CS283 Lecture 2 - Part 2 - Block Ciphers - 20090915

# CS283 Lecture 2 - Part 2 - Block Ciphers - 20090915 -...

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GWU CS 172/283 Autumn 2009 Some of this slide set is from: H. M. Heys, "A Tutorial on Linear and Differential Cryptanalysis", Technical Report CORR 2001-17, Centre for Applied Cryptographic Research, Department of Combinatorics and Optimization, University of Waterloo, Mar. 2001. (Also appears in Cryptologia, vol. XXVI, no. 3, pp. 189-221, 2002.) Lecture 2 – Part 2 - Block Ciphers: DES, SPNs, AES

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Block Cipher – Definition (From Bishop New 10,2) Let E be an encipherment algorithm, and let E k (b) be the encipherment of message b with key k. Let a message m = b 1 b 2 …. , where each b i is of a fixed length. Then a block cipher is a cipher for which E k (m) = E k (b 1 )E k (b 2 )…… GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 02 – Part 2 - Rev 20090915 2
GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 02 – Part 2 - Rev 20090915 3 Block Cipher (electronic Code book) Black Box 0 1 1 0 . Cryptovariable 0 1 1 0 0 1 0 Plain Text in fixed length blocks 0 1 1 0 0 1 0 1 1 0 1 0 1 0 Ciphertext in fixed length blocks

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GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 02 – Part 2 - Rev 20090915 4 Block Ciphers Implement a Permutation Plain Text (PT) Cipher Text (CT) 0 0 0 0 1 0 0 0 0 0 0 1 0 0 1 0 0 0 1 0 1 0 0 1 0 0 1 1 0 0 0 0 0 1 0 0 1 0 1 0 0 1 0 1 0 1 0 0 0 1 1 0 1 0 1 1 0 1 1 1 0 0 1 1 1 0 0 0 1 1 0 0 1 0 0 1 0 1 0 1 1 0 1 0 1 1 0 1 1 0 1 1 1 1 1 1 1 1 0 0 0 1 1 0 1 1 0 1 1 1 1 0 1 1 1 0 0 1 1 1 1 1 1 1 0 0 0 1 NOTE: The permutation is on the four-bit words, not on the bits in the words. This PT Always encrypts to this CT
GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 02 – Part 2 - Rev 20090915 5 Block Decryption Implements the Reverse Permutation Plain Text (PT) Cipher Text (CT) 0 0 0 0 1 0 0 0 0 0 0 1 0 0 1 0 0 0 1 0 1 0 0 1 0 0 1 1 0 0 0 0 0 1 0 0 1 0 1 0 0 1 0 1 0 1 0 0 0 1 1 0 1 0 1 1 0 1 1 1 0 0 1 1 1 0 0 0 1 1 0 0 1 0 0 1 0 1 0 1 1 0 1 0 1 1 0 1 1 0 1 1 1 1 1 1 1 1 0 0 0 1 1 0 1 1 0 1 1 1 1 0 1 1 1 0 0 1 1 1 1 1 1 1 0 0 0 1 Always decrypts to this PT This CT

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Data Encryption Standard GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 02 – Part 2 - Rev 20090915 6 Designed as a US Federal Standard for encryption of sensitive but non-classified information Bit oriented Developed by a team at IBM during the period 1973–1974 and based on an earlier algorithm, Horst Feistel's Lucifer cipher. Symmetric Key (i.e. encyption and decryption use same key) Uses both bit transposition and bit substitution and is sometimes referred to as a product cipher
Data Encryption Standard (cont’d) GWU CS 172/283 - Autumn 2009 Holmblad - Lecture 02 – Part 2 - Rev 20090915 7 Input, output, and (input) key are each 64 bits long A block is defined as 64 bits The cipher consists of 16 iterations, called rounds Each round uses a separate 48 bit key (called a round key) Round keys are generated from the key block by dropping the parity bits of each of the 8 bytes, thus reducing the effective (input) key size to 56 bits, permuting the bits, and then extracting 48 bits.

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