# Introduction

Cryptography -> Network Security -> Ethical and Legal Issue

## Classical Encryption Techniques

- Substitution Ciphers
- Transposition Ciphers
- One-Time Pad
- Rotor Machine

## Secret Key Cryptography

- 雙方的 key 相同。
- Block Ciphers
- Example
- FEAL, IDEA
- RC5, RC6
- Skipjack
- CAST Blowfish
- AES
- DES, TripleDES, Lucifer
- Modes of operation
- ECB (Electronic CodeBook)
- CBC(Cipher Block Chaining)
- CFB(CipherFeedBack)
- OFB(Output FeedBack)
- CTR（Counter)
- XTS-AES
- AEAD (authenticated encryption with associated data)
- 同時認證與加密
- Stream Cipher