北大密码学课程的课件~密码学基础1-4 讲得通俗易懂。适合初学者自学使用。也可用来辅助读加解密程序或者编程。-Beijing University courses courseware cryptography Cryptography ~ 1-4 speak basic user-friendly. Suitable for beginners to use self-study. Can also be used to support encryption and decryption