Comment by pregnenolone | Hacker News Reader

Comment by pregnenolone | Hacker News Reader

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> Since there's a ton of misunderstanding in this thread

True. It's unfortunate, amd a lot of false information being spread there.

> the KEK is stored inside the TPM

That's not how it works. The KEK is not stored inside the TPM, but encrypted/decrypted by the TPM.

> You could say "why not make the KEK be a hash-mixed combination of a PIN and something inside the TPM?".

Bitlocker does that. Cryptenroll doesn't (https://github.com/systemd/systemd/pull/27502), which is bad but has not been fixed.

TPMs are a nice idea, but there are a few problems:

- The KEK should also depend on the PIN. Cryptpentroll does not do this at all and Bitlocker limits the PIN to 20 characters.

- There are various manufacturers of TPMs and all of them have different implementations. Some of them had been broken in the past, which is why it's important to make secrets PIN-dependent.

I seriously doubt the author found a way to bypass PIN protected setups in general. This should only be possible in combination with a vendor/model specific vulnerability. Maybe an fTPM?

As of this moment, I would rather look at it as a convenience feature. A high entropy password + a proper KDF (not possible on Windows) like scrypt or argon2 is the better choice. Encryption should be handled by SoC engines like on Macs, iPhones or some Android phones to mitigate other attacks and preserve performance anyway. Panther Lake CPUs with vPro support do on Windows.

> That's not how it works. The KEK is not stored inside the TPM, but encrypted/decrypted by the TPM.

No, the KEK is stored inside the TPM, and the DEK is decrypted by it. If you have three layers (two KEKs, one encrypted with the other) then sure. But at the end of the day one private key is actually stored inside the TPM, taking up as much space as the length of the key.

To be specific, Bitlocker uses a stored key, not a derived key.

> Bitlocker does that. Cryptenroll doesn't (https://github.com/systemd/systemd/pull/27502), which is bad but has not been fixed.

I don't think Bitlocker does that, because it's possible to set up admin PIN recovery techniques. That would be impossible if the PIN were part of the KEK.

> - The KEK should also depend on the PIN. Cryptpentroll does not do this at all and Bitlocker limits the PIN to 20 characters.

I agree.

> There are various manufacturers of TPMs and all of them have different implementations. Some of them had been broken in the past, which is why it's important to make secrets PIN-dependent.

I agree. Perhaps consider using a FIDO2 token (supported by cryptsetup) instead of a TPM. There are open-source implementations of FIDO2 and open-hardware ones too. There are even open-source implementations of FIDO2 where the key is in fact derived from the user's PIN (plus a stored secret). If you did that, you get the proper security properties even without cryptsetup mixing the two methods.

> I seriously doubt the author found a way to bypass PIN protected setups in general.

I agree - I think the author probably just found a flaw in the windows recovery environment and is talking up how a PIN only helps you if the attacker does not know your PIN (in other words, acting as if the PIN provides no threat resistance when really it provides an additional layer).

> As of this moment, I would rather look at it as a convenience feature. A high entropy password + a proper KDF (not possible on Windows) like scrypt or argon2 is the better choice. Encryption should be handled by SoC engines like on Macs, iPhones or some Android phones to mitigate other attacks and preserve performance anyway. Panther Lake CPUs with vPro support do on Windows.

I think the best you can do right now is to layer a password with a hardware device. I don't think saying that the hardware devices are flawed means they are not useful as PART OF the security setup. It would certainly be nice if the software did this automatically/easily and it's unclear to me why it does not.

https://blog.scrt.ch/2024/10/28/privilege-escalation-through...

https://post-cyberlabs.github.io/Offensive-security-publicat...

Yes, the PIN is entangled with the key material. The admin PIN recovery techniques all involve enrolling additional unlocking methods.

Besides this, I completely agree with this post, especially:

> I think the best you can do right now is to layer a password with a hardware device. I don't think saying that the hardware devices are flawed means they are not useful as PART OF the security setup. It would certainly be nice if the software did this automatically/easily and it's unclear to me why it does not.

I've always found it silly that Microsoft's default BitLocker implementation doesn't make it easy to do the obvious thing that FileVault does and use the user's password as an additional wrapper to encrypt the KEK. Yes, then there's trouble when the user forgets their password and password-change rotation is a little annoying, but if you're in an enterprise scenario you need backup keys anyway, and your users are still as likely to get BitLocker'd by PCRs changing (ie due to EFI updates) than forgetting a password anyway.

> No, the KEK is stored inside the TPM

No. Technically, TPMs can store secrets (limited due to little nvram), but no FDE implementation as far as I'm aware does this. TPMs wrap/encrypt the key and return an encrypted blob which will be stored in the header which will be decrypted upon request. The actual KEK is never stored on the TPM.

> I don't think Bitlocker does that

It does, which is why cryptsetup was much more affected by the faulTPM exploit: https://arxiv.org/abs/2304.14717

> Perhaps consider using a FIDO2 token (supported by cryptsetup) instead of a TPM. There are open-source implementations of FIDO2 and open-hardware ones too.

Not only do they do the same thing, they often use the same cheap chips. I don't see the benefit in using a separate device when you can have one SoC because there are too many downsides. Not only are dedicated devices vulnerable to sniffing and fault injection attacks, the extraction of secrets is actually feasible with the right kind of equipment. SoC solutions make this impossible (manufacturing levels are simply too small). Apple, Google, and Intel (Panther Lake) have solved this the right way. Open Source security can and should be shifted to PIN dependent key enrollment to protect against backdoored or flawed secure elements.

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