DES | DATA ENCRYPTION STANDARD

DES (DATA ENCRYPTION STANDARD)

This is a symmetric key algorithm.

Input is taken à 64 bits

Key is taken à 56 bits

Output is taken à 64 bits

In case of this symmetric key algorithm, the size of the output is either equal or lesser than the size of the input. Generally the size of the output is longer than the input.

16 rounds processing to get it done.

In 1976, a Professor from Santa Clara University Edward found Simplified DES to make a symmetric more simple.

Input à 5 bits

Key à10 bits

Output à 8 bits

P10 : Permutation applied on 10 bits

 P8 : Permutation applied on 8 bits

IP : Initial Permutation

Fk : Complex function

SW : Switch function

Ip^-1 : Inverse of initial Permutation

Let’s try the exercise !

10 bit keyàp10àShiftàp8àShiftàp8

10 bit key = 1010000010

Step 1 à Apply  P10 on the 10 bit key

Given p10 =  3 5 2 7 4 10 1 9 8 6

#      = 1 2 3 4 5 6 7 8 9 10

Key = 1 0 1 0 0 0 0 0 1  0

Answer P10 à 10000 01100

Step 2 : Apply left circular shift separately on second 5 bit

1 0 0 0  0         0 1 1 0 0

1 2 3 4  5         1 2 3 4 5

Start from #2  which is 0 and 3^rd which is 0 (reverse) and 4^th is 0  and 5 which is 0 and 1 which 1

And the result will be 00001 and the rest also start from #2 so the result is 11000

Answer Shift à 0000111000

Step 3 à Apply p8 on the output step 2

Given P8 : 6 3 7 4 8 5 10 9

#      = 1 2 3 4 5 6 7 8 9 10

Key = 0 0 0 0 1 1 1 0 0 0

Answer P8 à 1 0 1 0 0  1 0 0  (k1 Subkey)

Step 4 à Apply left cecular  by 2 bits on output of step 2

0 0 0 0 1               1 1 0 0 0

1 2 3 4 5               1 2 3 4 5

Start from #2  which is 0 and 3^rd which is 0 (reverse) and 4^th is 0  and 5 which is 0 and 1 which 1

And the result will be 00100 and the rest also start from #2 so the result is 00011
Answe Shift à 00100 00011 (k2)

Step 5 à Apply p8 on output of step 4

Given p8 : 6 3 7 4 8 5 10 9

#      = 1 2 3 4 5 6 7 8 9 10

Key = 0 0 1 0 0 0 0 0 1  1

Final Cipher text à 0 1 0 0 0 0 1

Encryption

8 bits Plain text à IPà FKàSWàFKàIP à 8 bits Cipher text

Suppose : PT : 1011101

Step 1 : Initial Permutation

Given à 2 6 3 1 4 8 5 7

#      = 1 2 3 4 5 6 7 8

Key = 1 0 1 1 1 1 0 1

Answer p8 à 0 1 1 1 1 1 1 0

Step 2 : Complex function is combination of substitution and permutation.

0 1 1 1                   1 1 1 0

    L                            R

F_k (L₁R) = L + F (R,SK),R)

Where + is Exclusive OR

F  is Mapping Function

R is Right side

SK is Subkey à K₁

L is left side

Let’s see the result k1 and k2 in previous steps

K1 : 10100100

K2 : 01000011

Suppose output of F (1110,10100100) = 1101

L = 0111

Fk (0111,1110)=(0111+1101,1110)

XOR

0 1 1 1

1 1 0 1

1 0 1 0   à output

Fk = ( 1010,  1110)

             L         R

Step 3 à Switch function

 ( the left L is shifted to R and R is shifted to L / interchanging of L&R)

( 1010,  1110)

    R         L

Step 4 : Complec funcion

Fk (1110,1010)= (1110+F (1010,01000011),1010)

Fk(1010,01000011)= 1100

Fk(1110,1010)=(1110+1100,1010)

                       = (0010,1010)

XOR

1 1 1 0

1 1 0 0

0 0 1 0

Step 5 : Apply IP^-1 to output of step 4

Geven : 4 1  3 5 7 2 8 6

#      = 1 2 3 4 5 6 7 8

Key = 0 0 1 0 1 0 1 0

Final Cipher text à 0 0 1 1 1 0 0 0