There are two ways to "use a PIN".

Since there's a ton of misunderstanding in this thread, I'm going to go into how disk encryption works conceptually.

First, there's a symmetric key to encrypt blocks on the disk. Since you want to be able to change your unlocking password/mechanism without re-encrypting everything on the disk, this has nothing to do with unlocking the disk. This is what you want to get BY unlocking the disk. Let's call this the "data encryption key".

Then, there's something you use to encrypt the data encryption key. Let's call this the "key encryption key" (abbreviated KEK from here on in).

When you use a TPM, the KEK is stored inside the TPM. When you use a TPM PIN, the TPM refuses to release the KEK for use by the OS unless that PIN is provided.

You could say "why not make the KEK be a hash-mixed combination of a PIN and something inside the TPM?". One could do that! But that's not how Bitlocker works. There is a reason it doesn't work that way: the TPM is supposed to let company admins in charge of the device access it even if the original PIN is forgotten, by using other policies letting them get at the KEK. I personally set my own devices up such that the passphrase IS part of the KEK itself.

Interestingly, LUKS does not have a composite key mode natively that lets you combine a password with TPM material, but there are some good reasons not to use JUST a password:

1. The strength of your disk encryption reduces to the strength of the password, where a TPM can have a 256-bit truly random key

2. If someone keylogs the password, or tricks you into disclosing it, they can later decrypt your drive from anywhere, where a TPM binds the attack to those with posession of the TPM

3. There is no protection against brute force attacks (rate limiting), where a TPM does - or tries to - impose a rate limit

Now, let's go on to what YellowKey attacks.

A TPM can have inside itself "registers", called PCRs. These PCRs can be updated but not reset - think of it like you can add numbers to them but not subtract, and they only go back to zero when you reboot.

Using a passwordless encrypted boot, the TPM is configured to only release the key when the PCRs are in the exact correct state. As the OS boots it adds numbers to those PCRs. If you boot "the wrong" software, the numbers in those registers won't match the expectations, and you cannot unlock the disk.

Speculation on my part: the reason there's an exploit here is that the Windows Recovery Environment apparently can match the PCR values for the booted OS, causing the TPM to release the key, but WinRE doesn't require you to get your password right before it gives you access to the data. So far as I know, protecting the TPM key with a PIN would mitigate this issue, but it's still bad.

Or maybe the exploit actually does something inside the TPM itself, causing it to unconditionally release the key even when protected by a PIN: that would be even worse, but **NOT*** a problem with Windows. That would be a problem with the TPM.

Microsoft BitLocker – YellowKey zero-day exploit