The recommended way to find out which public key you need to verify the Ubuntu hashes is to attempt to check a signature of the hash file (both of which were downloaded in the clear). This produces an error indicating which key was used to sign the hashes:
gpg: Signature made Thu Apr 5 22:19:36 2018 EDT using DSA key ID FBB75451 gpg: Can't check signature: No public key gpg: Signature made Thu Apr 5 22:19:36 2018 EDT using RSA key ID EFE21092 gpg: Can't check signature: No public key
My question is: if everything is downloaded in cleartext and an attacker can modify the ISO and the hash file, couldn't they also sign the hash file with their own public key and then publish that key to the key server? I would download their signature which would error out their key which I would naively download from the key server. Don't I need to at least have secure access to the key ID for their public key?
How can I securely download lubuntu if I don't already have the Ubuntu signing keys on my machine?