Enchrypt and dechrypt a message

Hello everyone,
I am wondering if there is a easy was to enchrypt a message that is going to be send to an other arduino via xbee, eRIC or somethink like that... The decryptionpart would be interresting too. And I am wondering if there is a easy to use library online...

Depends on how strong you want it. Rot13 is pretty quick, uses few resources but is easily breakable. One Time Pad uses few resources and is unbreakable but it is difficult to share the pad.

Or are you looking for something like public key?

Well the modules I am probably going to use are standard industrial units and so i want that not everyone can read the message without investing a minimum of effort, because if someone wants to read he will aleays can... So I am just looking for a way to kerp ideots out of a network

TEA (or tea) is pretty good for this. "Tiny Encryption Algorithm." I used it in a PHP project a long time ago but it should be good on the Arduino.