说明：  本文介绍了一种基于DES和RSA算法的安全电子邮件系统（SEM）。本文介绍了加密解密的过程、安全密钥的管理以及一些算法的实现。
(This paper introduces a secure electronic mail system ( SEM), whose design and implementation are based on DES and RSA algorithms. The procedure of encryption and decryption, the management of secure key and the implementation of some algorithms are described in this thesis.)