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I want to know if I encrypt my flash with Truecrypt "Encrypt a non-system partition/drive" option and then mount it and then make a container with FreeOTFE on the Truecrypt encrypted drive and then put my data on the FreeOTFE container to make it super secure.

Is this a secure way or does it make it more weaker opening some possible attacks because the data is encrypted with FreeOTFE then with Truecrypt?

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The best way to examine this kind of setup is to consider your attack scenario. There are two main ones that I can think of:

  1. An attacker has access to your encrypted device, and is attempting to brute-force the password or key.
  2. You are being forced, either by law or threats, to divulge your password.

Your nested encryption doesn't really solve either issue.

If the attacker can break the password on the TC volume, you're either using a bad password or they have the resources to attempt a ridiculous number of passwords per second. This only means that you're giving them a second volume to crack, which just doubles the attack time rather than providing any significant protection. You'd be better off just using a better password on a TrueCrypt volume.

If you're being forced to give up your password, you certainly wouldn't be able to get away with giving up the TC volume password, but not the inner FreeOTFE volume. Again, no benefit here.

The only differences I can see are the following:

  • If a flaw is found in the cryptographic mechanisms in either piece of software, but not both, you still retain a reasonable expectation of security.
  • You increase your attack surface for various local vulnerabilities slightly, in the case where untrusted files are placed on the internal volume. This is unlikely to be exploitable, but it's worth thinking about.
  • The two systems are not designed to work in unison, which might lead to interesting specially-tailored attacks. For example, having a block encrypted by one piece of software and then another might result in a timing attack, or other side-channel attacks.

My advice would be to stick with TrueCrypt and just use cascaded ciphers and a decent password, which gives you reasonable protection against vulnerabilities found in any single cipher.

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  • 'If a flaw is found in the cryptographic mechanisms in either piece of software, but not both, you still retain a reasonable expectation of security.' thats what I make it for i truecrypt cracked or if it had aback door or something the attacker still had to pass freeotfe witch he dont know that I use it he might think it just another truecrypt container. right
    – illsecure
    Jan 10, 2013 at 11:23
  • 'The two systems are not designed to work in unison, which might lead to interesting specially-tailored attacks. For example, having a block encrypted by one piece of software and then another might result in a timing attack, or other side-channel attacks.' can u simply explain what that mean?
    – illsecure
    Jan 10, 2013 at 11:24
  • "if it had aback door or something the attacker still had to pass freeotfe" - no. If the attacker controls TrueCrypt, your machine is entirely compromised. He can just steal the files from your mounted FreeOTFE container.
    – Polynomial
    Jan 10, 2013 at 11:45
  • IYO what you think is better truecrypt or freeotfe because freeotfe had more ciphers and i can change the salt and add some bits the end of the container, what u prefer?
    – illsecure
    Jan 10, 2013 at 11:48
  • As far as the layered systems issue goes, it's rather complicated. But let's imagine I build a block cipher that computes SHA256(password+block_number) for each block, then xors each plaintext block with the hash for that block. In itself, this is reasonably good crypto as far as self-built ciphers go. In the way it's designed, it works great. But someone decides it's not strong enough, so decides to do two rounds of it. Unfortunately, this results in the ciphertext being identical to the plaintext! Whoops. So using a good cipher in a weird way can result in unexpected problems.
    – Polynomial
    Jan 10, 2013 at 11:51
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Using more than one layer of encryption adds more complexity and very little security.

Modern encryption ciphers rarely suffer catastrophic failures but software implementations are not that perfect. Using multiple disk encryption tools may decrease your risk in case one of them will have a known flaw or backdoor that will make your stored data vulnerable. By the same line of reasoning, running multiple encryption tools increases the risk that one of those tools might have an active backdoor or exploit that would compromise your whole system including the data you want to protect. That backdoor might be placed by the author or by attackers breaking into the author's computer or the website that servers the tool. Even the act of downloading (multiple) encryption tools might put you on the list of suspicious citizens (in some countries) or interesting targets (you have something important to protect).

The overall confidentiality of your data depends on many factors, not just encryption. Focusing on doing one layer of encryption right provides most of the security encryption can provide. The effort of using multiple encryption levels can be directed to improving other factors such as physical security, computing environment security (OS, Browser, etc) and key security.

Reducing the complexity of your whole system and managing the overall risk is the right mentality.

And remember, encryption is both a blessing and a curse for security.

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  • Care to explain why it adds very little security? Just as an FYI for @illsecure
    – Hammo
    Jan 10, 2013 at 3:41
  • @Cristian Dobre, can you explain why it adds very little security?
    – illsecure
    Jan 10, 2013 at 11:21
  • I am editing my answer with the explanation.
    – Cristian Dobre
    Jan 11, 2013 at 16:29
  • you are right 'downloading (multiple) encryption tools might put you on the list of suspicious citizens' how I never thought of that I'll keep that in my mind from now, thanks
    – illsecure
    Jan 12, 2013 at 13:00
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There is a tool to perform dictionary attacks on Free OTFE volumes -http://code.google.com/p/luks-volume-cracker/ however due to the intentional complexity of key generation it can only try around 3 keys / second.

There are memory key stealing attacks available for both TrueCrypt and FreeOTFE, and more will be released later this year as part of the DC3 Forensics competition.

The underlying algorithms however, are provably secure.

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