Cryptography and Cryptography and Cryptography and Cryptography and Cryptography and Cryptography and Cryptography and Cryptography and Network SecurityNetwork SecurityNetwork SecurityNetwork SecurityNetwork SecurityNetwork SecurityNetwork SecurityNetwork Security Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter 3 3 3 3 3 3 3 3 F Fif ifth Editionth Edition by William Stallingsby William Stallings - - Lecture slides by Lawrie BrownLecture slides by Lawrie Brown Zhang JingZhang Jing Chapter 3 Chapter 3 Chapter 3 Chapter 3 Chapter 3 Chapter 3 Chapter 3 Chapter 3 Block Ciphers and Block Ciphers and Block Ciphers and Block Ciphers and Block Ciphers and Block Ciphers and Block Ciphers and Block Ciphers and the Data Encryption the Data Encryption the Data Encryption the Data Encryption the Data Encryption the Data Encryption the Data Encryption the Data Encryption StandardStandardStandardStandardStandardStandardStandardStandard(DES)(DES)(DES)(DES)(DES)(DES)(DES)(DES) Modern Block CiphersModern Block CiphersModern Block CiphersModern Block CiphersModern Block CiphersModern Block CiphersModern Block CiphersModern Block Ciphers one of the most widely used types of one of the most widely used types of cryptographic algorithms cryptographic algorithms provide secrecy /authentication servicesprovide secrecy /authentication services focus on DES (Data Encryption Standard)focus on DES (Data Encryption Standard) to illustrate block cipher design principlesto illustrate block cipher design principles Block vs Stream CiphersBlock vs Stream CiphersBlock vs Stream CiphersBlock vs Stream CiphersBlock vs Stream CiphersBlock vs Stream CiphersBlock vs Stream CiphersBlock vs Stream Ciphers block ciphers process messages in blocks, block ciphers process messages in blocks, each of which is then en/decrypted each of which is then en/decrypted like a substitution on very big characterslike a substitution on very big characters 64-bits or more 64-bits or more stream ciphers stream ciphers process messages a bit or process messages a bit or byte at a time when en/decryptingbyte at a time when en/decrypting many current ciphers are block ciphersmany current ciphers are block ciphers broader range of applicationsbroader range of applications Block vs Stream CiphersBlock vs Stream CiphersBlock vs Stream CiphersBlock vs Stream CiphersBlock vs Stream CiphersBlock vs Stream CiphersBlock vs Stream CiphersBlock vs Stream Ciphers Block Cipher PrinciplesBlock Cipher PrinciplesBlock Cipher PrinciplesBlock Cipher PrinciplesBlock Cipher PrinciplesBlock Cipher PrinciplesBlock Cipher PrinciplesBlock Cipher Principles most symmetric block ciphers are based on a most symmetric block ciphers are based on a Feistel Cipher StructureFeistel Cipher StructureFeistel Cipher StructureFeistel Cipher StructureFeistel Cipher StructureFeistel Cipher StructureFeistel Cipher StructureFeistel Cipher Structure needed since must be able to needed since must be able to decryptdecryptdecryptdecryptdecryptdecryptdecryptdecrypt ciphertext ciphertext to recover messages efficientlyto recover messages efficiently block ciphers look like an extremely large block ciphers look like an extremely large substitution substitution would need table of 2would need table of 264 64 entries for a 64-bit block entries for a 64-bit block instead create from smaller building blocks instead create from smaller building blocks using idea of a product cipherusing idea of a product cipher Ideal Block CipherIdeal Block CipherIdeal Block CipherIdeal Block CipherIdeal Block CipherIdeal Block CipherIdeal Block CipherIdeal Block Cipher Claude Shannon and Substitution-Claude Shannon and Substitution-Claude Shannon and Substitution-Claude Shannon and Substitution-Claude Shannon and Substitution-Claude Shannon and Substitution-Claude Shannon and Substitution-Claude Shannon and Substitution- Permutation CiphersPermutation CiphersPermutation CiphersPermutation CiphersPermutation CiphersPermutation CiphersPermutation CiphersPermutation Ciphers Claude Shannon introduced idea of substitution-Claude Shannon introduced idea of substitution- permutation (S-P) networks in 1949 paperpermutation (S-P) networks in 1949 paper form basis of modern block ciphers form basis of modern block ciphers S-P nets are based on the two primitive S-P nets are based on the two primitive cryptographic operations seen before: cryptographic operations seen before: substitutionsubstitution (S-box) (S-box) permutation permutation (P-box)(P-box) provide provide confusionconfusion ; ; ; ; ; ; ; R R R R R R R R i i i i i i i i = L = L = L = L = L = L = L = Li-1 i-1i-1i-1i-1i-1i-1i-1 F(RF(RF(RF(RF(RF(RF(RF(Ri-1 i-1i-1i-1i-1i-1i-1i-1,K ,K,K,K,K,K,K,K i i i i i i i i ) ) ) ) ) ) ) ) Feistel Cipher Design ElementsFeistel Cipher Design ElementsFeistel Cipher Design ElementsFeistel Cipher Design ElementsFeistel Cipher Desig