Data Encryption Standard is a symmetric-key calculation for the encoding the data. It goes

under the square figure calculation which pursues a Feistel structure. Here is the square outline of the Data Encryption Standard.

**Fig1**: DES Algorithm Block Diagram [Image Source: Cryptography and Network Security Principles and Practices fourth Ed by William Stallings]

**Clarification for the above graph**: Each character of plain content changed over into paired organization.

Each time we take 64 bits from that and give as a contribution to DES calculation, at that point,

it handled through 16 adjusts and afterwards changed over to figure content.

**Starting Permutation**: 64 piece plain content goes under the introductory stage and afterwards given to cycle 1.

Since beginning stage step getting 64 bits, it contains a 1×64 grid which contains numbers from 1 to 64 however

in the rearranged request. From that point onward, we orchestrate our unique 64 piece message in the

request referenced in that lattice. [You can see the lattice in beneath code]

After starting stage, 64 piece content went through 16 adjusts. In each round, it prepared with 48 piece key.

That implies we need all-out 16 subkeys, one for each round. See underneath the graph, it

will show what’s going on in each round of calculation.

**Fig2**: Single Round of DES Algorithm. [Image Source: Cryptography and Network Security Principles and Practices fourth Ed by William Stallings]

Round I: In each round 64bit content separated into two 32bit parts. Left and Right.

You can find in chart Li-1 and Ri-1. As calculation says, Right 32bits goes under Expansion Permutation.

Development Permutation: Right side 32bit piece of content given to the extension stage.

It will create a 48bit book as yield. for example, 16bits included this progression.

A few bits beneath 32 are rehashed and masterminded in a 1×48 lattice structure.

We revamp 32bit content by following the request for that grid. [See the grid in underneath code]

After the extension stage, we need to XOR the yield 48bit with a 48bit subkey.

Let perceive how that 48bit sub key producing from 64bit unique key.

**Permutated Choice 1:** Initially we take a 64 piece key and afterwards apply to permutated decision 1.

It contains a 1×56 framework yet with rearranged 1 to 64 numbers aside from products of number 8.

for example 8, 16, 24, 32, 40, 48, 56, 64 will be disposed of. Staying 64-8 = 56 number will be there

in 1×56 network. We revise key in lattice determined request. [You can see the grid in beneath code]

**Left Circular Shift**: 56bit key from permutated decision 1 given to left roundabout move activity.

Here that 56bit key partitioned into equivalent parts of each 28bit. These 28bits moved relies

on the round number. We as of now have the data that in each round what number of bits circularly we need to move.

You can see this data in movements exhibit in code.

**Permutated Choice 2**: Result of Left roundabout move 56bit key given to permutated decision 2.

This progression will deliver 48bit subkey. For this, it has a 1×48 network, where out of 56, some arbitrary 8 bits will be

disposed of. What’s more, staying 48 will be there. As indicated by this bit positions we need to adjust the key.

You can see this grid in beneath code.

Presently yield of permutated decision 2 will be Xor with a yield of the development stage,

which results in a 48bit one. This 48bit again decreased to 32bit utilizing Substitution boxes [called S box].

Substitution boxes [S box]: In DES calculation we have 8 S boxes. Contribution for S box is 48bit.

Furthermore, the yield from S box is 32 piece. The info 48 pieces will be partitioned similarly to 8 s boxes

from s1, s2, … s8. So every s box will get 48/8= 6 bits as info. Each S box decrease 6 bits to 4 bits. i.e

contribution for every S box is 6 bits and yield is 4 bits. At long last, 8*4 = 32 piece. Which is the last yield of S box activity?

Let perceive how 6bits changed over to 4 bits from S box. S box is a 4×16 framework containing

numbers in run 0 to 15. Take an example, accept input 6 bits for S box are 011011. In this first and last piece

together speaks to the column number. Since greatest number with two bits is 3, S box additionally contains

0 to 3 lines aggregate of 4. Also, centre 4 numbers together speak to segment number. Since most extreme number with 4 bits is 15, S box additionally contains

segments 0 to 15 aggregate of 16. So here first and last piece = 01, for example, column number 1 and centre 4 bits 1101= 13 for example section number 13.

So for this information, the number situated at push 1 and section 13 will be picked. As referenced before S

enclose just contains number range 0 to 15. All can be spoken to in 4 bits. So picked number 4 bits are yield for the S box. See the code for all S boxes.

**Change**: After getting yield from all S boxes, we are applying again stage. Here likewise a

lattice with various game plans will be there, we need to mastermind as indicated by that.

**Last XOR**: After this stage, take the left half which at first separated 64bit content to two parts.

Do XOR with this stage yield to left 32bit part. This outcome is the new Right part. What’s more, Right

32bit part which went through all stage will become a new Left Part. These 2 sections will be the

contributions for the second round. Same as keys likewise, the parts before the left move are next round information keys.

This clarification for a solitary round for a 62bit plain book. Like this, it goes through a complete 16 adjusts.

**32 piece swap:** After the culmination of 16 adjusts, last 64 bits isolated into two 32 piece parts and they swap one another.

**Backwards Initial Permutation**: Here additionally a framework will be there, in which bits

are simply rearranged. No including or subtracting bits. See the code for this framework.

#### Program for DES Algorithm in C

```
#include <stdio.h>
int Original_key [64] = { // you can change key if required
0, 0, 0, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0,
0, 1, 0, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 0, 0, 1,
1, 0, 0, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 0, 0,
1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 1
};
int Permutated_Choice1[56] = {
57, 49, 41, 33, 25, 17, 9,
1, 58, 50, 42, 34, 26, 18,
10, 2, 59, 51, 43, 35, 27,
19, 11, 3, 60, 52, 44, 36,
63, 55, 47, 39, 31, 23, 15,
7, 62, 54, 46, 38, 30, 22,
14, 6, 61, 53, 45, 37, 29,
21, 13, 5, 28, 20, 12, 4
};
int Permutated_Choice2[48] = {
14, 17, 11, 24, 1, 5,
3, 28, 15, 6, 21, 10,
23, 19, 12, 4, 26, 8,
16, 7, 27, 20, 13, 2,
41, 52, 31, 37, 47, 55,
30, 40, 51, 45, 33, 48,
44, 49, 39, 56, 34, 53,
46, 42, 50, 36, 29, 32
};
int Iintial_Permutation [64] = {
58, 50, 42, 34, 26, 18, 10, 2,
60, 52, 44, 36, 28, 20, 12, 4,
62, 54, 46, 38, 30, 22, 14, 6,
64, 56, 48, 40, 32, 24, 16, 8,
57, 49, 41, 33, 25, 17, 9, 1,
59, 51, 43, 35, 27, 19, 11, 3,
61, 53, 45, 37, 29, 21, 13, 5,
63, 55, 47, 39, 31, 23, 15, 7
};
int Final_Permutation[] =
{
40, 8, 48, 16, 56, 24, 64, 32,
39, 7, 47, 15, 55, 23, 63, 31,
38, 6, 46, 14, 54, 22, 62, 30,
37, 5, 45, 13, 53, 21, 61, 29,
36, 4, 44, 12, 52, 20, 60, 28,
35, 3, 43, 11, 51, 19, 59, 27,
34, 2, 42, 10, 50, 18, 58, 26,
33, 1, 41, 9, 49, 17, 57, 25
};
int P[] =
{
16, 7, 20, 21,
29, 12, 28, 17,
1, 15, 23, 26,
5, 18, 31, 10,
2, 8, 24, 14,
32, 27, 3, 9,
19, 13, 30, 6,
22, 11, 4, 25
};
int E[] =
{
32, 1, 2, 3, 4, 5,
4, 5, 6, 7, 8, 9,
8, 9, 10, 11, 12, 13,
12, 13, 14, 15, 16, 17,
16, 17, 18, 19, 20, 21,
20, 21, 22, 23, 24, 25,
24, 25, 26, 27, 28, 29,
28, 29, 30, 31, 32, 1
};
int S1[4][16] =
{
14, 4, 13, 1, 2, 15, 11, 8, 3, 10, 6, 12, 5, 9, 0, 7,
0, 15, 7, 4, 14, 2, 13, 1, 10, 6, 12, 11, 9, 5, 3, 8,
4, 1, 14, 8, 13, 6, 2, 11, 15, 12, 9, 7, 3, 10, 5, 0,
15, 12, 8, 2, 4, 9, 1, 7, 5, 11, 3, 14, 10, 0, 6, 13
};
int S2[4][16] =
{
15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10,
3, 13, 4, 7, 15, 2, 8, 14, 12, 0, 1, 10, 6, 9, 11, 5,
0, 14, 7, 11, 10, 4, 13, 1, 5, 8, 12, 6, 9, 3, 2, 15,
13, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14, 9
};
int S3[4][16] =
{
10, 0, 9, 14, 6, 3, 15, 5, 1, 13, 12, 7, 11, 4, 2, 8,
13, 7, 0, 9, 3, 4, 6, 10, 2, 8, 5, 14, 12, 11, 15, 1,
13, 6, 4, 9, 8, 15, 3, 0, 11, 1, 2, 12, 5, 10, 14, 7,
1, 10, 13, 0, 6, 9, 8, 7, 4, 15, 14, 3, 11, 5, 2, 12
};
int S4[4][16] =
{
7, 13, 14, 3, 0, 6, 9, 10, 1, 2, 8, 5, 11, 12, 4, 15,
13, 8, 11, 5, 6, 15, 0, 3, 4, 7, 2, 12, 1, 10, 14, 9,
10, 6, 9, 0, 12, 11, 7, 13, 15, 1, 3, 14, 5, 2, 8, 4,
3, 15, 0, 6, 10, 1, 13, 8, 9, 4, 5, 11, 12, 7, 2, 14
};
int S5[4][16] =
{
2, 12, 4, 1, 7, 10, 11, 6, 8, 5, 3, 15, 13, 0, 14, 9,
14, 11, 2, 12, 4, 7, 13, 1, 5, 0, 15, 10, 3, 9, 8, 6,
4, 2, 1, 11, 10, 13, 7, 8, 15, 9, 12, 5, 6, 3, 0, 14,
11, 8, 12, 7, 1, 14, 2, 13, 6, 15, 0, 9, 10, 4, 5, 3
};
int S6[4][16] =
{
12, 1, 10, 15, 9, 2, 6, 8, 0, 13, 3, 4, 14, 7, 5, 11,
10, 15, 4, 2, 7, 12, 9, 5, 6, 1, 13, 14, 0, 11, 3, 8,
9, 14, 15, 5, 2, 8, 12, 3, 7, 0, 4, 10, 1, 13, 11, 6,
4, 3, 2, 12, 9, 5, 15, 10, 11, 14, 1, 7, 6, 0, 8, 13
};
int S7[4][16]=
{
4, 11, 2, 14, 15, 0, 8, 13, 3, 12, 9, 7, 5, 10, 6, 1,
13, 0, 11, 7, 4, 9, 1, 10, 14, 3, 5, 12, 2, 15, 8, 6,
1, 4, 11, 13, 12, 3, 7, 14, 10, 15, 6, 8, 0, 5, 9, 2,
6, 11, 13, 8, 1, 4, 10, 7, 9, 5, 0, 15, 14, 2, 3, 12
};
int S8[4][16]=
{
13, 2, 8, 4, 6, 15, 11, 1, 10, 9, 3, 14, 5, 0, 12, 7,
1, 15, 13, 8, 10, 3, 7, 4, 12, 5, 6, 11, 0, 14, 9, 2,
7, 11, 4, 1, 9, 12, 14, 2, 0, 6, 10, 13, 15, 3, 5, 8,
2, 1, 14, 7, 4, 10, 8, 13, 15, 12, 9, 0, 3, 5, 6, 11
};
int shifts_for_each_round[16] = { 1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1 };
int _56bit_key[56];
int _48bit_key[17][48];
int text_to_bits[99999], bits_size=0;
int Left32[17][32], Right32[17][32];
int EXPtext[48];
int XORtext[48];
int X[8][6];
int X2[32];
int R[32];
int chiper_text[64];
int encrypted_text[64];
int XOR(int a, int b) {
return (a ^ b);
}
void Dec_to_Binary(int n)
{
int binaryNum[1000];
int i = 0;
while (n > 0) {
binaryNum[i] = n % 2;
n = n / 2;
i++;
}
for (int j = i - 1; j >= 0; j--) {
text_to_bits[bits_size++] = binaryNum[j];
}
}
int F1(int i)
{
int r, c, b[6];
for (int j = 0; j < 6; j++)
b[j] = X[i][j];
r = b[0] * 2 + b[5];
c = 8 * b[1] + 4 * b[2] + 2 * b[3] + b[4];
if (i == 0)
return S1[r][c];
else if (i == 1)
return S2[r][c];
else if (i == 2)
return S3[r][c];
else if (i == 3)
return S4[r][c];
else if (i == 4)
return S5[r][c];
else if (i == 5)
return S6[r][c];
else if (i == 6)
return S7[r][c];
else if (i == 7)
return S8[r][c];
}
int PBox(int pos, int bit)
{
int i;
for (i = 0; i < 32; i++)
if (P[i] == pos + 1)
break;
R[i] = bit;
}
int ToBits(int value)
{
int k, j, m;
static int i;
if (i % 32 == 0)
i = 0;
for (j = 3; j >= 0; j--)
{
m = 1 << j;
k = value & m;
if (k == 0)
X2[3 - j + i] = '0' - 48;
else
X2[3 - j + i] = '1' - 48;
}
i = i + 4;
}
int SBox(int XORtext[])
{
int k = 0;
for (int i = 0; i < 8; i++)
for (int j = 0; j < 6; j++)
X[i][j] = XORtext[k++];
int value;
for (int i = 0; i < 8; i++)
{
value = F1(i);
ToBits(value);
}
}
void expansion_function(int pos, int bit)
{
for (int i = 0; i < 48; i++)
if (E[i] == pos + 1)
EXPtext[i] = bit;
}
void cipher(int Round, int mode)
{
for (int i = 0; i < 32; i++)
expansion_function(i, Right32[Round - 1][i]);
for (int i = 0; i < 48; i++)
{
if (mode == 0)
XORtext[i] = XOR(EXPtext[i], _48bit_key[Round][i]);
else
XORtext[i] = XOR(EXPtext[i], _48bit_key[17 - Round][i]);
}
SBox(XORtext);
for (int i = 0; i < 32; i++)
PBox(i, X2[i]);
for (int i = 0; i < 32; i++)
Right32[Round][i] = XOR(Left32[Round - 1][i], R[i]);
}
void finalPermutation(int pos, int bit)
{
int i;
for (i = 0; i < 64; i++)
if (Final_Permutation[i] == pos + 1)
break;
encrypted_text[i] = bit;
}
void Encrypt_each_64_bit (int plain_bits [])
{
int IP_result [64] , index=0;
for (int i = 0; i < 64; i++) {
IP_result [i] = plain_bits[ Iintial_Permutation[i] ];
}
for (int i = 0; i < 32; i++)
Left32[0][i] = IP_result[i];
for (int i = 32; i < 64; i++)
Right32[0][i - 32] = IP_result[i];
for (int k = 1; k < 17; k++)
{ // processing through all 16 rounds
cipher(k, 0);
for (int i = 0; i < 32; i++)
Left32[k][i] = Right32[k - 1][i]; // right part comes as it is to next round left part
}
for (int i = 0; i < 64; i++)
{ // 32bit swap as well as Final Inverse Permutation
if (i < 32)
chiper_text[i] = Right32[16][i];
else
chiper_text[i] = Left32[16][i - 32];
finalPermutation(i, chiper_text[i]);
}
for (int i = 0; i < 64; i++)
printf("%d", encrypted_text[i]);
}
void convert_Text_to_bits(char *plain_text){
for(int i=0;plain_text[i];i++){
int asci = plain_text[i];
Dec_to_Binary(asci);
}
}
void key56to48(int round, int pos, int bit)
{
int i;
for (i = 0; i < 56; i++)
if (Permutated_Choice2[i] == pos + 1)
break;
_48bit_key[round][i] = bit;
}
int key64to56(int pos, int bit)
{
int i;
for (i = 0; i < 56; i++)
if (Permutated_Choice1[i] == pos + 1)
break;
_56bit_key[i] = bit;
}
void key64to48(int key[])
{
int k, backup[17][2];
int CD[17][56];
int C[17][28], D[17][28];
for (int i = 0; i < 64; i++)
key64to56(i, key[i]);
for (int i = 0; i < 56; i++)
if (i < 28)
C[0][i] = _56bit_key[i];
else
D[0][i - 28] = _56bit_key[i];
for (int x = 1; x < 17; x++)
{
int shift = shifts_for_each_round[x - 1];
for (int i = 0; i < shift; i++)
backup[x - 1][i] = C[x - 1][i];
for (int i = 0; i < (28 - shift); i++)
C[x][i] = C[x - 1][i + shift];
k = 0;
for (int i = 28 - shift; i < 28; i++)
C[x][i] = backup[x - 1][k++];
for (int i = 0; i < shift; i++)
backup[x - 1][i] = D[x - 1][i];
for (int i = 0; i < (28 - shift); i++)
D[x][i] = D[x - 1][i + shift];
k = 0;
for (int i = 28 - shift; i < 28; i++)
D[x][i] = backup[x - 1][k++];
}
for (int j = 0; j < 17; j++)
{
for (int i = 0; i < 28; i++)
CD[j][i] = C[j][i];
for (int i = 28; i < 56; i++)
CD[j][i] = D[j][i - 28];
}
for (int j = 1; j < 17; j++)
for (int i = 0; i < 56; i++)
key56to48(j, i, CD[j][i]);
}
int main(){
char plain_text[] = "tomarrow we wiil be declaring war";
convert_Text_to_bits(plain_text);
key64to48(Original_key); // it creates all keys for all rounds
int _64bit_sets = bits_size/64;
printf("Decrypted output is\n");
for(int i=0;i<= _64bit_sets ;i++) {
Encrypt_each_64_bit (text_to_bits + 64*i);
}
return 0;
}
```

**Output**

```
Decrypted output is
0000111001101001001100011010111010010110111010111111111000010111001011111011111101010011011101011011000000111011100100000010110101000101011000011001000000101000001010011110101001011000111010011001110010110011011110110001101110000000001000001001000110111010
```