I have setup my fedora to use LUKS encryoted partitions. But entering two passwords gets quite tiring, as I shutdown my laptop quite often to get the benefit of LUKS (I am assuming nothing is encrypted when in suspend, please correctme if I am wrong)
I am thinking about setting up TPM autodecrypt. However, I was wondering does the decryption happen on boot or after I login?
If it happens on boot, then it seems like the benefit is pretty limited compare to a unencrypted drive. Since the attacker can simply boot my laptop and get the unecrypted drive.
Am I missing something here? I was wondering is there a way for me to enter my password once and unlock everything, from disk to gnome keyring?
The idea behind TPM-locked boot is that you can boot into your system unattended, but it stops booting into any other system. Typically no password is needed, but you can also assign an additional (non-user) password if you want.
This is nice if you trust your system to be basically secure. Nothing else can access its filesystems, so no external tool can be used to break into it. Rescue disks can not access any data without knowing a special rescue key – so make sure to set one up! A nice side effiect is that the key is only available while setting up disks in the initrd and totally inaccessible at any other time. That makes it very hard to extract the password once the system is running.
You can encrypt the home directories of users using other services like systemd-homed. That will prevent anyone from accessing any data in the user’s directory while that user is logged out. Homed will basically use your password to unlock your disk and if that works, then the password is accepted. So you do not need that user to be listed in the traditional /etc/passwd file, which is useful as you can just copy the users homedir image file onto another system to move a user account over.
The TPM releases the key to the OS at boot time. Without that, there would be no way for the OS to load (assuming the root FS is encrypted).
The key is bound to PCRs in the TPM, which control under what conditions the key can be released. For example, it can be tied to secure boot, bios settings, etc.
The data on disk doesn’t get decrypted at any time. Even if they boot it, they would still need to log in somehow.
There are attacks through DMA or extracting the decryption key from RAM, but those are not going to happen by a casual laptop thief.
…or extracting the decryption key from RAM.
Moral of the story:
Always shutdown the computer when you are not using it, kids.
I see your decryption key extraction and offer you a 5 dollar wrench.
The wrench also comes with DMA (direct mechanical assault), RDMA (remote direct mechanical assault via throwing), and DDIO (deals damage if opposing) capabilities. It’s a real NSA bargain!
But if the key is fully wrench-safe inside the TPM. You do not know it, you can not get convinced to give it up – even after repeated wrench use.
Of course the recovery key that typically goes with it and you logging password is not wrench safe, so that does not protect the system fully, while getting you a matching set of broken kneecaps.
Pretty sure the NSA doesn’t want the recovery key, they want the information the recovery key is protecting.
you don’t know it, you can not get convinced to give it up
It’s ok that you don’t know it, just log into your computer please and we’ll take care of setting a new key. Or you can have a conversation with Mr Wrench again.
on my laptop I have tpm to decrypt my drive and I’ve also enabled secure boot and set a bios password, so if someone steals my laptop it’s basically bricked,
secure boots there to prevent any potential tampering if someone were to take the drive out then put it back in
I feel this setup is secure enough for me, if you’ve got some nation state after you, or some guy with a wrench theres probably nothing you can do
I was wondering what security does the TPM password provides. If I understand correctly, it will prevent attacker from getting the data on the disk.
However, theoretically, attacker can boot the laptop and disconnect the disk to get a decrypted disk. Realistically, I think my laptop will shutdown when the chasis is opened (unless configured otherwise in the bios). So it should be safe?
However, theoretically, attacker can boot the laptop and disconnect the disk to get a decrypted disk.
This is not how this works. The bits are never decrypted on your disk / partition. If that was the case, a power loss would leave your device decrypted as well, and that is something a potential attacker might have control over.
What actually happens is that encrypted data is read from disk by the CPU, decrypted by the CPU and then written to RAM unencrypted. Unencrypted data should NEVER be written to non-volatile memory (the necessary exceptions, like the boot image, apply).
Oh! That makes much more sense! Thanks!
Then I guess there is not much point in encrypting both the full disk and the home dir together then (if I trust gnome login screen cannot be by-passed), since the data is always encrypted when they are on the disk.
In a single user context where the only user is also the administrator, full disk encryption has no disadvantages to home directory encryption AFAIK.
My laptop has an option to demand a bios password if a storage unit is removed and if the bottom of the laptop is removed.
Very useful.
I’m assuming the two passwords are the LUKS one and then your login, which you can remove by just configuring your login manager to do auto-login (no attacker will be able to access your data anyway if it’s encrypted, logged in or not).
Unfortunately, the gnome keyring will not be able to be unlocked in this way.
