[Encryption with Arduino?] Local Linux WebServer to Arduino commands

I'm a student in the Federal University of Technology - Paraná (Brazil), and I'm currently working in a project to control my Arduino. The project's main goal is to control multiple Arduinos that will lock doors. To do so, I'll create a local web server in Linux, so that authorized students will be able to open labs and rooms. The Arduino should be connected to the local network, and receive commands to open or close.
I didn't think an Arduino could handle all the needed processes to handle all that, so I'm exporting the processing role to an external server, and the Arduino meerly receives an "open" or "close".
My worry is if anybody could use a sniffing process to catch any commands I send to the Arduino. Therefore I'd like to know any method you know to safely send commands over local network.

P.S.: I haven't had any encryption or network lessons. I'm meerly asking for way to go and study for myself [books and papers are welcome as well). The Server I'm making with Linux is a simple one, for now...

TEA, Xtea and AES256 have been implemented on Arduino. Probably others too, Google arduino encryption to find them.

If the main concern is sniffing the network and possible replay attacks. Some simple code could be used to make the lock and unlock commands have a rolling code. Perhaps each Arduino would have a unique ID and a counter incrementing the number of commands sent to it. Using these the command required to lock or unlock could be obfuscated enough to prevent some low-level pranksters. Much more than that won't be useful however because someone who is motivated enough would simply break down the door.