Abstract
Every few years, computer security has to re-invite itself. New technologies and new applications bring new threats, and force use to invent new protection mechanisms. Cryptography became important when businesses started to build net worked computer systems. The expressive and representation power of Petri Nets render them ideally suited to the modeling of many complex event systems and to reveal important information about the structure and dynamic behavior of the modeled system. In this work Petri Nets is used to design a secret key cryptosystem to encrypt the secret message before send to the receiver then the receiver used it to decrypt received massage.