Unformatted text preview: lves. The halves are XORed and operated on by function fk, as indicated in the diagram. Figure 13.6 is a block diagram of function fk. The two 32-bit inputs are broken up into 8-bit blocks and combined and substituted as shown. S0 and S1 are defined as just shown. The 16-bit key blocks are then used in the encryption/decryption algorithm. On a 10 megahertz 80286 microprocessor, an assembly-language implementation of FEAL-32 can encrypt data at a speed of 220 kilobits per second. FEAL-64 can encrypt data at a speed of 120 kilobits per second . Figure 13.4 Function f. Figure 13.5 Key processing part of FEAL. Figure 13.6 Function fK. Cryptanalysis of FEAL
FEAL-4, FEAL with four rounds, was successfully cryptanalyzed with a chosen-plaintext attack in  and later demolished . This later attack, by Sean Murphy, was the first published differential-cryptanalysis attack and required only 20 chosen plaintexts. The designers retaliated with 8-round FEAL [1436,1437,1108] which Biham and Shamir cryptanalyzed at the SECURICOM ’89 conference. Another chosen-plaintext attack, using only 10,000 blocks, against FEAL-8  forced the designers to throw up their hands and define FEAL-N [1102,1104], with a variable number of rounds (greater than 8, of course). Biham and Shamir used differential cryptanalysis against FEAL-N; they could break it more quickly than by brute force (with fever than 264 chosen plaintext encryptions) for N less than 32 . FEAL-16 required 228 chosen plaintexts or 246.5 known plaintexts to break. FEAL-8 required 2000 chosen plaintexts or 237.5 known plaintexts to break. FEAL-4 could be broken with just eight carefully selected chosen plaintexts. The FEAL designers also defined FEAL-NX, a modification of FEAL, that accepts 128-bit keys (see Figure 13.7)[1103,1104]. Biham and Shamir showed that FEAL-NX with a 128-bit key is just as easy to break as FEAL-N with a 64-bit key, for any value of N . Recently FEAL-N(X)S has been proposed, which strengthens FEAL with...
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