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# 数学代写|密码学代写Cryptography Theory代考|Basic model of a cryptosystem

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## 数学代写|密码学Cryptography Theory代考|Basic model of a cryptosystem

We now examine a simple model for a cryptosystem providing confidentiality. This basic model is depicted in Figure 1.2. We make two restrictions in order to keep things as straightforward as possible. Please keep them in mind throughout the discussion.

The only security service required for this cryptosystem is confidentiality. Hence, the cryptographic primitive used within this cryptosystem is one that provides data confidentiality, such as a block cipher, a stream cipher, or a public-key encryption scheme. Athough the rest of this chapter will focus on encryption and encryption algorithms, most of the issues we address are relevant to other types of cryptographic primitive.

The basic model we describe is for a communications environment (in other words, Alice sending information to Bob across a communication channel of some sort). This basic model will look slightly different if we want data confidentiality in a different environment, such as for secure data storage.

Figure 1.2 depicts a sender who wishes to transfer some data to a receiver in such a way that any party intercepting the transmitted data cannot determine the content. The various components of the model are as follows:

The plaintext is the raw data to be protected during transmission from sender to receiver. Raw data of this type is sometimes referred to as being in the clear. This is also often (ambiguously) referred to as the message. The intention is that at the end of the process only the sender and the receiver will know the plaintext. In particular, an interceptor cannot determine the plaintext.

The ciphertext is the scrambled version of the plaintext that results from applying the encryption algorithm (and the encryption key) to the plaintext. It is sometimes referred to as the cryptogram. The ciphertext is not a secret and can be obtained by anyone who has access to the communication
channel. In certain contexts this access is referred to as eavesdropping.

## 数学代写|密码学Cryptography Theory代考|Codes

The word ‘code’ is not one we will be using within the context of cryptography, although it is a term often associated informally with cryptography. There are many different interpretations of the concept of a code.

Most generally, the term code is often used for any scheme where data is replaced by alternative data before being sent over a communication channel. This replacement is usually dictated by the contents of a codebook, which states precisely which replacement data to use. A good example is Morse Code, which replaces the letters of the alphabet with short sequences of dots and dashes. Note that Morse Code has nothing to do with secrecy, since the codebook in this case is well known. Morse Code was designed to efficiently transmit messages over telegraph wires. Another example of a code is ASCII, which provides a means of converting keyboard symbols into data suitable for processing on a computer (see the Mathematics Appendix).

If a codebook is kept secret, and is only known by the sender and the receiver of some data, then the resulting code can be regarded as a type of cryptosystem. In this case, the encryption algorithm is simply to replace the plaintext with its matching ciphertext entry in the codebook. The decryption algorithm is the reverse process. The encryption (and decryption) key is the codebook specification itself. For example, Morse Code is not a cryptosystem because there is only one way of replacing letters by dots and dashes. However, if the rule for replacing letters by dots and dashes was kept secret from everyone except a chosen sender and receiver, then we could regard this as a cryptosystem.

In general, cryptosystems based on codebooks only tend to be referred to as codes when the codebook describes ways of replacing dictionary words by other words. Thus, the term code is most likely to be encountered in reference to historical cryptosystems or recreational puzzles. The types of cryptosystem we will be most interested in do not convert words into words, but rather convert sequences of ones and zeros into other sequences of ones and zeros. While we could produce ‘codebooks’ for these modern cryptosystems, the codebooks would have to be so large that they would be impractical to use.

## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。