t Types of Cryptanalytic Attacks chosen ciphertext Encryption algorithm Ciphertext to be decoded Purported ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key chosen text Encryption algorithm Ciphertext to be decoded Plaintext message chosen by cryptanalyst, together with its corresponding Ciphertext with the secret key Purported cipehrtext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key 復大软件学院 LiT
LiJT 11 Types of Cryptanalytic Attacks • chosen ciphertext – Encryption algorithm – Ciphertext to be decoded – Purported ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key • chosen text – Encryption algorithm – Ciphertext to be decoded – Plaintext message chosen by cryptanalyst, together with its corresponding Ciphertext with the secret key – Purported cipehrtext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key
Monoalphabetic Cipher K: Plain: abcdefghijklmnopqrstuvwxyz Cipher: DKVQF工 BUWPESOⅩ HTMYAUOLRGZN Plaintext ifwewishtoreplaceletters Ciphertext: WIRFRWAJUHYETSDVESEUUFYA hence key is 26 letters long 12 復大软件学院 LiT
LiJT 12 Monoalphabetic Cipher K: Plain: abcdefghijklmnopqrstuvwxyz Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN Plaintext: ifwewishtoreplaceletters Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA • hence key is 26 letters long
An Improvement Homophone Assign each letter a number of different cipher symbols The number of symbols assigned to each letter is proportional to the relative frequency of that letter 復大软件学院 LiT
LiJT 13 An Improvement • Homophone • Assign each letter a number of different cipher symbols • The number of symbols assigned to each letter is proportional to the relative frequency of that letter
提高单字母表密码安全性 两个角度 “多”对“-”→ Playfair ”对“多”→ Vigenere 復大软件学院 LiT
LiJT 14 提高单字母表密码安全性 • 两个角度 –“多”对“一” Playfair – “一”对“多” Vigenère
Playfair Cipher not even the large number of keys in a monoalphabetic cipher provides security one approach to improving security was to encrypt multiple letters the Playfair Cipher is an example invented by Charles Wheatstone in 1854 but named after his friend Baron Playfair 15 復大软件学院 LiT
LiJT 15 Playfair Cipher • not even the large number of keys in a monoalphabetic cipher provides security • one approach to improving security was to encrypt multiple letters • the Playfair Cipher is an example • invented by Charles Wheatstone in 1854, but named after his friend Baron Playfair