3 - Lecture #3 SS G513 Network Security Cryptography and...

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Lecture #3 SS G513 Network Security
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Cryptography and Network Security Third Edition by William Stallings Lecture slides by Lawrie Brown
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Chapter 3 – Block Ciphers and the Data Encryption Standard All the afternoon Mungo had been working on 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 about that drop. It was obvious that they didn't care how often Mungo read their messages, so confident were they in the impenetrability of the code. Talking to Strange Men, Ruth Rendell
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Modern Block Ciphers will now look at modern block ciphers one of the most widely used types of cryptographic algorithms provide secrecy and/or authentication services in particular will introduce DES (Data Encryption Standard)
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Block vs Stream Ciphers block ciphers process messages in into 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 hence are focus of course
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Block Cipher Principles most symmetric block ciphers are based on a Feistel Cipher Structure needed since must be able to decrypt ciphertext to recover messages efficiently block ciphers look like an extremely large substitution would need table of 2 64 entries for a 64-bit block instead create from smaller building blocks using idea of a product cipher
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Claude Shannon and Substitution- Permutation Ciphers in 1949 Claude Shannon introduced idea of substitution-permutation (S-P) networks modern substitution-transposition product cipher these form the basis of modern block ciphers S-P networks are based on the two primitive cryptographic operations we have seen before: substitution (S-box) permutation (P-box) provide confusion and diffusion of message
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Confusion and Diffusion cipher needs to completely obscure statistical properties of original message a one-time pad does this more practically Shannon suggested combining elements to obtain: diffusion – dissipates statistical structure of plaintext over bulk of ciphertext confusion – makes relationship between ciphertext and key as complex as possible
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Feistel Cipher Structure Horst Feistel devised the feistel cipher based on concept of invertible product cipher
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3 - Lecture #3 SS G513 Network Security Cryptography and...

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