# 数学代写|密码学代写cryptography theory代考|ClS455

#### Doug I. Jones

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• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础
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## 数学代写|密码学代写cryptography theory代考|Message Integrity

One common use of hashing algorithms is in ensuring integrity of messages (Easttom 2019). It should be fairly obvious that messages can be altered in transit, either intentionally or accidentally. Hashing algorithms can be used to detect that such an alteration has occurred. Consider the simple example of an email message. If you put the body of the message into a hashing algorithm, let’s just say SHA-1, the output is a 160-bit hash. That hash can be appended at the end of the message.
When the message is received, the recipient can re-calculate the cryptographic hash of the message and compare that result to the hash that was attached to the message. If the two do not match exactly, this indicates that there has been some alteration in the message and the message contents are no longer reliable.

Cryptographic hashes are also used in file integrity systems. For example, the very popular TripWire product (both the open source Linux version and the Windows version) creates a cryptographic hash of key files (as designated by the TripWire administrator). At any time, a hash of the current file can be compared to the previously computed cryptographic hash to determine if there has been any change in the file. This can detect anything from a simple edit of a file such as a spreadsheet to an executable that has been infected with a Trojan Horse.

It is likely that many readers already know what a Trojan Horse is, but in case you don’t it is a program or file that has had malware attached to it. Often wrapper programs are used to tie a virus or spyware to a legitimate program. When the user executes the legitimate program, he or she does not realize that they also just launched the malware.

Cryptographic hashes also provide a level of security against insider threats. Consider the possibility that someone with access to a system, for example, a network administrator, has ill intent. Such a person might simply read a user’s password from the database, then use that user’s login credentials to accomplish some attack on the system. Then, should the attack become known, it is the end user who will be a suspect, not the administrator who actually perpetrated the breach. One way to avoid this is to store passwords in a cryptographic hash. When the user logs into the system, whatever password they typed in is hashed, then compared to the hash in the database. If it matches exactly, then the user is logged into the system.

Given that the database only stores a hash of the password, and hashes are not reversible, even a network administrator or database administrator cannot retrieve the password from the database. If someone attempted to type in the hash as a password, the system will hash whatever input is placed into the password field, thus yielding a different hash than is stored in the database. The storing of passwords as a hash is widely used and strongly recommended.

Then store that in the SAM (Security Accounts Manager) file in the Windows System directory (Easttom 2017). When you log on, Windows cannot “un hash” your password. What Windows does is take whatever password you type in, hash it, then compare that result with what is in the SAM file. If they match (exactly), then you can login.、

It is worth noting here, that there are methods for circumventing this security and retrieving passwords. A rainbow table is one such mechanism. A rainbow table is a table of pre-computed hashes. Windows uses the NTLMv2 hashing algorithm to store passwords. Imagine you make a table of all common 8-character passwords in one column, and the NTLMv2 hash of them in the second column. Then you repeat this for all common 9-character passwords. Then for all 10-character passwords. You can take this as far as you like, and your computing resources will support. Then if you can extract a hash from the target machine, you search the tables for a match. If you find a match in the second column, whatever is in the first column must be that person’s password. It will look something like what is shown in Fig. 9.1.

If an attacker is able to get the windows SAM file, then he or she can take the hashes and search the rainbow table seeking a match. There are even rainbow tables available online:
https://crackstation.net/
http://rainbowtables.it64.com/

# 密码学代写

## 数学代写|密码学代写cryptography theory代考|Message Integrity

. Message Integrity . Message Integrity . Message Integrity

，然后将其存储在Windows系统目录(Easttom 2017)的SAM(安全帐户管理器)文件中。当你登录时，Windows不能“un hash”你的密码。Windows所做的就是获取您输入的任何密码，哈希它，然后将结果与SAM文件中的结果进行比较。如果它们完全匹配，那么您可以登录。，

https://crackstation.net/
http://rainbowtables.it64.com/

## 有限元方法代写

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## 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。

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