SSL and man-in-the-middle misunderstanding

Question

I've read tons of documentation related to this problem but I still can't get all the pieces together, so I'd like to ask a couple of questions.

1. First of all I'll describe briefly the authentication procedure as I understand it, as I may be mistaken in that regard: A client starts a connection, which a server responds to with a combination of public key, some metadata and digital signature of a trusted authority. Then the client takes the decision if she trusts the server, encrypts some random session key with the public key and sends it back. This session key can be decrypted only with private key stored on the server. Server does this and then the HTTPS session begins.

2. So, if I'm correct above, the question is how the man-in-the-middle attack can occur in such scenario? I mean, even if somebody intercepts the server (e.g. www.server.com) response with public key and has some means to make me think that he is www.server.com, he still wouldn't be able to decrypt my session key without the private key.

3. Speaking about the mutual authentication, is it all about the server confidence about the client identity? I mean, the client can already be sure that she is communicating with the right server, but now the server wants to find out who the client is, right?

4. And the last question is about the alternative to the mutual authentication. If I act as a client in the situation described, what if I send a login/password in the HTTP header after the SSL session is established? As I see it, this information can't be intercepted because the connection is already secured and the server can rely on it for my identification. Am I wrong? What are the downsides of such an approach compared with mutual authentication (only security issues are important, not the implementation complexity)?

Answer 1

Man-in-the-middle attacks on SSL are really only possible if one of SSL's preconditions is broken, here are some examples;

• The server key has been stolen - means the attacker can appear to be the server, and there is no way for the client to know.

• The client trusts an untrustworthy CA (or one that has had it's root key stolen) - whoever holds a trusted CA key can generate a certificate pretending to be the server and the client will trust it. With the number of CAs pre-existing in browsers today, this may be a real problem. This means that the server certificate would appear to change to another valid one, which is something most clients will hide from you.

• The client doesn't bother to validate the certificate correctly against its list of trusted CA's - anyone can create a CA. With no validation, "Ben's Cars and Certificates" will appear to be just as valid as Verisign.

• The client has been attacked and a fake CA has been injected in his trusted root authorities - allows the attacker to generate any cert he likes, and the client will trust it. Malware tends to do this to for example redirect you to fake banking sites.

Especially #2 is rather nasty, even if you pay for a highly trusted certificate, your site will not be in any way locked to that certificate, you have to trust all CAs in the client's browser since any of them can generate a fake cert for your site that is just as valid. It also does not require access to either the server or the client.