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Chapter: Cryptography and Network Security

Multiple Encryption & DES

1 Double-DES 2 Triple-DES with Two-Keys 3 Triple-DES with Three-Keys

MULTIPLE ENCRYPTION & DES

 

clear a 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

 

1 Double-DES

 

could use 2 DES encrypts on each block

 

= EK2(EK1(P))

 

issue of reduction to single stage and have “meet-in-the-middle” attack

works whenever use a cipher twice

 

since X = EK1(P) = DK2(C)

 

attack by encrypting P with all keys and store

 

then decrypt C with keys and match X value

 

can show takes O(256) steps

 

2 Triple-DES with Two-Keys

 

hence must use 3 encryptions

would seem to need 3 distinct keys

 

but can use 2 keys with E-D-E sequence

 

= EK1(DK2(EK1(P)))

 

nb encrypt & decrypt equivalent in security

 

if K1=K2 then can work with single DES

 

standardized in ANSI X9.17 & ISO8732

 

no current known practical attacks

 

3 Triple-DES with Three-Keys

 

although are no practical attacks on two key Triple-DES have some indications

 

can use Triple-DES with Three-Keys to avoid even these

 

C = EK3(DK2(EK1(P)))

 

has been adopted by some Internet applications

 

eg PGP, S/MIME


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