No, what you describe is for HTTP Authentication, and that's not what you need.
And as far as the Man in the middle Attack goes: every request that is unencrypted is prone to the man in the middle attack (and even some encryption methods are prone to the man in the middle attack).
What you proposed was to do a request like:
The problem with this is that people can see the URL, see what it returns and they're in.
If you were to do this very same thing using SSL, it still wouldn't work, as the URL in the request is not encrypted, only the data sent to and from the server is.
That's why I proposed a POST request, where the sent data is encrypted
That way the post-data is encrtyped on your server before it is sent and gets sent to other server that way. No man in the middle is able to hack that system. (unless he's got several billion pc's at his disposal; eventually everything can be hacked :))