How are you intending the username/password to be handled?
Who does the authentication?

Who adds the username and password to the data stream?

MacOS is really not the operating system for doing this kind of stuff. It can be done, but it's not what the OS was designed for. Use a UNIX/Linux style box and you will have loads of options.

What is computer C with port Y on it?

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I was thinking something like a text file with the auth info.

Computer B would authenticate with Computer C

Computer B does.

Don't really have a box handy to run UNIX. Unless OpenBSD was used on the Mac, which I cannot find. Or it would have to be a pretty lightweight UNIX system. I'm not good with UNIX at all, so that might be a little challenging.

Computer C is the "final destination", which is located at a friends house. Basically Computer A can only send data on one port, and Computer C can only accept data on one port. Computer A cannot be changed to the same port that C runs on, and vice-versa. So there needs to be a "bridge" between them, so that they can talk to eachother. Complicated, I know, but that's how it is.