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Unformatted text preview: Block Ciphers Block Ciphers • Encryption/Decryption of one block of data at a time • Need to partition message into blocks • Padding last block of a message, if necessary M C K Encryption Block Ciphers ( Block Ciphers ( contd contd … … ) ) • Plaintext: M = M 1 , M 2 , …, M L • Each block is encrypted with the same encryption function E K () and with the same key K C i = E K (M i ) Block Ciphers ( Block Ciphers ( contd contd … … ) ) • K should not be too small or too large • Typical sizes for K were 64 bits, currently 128 or more bits • Must have C ≥ M, Typically C = M • Typical sizes for Mi were 64 bits, currently 128 / 192 / 256 bits Block Ciphers ( Block Ciphers ( contd contd … … ) ) • Some permutations from {0,1} N to {0,1} N are weak • Let M = b 1 b 2 . . . b N (binary representation of M) Examples of Weak Block Ciphers: • E (b 1 b 2 . . . b N1 b N ) = b 1 b 2 . . . b N1 b N (identity permutation) • E 1 (b 1 b 2 . . . b N1 b N ) = b 2 b 3 . . . b N b 1 (cyclic shift of the bits) • E 2 (b 1 b 2 . . . b N1 b N ) = b 2 b 1 . . . b N b N1 (bit transpositions) Block Ciphers ( Block Ciphers ( contd contd … … ) ) • C i p h e r b l o c k s s h o u l d l ook random when messages change • Cipher blocks should look random when keys change • Each bit in the plaintext block should influence all the bits in the cipher block (” avalanche effect ”) Feistel...
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 Fall '09
 ManikLalDas
 Data Encryption Standard, block ciphers, Horst Feistel

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