Using a weak password on a truecrypt volume

Question

I have put together a portable apps USB stick for when I need to use random site computers, I also threw truecrypt on with an encrypted volume for some word documents.

The password I am using is much weaker than what is recommended (just a 4 digit number). The way I was thinking about it is this is still at least as secure as the windows xp laptop I cart around anyway, which has the same documents on it. My aim was to make it at least a little wrong for someone to access the documents, sort of like a padlock to keep out the honest criminals.

Is my logic flawed here?

Answer 1

Formally, yes. You have identified a level of residual risk you are prepared to accept, based on a threat analysis, and have implemented controls to manage the risk down to this level.

Informally, as others have pointed out, if a control can be made very much stronger with very little cost, then you should do it anyway. Risk analysis and threat modelling are not very precise sciences.

Something like Courage8366gapor is not hugely more difficult to remember or type compared to a 4 digit PIN code, but the PIN takes under a second to brute force and the passphrase would take somewhere between 2 days and ten times the age of the Earth.

Answer 2

This states truecrypt uses a hash-function like SHA-1 and keystrengthen it by a factor of ~2000 times. So if your attacker knew it was a four digit number; it would take your attacker under 10000 tries to crack, which means they would need about 2000×10000=20 million sha1 hashes. A GPU can generate sha1 hashes at about a billion per second; so your solution would not survive the most basic password cracking attempt.

If security of your data is a requirement, I'd recommend that instead of a four-digit number (13 bits of entropy: 2^13.3 ≅ 10000); using a four-word (or longer) random diceware passphrase (~52 bits: 2⁵² ≅ (6⁵)⁴ ≅ 4×10¹⁵). Instead of surviving for ~1/50th of a second, your passphrase would survive for about ~200 years of a dedicated GPU attack. This is in the realm of breakable in months by very large organizations that are very interested in getting your data (think NSA with a server farm of thousands of GPUs dedicated to cracking your encryption), but protected against all lesser brute-force threats (and adding a couple more words to the passphrase would protect against very large organizations). (EDIT: There are other methods that could be used; e.g., covert keyloggers to steal your passphrase).

As the preface to Applied Cryptography puts it:

There are two kinds of cryptography in this world: cryptography that will stop your kid sister from reading your files, and cryptography that will stop major governments from reading your files.

Your current solution prevents against the "kid sister" attacks of the world, but would not be much more difficult to upgrade to the "major government" kind if desired.

Answer 3

One argument against this is that you should not get into the habit (whether or not the risk tradeoff is acceptable) of choosing weak passwords. Since you're security-aware enough to use Truecrypt, it seems the marginal cost of using a decent passphrase would be just about zero.

Answer 4

In general, I think that having little security is worst than having no security at all.

Because when you have no security, you know that you have no security, and wouldn't trust it to store important things. When you have little security, you might ending up having to draw the line between important stuff vs. not-so-important-stuff vs. just-keep-curious-away. And if you fail to determine the exact point of that, you can "leak" information that you wouldn't want, really.

So, if you want to go secure, make sure you go secure, and choose a good password.

Answer 5

My aim was to make it at least a little wrong for someone to access the documents, sort of like a padlock to keep out the honest criminals.

Firstly honest criminals are quite a rare breed and on the balance of probabilities you are more likely than not to meet the other more common variant, the dishonest variety.

Secondly it is much easier to lose a USB stick than a laptop due to its size and portability by either accidentally leaving it in a cyber cafe computer or just straight having it stolen from you.

So on the balance of probabilities again there is a high chance of your USB stick ending up in the hands of someone who is not an honest criminal than say a laptop so the security level must be higher.

As a general rule though you need to use a strong password on both your laptop and USB stick as good security is not based on whether or not your computer has been stolen yet, if its portable and you move it around, then it is more at risk than a stationary desktop computer.