Comment by jjmarr
3 hours ago
Because the TPM effectively rate limits brute forcing of the key.
https://learn.microsoft.com/en-us/windows/security/hardware-...
> For example, when BitLocker is used with a TPM + PIN configuration, the number of PIN guesses is limited over time. A TPM 2.0 in this example could be configured to allow only 32 PIN guesses immediately, and then only one more guess every two hours. This totals a maximum of about 4,415 guesses per year. If the PIN is four digits, all 9999 possible PIN combinations could be attempted in a little over two years.
In that case, the median would still be just over a month, if the PINs were entered in order of how commonly they are used. Even the worst case of two years is still soon enough for a lot of data still be useful.
Also, how is the time limit enforced? With hardware access, it would be easy to change time or increase the clock rate, as well as many other side-channel attacks that could eliminate the wait altogether.
Most enterprises require a 12 digit code, to meet a specific security standard. Bruteforcing that, with hardware access restricted by TPM, would take a very, very long time.
You're also not restricted to 4 digits. A full passphrase is an option.
The time limit is enforced by the TPM itself which defends against tampering.
> the TPM effectively rate limits
I had a friend working at trusted compute at Microsoft, and he had so many stories.
These TPM firmwares are often written by shitty companies that have no fxcking clue what they are doing.
Most TPM implementations are a clown show, companies just want to check a box on paper so they say "look! We have a TPM!" and move on.