PPT On DES

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    Muhammad R

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The Data Encryption Standard was once a predominant symmetric-key algorithm for the encryption of electronic data. It was highly influential in the advancement of modern cryptography in the academic world.This PPT will give a brief idea about DES

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    Data Encryption Standard (DES)
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    Objectives a To review a short history of DES a To define the basic structure of DES a To describe the details of building elements of DES a To describe the round keys generation process a To analyze DES
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    INTRODUCTION The Data Encryption Standard (DES) is a symmetric-key block cipher published by the National Institute of Standards and Technology (NIST).
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    QC History In 1973, NIST published a request for proposals for a national symmetric-key cryptosystem, A proposal from IBM, a modification of a project called Lucifer, was accepted as DES, DES was published in the Federal Register in March 1975 as a draft of the Federal Information Processing Standard (FIPS).
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    QC Overview DES is a block cipher, as shown in Figure. Encryption and decryption with DES 64-bit plaintext DES 56-bit key cipher 64-bit ciphertext 64-bit plaintext DES reverse cipher 64-bit ciphertext
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    DES STRUCTURE The encryption process is made of two permutations (P-boxes), which we call initial and final permutations, and sixteen Feistel rounds.
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    Continue General structure of DES 64-bit plaintext Initial permutation Round I Round 2 Round 16 Final permutation 64-bit ciphertext DES 48-bit 48-bit 48-bit 56-bit cipher key
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    Initial and Final Permutations Initial and final permutation steps in DES 1 1 1 1 2 2 2 2 25 25 16 Rounds 25 25 40 40 40 40 58 58 58 58 64 Initial Permutation 64 64 Final Permutation 64
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    Continue Initial and final permutation tables Initial Permutation Final Permutation 58 60 62 64 57 59 61 63 50 52 54 56 49 51 53 26 18 64 32 63 31 62 30 13 53 16 08 61 29 09 01 60 28 43 35 59 27 11 03 45 37 29 21 13 05 42 10 50 18 15 07 42 44 48 34 46 38 30 40 41 33 25 47 39 31 36 28 20 32 27 10 12 14 02 04 06 22 24 17 19 23 40 39 38 37 36 35 34 33 08 07 06 05 04 03 02 48 46 45 43 41 16 47 15 55 14 44 12 52 11 09 56 54 51 49 24 23 22 21 20 19 58 26 17 57 25 How to read this table? The 58th bit of input x will be the IPO), first bit of output the 50th bit of x is the second bit of IP(x) , etc. The initial and final permutations are straight P-boxes that are inverses of each other. They have no cryptography significance in DES.
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    Continued Example 1 Find the output of the initial permutation box when the input is given in hexadecimal as: oxoooo 0080 0000 0002 Solution Only bit 25 and bit 64 are Is; the other bits are Oso In the final permutation, bit 25 becomes bit 64 and bit 63 becomes bit 15. The result is ox0002 0000 0000 0001
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    Continued Example 2 Prove that the initial and final permutations are the inverse of each other by finding the output of the final permutation if the input is 0000 0000 0001 Solution The input has only two Is; the output must also have only two Is. Using Table 6.1, we can find the output related to these two bits. Bit 15 in the input becomes bit 63 in the output. Bit 64 in the input becomes bit 25 in the output. So the output has only two Is, bit 25 and bit 63. The result in hexadecimal is oxoooo 0080 0000 0002
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    QC Rounds DES uses 16 rounds. Each round of DES is a Feistel cipher. A round in DES (encryption site) 01 32 bits 32 bits RI_I, 32 bits 1-1 32 bits
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    Continued DES Function The heart ofDES is the DES function. The DES function applies a 48-bit key to the rightmost 32 bits to produce a 32-bit output. In f(RI l, KI) 32 bits Expansion P-box DES function 48 bits XOR + 48 bits S-Boxes 32 bits Straight P-box 32 bits Out KI (48 bits)
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    Continue Expansion P-box Since Rl_l is a 32-bit input and is a 48-bit key, we first need to expand Rl_l to 48 bits. Expansion permutation From bit 32 32-bit input sausaxasa» sasmsns7'i usasausm 48-bit output From bit I
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    Continue Although the relationship between the input and output can be defined mathematically, Expansion P-box table 32 04 08 12 16 20 24 28 01 05 09 17 21 25 29 02 06 10 14 18 22 26 31 03 07 11 15 19 23 27 31 04 08 12 16 20 24 28 32 05 09 13 17 21 25 29 01
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    Continue Whitener (XOR) After the expansion permutation, DES uses the XOR operation on the expanded right section and the round key. Note that both the right section and the key are 48- bits in length. Also note that the round key is used only in this operation.
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    Continue S-Boxes The S-boxes do the real mixing (confusion). DES uses 8 S-boxes, each with a 6-bit input and a 4-bit output. S-boxes 48-bit input Array of S-Boxes 32-bit output
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    Continue bit I S-box rule bit 2 bit 3 bit 4 0123 --r--r--r--r S-box bit I Table ent bit 2 bit 3 bit 5 bit 6 15 bit 4
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    Continue Table shows the permutation for S-box 1. For the rest of the boxes see the textbook. S-box 1 14 oo 04 15 04 15 01 12 13 07 14 08 01 04 08 02 02 14 13 04 15 02 06 09 11 13 02 01 08 10 11 07 03 03 15 05 10 06 12 11 10 06 12 09 03 n 12 11 07 14 12 05 09 03 10 13 09 05 10 03 05 06 .15 07 08 13
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    Continued Example The input to S-box 1 is 10001 Solution . What is the output? If we write the first and the sixth bits together, we get 11 in binary, which is 3 in decimal. The remaining bits are 0001 in binary, which is 1 in decimal. We look for the value in row 3, column 1, in Table 6.3 (S-box 1). The result is 12 in decimal, which in binary is 1100, So the input 100011 yields the output 1100
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    Continued Example The input to S-box 8 is 000000. What is the output? Solution If we write the first and the sixth bits together, we get 00 in binary, which is 0 in decimal. The remaining bits are 0000 in binary, which is 0 in decimal, We look for the value in row 0, column 0, in Table for (S-box 8). The result is 13 in decimal, which is 1101 in binary. So the input 000000 yields the output 1101
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    Continue Straight Permutation Straight permutation table 16 01 02 19 07 15 08 20 23 24 30 21 26 14 06 29 05 32 22 12 18 27 11 28 03 04 17 10 09 25

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