*PROGRAM*
#include<stdio.h>
#include<conio.h>
#include<math.h>
float encrypt[3][1], decrypt[3][1], a[3][3], b[3][3], mes[3][1], c[3][3];
int encryption();
void decryption();
void Key();
void inverse();
int main()
{
int choice;
do
{
printf("\n\n1. Encryption");
printf("\n2. Decryption");
printf("\n3. Exit");
printf("\nEnter your Choice: ");
scanf("%d",&choice);
switch(choice)
{
case 1:
encryption();
break;
case 2:
decryption();
break;
case 3:
exit(0);
default:
printf("Wrong choice!!!");
}
}while(choice!=3);
return 0;
}
int encryption() {
int i, j, k;
Key();
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((char)(fmod(encrypt[i][0], 26) + 97));
return 0;
}
void decryption() {
int i, j, k;
Key();
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((char)(fmod(decrypt[i][0], 26) + 97));
printf("\n");
}
void Key() {
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("%d %d",&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];
}
*OUTPUT*
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