ISSN: 2250-3005 - ijcer

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International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8The 58th bit of M becomes the first bit of IP. The 50th bit of M becomes the second bit of IP. The 7th bit of M is thelast bit of IP. Example: Applying the initial permutation to the block of text M, given previously, we getM = 0000 00010010 00110100 01010110 01111000 1001101010111100110111101111IP=1100110000000000110011001111111111110000 101010101111000010101010Here the 58th bit of M is "1", which becomes the first bit of IP. The 50th bit of M is "1", which becomes the second bit of IP.The 7th bit of M is "0", which becomes the last bit of IP. Next divide the permuted block IP into a left half L0 of 32 bits, and aright half R0 of 32 bits.Example: From IP, we get L0 and R0L0 = 11001100000000001100110011111111R0 = 11110000101010101111000010101010We now proceed through 4 iterations, for 1
International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8DES Fiestel DiagramExample: For the first round, we obtain as the output of the eight S boxes:K1+E(R0)=100101010001100001010101011101101000010111000111S1(B1)S2(B2)S3(B3)S4(B4)S5(B5)S6(B6)S7(B7)S8(B8)=010111001000 00101011 010110010111The final stage in the calculation of f is to do a permutation P of the S-box output to obtain the final value of f: f =P(S1(B1)S2(B2)...S8(B8)) P yields a 32-bit output from a 32-bit input by permuting the bits of the input block. Example:From the output of the eight Sboxes:S1(B1)S2(B2)S3(B3)S4(B4)S5(B5)S6(B6)S7(B7)S8(B8) = 010111001000 00101011 01011001 0111we get f = 0010 0011010010101010100110111011R1=L0+f(R0,K1)= 1100110000000000110011001111 1111+ 00100011010010101010100110111011= 11101111010010100110010101000100In the next round, we will have L2 = R1, which is the block we just calculated, and then we must calculate R2 =L1 + f(R1,K2), and so on for 4 rounds. At the end of the sixteenth round we have the blocks L4 and R4. We then reverse the order of thetwo blocks into the 64-bit block R16L16 and apply a final permutation IP-1 as defined by the following table:Table 4: IP^ (-1)40 8 48 16 56 24 64 3239 7 47 15 55 23 63 3138 6 46 14 54 22 62 3037 5 45 13 53 21 61 2936 4 44 12 52 20 60 2835 3 43 11 51 19 59 2734 2 42 10 50 18 58 2633 1 41 9 49 17 57 25That is, the output of the algorithm has bit 40 of the pre output block asHow “FOUR-ROUND DES” WorksThis section briefly gives an overview of Four-Rounded DES Algorithm. Four-Rounded DES is composedof substitutions and permutations which take place in four rounds. It is a symmetric cryptosystem which me ansthat both the parties use the same key. Hence, the key must be kept secret.In DES, 64-bit data is divided into left and right halves. In each round, a main function F is applied on right half ofthe data and a sub-key (Ki) of 48 bits. During this process eight S-boxes are used which convert each 6-bit block into 4-bitblock generating 32-bit data. Finally, the left half of data is X-NORed with 32-bit output of the main function.||Issn 2250-3005(online)|| ||December||2012|| Page 198
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ISSN: 2250-3005 - ijcer

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