DESpseodo Code Intro

8/3/2019 DESpseodo Code Intro.

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DATA ENCRYPTION

STANDARD(DES)

by Engr. Khurram Hashmi


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Sequence

Cryptography

Block Ciphers

DES

Encryption Algorithm

Feistel Function

Pseudo Code


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Cryptography


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Block Ciphers

Block ofplain Text

Size: (M)

CipherEncryption

Algorithm

Block ofcipher Text

Size : (M)

Secret key


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Data Encryption Standard

It is a block Cipher

Data is encrypted block by block i.e. 64 bits each

time

plain Text64 bit blocks

CipherEncryptionAlgorithm

16 cycles

Cipher Text64 bit blocks

Secret key64 bits

N Blocks N Blocks

56 bits used


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Block Division

Divides the Data into N blocks of 64 bit length

Block _size = 64 bit

Assuming that each location in an array stores 1 bit

Input>> input_data [any length ]

Loop: (count bits: 0 to data end)

Initialize: N=0, until all data stored

{

Loop: (count bits: 0 to 63),

initialize: i=0, until : i=63

{

Input_data_block N [ i]= Input_ data[ i]

Increment : i

}

Increment: N a number of blocks

}

128 bit

64 bit 64 bit


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Data is processed as 32 bithalf-blocks

Takes 16 rounds to complete

In each round, a new sub-key from the key schedulerisutilized

Data is swapped after each

round except the last Initial and final permutations

are Inverses of each other

48 bitSub-key

48 bitSub-key

32bit32bit

32bit32bit

XOR


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Encryption Algorithm-Splitting Half

Input _data_block[64 bits]


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Initial Permutation

Loop:( counts :16 rounds)

{

Fiestel Function executed with Inputs:

Half_block_1 [32 bits]

Subkey_N [48 bits]

Cipher_text[32 bit] = Fiestel_output [32 bit] Half_block_2 [32bits]

Such that output is swapped each time before next round except the last round

}

Final Permutation

Output >> Cipher_text [64 bits]

Initial and Final permutations are inverses of each other



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Feistel function

XOR 6-bit input

4-bit output

Enables Swap

Expansion

The S-BoxesNon LinearTransformation

32 bit Data Output

48 bit data


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Feistel function

Expansion:

Loop(until data expansion is complete)

{

halfblock_expnaded[48 bits]


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Feistel function

Input to S-boxes:

the input for each s-box can be stored in a separate array of 6 locations

Loop: (counts 8 cycles) % thereby accounting for the 48 bits: 6*8=48

{

Loop : (counts 6 bits: 0 to 5)

Initialize: i=0, until i=5

{

S-box_input_N [i] = array [i]

increment: i

}

}

Array [48 bit]

S input S input2 S input 8


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Feistel function

Substitution (S-Boxes):

Feistal_output [32 bit array] %will store the output of the s-boxes

Loop: (counts : 8 cycles) %the eight boxes

{

Loop:(counts 4 bits) %Each box gives output of 4 bits

{

Look_up_table : non linear transformation executed

S-box_input_N [i] > transformed> SN- box output [4 bit]

Feistal_output [ i ]


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Look up table

S5

Middle 4 bits of input

0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111

Outer

bits

00 0010 1100 0100 0001 0111 1010 1011 0110 1000 0101 0011 1111 1101 0000 1110 1001

01 1110 1011 0010 1100 0100 0111 1101 0001 0101 0000 1111 1010 0011 1001 1000 0110

10 0100 0010 0001 1011 1010 1101 0111 1000 1111 1001 1100 0101 0110 0011 0000 1110

11 1011 1000 1100 0111 0001 1110 0010 1101 0110 1111 0000 1001 1010 0100 0101 0011

Input: 011011 Output: 1001

Non Linear Transformation


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Feistel function

Output Permutation:

Such that in the next round a Swap occurs

Loop: ( )

{

Outputfixed Permutation on

Feistel_output [32 bit]

}

Output >> Feistel_output [32 bit array]


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Encryption

And Decryption is Similar. . .

plain Text64 bit blocks

CipherEncryptionAlgorithm

16 cycles

Cipher Text64 bit blocks

Secret key64 bits

N Blocks N Blocks

56 bits used


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Questions

Thank You

Documents

DESpseodo Code Intro