• Document: Cryptography and Network Security Chapter 3
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Chapter 3 – Block Ciphers and the Cryptography and Data Encryption Standard Network Security All the afternoon Mungo had been working on Chapter 3 Stern's code, principally with the aid of the latest messages which he had copied down at the Nevin Square drop. Stern was very confident. He must be well aware London Central knew Fifth Edition about that drop. It was obvious that they didn't care how often Mungo read their messages, so by William Stallings confident were they in the impenetrability of the Lecture slides by Lawrie Brown code. —Talking to Strange Men, Ruth Rendell (with edits by RHB) Outline Modern Block Ciphers • will consider: • now look at modern block ciphers – block vs stream ciphers • one of the most widely used types of – Feistel cipher design & structure cryptographic algorithms – DES • provide secrecy /authentication services • details • strength • focus on DES (Data Encryption Standard) to illustrate block cipher design principles – Differential & Linear Cryptanalysis – block cipher design principles Block vs Stream Ciphers Block vs Stream Ciphers • block ciphers process messages in blocks, each of which is then en/decrypted • like a substitution on very big characters – 64-bits or more • stream ciphers process messages a bit or byte at a time when en/decrypting • many current ciphers are block ciphers – better analysed – broader range of applications Block Cipher Principles Ideal Block Cipher • most symmetric block ciphers are based on a Feistel Cipher Structure (more below) • needed since must be able to decrypt ciphertext to recover messages efficiently • block ciphers look like an extremely large substitution • for a 64-bit block would need table of 264 entries • this 264 entry table would be the key • instead create from smaller building blocks, using idea of a product cipher, and a much smaller key Claude Shannon and Substitution- Confusion and Diffusion Permutation Ciphers • Claude Shannon introduced idea of substitution- • cipher needs to completely obscure permutation (S-P) networks in 1949 paper statistical properties of original message • form basis of modern block ciphers • a one-time pad does this • S-P nets are based on the two primitive • more practically Shannon suggested cryptographic operations seen before: combining S and P elements to obtain: – substitution (S- (S-box) • diffusion – dissipates statistical structure – permutation (P- (P-box) of plaintext over bulk of ciphertext • provide confusion & diffusion of message & key • confusion – makes relationship between ciphertext and key as complex as possible Feistel Cipher Structure Feistel Cipher Structure • Horst Feistel devised the Feistel Cipher – based on concept of invertible product cipher • partitions input block into two halves – process through multiple rounds which – perform a substitution on left data half – based on round function of right half & subkey – then have permutation swapping halves • implements Shannon’s S-P net concept Feistel Cipher Design Elements • block size • key size • number of rounds • subkey generation algorithm • round function • fast software en/decryption • ease of analysis Data Encryption Standard (DES) DES History • most widely used block cipher in world • IBM developed Lucifer cipher • adopted in 1977 by NBS (now NIST) – by team led by Feistel in late 60’s – as FIPS PUB 46 – used 64-bit data blocks with 128-bit key • encrypts 64-bit data

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