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Comment by cronos | Hacker News Reader

Windows uses TPM for Bitlocker. A very common scenario where TPMs get reset is BIOS updates (when a TPM is implemented in firmware). AFAIK, Windows cheats here because it also manages BIOS updates. When an update happens, it takes extra steps to preserve the Bitlocker encryption key in plaintext, and re-seals it to the TPM after the update completes.

Apart from Windows, there are many setups that fail in fun ways: Kubernetes pods that migrate from one VM with a TPM to another one, hypervisors that mount a virtual TPM to VMs, containers or VM images that do Tailscale registration on one machine and then get replicated to others, etc.

Tailscale already did some attempts at cleverness when deciding whether to enable features using a TPM (e.g. probing for TPM health/version on startup, disabling node state encryption on Kubernetes pods), but there was still a long tail of edge cases.

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> Bitlocker encryption key in plaintext

Actually, this is not the case. BitLocker wraps the key, meaning even if the TPM were compromised, one would still have to brute-force the PIN for the actual key. It’s cryptsetup on Linux that stores the key on the TPM in plaintext. This vulnerability has been known for quite a while and nothing has been done about it so far.

https://arxiv.org/abs/2304.14717

https://github.com/systemd/systemd/issues/37386

https://github.com/systemd/systemd/pull/27502

> Windows cheats here

Slightly off-topic: it also cheats in how TPM works for Bitlocker when you do TPM + PIN. One would assume PIN becomes part of the encryption key, but in reality, it's just used as the auth for TPM to release the key. So while it sounds like a two-factor solution, in reality it's just single factor.

So the Bitlocker without TPM is actually a better idea and Windows makes it very painful to do if TPM is on.

I don’t know much about the TPM but if it’s anything like Apple’s Secure Enclave, it should require exponentially longer time after each incorrect PIN past the first one, making it so you can’t reasonably brute force it without getting lucky.

I’m not sure how the typical “two factor” best practices would interpret one of the factors basically self destructing after 10 guesses, but IMO it’s a pretty decent system if done right.

That's not the issue. The TPM isn't blinded in the above description meaning that if someone cracks the TPM they can get your key. Ideally both factors are always required to access the secret.

If you're wondering, yes this is a security issue in practice. There have been TPM vulnerabilities in the past that enabled exfiltration of secrets.

Aren't PINs usually short, and might even be really be made out of just digits in the first place? So would there be real security benefits in adding that to the key?

You can make PINs as complex as you want, there's only a maximum length limitation of 20 characters. There's no difference between passwords and PINs in Windows except that Windows calls it a PIN if it's used with TPM. And yes, it does nudge you in the direction of making it simple because "TPM guarantees security", but you don't have to.

> Windows cheats here because it also manages BIOS updates

Is this (relatively) new?

I don't use TPM and I rarely update BIOS unless I really need to, but I thought there was an option on my BIOS/UEFI to use USB drive to update it. How would Windows know about it?

Window can get BIOS updates through windows update, if the OEM participates and packages them. I haven't seen BIOS updates through windows update on my systems where I built it from components, I've only seen it on integrated systems from major builders (HP, Lenovo, etc).

The BIOS update instructions for my retail packaged motherboard indicate to turn off BitLocker before doing upgrades to prevent loss of TPM turning into a loss of access, but it'd be easier if it were automated.

You can update with a USB drive, but if you have bitlocker enabled and don't temporarily disable it before the BIOS update, you'll need to reformat and reinstall Windows.

No, you can save a recovery key to a file or enter it from a printed one.

I believe you can also get it from your online Microsoft account if that's what you logged in with once. I ran into this a while ago and had to do it that way. I didn't even know I'd set up Bitlocker.

On Windows, certificates can also be stored in the TPM.

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