Cryptology_Ch02

Chapter 02 Cryptology

无密钥体制

1. A将密文加密,发给B
2. B再将密文加密,发给A
3. A解密自己的密文,发回B
4. B解密自己的密文

特点: 1. 不需要交换秘钥 2. 未找到安全算子

单钥/对称体制

1949,Shannon

1. form basis of modern block ciphers
2. S-P nets are based on the two primitive cryptographic operations seen before:
3. substitution (S-box)
4. permutation (P-box)
5. provide confusion & diffusion of message & key

S-box

Substitution

P-box

Permutation

Confusion 使密码和密文之间的联系复杂

makes relationship between ciphertext and key as complex as possible

Diffusion 模糊明文统计结构

issipates statistical structure of plaintext over bulk of ciphertext

Feistel Cipher structure

1. data split in 2 halves,
2. processed through a number of rounds which perform a substitution on left half using output of round function on right half & key,
3. and a permutation which swaps halves, as listed previously.