Multiple Encryption & DES

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

= E_K2(E_K1(P))

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

works whenever use a cipher twice

since X = E_K1(P) = D_K2(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

= E_K1(D_K2(E_K1(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 = E_K3(D_K2(E_K1(P)))

has been adopted by some Internet applications

eg PGP, S/MIME