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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 60s 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

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