Did you read the rest of the comment? Specially the part below: > That's where CAs come into play [...] Unless you're somehow able to collect all signatures r

Did you read the rest of the comment? Specially the part below: > That's where CAs come into play [...] Unless you're somehow able to collect all signatures realiably without anyone MITMing while you collect the signatures over the internet. That's why CAs exist. They're a reliable and secure way to collect signatures. They're secure because they're authorities whose certificates are in your system and not sent over the wire. That's why they're preloaded in your web browser/OS, so they can't be tampered with in the first place. Why have CAs and not just signatures for domains? 1. The list of certificates for the whole internet is HUUUUGE. 2. Certificates change. 3. Certificates get revoked. 4. New certificates are issued. How could you update your certificate list over the wire making sure you're getting the correct certificates? Talking with a known authority, i.e. the CAs. Of course, it can still be tampered if you download certificates (or programs bundled with certificates) from untrusted sources and/or not check the signature before installing.