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Unformatted text preview: Block Ciphers and DES W . Stallings, L. Brown, A. Rezgui, P. Krishnamurthy 2 Introduction • What do we need? – An encryption algorithm that either costs a lot to break or takes a lot of time to break • Computational security – The cost of breaking the ciphertext exceeds the value of the encrypted information – The time required to break the ciphertext exceeds the useful lifetime of the information 3 Modern Block Ciphers • now look at modern block ciphers • one of the most widely used types of cryptographic algorithms • provide secrecy/authentication services • focus on DES (Data Encryption Standard) • to illustrate block cipher design principles 4 Block Cipher • one pass through: one input bit affects eight output bits 64bit input T 1 8bits 8 bits 8bits 8 bits 8bits 8 bits 8bits 8 bits 8bits 8 bits 8bits 8 bits 8bits 8 bits 8bits 8 bits 64bit scrambler 64bit output loop for n rounds T 2 T 3 T 4 T 6 T 5 T 7 T 8 • multiple passes: each input bit affects all output bits • block ciphers: DES, 3DES, AES 5 Block vs. Stream Ciphers • block ciphers process messages in blocks, each of which is then en/decrypted • like a substitution on very big characters – 64bits or more • stream ciphers process messages a bit or byte at a time when en/decrypting • many current ciphers are block ciphers • broader range of applications 6 Goal of modern encryption schemes • Oscar can recover the key to the encryption algorithm by brute force search alone and not by any shortcuts • The number of possible keys to be tested should be so large as to make brute force search infeasible • Example: Data Encryption Standard has 56 bit keys 2 56 possible keys = 7.2 x 10 16 keys – If each key attempt took 100ms, a worst case brute force attack would still take 228,493,131 years. 7 Block Cipher Principles • most symmetric block ciphers are based on a Feistel Cipher Structure – Read Chapter 3 of textbook • 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 64bit block • instead create from smaller building blocks • using idea of a product cipher 8 Data Encryption Standard (DES) • most widely used block cipher in world • adopted in 1977 by NBS (now NIST) – as FIPS PUB 46 • encrypts 64bit data using 56bit key • has widespread use • has been considerable controversy over its security 9 DES History • IBM developed Lucifer cipher – by team led by Feistel in late 60’s – used 64bit data blocks with 128bit key • then redeveloped as a commercial cipher with input from NSA and others • in 1973 NBS issued request for proposals for a national cipher standard • IBM submitted their revised Lucifer which was eventually accepted as the DES 10 DES Design Controversy • although DES standard is public • was considerable controversy over design – in choice of 56bit key (vs Lucifer 128bit)...
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 Fall '09
 REZGUI
 Advanced Encryption Standard, Block cipher, Data Encryption Standard, initial permutation

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