密码学中，密码体制（Security System）是满足以下条件的五元组(P, C, K, E, D):

第四点保证了使用加密方式对明文进行加密，也可用相应的解密方式对密文进行解密，得到明文。

如果密钥空间和明文空间一样大，那幺这个加密方式就是一个置换。且加密方式必须保证是一个单射函数，即不同明文加密后不可对应相同密文。

A security alarm is a system designed to detect intrusion – unauthorized entry – into a building or area. Security alarms are used in residential, commercial, industrial, and military properties for protection against burglary (theft) or property damage, as well as personal protection against intruders. Car alarms likewise protect vehicles and their contents. Prisons also use security systems for control of inmates.