But what is the threat model where a TPM-stored LUKS key protects against?
I fail to see it. It doesn’t protect against any physical attacks at all, and it also doesn’t protect against attacks while the system is powered on.
It seems to only protect against the attack of someone stealing your hard drive out of your system but leaving the entire system behind. Maybe for encrypting extra data drives that will want to be resold later? But that is more a datacenter threat model
Aside from the full scenarios provided in the other comments, there is another, more simple reason to still prefer it: your daily workflow is not intercepted, but if your disk dies, you can throw it out and replace it without a second thought, since it was encrypted all the time.
It does protect against physical attacks. PCRs are used to tie keys to specific hardware and software configurations and versions, boot paths, kernel command line arguments, etc and will lock out if changed. One of the reasons Ubuntu waited so long for official support was to set up the infrastructure for unified kernels and signing, the kernel and initrd are unified and signed and verified before it will unlock to protect against sophisticated attacks that most people will never encounter. For most people worried about theft, having it lock out when the boot order is changed would be enough. And when running, brute forcing the login process is slow and can be made even more painful with lockouts.
The TPM functions very differently than putting keys on a permanently attached usb drive.
Let’s say you have LUKS with TPM enabled. Verified or Safe Boot is also on, so someone cannot access your decrypted files just with some live usb distro. And let’s say you also have a password set for changing BIOS/UEFI settings so the attacker cannot disable safe boot without a password. So the attacker might want to steal the hard drive in this case to get the data, but that’s also not possible because the hard drive is encrypted and the key is in the TPM. The way TPM also checks for integrity makes the system tamper-proof. So any attempt at tampering with the hardware will trigger the tamper protection and ask the user for a recovery passphrase to let the TPM hand over the decryption key again. So actually TPM is useless without safe boot and without a password for the user account in your OS. The only thing protecting your data from the attacker in this case is your user password… unless the user has one of these raspberry pi pico devices that are programmed to take the TPM keys sent to the CPU at boot in plaintext. Though I am pretty sure this is also fixed or will be fixed in the future. Maybe public key cryptography could help. I haven’t researched that yet.
TL;DR: TPM is pretty useless without safe boot, but is pretty decent with safe boot. TPM is not the securest thing in the world but for a regular user it’s probably the best choice without sacrificing convenience.
Just wanted to add that your BIOS password can be circumvented by taking out the CMOS battery. That will clear all your settings and allow unrestricted access. A BIOS password should absolutely never be used as a form of security, it is trivial to bypass.
Granted, I don’t believe that the TPM will give the key if secure boot were disabled, I just wanted to mention that BIOS passwords don’t do anything against any real attack.
I also want to add that the TPM will request the recovery key if the BIOS goes back to factory defaults. I also think changing the secure boot setting might trigger it. If that’s the case then a BIOS password is pretty useless.
I believe that the TPM will refuse to provide keys after secure boot is disabled, but I didn’t intend to imply that it could be used to bypass TPM decryption or anything. Just as an aside that BIOS passwords are effectively useless at preventing access to the BIOS.
The threat ia that someone who uses a shitty password and then their laptop gets taken by a sophisticated actor that could crack their stupid password in an hour without the TPM wiping itself after X failed attempts.
The solution, of course, is to not use shitty passwords
You still have a scenario where the encryption key is still on your physical device, LUKS with or without TPM, or … some kind of TPM based Linux encryption solution I have never heard of?
Does Windows Secure Boot work on Linux via the TPM?
No…
Am I missing something?
Theres no point in involving TPM in securing a linux computer.
In a scenario where you’ve physically lost your computer, using TPM or not it wont matter if your pc gets into the hands of someone who can attempt to brute force the keys.
If your pc is remotely compromised to the point it has something on it that can grab your keys, it also will not matter if you are using TPM in some way.
The only practical use of full disk encryption is if your linux pc and or laptop gets stolen and falls into the hands of a non tech savvy person, and in that scenario, going through the trouble of correctly binding LUKS to TPM will have just been a waste of time.
Thus, you should probably just use LUKS and not bother routing it through TPM.
It’s not a new feature, it’s convenient and also has use cases outside of convenience (it’s also generally going to make stronger keys than any passphrase). Here is one way that has existed for years, except Ubuntu specifically patches it out: www.freedesktop.org/…/systemd-cryptenroll.html
It’s not a lot of work, it’s one command and a one word update in the crypttab.
Secure boot is generally a requirement to use the TPM.
This person asked if they can make PopOS secure via TPM.
I am saying that while yes, you can, there isnt much point, because setting up LUKS to work with TPM is inconvenient, easy to fuck up, and basically offers no additional protection against all but extremely implausible security scenarios for basically everyone other than bladed server room admins worried about corporate espionage who are for some reason running bare metal PopOS on their server racks.
Like the only actual use case I can see for this is /maybe/ having a LUKS encrypted portable backup drive, but even then you can still base the encryption key in the actual main pc’s harddrive without using tpm, though at /that and only that point/ are we approaching parity between the difficulty of using or not using tpm to accomplish this.
You don’t seem to understand how TPM works at all. You cannot extract keys from the TPM, it provides protection against any attack that involves removing the hard drive from the computer it is installed in. This is not like storing an encryption key on a USB drive, as you seem to think. I recommend you actually do some reading on TPM before you attempt to talk with any authority. I don’t personally think it’s a great solution (for me, at least), but not for any of the reasons you’ve listed in your comments.
LUKS encrypted portable backup drive
You can’t use TPM-based encryption on a portable drive, that isn’t even possible. That’s exactly the point of TPM to begin with. You know, the whole Trusted Platform Module? That exists to ensure your hard drive (or whatever other use you have for the TPM) cannot boot or be read by any machine other than the one it was set up with. That’s the entire premise of establishing a root of trust. What are you on about?
You didn’t know you could use it 30 minutes ago. It seems like you don’t know how it’s set up, what protection it does or does not offer, what the use cases might be, nor where any vulnerabilities may be. I’m wondering why you remain actively involved in the conversation with an opinion rather than sitting back and learning something new.
It offers convenience of not putting in an encryption passphrase at every boot, with reasonable security against a lost or stolen machine that nobody can just boot up a live usb and access the data. Its end-user behavior is like every other consumer operating system.
I think it even increases the security by not asking for the passphrase. Because the moment it asks, you know your machine has been tampered with and that you should be alert.
TPM isn’t an encryption algorithm. TPM just holds the decryption key (in my case the LUKS decryption key) and hands it to the CPU if all checks pass for convenience. No key is stored in the storage in plaintext. TPM isn’t the most secure thing but at least its better than nothing at all.
The point is to have the system automatically unlock without the need for a boot password. This provides decent security if secure boot is enabled, but requires very little from the user. It isn’t a stopper for high threats, but a simple theft will mean the data is safe. It also ensures that if the drive is separated from the host machine, it is useless without a copy of they key. It doesn’t stop all threats, but stops a lot of them, and all of the most common.
Oh ok so the use case here is if this casual linux user asking this question has only their harddrive stolen from their pc or their laptop in their home or apartment or workplace, not their whole pc.
Mhm that seems likely.
I guess this maybe makes sense if youre running like a server room, but chances are low thats the actual context of this question.
Why would you run PopOS on a large operation’s servers?
While i am personally also not a huge fan of TPM for FDE it is still a valid use. Why? In order to access data on the disk you would still need to bypass the login screen which is non trivial. Also another use case is encrypting the drive so when you sell it or dispose of it you do not need to worry about wiping it at least once to get rid of all data.
TPM has its weaknesses but pls don’t talk down to someone who wants to use it when you do not understand his use case.
It should stop issues with full device theft as well, if done correctly, because if secure boot isn’t on and working, it will refuse to give the key. Which means, if it was setup correctly, the computer cannot be accessed without know the users name and password. This is the general accepted stack for Microsoft’s BitLocker. It becomes completely transparent to the user, but puts a decent blocker to access in cases of theft. There are ways around it like freezing RAM or packet sniffing an external TPM, but those are high level attacks.
If the TPM is not integrated in the CPU and rather a separate Chip on the MB, the communication can be easily sniffed since it’s not encrypted. See here youtu.be/wTl4vEednkQ?si=26A0NK-cVtP3uKgk
Shown how cheap it is i would not say it is high level.
Nobody thinks it needs to be involved. They want it involved so the drive is automatically unlocked at boot, but inaccessible if someone removes it from the machine to try and bypass login (and in the future, if someone tries tampering). Especially useful in machines you want useable without being physically present.
It’s not cajoling anything, it’s a built in feature you configure, although Ubuntu currently goes out of their way to remove the support from some tools.
Note that, as I understand it, using TPM will only protect data on your encrypted disk if it is removed from your computer. If someone steals your entire computer, the disk will be decrypted on boot.
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