3 ms·

The TPM is a computer running code that says "after 3 bad password guesses, forget the underlying key forever". That's the value it adds here. It's assumed th

The TPM is a computer running code that says "after 3 bad password guesses, forget the underlying key forever". That's the value it adds here. It's assumed that typing the password is the only way to get the actual encryption key out of the device, so the only attack is to guess the underlying encryption key, which is much harder than guessing a user's password (that they probably typed into totally-legit-blog.example.com to post a comment once). In the real world, it's not clear that this is strictly better than using a slow password-based key derivation function. With a key derivation function, the attacker gets unlimited tries for all of time, so it is technically slightly less secure than "the key blows up in a ball of fire after 3 guesses". I don't have data on whether or not passwords are successfully guessable on average in 3 attempts, but they probably are for certain users. (You look at data breaches and if they use the same password on all of them, it's probably their TPM unlock password too.) How secure TPMs are is also up for debate. I think I had an early one that was just a standard 8 bit microcontroller speaking the protocol the right way, without any actual security. That loophole is likely plugged for DRM reasons (software can ask the TPM to certify itself), but assuming that a computer running code you've never read has 0 bugs is unwise.

wat10000 2y ago
A theoretically perfect TPM is way better. The strength of your KDF is limited by the user experience. If you want to limit an attacker to trying one password per minute, then you have to make the user wait one minute to log in, if you assume the attacker only has the same amount of computing power you do. A TPM can just say “you have ten tries and that’s it.” Of course, a real TPM won’t be perfect. But there’s no reason you can’t do both. Have the TPM use a good KDF and then an attacker has to break both.
- cafeinux 2y ago
  When will come the day a TPM deletes itself after n wrong passwords, we will feel a great disturbance in the force, as if millions of IT support voices suddenly cry out in terror, and are suddenly hit with "my cat walked on my keyboard".

wat10000 2y ago

A theoretically perfect TPM is way better. The strength of your KDF is limited by the user experience. If you want to limit an attacker to trying one password per minute, then you have to make the user wait one minute to log in, if you assume the attacker only has the same amount of computing power you do. A TPM can just say “you have ten tries and that’s it.” Of course, a real TPM won’t be perfect. But there’s no reason you can’t do both. Have the TPM use a good KDF and then an attacker has to break both.

cafeinux 2y ago
When will come the day a TPM deletes itself after n wrong passwords, we will feel a great disturbance in the force, as if millions of IT support voices suddenly cry out in terror, and are suddenly hit with "my cat walked on my keyboard".