I’m not sure where this is coming from??
LUKS on the Librem 5 is perfectly happy with a LUKS password that contains ASCII letters (uppercase and lowercase) and ASCII digits and ASCII punctuation characters.
I know this for a fact because I have intentionally used a LUKS password that contains at least one ASCII character from each of those 4 subsets. I have zero problems to enter the LUKS password during boot.
I guess the safe approach is to
- Start out with an all digits LUKS password (preferably keeping your phone off the internet, just in case).
- Finish setting up your phone.
- Use Jumpdrive to make an image of the phone’s disk. (All users should know how to do this and all users should do this, regardless of LUKS password issues.)
- Change the LUKS password to use any characters from the set of printable ASCII characters (longer and stronger).
- Reboot the phone to confirm that you can successfully get past the LUKS password entry.
That may not be possible because you can customise the keyboard that you use once you have logged in to the phone. I mean I guess with enough work this could be done - but if you just tell people to use printable ASCII only and you give people exactly one LUKS keyboard that can generate any printable ASCII character then that is good enough.
I believe that the keyboard is from package osk-sdl and that it is therefore not created by Purism and is more or less part of LUKS (i.e. in any case a standard Debian package).