modes of block cipher
TRANSCRIPT
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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$=