One of my internal harddisks is encrypted with LUKS and used for backups only. I have an entry in fstab with the noauto option. After login I can decrypt and mount this volume with one click in the GUI Files application very easily. However I often forget this click and then don't have backups running. Can this be done automatically after login with a script? Cryptsetup with a keyfile can work, but requires sudo and thus another manual PW entry. /etc/crypttab is processed too early during booting. Any other way to automatically decrypt? (I'm on Ubuntu 18.04)
The Arch Wiki gives a pretty good overview:
As long as the keyfile is accessible during boot time, it should be possible to do what you want to do. I don't know what you mean by "/etc/crypttab is processed too early during booting" - I unlock my home partition just fine as described in the link.
Then of course you have to ask yourself what use an encrypted partition is that is unlocked on booting without entering any password. Probably you will want to put the keyfile on a removable thumbdrive.