//C++ program for encryption and decryption #include #include #include #include using namespace std; int x, y, n, t, i, flag; long int e[50], d[50], temp[50], j; char en[50], m[50]; char msg[100]; int prime(long int); //function to check for prime number void encryption_key(); long int cd(long int); void encrypt(); void decrypt(); int main() { cout << "\nENTER FIRST PRIME NUMBER\n"; cin >> x; //checking whether input is prime or not flag = prime(x); if(flag == 0) { cout << "\nINVALID INPUT\n"; exit(0); } cout << "\nENTER SECOND PRIME NUMBER\n"; cin >> y; flag = prime(y); if(flag == 0 || x == y) { cout << "\nINVALID INPUT\n"; exit(0); } cout << "\nENTER MESSAGE OR STRING TO ENCRYPT\n"; cin >> msg; for(i = 0; msg[i] != NULL; i++) m[i] = msg[i]; n = x * y; t = (x - 1) * (y - 1); encryption_key(); cout << "\nPOSSIBLE VALUES OF e AND d ARE\n"; for(i = 0; i < j - 1; i++) cout << "\n" << e[i] << "\t" << d[i]; encrypt(); decrypt(); return 0; } //end of the main program int prime(long int pr) { int i; j = sqrt(pr); for(i = 2; i <= j; i++) { if(pr % i == 0) return 0; } return 1; } //function to generate encryption key void encryption_key() { int k; k = 0; for(i = 2; i < t; i++) { if(t % i == 0) continue; flag = prime(i); if(flag == 1 && i != x && i != y) { e[k] = i; flag = cd(e[k]); if(flag > 0) { d[k] = flag; k++; } if(k == 99) break; } } } long int cd(long int a) { long int k = 1; while(1) { k = k + t; if(k % a == 0) return(k/a); } } //function to encrypt the message void encrypt() { long int pt, ct, key = e[0], k, len; i = 0; len = strlen(msg); while(i != len) { pt = m[i]; pt = pt - 96; k = 1; for(j = 0; j < key; j++) { k = k * pt; k = k % n; } temp[i] = k; ct= k + 96; en[i] = ct; i++; } en[i] = -1; cout << "\n\nTHE ENCRYPTED MESSAGE IS\n"; for(i=0; en[i] != -1; i++) cout << en[i]; } //function to decrypt the message void decrypt() { long int pt, ct, key = d[0], k; i = 0; while(en[i] != -1) { ct = temp[i]; k = 1; for(j = 0; j < key; j++) { k = k * ct; k = k % n; } pt = k + 96; m[i] = pt; i++; } m[i] = -1; cout << "\n\nTHE DECRYPTED MESSAGE IS\n"; for(i = 0; m[i] != -1; i++) cout << m[i]; cout << endl; }