密码学基础(03) Modern Block Ciphers 復大辱软件学院 LiJT
LiJT 1 密码学基础（03） Modern Block Ciphers
Outline-Modern Block Ciphers now look at modern block ciphers one of the most widely used types of cryptographic algorithms provide confidentiallauthentication services focus on DES (Data Encryption Standard) to illustrate block cipher design principles 復大辱软件学院 LiST
LiJT 2 Outline-Modern Block Ciphers • now look at modern block ciphers • one of the most widely used types of cryptographic algorithms • provide confidential/authentication services • focus on DES (Data Encryption Standard) • to illustrate block cipher design principles
ef Review: Symmetric Cipher Model Secret key shared by Secret key shared by sender and recipient sender and recipient Transmitted ciphertext Plaintext Plaintext Input Encryption algorithm Decryption algorithm output (e.g, DES) (reverse of encryption algorithm) 3 復大辱软件学院 LiST
LiJT 3 Review: Symmetric Cipher Model
Review Monoalphabetic Cipher Playfair Vigenere Cipher 復大辱软件学院 LiJT
LiJT 4 Review • Monoalphabetic Cipher • Playfair • Vigenère Cipher
o Review: Product Ciphers ciphers using substitutions or transpositions are not secure because of language characteristics hence consider using several ciphers in succession to make harder but 'o substitutions make a more complex substitution two transpositions make more complex transposition but a substitution followed by a transposition makes a new much harder cipher this is bridge from classical to modern ciphers 復大辱软件学院 LiJT
LiJT 5 Review: Product Ciphers • ciphers using substitutions or transpositions are not secure because of language characteristics • hence consider using several ciphers in succession to make harder, but: – two substitutions make a more complex substitution – two transpositions make more complex transposition – but a substitution followed by a transposition makes a new much harder cipher • this is bridge from classical to modern ciphers