Cryptographyy

Cryptographyy - ComputerSecurity Cryptographyanintroduction...

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11/07/11 1 Computer Security Cryptography –an introduction
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11/07/11 2 Encryption key K E key K D x plaintext y ciphertext original plaintext x . encryption decryption Eavesdropper
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11/07/11 3 Encryption A cryptosystem involves an encryption algorithm E , and a a decryption algorithm D Both algorithms make use of a key. Let K E be the encryption key and K D the decryption key. For symmetric cryptosystems the same key is used both encryption and decryption: K E = K D .
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11/07/11 4 Encryption If P is the plaintext message, C the ciphertext, then for symmetric cryptosystems: C = E ( K,P ) and P = D ( K,E ( K,P )) = D ( K,C ) For an asymmetric cryptosystem C = E ( K E ,P ) and P = D ( K D ,E ( K E ,P )) = D ( K D ,C )
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11/07/11 5 Kerchoffs’ assumption The adversary knows all details of the encrypting function except the secret key
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11/07/11 6 Symmetric key encryption There are two types of cipher systems: Stream ciphers, Block ciphers.
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11/07/11 7 Stream ciphers Encryption x = ISSOPMI y = wdhuvad Key K E
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11/07/11 8 Block ciphers Encryption x = XNE OIG TPH YRK … y = . Key K E wdm . hut vap dgd …
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11/07/11 9 Block ciphers An overview of the DES Algorithm DES is an iterated block cipher with 16 rounds, block length 64 bits and key length 56 bits
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11/07/11 10 Iterating Block ciphers 1. Iterated block cipher Random (binary) key K round keys : K 1 ,..., K Nr , 2. Round function g            w r = g(w r-1 , K r ), where w r-1 is the previous state
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11/07/11 11 Iterated cipher … Encryption operation: w 0 x ( x = plaintext ) w 1 = g(w 0 , K 1 ), w 2 = g(w 1 , K 2 ), w Nr = g(w Nr-1 , K Nr ), y w Nr ( y = ciphertext )
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11/07/11 12 Iterated cipher … For decryption we must have: g ( . , K ) must be invertible for all K Then decryption is the reverse of encryption (bottom-up)
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11/07/11 13 Data Encryption Standard DES is a special type of iterated cipher called a Feistel cipher . Block length 64 bits Key length 56 bits Ciphertext length 64 bits
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11/07/11 14 DES The round function is: g([L i-1 ,R i-1 ]),K i ) = (L i ,R i ), where L i = R i-1 and R i = L i-1 XOR f (R i-1 , K i ).
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11/07/11 15 DES round encryption
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11/07/11 16 DES inner function
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11/07/11 17 DES computation path
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18 Inner function f Combine 32 bit input and 48 bit key into 32 bit output Expand 32 bit input to 48 bits XOR the 48 bit key with the expanded 48 bit input
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This note was uploaded on 11/07/2011 for the course CIS 4360 taught by Professor Mikeburmester during the Fall '11 term at FSU.

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Cryptographyy - ComputerSecurity Cryptographyanintroduction...

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