ieo Types of Cryptanalytic Attacks ciphertext only Encryption algorithm Ciphertext to be decoded known plaintext Encryption algorithm Ciphertext to be decoded One or more plaintext-ciphertext pairs formed with the secret key chosen plaintext Encryption algorithm Ciphertext to be decoded Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key 復大辱软件学院 LiST
LiJT 11 Types of Cryptanalytic Attacks • ciphertext only – Encryption algorithm – Ciphertext to be decoded • known plaintext – Encryption algorithm – Ciphertext to be decoded – One or more plaintext-ciphertext pairs formed with the secret key • chosen plaintext – Encryption algorithm – Ciphertext to be decoded – Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key
ieo 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 12 復大辱软件学院 LiST
LiJT 12 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
o2 Monoalphabetic Cipher K Plain: abcdefghijklmnopqrstuvwxyz Cipher: DKVQF工 BUWPESOⅩ HTMYAUOLRGZN Plaintext ifwewishtoreplaceletters Ciphertext: WIRERWAJUHYETSDVESEUUFYA hence key is 26 letters long 復大辱软件学院 LiST
LiJT 13 Monoalphabetic Cipher K: Plain: abcdefghijklmnopqrstuvwxyz Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN Plaintext: ifwewishtoreplaceletters Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA • hence key is 26 letters long
提高单字母表密码安全性 两个角度 “多”对“-”> Playfair “-”对“多”→ Vigenere 復大辱软件学院 LiST
LiJT 14 提高单字母表密码安全性 • 两个角度 –“多”对“一” à Playfair – “一”对“多” à 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 復大辱软件学院 LiST
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