, was illegally detained under the UK’s Terrorism Act. ‘TrueCrypt,’ said Detective Superintendent Caroline Goode, ‘renders the material extremely difficult to access.’

We have written a couple of articles on how to use TrueCrypt (Part 1 and Part 2), but we feel it important to bring to our readers attention a possibly fatal conceptual flaw in this otherwise exceptional program, and an issue that rarely gets raised when TrueCrypt gets discussed.

*Strictly speaking, some elements of TrueCrypt are source available, rather that truly FOSS (Free Open Source Software). However, we don’t think makes much difference to most end-users.

The Problem

In many ways the fact that TrueCrypt provides ‘plausible deniability’ is one of its greatest strengths, as it is impossible to prove that a hidden volume exists. In most situations, if an adversary (such as law enforcement officials) cannot prove that a volume exists due to lack of evidence, then there can be no case against you.

However, the flip side of this is that it is also impossible to prove that a hidden volume does not exist inside a TrueCrypt container. There are situations where the mere lack of proof that a volume actually exists will not deter an adversary who believes or suspects that it does, and in such situations the fact that any TrueCrypt container could potentially contain a hidden folder could be extremely dangerous.

Consider the situation in a country where the law provides no presumption of innocence, and the burden of proof lies with the accused. In such a situation, while an adversary may not be able to prove that a hidden volume exists, he could also not prove that it doesn’t, and could consequently face jail or worse if the adversary felt that he hiding information (even if he was not).

A similar situation might exist for a Mafia whistleblower facing torture if he does not reveal the password to a suspected hidden volume. Without being able to prove that such volume does not exist, he could be in deep trouble.

In the UK for example, you can be jailed for refusing to hand over encryption keys to data the police believe contains criminal content. However (at least at present), the police do have to demonstrate that some encrypted data exists that you have keys to. If there is no evidence that such data even exists, there is no case.

A TrueCrypt hidden volume makes it impossible to know (and therefore prove) that hidden data exists, which is what lies at the heart of the concept of ‘plausible deniability’. The problem comes when you are dealing with an adversary who doesn’t care about proving the data exists, and will imprison or torture you simply on the grounds of suspicion.

In such a situation, using TrueCrypt to protect your files may be especially dangerous, since if no hidden volume actually exists then you can neither prove this to be the case, nor surrender the non-existent keys. Is your adversary chooses not believe you, then you have nowhere to go.

A potential workaround

At least in theory*, one possible solution to this problem could be to create a hidden volume even if you do not plan to use it. In this way, should you decide that the consequences of revealing your TrueCrypt protected data outweigh the benefits of keeping it secret, you can also reveal your hidden container key to show that there is no further hidden data (only one hidden volume TrueCrypt can be created in each TrueCrypt volume).

*Please note that this is a personal suggestion, and while we have run tests to confirm that only one hidden volume can exist in a TrueCrypt container (if you create a second, the first is deleted), the above idea is just that, and we take no responsibly on how events might play out in practice.


TrueCrypt is probably the best means currently available to keep data safe, and its hidden volume feature could be a life saver in many situations. However, the fact that the existence of a hidden volume cannot be proven one way or another, and may exist within any TrueCrypt container, can under certain circumstances be viewed as a dangerous liability that all users of TrueCrypt should be aware of.

Douglas Crawford

I am a freelance writer, technology enthusiast, and lover of life who enjoys spinning words and sharing knowledge for a living. You can now follow me on Twitter - @douglasjcrawf.

5 responses to “A word on the dangers of TrueCrypt’s hidden volumes feature

  1. It depends on how much info the attacker or government has,in most western countries you are usually better off, or if there are limits to torture or in certain situations. Yes, someone could keep torturing until you reveal it. I wonder if there is anyway to create hidden volumes within hidden volumes, a smart idea would be to use a key or delay tactic, similar to salt, for instance you have a password and keyfile, you can explain to the government that the keyfile or file is located somwhere else. You can then offer to compile the keyfile, in terms of words it can be anything long, an indexed poem,encyclopedia,peoples ages added up with places of births. It can even be a photo or audio or any file though whether that’s recommended is uncertain because you can lose it.

    Thus, the password alone will not be enough, remember you can create a normal volume within a file within truecrypt or newer programs. This buys time and explanation,then you can explain that hidden files have their own keyfiles. I wonder what the best approach would be, being forced to compile words would be long but can be forced, if they keyfile were some secret recording or rare album in uncompressed format then you could buy time. The keyfile would have to be something that can be recreated at the pace you want.

    1. Hi alex,

      The problem, however, is not so much if you do have a hidden volume (which you can choose to reveal when the pressure gets too much), but if you don’t have a hidden volume, but your adversary does not believe you when you say this. It is impossible to prove that a TrureCrypt (or VeraCrypt) volume does not contain a hidden volume. And this can, potentially, be very problematic.

  2. Well if you don’t want a hidden volume and also don’t want to be accused of having one… just create a small empty one anyway. Since only one can exist on a drive, the existence of one is proof that none other exists.

  3. In that situation you must consider that absence of Truecrypt hidden volume may not be the last thing you have to prove. After all you can be arbitrarily accused about infinite number of issues.

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