Monterey not mounting encrypted volumes

Apple Silicon Macs require that all executables are signed. When you run an unsigned executable, it is not allowed to run - and it will show up in the shell as "killed".

As you compiled the tool yourself, you have probably left out codesigning. You can do that after compiling by running this command in the Terminal:

codesign -s - /Library/PrivilegedHelperTools/au.com.openwall.BootUnlock/BootUnlock

The "-" means that it will perform an ad-hoc signing. The resulting signature does not use a specific identity and the program will only be able to run on your own local machine. If you have either a developer subscription with Apple, or you have created a free certificate with your Apple ID, you can use that for signing instead.

UPDATE:

I have found the source repository for the program in question, and can see that the BootUnlock is not a new, unique binary - but rather just a copy of the built-in /usr/bin/security program. As that program is known to work and is codesigned appropriately, it would be a good idea to change the script to simply call /usr/bin/security instead of BootUnlock.