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Fourth Editionby William Stallings

Lecture slides by LawrieBrown

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"I am fairly familiar with all the forms of secret writings, andam myself the author of a trifling monograph upon thesubject, in which I analyze one hundred and sixtyseparate ciphers," said Holmes.

The Adventure of the Dancing Men, Sir ArthurConan Doyle

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Replacement for DES was needed

theoretical attacks that can break it

demonstrated exhaustive key search attacks

AES is a new cipher alternative

prior to this alternative was to use multiple encryption

with DES implementations

Triple-DES is the chosen form

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could use 2 DES encrypts on each block

C = EK2(E

K1(P))

issue of reduction to sinle stae

and have !meet-in-the-middle" attack

works whenever use a cipher twice

since X = EK1(P) = D

K2(C)

attack by encryptin # with all keys and store

then decrypt $ with keys and match % value

can show takes O(256)steps

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hence must use & encryptions

would seem to need & distinct keys but can use 2 keys with E-D-E se'uence

C = EK1(D

K2(E

K1(P)))

nb encrypt ( decrypt e'uivalent in security if K1=K2then can work with sinle DES

standardi)ed in A*S+ %,./ ( +S01/&2

no current known practical attacks

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althouh are no practical attacks on two-key Triple-DES

have some indications

can use Triple-DES with Three-eys to avoid even

these

C = EK3(D

K2(E

K1(P)))

has been adopted by some +nternet applications3 e

#4#3 S56+6E

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block ciphers encrypt fixed si)e blocks

e DES encrypts 78-bit blocks with 97-bit key need some way to en5decrypt arbitrary amounts of data

in practise

ANSI X3.106-1983 Modes o !se :now ;+#S 1.

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&lectronic Code"ook Mode :E$=