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Unformatted text preview: he key pair K1 and K3 are possible candidates for the right key. Repeat the attack a few times, and only one candidate will remain. This shows that xDES1 is not an ideal solution. Even worse, there is a chosen plaintext attack that proves xDES1 is not much stronger than the underlying block cipher . xDES2 extends this idea to a 5-round algorithm with a block size 4 times that of the underlying block cipher and a key size 10 times that of the underlying block cipher. Figure 15.4 is one round of xDES2; each of the four sub-blocks are the size of the underlying block ciphers and all 10 keys are independent. This scheme is also faster than triple encryption: Ten encryptions are used to encrypt a block four times the size of the underlying block cipher. However, it is vulnerable to differential cryptanalysis  and should not be used. The scheme is even vulnerable if DES with independent round keys is used. Figure 15.4 One round of xDES2. For i e 3, xDESi is probably too big to be useful as a block algorithm. For example, the block size for xDES3 is 6 times that of the underlying cipher, the key size is 21 times, and 21 encryptions are required to encrypt a block 6 times that of the underlying block cipher. Triple encryption is faster. Quintuple Encryption
If triple encryption isn’t secure enough—perhaps you need to encrypt triple-encryption keys using an even stronger algorithm—then higher multiples might be in order. Quintuple encryption is very strong against meet-in-the-middle attacks. (Similar arguments to the ones used with double encryption can show that quadruple encryption provides minimal security improvements over triple encryption.) C = EK1(DK2(EK3(DK2(EK1(P))))) P = DK1(EK2(DK3(EK2(DK1(C))))) This construction is backwards compatible with triple encryption if K2 = K3, and is backwards compatible with single encryption if K1 = K2 = K3. Of course, it would be even stronger if all five keys were independent. Previous Table of Contents Next Products | Contact Us | About Us | Privacy | Ad Info | Home Us...
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- Fall '10