"Protect the Data" from the Evil Maid

I recently posted "Protect the Data" from Whom?. I wrote:

[P]rivate citizens (and most organizations who are not nation-state actors) do not have a chance to win against a sufficiently motivated and resourced high-end threat.

Joanna Rutkowska provides a great example of the importance of knowing the adversary in her post Evil Maid goes after TrueCrypt!, a follow-up to her January post Why do I miss Microsoft BitLocker?

Her post describes how she and Alex Tereshkin implemented a physical attack against laptops with TrueCrypt full disk encryption. They implemented the attack (called "Evil Maid") as a bootable USB image that an intruder would use to boot a target laptop. Evil Maid hooks the TrueCrypt function that asks the user for a passphrase on boot, then stores the passphrase for later physical retrieval.

The scenario is this:

1. User leaves laptop alone in hotel room.


2. Attacker enters room, boots laptop with Evil Maid, and compromises TrueCrypt loader. Attacker leaves.


3. User returns to hotel room, boots laptop, enters TrueCrypt passphrase. Game over.


4. User leaves laptop alone in hotel room again.


5. Attacker enters room again, boots laptop with Evil Maid again, and retrieves passphrase.

Joanna recommends implementing a product that supports Trusted Platform Module (TPM), like Microsoft BitLocker. A detection-oriented workaround is to calculate hashes of selected disk sectors and partitions and decide that mismatches indicate an intrusion has occurred. That approach still misses BIOS-based attacks but it's the best one can do without TPM support.