I am using SSLStrip against my own computer, but am not being redirected to HTTP. Skype automatically starts up and logs in at my computer's startup, so I was wondering whether the fact that port 80 is already in use (by Skype) defeats SSLStrip. It is my understanding that except for the fact that they are being redirected to a non-encrypted connection, it is impossible for a victim (or their AV) to tell SSLStrip is being used against them. If that is correct I would like to know if anyone has a way to cause Skype to restart on the victim computer so I can get port 80.
SSLStrip does not downgrade from HTTPS. It tries to prevent upgrade to HTTPS.
I think this is a misunderstanding of how SSLStrip works.
The idea is that SSLStrip stops plain text HTTP sites from upgrading to HTTPS. It will not work if you start by entering "https://" in your browser right away.
What it does is that it installs itself as a man-in-the-middles and then transparently forwards traffic largely without changing it. HOWEVER: If the HTTP page contains any HTTPS links then SSLStrip will "strip" these HTTPS links out and replace them with plain old HTTP.
So if your web server does not even speak plain text HTTP, on default port 80 or elsewhere, then you can't upgrade to HTTPS either. And in that case SSLStrip is not the right tool for whatever you're trying to do.
SSLstrip works by listening for https traffic and redirecting it to http.
For one thing Skype does not use http it uses its own proprietary protocol. For another they do not offer a non-encrypted version of their protocol the way https does with http so the concept of an SSLstrip type attack will not work.
For those reasons SSLstrip will not work against Skype.