"This device needs to be reencrypted"

Hardware Librem 5
1

Because of a misconfigured flashing station, some of the recently shipped Librem 5s and Liberty Phones may have ended up being flashed before shipment with an OS image that’s effectively unencrypted. The device would still appear encrypted - it would ask for encryption password as boot and you could change that password - but the actual data on disk would remain plaintext.

A PureOS update to check for and remedy this is about to migrate into crimson repositories. If your device happens to be affected, in a few days you should see this screen after performing system update right after entering your disk encryption password:

IMG_5204
IMG_52041920×2560 689 KB

Don’t panic, this is a legit message coming from PureOS.

After typing your disk encryption password in again, it will reencrypt the entire contents of the disk. On a Librem 5 with 32GB eMMC it should take about 15-20 minutes; on a Liberty Phone with 128GB eMMC it will take about 60-75 minutes. Once started, the process can’t be stopped until it’s finished, so make sure to do it when you won’t need to use the phone during this time. If you’d rather perform this later, you can skip it until the next boot by entering an empty password.

If you don’t want to wait for the update to arrive or want to perform online reencryption instead, you can run sudo cryptsetup reencrypt /dev/mmcblk0p2 on your phone. At first sudo will ask you for your user password (the one that’s typed in on the lockscreen), and then cryptsetup will ask for the disk encryption passphrase (the one typed in at boot). Online reencryption may take even longer, but it will let you use the phone during the process.

Note that this issue only affects the image that has been installed out-of-the-box on some phones that have shipped with crimson. Phones that shipped with byzantium are unaffected; neither are phones that have ever been reflashed by the user after shipment.

You can check whether your device is currently affected by running sudo cryptsetup luksDump /dev/mmcblk0p2 | grep cipher. If the output is cipher: cipher_null-ecb, your device needs to be reencrypted - you can either perform online reencryption or wait for the update to arrive shortly. If it’s something else, such as cipher: aes-xts-plain64, your device isn’t affected and you should not see the reencryption dialog after the upgrade.

2

For anybody who used the same luks password for a long time, but is unaffected, can we reencrypt once annually this way to rotate keys or does it only work on the unencrypted configuration (specific case of issue)?

3

You can perform reencryption using the mentioned command whenever you want, just be aware that it rewrites the entire disk so you may want to not wear your eMMC out too much with frequent reencryptions.

4

reencrypt isn’t about the LUKS unlock passphrase (the thing that you type) though. It is about the underlying disk encryption master key (aka the volume key), which will have been randomly generated as a one-off and is a value that you will never see unless you specifically use cryptsetup luksDump ... (but see the warnings in the man page about doing that).

If you use the same LUKS unlock passphrase “forever” and you have reason to believe that that passphrase has been compromised then reencrypt will achieve very little - because being in possession of the unchanged LUKS unlock passphrase and the phone itself allows you to obtain the current disk encryption master key, no matter how many times you reencrypt.

Hypothetically speaking in the scenario of the previous paragraph, you would have to change both the LUKS unlock passphrase and the disk encryption master key (the latter using reencrypt, the former using luksChangeKey).

However the disk encryption master key may not be relevant at all if using the “null” cipher. :open_mouth: