Yeah, but the checksums are GPG signed, so you can't change the ISO and then create a valid signature for the new checksum without also having the GPG private k

Yeah, but the checksums are GPG signed, so you can't change the ISO and then create a valid signature for the new checksum without also having the GPG private key matching the public key you use to check the signature. Having said that, it seems keyserver.ubuntu.com is happy enough to allow connections via http instead of https, so there's a valid avenue to serve up a bogus public key... That's a much smaller window of opportunity since if you're already a Ubuntu user you'll have a pre-existing copy of ubuntu-archive-keyring.gpg already, rather than trying to download a possibly mitm-ed public key at the same time as you download the ISO. But I must admit I boggled a little bit when I saw that keyserver.ubuntu.com happily server their public key over http instead of just https...