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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



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



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