Check out example codes for "hill cipher encryption in c". It will help you in understanding the concepts better.

Code Example 1

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

float encrypt[3][1], decrypt[3][1], a[3][3], b[3][3], mes[3][1], c[3][3];

void encryption();	//encrypts the message
void decryption();	//decrypts the message
void getKeyMessage();	//gets key and message from user
void inverse();		//finds inverse of key matrix

void main() {
getKeyMessage();
encryption();
decryption();
}

void encryption() {
int i, j, k;

for(i = 0; i < 3; i++)
for(j = 0; j < 1; j++)
for(k = 0; k < 3; k++)
encrypt[i][j] = encrypt[i][j] + a[i][k] * mes[k][j];

printf("\nEncrypted string is: ");
for(i = 0; i < 3; i++)
printf("%c", (char)(fmod(encrypt[i][0], 26) + 97));

}

void decryption() {
int i, j, k;

inverse();

for(i = 0; i < 3; i++)
for(j = 0; j < 1; j++)
for(k = 0; k < 3; k++)
decrypt[i][j] = decrypt[i][j] + b[i][k] * encrypt[k][j];

printf("\nDecrypted string is: ");
for(i = 0; i < 3; i++)
printf("%c", (char)(fmod(decrypt[i][0], 26) + 97));

printf("\n");
}

void getKeyMessage() {
int i, j;
char msg[3];

printf("Enter 3x3 matrix for key (It should be inversible):\n");

for(i = 0; i < 3; i++)
for(j = 0; j < 3; j++) {
scanf("%f", &a[i][j]);
c[i][j] = a[i][j];
}

printf("\nEnter a 3 letter string: ");
scanf("%s", msg);

for(i = 0; i < 3; i++)
mes[i][0] = msg[i] - 97;
}

void inverse() {
int i, j, k;
float p, q;

for(i = 0; i < 3; i++)
for(j = 0; j < 3; j++) {
if(i == j)
b[i][j]=1;
else
b[i][j]=0;
}

for(k = 0; k < 3; k++) {
for(i = 0; i < 3; i++) {
p = c[i][k];
q = c[k][k];

for(j = 0; j < 3; j++) {
if(i != k) {
c[i][j] = c[i][j]*q - p*c[k][j];
b[i][j] = b[i][j]*q - p*b[k][j];
}
}
}
}

for(i = 0; i < 3; i++)
for(j = 0; j < 3; j++)
b[i][j] = b[i][j] / c[i][i];

printf("\n\nInverse Matrix is:\n");
for(i = 0; i < 3; i++) {
for(j = 0; j < 3; j++)
printf("%d ", b[i][j]);

printf("\n");
}
}``````

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