Debian:
-------

See: https://ftp-master.debian.org/keys.html

gpg --no-options  --trust-model always --no-default-keyring --secret-keyring /dev/null --keyring ./debian-<codename>.gpg --recv-keys <key-id-from-above>



Ubuntu:
-------

get keys from installed system: apt-key list --fingerprint

gpg --no-options  --trust-model always --no-default-keyring --secret-keyring /dev/null --keyring ./ubuntu-archive.gpg --recv-keys <key-id-from-above>



Openwrt
-------

See: https://openwrt.org/docs/guide-user/security/signatures

gpg --no-options  --trust-model always --no-default-keyring --secret-keyring /dev/null --keyring ./openwrt-<yy.mm>.gpg --import  < (follow download link)