In other words, if I pointpkgat my chosen third-party and it downloads a package, are there mechanisms in place to confirm that the package is genuine? I can see the mirror provides a digests.txz file but a misbehaving mirror could potentially serve a bad package and a deliberately-crafted digests.txz, and I would be none the wiser.
However, on reflection, I presume this is handled for me bypkg-- if the packages came from a true mirror of pkg.freebsd.org,pkgis going to install these without issue; and if they were built by an independent ports-mgmt/poudriere, I would have to explicitly configure trusting a different key.
Are these package repositories using signing? Do they provide the public key to the packages? It's not mandatory. We have public repositories of packages with no key signing , ie 'signature-type: none' and private server repository that's signed, ie 'signature-type: pubkey'.
There's also checksums on packages to help verify authenticity and integrity.
Nothing's guarantee, though.