ISSN: 2250-3005 - ijcer

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International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8DES works by encrypting groups of 64 message bits, which is the same as 16 hexadecimal numbers. To do theencryption, DES uses "keys" where are also apparently 16 hexadecimal numbers long, or apparently 64 bits long. However,every 8th key bit is ignored in the DES algorithm, so that the effective key size is 56 bits. But, in any case, 64 bits (16hexadecimal digits) is the round number upon which DES is organized.For example, if we take the plaintext message "8787878787878787", and encrypt it with the DES key"0E329232EA6D0D73", we end up with the ciphertext "0000000000000000". If the ciphertext is decrypted with the samesecret DES key "0E329232EA6D0D73", the result is the original pla intext "8787878787878787".This example is neat and orderly because our plaintext was exactly 64 bits long. The same would be true if theplaintext happened to be a multiple of 64 bits. But most messages will not fall into this category. They will not be an exactmultiple of 64 bits (that is, an exact multiple of 16 hexadecimal numbers).For example, take the message "Your lips are smoother than vaseline". This plaintext message is 38 bytes (76hexadecimal digits) long. So this message must be padded with some extra bytes at the tail end for the encryption. Once theencrypted message has been decrypted, these extra bytes are thrown away. There are, of course, different padding schemes --different ways to add extra bytes. Here we will just add 0s at the end, so that the total message is a multiple of 8 bytes (or 16hexadecimal digits, or 64 bits).The plaintext message "Your lips are smoother than vaseline" is, in hexadecimal, "596F7572206C6970732061726520736D 6F6F746865722074 68616E2076617365 6C696E650D0A" .(Note here that the first 72 hexadecimal digits represent the English message, while "0D" is hexadecimal for Carriage Return,and "0A" is hexadecimal for Line Feed, showing that the message file has terminated.) We then pad this message with some 0son the end, to get a total of 80 hexadecimal digits:"596F7572206C6970 732061726520736D 6F6F746865722074 68616E2076617365 6C696E650D0A0000".If we then encrypt this plaintext message 64 bits (16 hexadecimal digits) at a time, using the same DES key"0E329232EA6D0D73" as before, we get the ciphertext:"C0999FDDE378D7ED 727DA00BCA5A84EE 47F269A4D6438190 9DD52F78F5358499 828AC9B453E0E653".This is the secret code that can be transmitted or stored. Decrypting the ciphertext restores the original message"Your lips are smoother than vaseline". (Think how much better off Bill Clinton would be today, if Monica Lewinsky hadused encryption on her Pentagon computer!)DES is a block cipher--meaning it operates on plaintext blocks of a given size (64-bits) and returns ciphertext blocks of thesame size. Thus DES results in a permutationamong the 2^64 (read this as: "2 to the 64th power") possible arrangements of 64bits, each of which may be either 0 or 1. Each block of 64 bits is divided into two blocks of 32 bits each, a left halfblock L and a right half R. (This division is only used in certain operations.)Example: Let M be the plain text message M = 0123456789ABCDEF, where M is in hexadecimal (base 16) format.Rewriting M in binary format, we get the 64-bit block of text:M = 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111L = 0000 0001 0010 0011 0100 0101 0110 0111R = 1000 1001 1010 1011 1100 1101 1110 1111The first bit of M is "0". The last bit is "1". We read from left to right.DES operates on the 64-bit blocks using key sizes of 56- bits. The keys are actually stored as being 64 bits long, but every 8thbit in the key is not used (i.e. bits numbered 8, 16, 24, 32, 40, 48, 56, and 64). However, we will nevertheless number the b itsfrom 1 to 64, going left to right, in the following calculations. But, as you will see, the eight bits just mentioned get eli minatedwhen we create subkeys.Example: Let K be the hexadecimal key K = 133457799BBCDFF1. This gives us as the binary key (setting 1 = 0001,3 = 0011, etc., and grouping together every eight bits, of which the last one in each group will be unused):K = 00010011 00110100 01010111 01111001 10011011 10111100 11011111 11110001The DES algorithm uses the following steps:Step 1: Create 4 sub-keys, each of which is 48- bits long. The 64-bit key is permuted according to the following table, PC-1.Since the first entry in the table is "57", this means that the 57th bit of the original key K becomes the first bit of the p ermutedkey K+. The 49th bit of the original key becomesthesecond bit of the permuted key. The 4th bit of the original key is the lastbit of the permuted key. Note only 56 bits of the original key appear in the permuted key. Example: From the original 64-bitkey||Issn 2250-3005(online)|| ||December||2012|| Page 195
International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8K =11111111111111100000000000000001010101 0101010100101010101010101 we get the 56-bit permutationK+=00110011110000110011001111000011001111000011001100110011Next, split this key into left and right halves, C0 and D0, where each half has 28 bits. Example: From the permuted key K+,we getC0 = 0011001111000011001100111100D0 = 0011001111000011001100110011Table 1: PC-157 49 41 33 25 17 91 58 50 42 34 26 1810 2 59 51 43 35 2719 11 3 60 52 44 3663 55 47 39 31 23 157 62 54 46 38 30 2214 6 61 53 45 37 2921 13 5 28 20 12 4With C0 and D0 defined, we now create sixteen blocks Cn andDn, 1
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ISSN: 2250-3005 - ijcer

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