Don't worry, you just don't understand how TLS works :-)
The CA never gets the private key. Instead they get a certificate signing request (CSR), which only contains the public key part. They sign that.
Oh, and then there is perfect forward secrecy, which basically means that even the servers private key is not the one used to encrypt the actual data (after the initial handshaking, and only for suitable cipher suites, subject to downgrade attacks).
Disclaimer: at least, thats how its properly done. Some CAs offer a "send us your cert and we'll sign it", and dumb people who shouldn't be admins use it because it's (slightly) easier to use.
But you got the conclusion right, the notion of CAs is problematic.
This is what kills CA security. Anyone at a employer with over 5 people in the IT dept probably has someone who can insert a CDROM but has no idea how to set up CA and SSL stuff installing intranet internal servers using https and a self signed cert.
So we're carefully raising a whole generation of users programmed to accept any self signed cert, after all "thats how the benefits website is at work" or "thats how the source code mgmt site is at work". Then they go home, and oddly enough their bank presents a new self signed cert, or at least they think its their bank, and much as they have to click thru 10 times a day at work, they click thru the same popup at home and then enter their uname pword and ...
Paradoxically as a budget weapon its excellent because you probably have good enough physical security at work and frankly its usually not something worth protecting anyway, but it is incredibly annoying so you can bring up at budget meetings that IT can't afford to fix the SSL cert errors on some meaningless server because they can't afford it, etc. Not technically true but J Random MBA managing something he knows nothing about, can't figure it out, so its a great budget weapon. Highly annoying but doesn't really hurt anything.
To fix this you'd need something like an enterprise programers union standard union contract rule that enterprise programmers will never, ever, ship enterprise software that allows a self signed key. Good luck defining enterprise software, I suppose.
And in the spirit of idiot proofing leads to better idiots, requiring no self signed keys means idiots will create their own root and train users to import any root they ever see anytime they see one. Then distribute a non-self signed key signed by the imaginary "Innitech CA services" root. What could possibly go wrong with training users to do that?
In the spirit of "idiot proofing leads to better idiots" of course that will not happen.
In fairness if you have a heterogeneous network of legacy windows, some macs for real work, legacy blackberry and both real smartphones, distributing it "everywhere" can get kinda hard.
Yes, but they can also do so if you use a self-signed certificate, by just self-signing their own. There's no way that's less secure than a CA-signed cert.
As far as I know, self-signed certs have to be approved on a case-by-case basis in most browsers. Thus if a site is hit by MITM, the cert will change and the browser will warn. Of course, that's assuming you've visited the site before and care to pay attention to the warning.
Besides geococcyxc's remark, how are you to know that the first certificate is legitimate? How are you to know that the new certificate after the old one has expired is legitimate?
Care to elaborate? I do not think you will get a warning if the MITM is done with a certificate signed by a valid CA, even if you have approved some self-signed certificate before for that site. At least I have never seen this in any browser.
The CA never gets the private key. Instead they get a certificate signing request (CSR), which only contains the public key part. They sign that.
Oh, and then there is perfect forward secrecy, which basically means that even the servers private key is not the one used to encrypt the actual data (after the initial handshaking, and only for suitable cipher suites, subject to downgrade attacks).
Disclaimer: at least, thats how its properly done. Some CAs offer a "send us your cert and we'll sign it", and dumb people who shouldn't be admins use it because it's (slightly) easier to use.
But you got the conclusion right, the notion of CAs is problematic.