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COT5937Lec16

# COT5937Lec16 - Cryptography(COT 5937 Lecture 16 Data...

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Cryptography(COT 5937) Lecture 16 7/10/01 Data Encryption Standard(DES) Here is the basic algorithm used for DES: To encrypt a plaintext x of 64 bits and a secret key K of 56 bits do the following: 1) Compute x 0 = IP(x), a fixed permutation of the bits in x. IP is specified in the text. 2) Let x i = L i R i , for 0 i 16, where L i is the 32 leftmost bits of x i and R i is the 32 rightmost bits of x i . Make the following sequence of computations: for (i=1 to 16) { L i = R i-1 R i = L i-1 f(R i-1 , K i ) } Essentially, each loop iteration is known as a Feistel round. (Feistel is the creator of DES.) DEs comprises 16 of these rounds. Each round encrypts ½ of the bits from the pervious round. The function f and the key for the ith round K i will be discussed in detail later in these notes. 3) y = IP -1 (R 16 L 16 ), this means applying the inverse permutation applied in step 1 to the string R 16 L 16 . (Notice the “reverse” order of the two blocks L 16 and R 16 .) In essence, you would repeat this process for every block of 64 bits that needs to be encrypted.

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