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Comment by candiddevmike | Hacker News Reader

What's the point of TPM-backed full disk encryption with no usability impact (meaning password/pin-less) for the average user who is more likely to get their device stolen vs some covert disk image shenanigan?

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If the device is stolen it can still enforce OS-level authentication (including potentially phoning home, invalidating its access to remote resources, or erasing itself), except now you can't bypass it by rebooting and running chntpw.

Will this stop a dedicated attacker? Probably not, although a fTPM with an up to date OS would require the attacker to find an exploit for this machine's early boot firmware (UEFI, etc) or burn a Windows zero-day, both of which are very costly.

It does however prevent your casual thief from watching a YT video "how to reset windows password using linux live cd" and then getting access to your sensitive data (browser's saved passwords, etc), so it's a major improvement.

I prefer to require entering a master password on boot manually and then configuring the OS to auto login to my non root user (with a different password than the disk). The longer and more complex your dependency chain for security, the more opportunity for it to be compromised. The encrypted “password on boot” partition then contains the keys to mount the other disks.

I’d really like Apple’s model on my machine where the root image is just the stock OS image unencrypted and the co-processor owns the responsibility of managing IO (and done efficiently) using my master key. TPM seems like it misses the mark from that perspective.

Using a decryption password on boot is less secure than TPM + measured boot/secure boot. Specifically, it’s vulnerable to a two-touch attack. In the first touch, the attacker replaces your bootloader with one that looks identical but steals your password. On the second touch, they now use the password to steal your data.

If the attacker can install a custom boot loader the system is already defective by design.

If the attacker can replace your bootloader, why can't they just get the decryption key from the kernel later? And if you did have Secure Boot, then using a password with encryption at rest is just as secure : you can't change the bootloader and you can't change the OS (since it's encrypted), so you can't exfiltrate the password. The end result is that the TPM doesn't have a practical benefit.

The bit about "two touches" seems to imply physical access, so in absence of TPM the attacker can replace your bootloader with little effort vs with TPM they'd need to break TPM.

You can fix this by asking for the password before letting the attacker replace the bootloader.

Sorry, I missed the bit about Secure Boot.

Yes, with Secure Boot and password your data is safe. But you have to type the password to boot your system, which is impractical for remote and headless systems, or even local systems that need to be available remotely.

You would still use the TPM to verify the software chain. But don’t use the TPM to Auto unlock disks. That’s the part that feels like a bad idea

The issue is that data disks and system disks get conflated. For the system disk (anything outside of /home) you generally only care about signing - which FDE does as a side-effect. Each user should have their own disk/partition/subvolume with a distinct key that is retrieved from the PAM.

This achieves two things: I know that I am typing my password into the OS that I or a trusted third party compiled (not one planted by a hacker), and my home directory gets decrypted as part of my normal login routine.

An attacker still needs to use some kind of semi-advanced attack in the boot chain or DMA to steal the user's data, instead of just plugging in a LiveUSB and going to town.

Yes, there are a lot of vulnerabilities in the Secure Boot process on most devices, because the surface area is huge, but the attacker still needs _some sort_ of vulnerability to gain a foothold.

I agree with the frustration in the gist - Secure Boot and TPM-sealed disk encryption aren't nearly as good as they could be, because the surface area is gigantic and sure to get exploited. But this is a classic Security Nerd vs Reality scenario: while it is absolutely _possible_ to pwn Secure Boot + TPM-sealed encryption in almost any scenario, using it still makes it _much harder_ for an attacker to do so, and most will give up.

For the typical user, losing their data is a greater risk than someone with physical control over their machine being able to access it. The logic board in your computer fails or you forget your password and all your data is gone.

And the default way of mitigating it is an even worse security risk. Now all your data is on some cloud somewhere, waiting for that vendor to get breached or your account to get phished which is now possible without physical control over your device. Plus, if you couldn't get into your computer because you lost access to your account, you also lost access to the data in the cloud.

Whereas if you really do have sensitive data, you still don't need a TPM and get better security without one. You keep a Yubikey in your pocket or memorize a strong passphrase and then the key physically isn't stored on your device.

If your data is this valuable, you certainly do backups? I suppose something like cloud backups is now built-in into windows, and would save your Documents (and maybe more) also by default.

We're talking about ordinary people here. Their data is valuable to them because it's their pictures of their grandkids and their draft of the Great American Novel and their recipe collection. They're not backing it up themselves, they don't even know how.

But it's also their copy of all their bank statements that include their routing number, which nobody who is physically in their house is going to use against them but is a serious fraud risk if it can be accessed remotely on some cloud.

Windows backups are subpoenable by half the governments on the planet, who have bad actors in them, and may also have exploits for dedicated attackers because they present a huge target.

If your threat model includes state-level actors, I wonder why you consider running Windows at all, or at least not in a highly secured transient VM.

I hate this. People are claiming "state-level" actors are all the same. Microsoft backups are subpoenable by local cops, hell, by your ex-wife in a divorce proceeding in some jurisdictions.

Yes, if the NSA has a decent reason to think you're going to nuke a sports game you'll still have a problem with very, very good security measures.

That doesn't mean there isn't a very large in-between zone where you're fine with better security measures.

You can store the full disk encryption key in the TPM and rate-limit PIN attempts using its secure non-volatile storage, as far as I know. That's very useful in case of loss/theft, given that users don't like typing long passwords or PINs for every login.

I'm not sure if this is what Windows actually does, though, or if the TPM just hands over the disk encryption key after Windows passes system attestation and then verifies the screen unlock PIN/password in software – that would be significantly less secure.

Why go pin less? I like the fact that the TPM can restrict retry counts and I do not need a rediculously long password.

The only thing I do not get is why this is not done by a simple SIM card like in any mobile phone. Then one could choose a TPM. Even more: I do not get why I cannot encrypt my android phone with my SIM card.

I have many machines, some headless.

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