Ubuntu 20.04 alternate gdm-login (screensaver unlock password)

gistfile1.txt

# apt install db5.3-util whois
# mkdir /var/local/screenpass
# chown root:shadow /var/local/screenpass
# chmod 2750 /var/local/screenpass
$ mkpasswd -m des
# db5.3_load -h /var/local/screenpass -t hash -T passwd.db
[user]
[despass]
^D
# cp /etc/pam.d/gdm-password /etc/pam.d/_gdm-password

Replace "@include common-auth" in /etc/pam.d/gdm-password with:
auth    [success=1 default=ignore]      pam_userdb.so crypt=crypt db=/var/local/screenpass/passwd
auth    requisite                       pam_deny.so
auth    required                        pam_permit.so