@gomp Why would you be taking the signature from the same website? Ever heard of PGP key servers?
That would be “a pre-shared trusted signature to check against”, and is seldom available (in the real world where people live - yes, there are imaginary/ideal worlds where PGP is widespread and widely used) :)
@gomp You mean, as seldom available as every apt install ever? https://superuser.com/a/990153
My bad for causing confusion: when I wrote “trusted signature” I should have said “trusted public key”.
The signatures in an apt repo need to be verified with some public key (you can think of signatures as hashes encrypted with some private key).
For the software you install from your distro’s “official” repo, that key came with the .iso back when you installed your system with (it may have been updated afterwards, but that’s beyond the point here).
When you install from third-party repos, you have to manually trust the key (IIRC in Ubuntu it’s something like curl <some-url> | sudo apt-key add -?). So, this key must be pre-shared (you usually get it from the dev’s website) and trusted.
