# Introduction

## Classical Encryption

- Substitution Ciphers
- Transposition Ciphers
- One-Time Pad
- Rotor Machine
- Ex. Enigma（恩尼格瑪密碼機）

## Secret-Key Cryptosystems

- Block Ciphers



- Stream Ciphers
- 速度快但不安全
- 產生出來的 $K$ 同時用於加密與解密
- Ex. RC4, A5, Salsa20, ChaCha20


## Public-Key Cryptosystems

- 有 public key 與 private key，互為加解密的關係。
- 只有使用者持有 private key


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- Diffie-Hellman Key Exchange Scheme (1976)


- Number Theory
- Euclidean Algorithm
- Modular Arithmetic
- Fermat’s and Euler’s Theorems
- Testing for Primality
- Chinese Remainder Theorem (CRT)
- Discrete Logarithms
- Common Security Basis
- Integer Factorization Problem (IFP)
- Discrete Logarithm Problem (DLP)
- Elliptic Curve Discrete Logarithm Problem (ECDLP)
- RSA (1977)


## Cryptographic Hash Functions

- Applications
- Message Authentication
- Digital Signature
- Blockchain
- Algorithms
- MD5 (broken in 2004), RIPEMD-160 (broken in 2004)
- SHA-1 (broken in 2017)
- 160 bits
- SHA-2: SHA-224 (for 2-Key 3DES), SHA-256, SHA-384, SHA-512
- SHA-3

## Message Authentication Codes

- Schemes
- DAA
- CMAC
- HMAC
- Poly1305
- Authenticated Encryption
- 同時加密與解密
- CCM
- GCM
- ChaCha 20 – Poly 1305

## Lightweight Cryptography

- A subfield of cryptography concerned with the development of cryptographic algorithms for resource-constrained devices
- Develop efficient implementations of conventional cryptographic algorithms as well as the design of new lightweight algorithms

## Quantum

- Quantum Threat
- Quantum Cryptanalysis
- Quantum Cryptography
- Quantum Key Distribution (QKD)
- Ex. BB84 Protocol
- Post-Quantum Cryptography (PQC)
- Types of PQC
- NIST Post-Quantum Cryptography Standardization
- Public-Key Encryption / KEM (Key Encapsulation Mechanism)
- Digital Signature Algorithms
- CRYSTALS-Kyber