Because they’ll use it and it’s I that verify by decrypting it with my private key? If someone makes him use a bad key then it just won’t work.
Sure, but the first time the other person would have to accept your self signed cert. There is no knowing that the cert presented the first time was actually from you and not someone else.
How would that matter?
Say my website sends you my homemade cert, if you don’t use it you cant communicate with me (or go unsecure).
Why myst some “trusted entity” emit tjose certificates? They are just a bunch of RSA keys!
Yes, there is no difference from your homemade cert compared to anyone else’s homemade cert.
So if someone else claims to be your website and uses a similar homemade cert there is no way to know that the site isn’t yours.
How do they know you verified by decrypting?
The client has to be able to verify. They can’t trust your result. Imagine a man-in-the-middle attack; if someone intercepted all traffic between you and the client, including the cert exchange, how would either party figure out that traffic was being intercepted?
Client connects to website, but gets intercepted. Attacker provides own self-signed certificate to client. Client asks you to verify the certificate, but attacker can intercept that too and just reply the certificate is “totes cool bro just trust me”. You are none the wiser either, because the attacker can just decrypt client traffic and pretend they are the client by re-encrypting the data themselves.
With a Certificate Authority, the client can take the received cert and ask the CA “did you sign this?”. The CA will then tell you they didn’t, exposing the attacker’s fake cert. This works, because the CA is already a trusted entity. That trust is being extended to your website’s certificate validity and thus the website identity.
I see where you’re getting at, or so I think:
A malevolent user takes over my website and installs his non-authorised certificate => danger!
But I mean he can use my certificate, it’s already there, installed and set up to work?
If he fully takes over your website there’s nothing you can do as a client to detect it. But that’s not the point of the certificate. The certificate is there to ensure you are communicating with the website/server you think you’re communicating with.
It ensures your communication is safe. In my example, the attacker doesn’t take over your website, he takes over some part of the network infrastructure between your website and the client, thus intercepting all the traffic. There’s a “man in the middle”, e.g. the website is safe, the client is safe, but the communication between them is not. The certificate ensures nobody is impersonating the website by intercepting all the traffic, ensuring the communication.
If the website does get compromised, the CA has the option to invalidate the certificate at your request, via some verification procedure. Thus it also defends against compromised servers, though it’s not the primary purpose for which they exist.
