Cryptocat is a server for secure file transfer. It is based on AES encryption with two layers of keys. The protocol has been secretly developed but after a few years the source code has been leaked.
Nevertheless, cryptocat is still in use on server seclab.dsi.unive.it at port 31337. You can connect to it using nc:
$ nc seclab.dsi.unive.it 31337
Of course you need to know the protocol and the key to successfully interact with cryptocat. Together with the source code hackers have been able to intercept two commands sent by the administrator. Apparently they are enough to leak a very secret information (FLAG) on the server … but you will need all of your crypto skills to complete the task!