第7章传统加密技术

ta
e e te a that e e a
at
VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX
e t ta t ha e ee a e th t a
EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ
e e e tat e the et
• proceeding with trial and error finally get:
it was disclosed yesterday that several informal but
• Mathematical expression: c = E(p) = (p + 3) mod 26 p = D(c) = (c – 3) mod 26
Cryptanalysis of Caesar Cipher
• 已知加密和解密算法 • 需测试的密钥只有25个 • 明文所用的语言是已知的，且其意义易于识别
第7章传统加密技术
Example Cryptanalysis*
• given ciphertext:
UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ
• Arabian scientists, 9th century • key concept - monoalphabetic substitution
do not change relative letter frequencies • human languages are redundant
密码表
DEFGH I J KLMNOPQ RST UVW XY Z ABC
veni，vidi，vici
YHAL YLGL YLFL
Caesar Cipher
• Mathematically give each letter a number
abcdefghijk l m 0 1 2 3 4 5 6 7 8 9 10 11 12 nopqrstuvwxyZ 13 14 15 16 17 18 19 20 21 22 23 24 25
– followed by T,R,N,I,O,A,S
• other letters like Z,J,K,Q,X are fairly rare • have tables of single, double & triple letter
frequencies for various languages
ciphertext letter • key is 26 letters long rather than just 1 letter • e. g.：
abcdefg hijklmn opq rst uvw xyz key: DKVQFIB JWPESCX HTM YAU OLR GZN Plaintext: ifwewishtoreplaceletters Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA
所以可以采用穷举攻击分析方法
第7章传统加密技术
单表代换密码（Monoalphabetic Cipher）
• rather than just shifting the alphabet • could shuffle (jumble) the letters arbitrarily • each plaintext letter maps to a different random
• count relative letter frequencies
第7章传统加密技术
• guess P as e and Z as t
• guess ZW is th and hence ZWP is the, and ZWSZ is that
UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ
第7章 传统加密技术
第7章 传统加密技术
主要内容
• 代换密码 – 单表代换密码 – 多表代换密码 – 一次一密 • 置换密码 – 栅栏密码 – 多步置换密码 转轮机Enigma
2.2 Substitution Ciphers（代换密码）
• where letters of plaintext are replaced by other letters or by numbers or symbols
Time required at 1 Time required at
decryption/µs
106 decryptions/µs
2 1026 µs
=
6.4 1012 years
6.4 106 years
• Secure ?
– !!!WRONG!!!
第7章传统加密技术
Cryptanalysis of monoalphabetic cipher
第7章传统加密技术
English Letter Frequencies
第7章传统加密技术
Language Redundancy
• letters are not equally commonly used • in English E is by far the most common
letter
• 代换法就是将明文字母替换成其他字母、数字或 符号的方法。
单表代换密码
• Caesar密码 • 公元前1世纪，凯撒大帝（视频）
第7章 传统加密技术
Caesar Cipher
• can define transforபைடு நூலகம்ation as: 明码表
ABCDEFG HI JK LMN OPQ RST UVW XYZ
第7章传统加密技术
Monoalphabetic Cipher Security
• Key space: 26!, i.e. about 4 x 1026 possibilities
Key Size (bits)
Number of Alternative
Keys
26 characters 26! = 4 1026 (permutation)