The system will prompt you for the block devices listed in
/etc/crypttab. See "man crypttab"
That seems the best for me. The most difficult was to find what to
write in /etc/crypttab but I managed to do that looking at
/dev/disk/by-uuid. I have now 3 lines like this one:
luks-${UUID} UUID=${UUID} none
where ${UUID} is what I found in /dev/disk/by-uuid that points to a
physical partition /dev/sdX1
The good surprise is that if all the encrypted partitions have the
same passphrase, it is asked only once! but if you make an error the
first time then, you have to type it for all of your partitions!
Thanks for your help,
F