原 PPT 日期： 2025-11-03
此文为codex改编往年课件而成
先把地图点亮
如果你是第一次接触这个主题，不用先背一堆名词。先抓住一个小问题：它解决什么麻烦？输入从哪里来？最后能留下什么证据？
密码学基础课介绍从古典密码到现代加密的核心思想。课堂目标不是手写完整算法，而是知道不同密码机制解决什么问题、不能解决什么问题。
下面按“概念 -> 例子 -> 可操作的小任务”的顺序拆开。读完不一定立刻变成高手，但至少能知道下一步该点亮哪块地图。
你会学到
- 理解对称加密、非对称加密和哈希的区别
- 认识 Vigenere、XOR 等基础思想
- 建立不要自创密码算法的安全意识
1. 密码学解决什么问题
密码学常见目标包括保密性、完整性、身份认证和不可否认性。不同算法负责不同目标，把它们混用会导致错误的安全感。
加密不等于安全。密钥管理、随机数、协议设计和实现细节同样会决定结果。
小提示：看到命令别只复制，顺手问一句：它读了什么、改了什么、留下了什么证据？
2. 对称与非对称加密
对称加密速度快，但双方要共享同一把密钥；非对称加密便于密钥交换和签名，但计算成本更高。现代协议通常把两者组合使用。
TLS 等协议不是只用一种算法，而是把密钥交换、身份验证、对称加密和完整性校验组织成流程。
小提示：如果结论只能靠“感觉”，那还没通关；补一条可复现的命令、截图或日志。
3. Vigenere 与 XOR
Vigenere 展示了“密钥重复使用”带来的模式问题，XOR 展示了位运算在加密和编码中的基础作用。它们适合帮助初学者理解密钥与明文的关系。
在 CTF 中看到 XOR，不要只想爆破，也要观察明文格式、文件头和重复周期。
小提示：报错不是敌人，它通常是在很诚实地告诉你哪一层没对上。
4. 作业与复习
复习密码学时建议按问题分类：我要隐藏内容、验证完整性、确认身份，还是交换密钥。先确认目标，再选择机制。
不要在真实项目中自创加密方案。学习可以复现，生产要使用成熟库和成熟协议。
小提示：工具是技能栏，不是自动胜利按钮；真正的主角仍然是你的判断链。
动手小任务
- 用自己的话解释哈希和加密的区别
- 完成一个简单 XOR 还原练习
- 列出 TLS 中至少两个密码学机制
Original PPT date: 2025-11-03
This article was adapted by Codex from previous course slides.
Overview
If this topic is new to you, do not start by memorizing every term. First ask a smaller question: what problem does it solve, where does input enter, and what evidence can we observe?
Cryptography basics introduce classical and modern ideas: confidentiality, integrity, authentication, and their limits.
The article follows a simple path: idea, example, and a small task you can reproduce safely.
Learning Goals
- Explain the main workflow behind Cryptography Basics.
- Use Cryptography, Hash, Symmetric Encryption to read commands, traffic, logs, or code with evidence.
- Stay inside authorized lab environments and document each step clearly.
1. What cryptography solves
Cryptography supports confidentiality, integrity, authentication, and non-repudiation, but only when used correctly.
Read it as a small investigation: what enters the system, what changes inside it, and what evidence proves the result?
Side note: Do not just copy the command. Ask what it reads, what it changes, and what evidence it leaves.
2. Symmetric and asymmetric encryption
Modern protocols combine symmetric and asymmetric techniques.
Read it as a small investigation: what enters the system, what changes inside it, and what evidence proves the result?
Side note: If a conclusion only feels right, it is not cleared yet. Add reproducible evidence.
3. Vigenere and XOR
Classical examples reveal how keys interact with plaintext and why patterns matter.
Read it as a small investigation: what enters the system, what changes inside it, and what evidence proves the result?
Side note: Errors are not the villain; they usually point at the layer that does not match.
4. Homework and review
Choose cryptographic mechanisms by security goal, not by name recognition.
Read it as a small investigation: what enters the system, what changes inside it, and what evidence proves the result?
Side note: Tools are skill slots, not an auto-win button. The real protagonist is your reasoning chain.
Practice
- Summarize the main workflow of Cryptography Basics in your own words.
- Reproduce one safe observation step and record the evidence.
- Explain one likely risk and one matching defense.