本文建立在密码学入门书籍Introduction to Modern Cryptography基础上，先对古典密码学中的密码和解法进行了介绍，随后给出现代密码学的三个原则，使读者大致了解了一个加密方案应该是怎样构造的。密码的安全性是最受关注的，因此本文在第二章介绍了安全性和攻击模型的定义，列举了基础的完美安全性和计算安全性，并在第三章分别举例一次一密（OTP）和流密码，讨论其安全性，最后简单讲述了应用最为广泛的流密码RC4密码。

Building on the introductory book on cryptography, Introduction to Modern Cryptography, this paper first introduces ciphers and solutions in classical cryptography, and then gives three principles of modern cryptography to give the readers a general idea of how an encryption scheme should be constructed. The security of ciphers is the most important concern, so this paper introduces the definition of security and attack models in Chapter 2, lists the basic perfect security and computational security, and discusses the security of the one-time pad (OTP) and stream ciphers with examples in Chapter 3, and finally briefly describes the most widely used stream cipher, RC4 cipher.