To Implement Simplified Data Encryption Standard.

 Program:

S-DES_KeyGeneration And Encryption

#include <stdio.h>

#include <string.h>

int s_matrix0[4][4] = {{1, 0, 3, 2},

                       {3, 2, 1, 0},

                       {0, 2, 1, 3},

                       {3, 1, 3, 2}};

int s_matrix1[4][4] = {{0, 1, 2, 3},

                       {2, 0, 1, 3},

                       {3, 0, 1, 0},

                       {2, 1, 0, 3}};

int s0 = 0, s1 = 0;

int row = 0, col = 0;

int s0_binary[2], s1_binary[2];

int result[2];

int to_digit(int a, int b)

{

    int output;

    if (a == 1 && b == 1)

        output = 3;

    if (a == 0 && b == 1)

        output = 1;

    if (a == 1 && b == 0)

        output = 2;

    if (a == 0 && b == 0)

        output = 0;

    return output;

}

void to_binary(int num)

{

    int x;

    if (num == 3)

    {

        for (x = 0; x < 2; x++)

            result[x] = 1;

    }

    else if (num == 1)

    {

        result[0] = 0;

        result[1] = 1;

    }

    else if (num == 2)

    {

        result[0] = 1;

        result[1] = 0;

    }

    else if (num == 0)

    {

        for (x = 0; x < 2; x++)

            result[x] = 0;

    }

}

void main()

{

    int i, j, index;

    int k1[8], k2[8];

    int afterp10[11], ls1[10], ls2[10], afterip[8], afterep[8], afterp4[4];

    int aftersboxesone[4], aftersboxestwo[4];

    int leftafterip[4], rightafterip[4];

    int leftafterep[4], rightafterep[4];

    int leftafterone[4], rightafterone[4];

    int afteripinverse[8];

    int afterone[8], aftertwo[8];

    int p10[10] = {3, 5, 2, 7, 4, 10, 1, 9, 8, 6};

    int p8[8] = {6, 3, 7, 4, 8, 5, 10, 9};

    int ip[8] = {2, 6, 3, 1, 4, 8, 5, 7};

    int ep[8] = {4, 1, 2, 3, 2, 3, 4, 1};

    int p4[4] = {2, 4, 3, 1};

    int ipinverse[8] = {4, 1, 3, 5, 7, 2, 8, 6};

    int key[11] = {1, 0, 1, 0, 0, 0, 0, 0, 1, 0};

    int plain[8] = {0, 1, 1, 0, 1, 1, 0, 1};

    printf("S-DES Key Generation and Encryption.\n");

    printf("\n----------KEY GENERATION----------\n");

    printf("\nEntered the 10-bit key is: ");

    for (i = 0; i < 10; i++)

        printf("%d ", key[i]);

    printf("\np10 permutation is defined as: ");

    for (i = 0; i <= 9; i++)

        printf("%d ", p10[i]);

    for (i = 0; i <= 9; i++)

    {

        index = p10[i];

        afterp10[i] = key[index - 1];

    }

    afterp10[i] = '\0';

    printf("\n\nAfter p10 = ");

    for (i = 0; i <= 9; i++)

        printf("%d ", afterp10[i]);

    for (i = 0; i < 5; i++)

    {

        if (i == 4)

            ls1[i] = afterp10[0];

        else

            ls1[i] = afterp10[i + 1];

    }

    for (i = 5; i < 10; i++)

    {

        if (i == 9)

            ls1[i] = afterp10[5];

        else

            ls1[i] = afterp10[i + 1];

    }

    printf("\n\nAfter LS-1 = ");

    for (i = 0; i < 10; i++)

        printf("%d ", ls1[i]);

    printf("\np8 permutation is defined as: ");

    for (i = 0; i < 8; i++)

        printf("%d ", p8[i]);

    index = 0;

    for (i = 0; i <= 9; i++)

    {

        index = p8[i];

        k1[i] = ls1[index - 1];

    }

    printf("\n\n---->Key-1 is: ");

    for (i = 0; i < 8; i++)

        printf("%d ", k1[i]);

    for (i = 0; i < 3; i++)

    {

        ls2[i] = ls1[i + 2];

    }

    ls2[3] = ls1[0];

    ls2[4] = ls1[1];

    for (i = 5; i < 8; i++)

    {

        ls2[i] = ls1[i + 2];

    }

    ls2[8] = ls1[5];

    ls2[9] = ls1[6];

    printf("\n\nAfter LS-2 = ");

    for (i = 0; i < 10; i++)

        printf("%d ", ls2[i]);

    printf("\np8 permutation is defined as: ");

    for (i = 0; i < 8; i++)

        printf("%d ", p8[i]);

    index = 0;

    for (i = 0; i <= 9; i++)

    {

        index = p8[i];

        k2[i] = ls2[index - 1];

    }

    printf("\n\n---->Key-2 is: ");

    for (i = 0; i < 8; i++)

        printf("%d ", k2[i]);

    printf("\n\n----------S-DES ENCRYPTION---------\n");

    printf("\n---->Entered the 8-bit plaintext is: ");

    for (i = 0; i < 8; i++)

        printf("%d ", plain[i]);

    printf("\nInitial permutation is defined as: ");

    for (i = 0; i < 8; i++)

        printf("%d ", ip[i]);

    for (i = 0; i < 8; i++)

    {

        index = ip[i];

        afterip[i] = plain[index - 1];

    }

    afterip[i] = '\0';

    printf("\nAfter ip = ");

    for (i = 0; i < 8; i++)

        printf("%d ", afterip[i]);

    printf("\n\nExpand permutation is defined as: ");

    for (i = 0; i < 8; i++)

        printf("%d ", ep[i]);

    for (j = 0; j < 4; j++)

        leftafterip[j] = afterip[j];

    for (j = 0; j < 4; j++)

        rightafterip[j] = afterip[j + 4];

    for (i = 0; i < 4; i++)

    {

        index = ep[i];

        afterep[i] = rightafterip[index - 1];

    }

    for (i = 4; i < 8; i++)

    {

        index = ep[i];

        afterep[i] = rightafterip[index - 1];

    }

    afterep[i] = '\0';

    printf("\nAfter ep = ");

    for (i = 0; i < 8; i++)

        printf("%d ", afterep[i]);

    for (i = 0; i < 8; i++)

        k1[i] = k1[i] ^ afterep[i];

    printf("\n\nAfter XOR operation with 1st Key= ");

    for (i = 0; i < 8; i++)

        printf("%d ", k1[i]);

    row = to_digit(k1[0], k1[3]);

    col = to_digit(k1[1], k1[2]);

    s0 = s_matrix0[row][col];

    to_binary(s0);

    for (j = 0; j < 2; j++)

        s0_binary[j] = result[j];

    row = to_digit(k1[4], k1[7]);

    col = to_digit(k1[5], k1[6]);

    s1 = s_matrix1[row][col];

    to_binary(s1);

    for (j = 0; j < 2; j++)

        s1_binary[j] = result[j];

    for (j = 0; j < 2; j++)

        aftersboxesone[j] = s0_binary[j];

    for (i = 0, j = 2; i < 2, j < 4; i++, j++)

        aftersboxesone[j] = s1_binary[i];

    printf("\n\nAfter first S-Boxes= ");

    for (i = 0; i < 4; i++)

        printf("%d ", aftersboxesone[i]);

    printf("\n\nP4 is defined as: ");

    for (i = 0; i < 4; i++)

        printf("%d ", p4[i]);

    for (i = 0; i < 4; i++)

    {

        index = p4[i];

        afterp4[i] = aftersboxesone[index - 1];

    }

    afterp4[i] = '\0';

    printf("\nAfter P4= ");

    for (i = 0; i < 4; i++)

        printf("%d ", afterp4[i]);

    for (i = 0; i < 4; i++)

        afterp4[i] = afterp4[i] ^ leftafterip[i];

    printf("\n\nAfter XOR operation with left nibble of after ip= ");

    for (i = 0; i < 4; i++)

        printf("%d ", afterp4[i]);

    for (i = 0; i < 4; i++)

        afterone[i] = rightafterip[i];

    for (i = 0, j = 4; i < 4, j < 8; i++, j++)

        afterone[j] = afterp4[i];

    afterone[j] = '\0';

    printf("\nAfter first part= ");

    for (i = 0; i < 8; i++)

        printf("%d ", afterone[i]);

    for (j = 0; j < 4; j++)

        leftafterone[j] = afterone[j];

    for (j = 0; j < 4; j++)

        rightafterone[j] = afterone[j + 4];

    printf("\n\nExpand permutation is defined as: ");

    for (i = 0; i < 8; i++)

        printf("%d ", ep[i]);

    for (i = 0; i < 4; i++)

    {

        index = ep[i];

        afterep[i] = rightafterone[index - 1];

    }

    for (i = 4; i < 8; i++)

    {

        index = ep[i];

        afterep[i] = rightafterone[index - 1];

    }

    afterep[i] = '\0';

    printf("\nAfter second ep = ");

    for (i = 0; i < 8; i++)

        printf("%d ", afterep[i]);

    for (i = 0; i < 8; i++)

        k2[i] = k2[i] ^ afterep[i];

    printf("\n\nAfter XOR operation with 2nd Key= ");

    for (i = 0; i < 8; i++)

        printf("%d ", k2[i]);

    row = to_digit(k2[0], k2[3]);

    col = to_digit(k2[1], k2[2]);

    s0 = s_matrix0[row][col];

    to_binary(s0);

    for (j = 0; j < 2; j++)

        s0_binary[j] = result[j];

    row = to_digit(k2[4], k2[7]);

    col = to_digit(k2[5], k2[6]);

    s1 = s_matrix1[row][col];

    to_binary(s1);

    for (j = 0; j < 2; j++)

        s1_binary[j] = result[j];

    for (j = 0; j < 2; j++)

        aftersboxestwo[j] = s0_binary[j];

    for (i = 0, j = 2; i < 2, j < 4; i++, j++)

        aftersboxestwo[j] = s1_binary[i];

    printf("\n\nAfter second S-Boxes= ");

    for (i = 0; i < 4; i++)

        printf("%d ", aftersboxestwo[i]);

    printf("\n\nP4 is defined as: ");

    for (i = 0; i < 4; i++)

        printf("%d ", p4[i]);

    for (i = 0; i < 4; i++)

    {

        index = p4[i];

        afterp4[i] = aftersboxestwo[index - 1];

    }

    afterp4[i] = '\0';

    printf("\nAfter P4= ");

    for (i = 0; i < 4; i++)

        printf("%d ", afterp4[i]);

    for (i = 0; i < 4; i++)

        afterp4[i] = afterp4[i] ^ leftafterone[i];

    printf("\n\nAfter XOR operation with left nibble of after first part= ");

    for (i = 0; i < 4; i++)

        printf("%d ", afterp4[i]);

    for (i = 0; i < 4; i++)

        aftertwo[i] = afterp4[i];

    for (i = 0, j = 4; i < 4, j < 8; i++, j++)

        aftertwo[j] = rightafterone[i];

    aftertwo[j] = '\0';

    printf("\n\nAfter second part= ");

    for (i = 0; i < 8; i++)

        printf("%d ", aftertwo[i]);

    printf("\n\nInverse Initial permutation is defined as: ");

    for (i = 0; i < 8; i++)

        printf("%d ", ipinverse[i]);

    for (i = 0; i < 8; i++)

    {

        index = ipinverse[i];

        afteripinverse[i] = aftertwo[index - 1];

    }

    afteripinverse[j] = '\0';

    printf("\n\n--->8-bit Ciphertext will be= ");

    for (i = 0; i < 8; i++)

        printf("%d ", afteripinverse[i]);

}

Output: