# week5 - SIT281 Introduction to Cryptography Introduction to...

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1 SIT281 Introduction to Cryptography Week 5 Objectives > In this lecture, we examine how DES, one of the most commonly used cryptosystems, works. > DES (Data Encryption Standard) is prevalent world- wide. > The major components, block cipher structure, Feistel design and substitution boxes, will be discussed. This is Chapter 4 of the text.

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2 Stream vs Block Ciphers > Stream ciphers process messages a bit or byte at a time when en/decrypting. > Linear Feedback Shift registers are examples of stream ciphers. > Stream ciphers are often used to produce keystream for audio or video streaming applications such as pay television. Stream vs Block Ciphers cont’d > Block ciphers process messages in blocks, each of which is then en/decrypted. > This works like a substitution on very big strings ± Usually 64-bits or more > Many current ciphers are block ciphers. > They are used because they offer a broad range of > They are used because they offer a broad range of applications. > The Hill cipher is an example of a block cipher; very large matrices can be used.
3 Block Cipher Principles > Most symmetric block ciphers are based on a Feistel Cipher Structure which we will see in a later slide. > This method allows us to decrypt ciphertext and recover messages efficiently. > Block ciphers act like an extremely large substitution and we would need to use a table of 2 64 entries for a 64-bit block, which is difficult to work with. > Instead we often construct it from smaller building blocks, and this is how DES works. Ideal Block Cipher

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4 The ideal block cipher above > A 4-bit input produces one of 16 possible input states, which is mapped by the substitution cipher into a unique one of 16 possible output states, each of which is represented by 4 ciphertext bits. > The encryption and decryption mappings can be defined by a table. > The previous picture illustrates a small 4-bit substitution to show that each possible input can be arbitrarily mapped to any output - which is why its complexity grows so rapidly. Claude Shannon and Substitution-Permutation Networks > Claude Shannon introduced the idea of substitution- permutation (S-P) networks in a1949 research paper. > His ideas now form the basis of all modern block cipher design. > S-P networks are based on the two primitive cryptographic operations we have seen before: ± substitution (S-box) ± permutation (P-box) > They provide confusion & diffusion of message & key.
5 Confusion and Diffusion We want the cipher to completely obscure statistical properties of the original (plaintext) message More practically Shannon suggested combining S & P elements to obtain: > Diffusion – dissipates statistical structure of plaintext over bulk of ciphertext > Confusion – makes relationship between ciphertext – makes relationship between ciphertext and key as complex as possible Thus, the terms diffusion and confusion capture the two basic building blocks for any cryptographic system.

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week5 - SIT281 Introduction to Cryptography Introduction to...

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