hill_cipher.c

#include<stdio.h>
#include<math.h>
#include<string.h>

char plain[20],plain2[20],encrypted[20],x[1],y[1],z[1];
char characters[27]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','\0'}, ciphertext[3], plaintext[3];
int i,j,key2[3][3],k,w,key[2][2],key3[2][2];
int number[3], cipher[3];
int choice,choice2;
int determinant;
char temp;

void encryption2x2(char ch1[], char ch2[])
{
    char plain[3]= {};
    plain[2] = '\0';

    strcat(plain,ch1);
    strcat(plain,ch2);
    printf("\n-----Two-pair character text is: ");
    puts(plain);

    number[0]=0;
    number[1]=0;
    cipher[0]=0;
    cipher[1]=0;
    ciphertext[0] = 0;
    ciphertext[1] = 0;

    for(i=0; i<=1; i++)
    {
        for(j=0; j<=25; j++)
        {
            if( plain[i] == characters[j])
            {
                number[i] = j;
            }
        }
    }

    printf("\nAlphabetical ordered number text will be-\n");
    for(k=0; k <= 1; k++)
        printf("%d  ", number[k]);

    for(i=0; i<=1; i++)
    {
        for(j=0; j<=1; j++)
        {
            cipher[i] = cipher[i] + ( key[i][j] * number[j] );
        }

        cipher[i] = cipher[i]%26;

        for(k=0; k<=25; k++)
        {
            if(cipher[i] == k)
                ciphertext[i] = characters[k];
        }
    }

    printf("\nGenerated Cipher text in alphabetical ordered number text will be-\n");
    for(i=0; i<=1; i++)
        printf("%d ", cipher[i]);

    printf("\n-----Cipher text of the pair will be: ");
    puts(ciphertext);
}

void key_inverse(int key3[2][2])
{
    printf("\nDeterminant of entered key matrix is: ");
    determinant = ( key3[1][1]*key3[0][0] ) - ( key3[0][1]*key3[1][0] ) ;
    determinant = determinant%26;
    printf("%d", determinant);

    printf("\n\nAdjoint of the entered key matrix will be- \n");
    key3[0][1] = -key3[0][1] + 26;
    key3[1][0] = -key3[1][0] + 26;
    temp = key3[0][0];
    key3[0][0] = key3[1][1];
    key3[1][1] = temp;

    for(i=0;i<=1;i++)
    {
        for(j=0;j<=1;j++)
        {
            printf("%d  ",key3[i][j]);
        }
        printf("\n");
    }

    int d;
    int x=0;
    for(x=1; x<25; x++){
        if( (x*determinant)%26 == 1)
            d = x;
    }
    printf("\n D-inverse will be: %d", d);

    for(i=0;i<=1;i++)
    {
        for(j=0;j<=1;j++)
        {
            key3[i][j] = (key3[i][j] * d)%26;
        }
    }

    printf("\nInverse of Key matrix will be- \n");
    for(i=0;i<=1;i++)
    {
        for(j=0;j<=1;j++)
        {
            printf("%d  ",key3[i][j]);
        }
        printf("\n");
    }
}
void decryption2x2(char ch1[], char ch2[])
{
    char encrypt[3]= {};
    encrypt[2] = '\0';

    strcat(encrypt,ch1);
    strcat(encrypt,ch2);
    printf("\n-----Two-pair encrypted character text is: ");
    puts(encrypt);

    number[0]=0;
    number[1]=0;
    cipher[0]=0;
    cipher[1]=0;
    ciphertext[0] = 0;
    ciphertext[1] = 0;

    for(i=0; i<=1; i++)
    {
        for(j=0; j<=25; j++)
        {
            if( encrypt[i] == characters[j])
            {
                number[i] = j;
            }
        }
    }

    printf("\nAlphabetical ordered number text will be-\n");
    for(k=0; k <= 1; k++)
        printf("%d  ", number[k]);

    for(i=0; i<=1; i++)
    {
        for(j=0; j<=1; j++)
        {
            cipher[i] = cipher[i] + ( key3[i][j] * number[j] );
        }

        cipher[i] = cipher[i]%26;

        for(k=0; k<=25; k++)
        {
            if(cipher[i] == k)
                plaintext[i] = characters[k];
        }
    }

    printf("\nGenerated Plain text in alphabetical ordered number text will be-\n");
    for(i=0; i<=1; i++)
        printf("%d ", cipher[i]);

    printf("\n-----Plain text of the pair will be: ");
    puts(plaintext);
}

void encryption3x3(char ch1[], char ch2[], char ch3[])
{
    char plain[4]= {};
    plain[3] = '\0';

    strcat(plain,ch1);
    strcat(plain,ch2);
    strcat(plain, ch3);
    printf("\n-----Three-pair character text is:-----\n");
    puts(plain);

    number[0]=0;
    number[1]=0;
    number[2]=0;
    cipher[0]=0;
    cipher[1]=0;
    cipher[2]=0;
    ciphertext[0] = 0;
    ciphertext[1] = 0;
    ciphertext[2] = 0;

    for(i=0; i<=2; i++)
    {
        for(j=0; j<=25; j++)
        {
            if( plain[i] == characters[j])
            {
                number[i] = j;
            }
        }
    }

    printf("\nAlphabetical ordered number text will be-\n");
    for(k=0; k <= 2; k++)
        printf("%d  ", number[k]);

    for(i=0; i<=2; i++)
    {
        for(j=0; j<=2; j++)
        {
            cipher[i] = cipher[i] + ( key2[i][j] * number[j] );
        }

        cipher[i] = cipher[i]%26;

        for(k=0; k<=25; k++)
        {
            if(cipher[i] == k)
                ciphertext[i] = characters[k];
        }
    }

    printf("\nGenerated Cipher text in alphabetical ordered number text will be-\n");
    for(i=0; i<=2; i++)
        printf("%d ", cipher[i]);

    printf("\nCipher text of the pair will be: ");
    puts(ciphertext);
}

void main()
{
    printf("Hill Cipher");
    do{
    printf("\nMAIN MENU");
    printf("\n1. 2x2 encryption/decryption \t2. 3x3 encryption \t3. EXIT");
    printf("\nEnter your choice:");
    scanf("%d", &choice);
    switch(choice)
    {
        case 1: do{
                printf("\n--------2x2 Hill Cipher-----------");
                printf("\n1. Encryption\t2. Decryption\t3. EXIT");
                printf("\nEnter your choice: ");
                scanf("%d", &choice2);
                switch(choice2)
                {
                    case 1: printf("\nEnter the plain text: ");
                            scanf("%s", &plain);

                            printf("\nEnter the 2*2 Key Matrix-");
                            for(i=0;i<=1;i++)
                            {
                                for(j=0;j<=1;j++)
                                {
                                    scanf("%d",&key[i][j]);
                                }
                            }
                            printf("\nKey is:\n");
                            for(i=0;i<=1;i++)
                            {
                                for(j=0;j<=1;j++)
                                {
                                    printf("%d  ",key[i][j]);
                                }
                                printf("\n");
                            }
                            printf("\n-------------------Encrypting each two-character pair of plain text-----------------------\n\n");

                            for(w=0; plain[w] != '\0'; ){

                                x[0] = plain[w];
                                x[1] = '\0';

                                y[0] = plain[w+1];
                                y[1] = '\0';

                                encryption2x2(x, y);
                                w = w + 2;
                            }
                            break;

                    case 2: printf("\nEnter the cipher text: ");
                            scanf("%s", &encrypted);

                            printf("\nEnter the 2*2 Key Matrix-");
                            for(i=0;i<=1;i++)
                            {
                                for(j=0;j<=1;j++)
                                {
                                    scanf("%d",&key3[i][j]);
                                }
                            }
                            printf("\nEntered Key is:\n");
                            for(i=0;i<=1;i++)
                            {
                                for(j=0;j<=1;j++)
                                {
                                    printf("%d  ",key3[i][j]);
                                }
                                printf("\n");
                            }

                            key_inverse(key3);

                            printf("\n-------------------Decrypting each two-character pair of plain text-----------------------\n\n");

                            for(w=0; encrypted[w] != '\0'; ){

                                x[0] = encrypted[w];
                                x[1] = '\0';

                                y[0] = encrypted[w+1];
                                y[1] = '\0';

                                decryption2x2(x, y);
                                w = w + 2;
                            }

                            break;
                }
                }while( choice2 != 3);
                break;


        case 2: printf("\n--------3x3 Hill Cipher-----------");
                printf("\nEnter the plain text: ");
                scanf("%s", &plain2);

                printf("\nEnter the 3*3 Key Matrix-");
                for(i=0;i<=2;i++)
                {
                    for(j=0;j<=2;j++)
                    {
                        scanf("%d",&key2[i][j]);
                    }
                }
                printf("\nKey is:\n");
                for(i=0;i<=2;i++)
                {
                    for(j=0;j<=2;j++)
                    {
                        printf("%d  ",key2[i][j]);
                    }
                    printf("\n");
                }

                printf("\n-------------------Encrypting each three-character pair of plain text-----------------------\n\n");

                for(w=0; plain2[w] != '\0'; ){

                    x[0] = plain2[w];
                    x[1] = '\0';

                    y[0] = plain2[w+1];
                    y[1] = '\0';

                    z[0] = plain2[w+2];
                    z[1] = '\0';

                    encryption3x3(x, y, z);
                    w = w + 3;
                }
                break;

        }
    }while(choice != 3);
}