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Brief Full Advanced Search Search Tips (Publisher: John Wiley & Sons, Inc.) Author(s): Bruce Schneier ISBN: 0471128457 Publication Date: 01/01/96 Search this book:
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----------- This is sometimes called encrypt-decrypt-encrypt (EDE) mode . If the block algorithm has an n-bit key, then this scheme has a 2n-bit key. The curious encrypt-decrypt-encrypt pattern was designed by IBM to preserve compatibility with conventional implementations of the algorithm: Setting the two keys equal to each other is identical to encrypting once with the key. There is no security inherent in the encrypt-decrypt-encrypt pattern, but this mode has been adopted to improve the DES algorithm in the X9.17 and ISO 8732 standards [55,761]. K1 and K2 alternate to prevent the meet-in-the-middle attack previously described. If C = EK2(EK1(EK1(P))), then a cryptanalyst could precompute EK1(EK1(P))) for every possible K1 and then proceed with the attack. It only requires 2n + 2 encryptions. Triple encryption with two keys is not susceptible to the same meet-in-the-middle attack described earlier. But Merkle and Hellman developed another time-memory trade-off that could break this technique in 2n - 1 steps using 2n blocks of memory . For each possible K2, decrypt 0 and store the result in memory. Then, decrypt 0 with each possible K1 to get P. Triple-encrypt P to get C, and then decrypt C with K1. If that decryption is a decryption of 0 with a K2 (stored in memory), the K1 K2 pair is a possible candidate. Check if it is right. If it’s not, keep looking. This is a chosen-plaintext attack, requiring an enormous amount of chosen plaintext to mount. It requires 2n time and memory, and 2m chosen plaintexts. It is not very practical, but it is a weakness. Paul...
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