RSA

//Program for RSA asymmetric cryptographic algorithm
//for demonstration values are relatively small compared to practical application
#include<stdio.h>
#include <string.h>
#include<stdbool.h>
#include<math.h>

bool prime (long n) {
    for (long i = 2; i < sqrt(n); i++) {
        if (n % i == 0) return false;
    }
    return true;
}

//to find gcd
long gcd(long a, long h) {
    long temp = 0;
    while (h != 0) {
        temp = a % h;
        a = h;
        h = temp;
    }
    return a;
}

//to fast pow
long binPow(long a, long b) {
    if (b == 0) return 1;
    else {
        if (b % 2 == 1) return a * (binPow(a, b - 1));
        else return binPow(a*a, b/2);
    }
}

long p = 0;
long q = 0;
long n = 0;
long d = 0;
long f = 0;
long e = 0;
long msg = 0;
long c = 0;
void creatKeyPairs() {
    printf("First prime number: ");
    do scanf(" %ld", &p); while(!prime(p));
    printf("First prime number: ");
    do scanf(" %ld", &q); while(!prime(q));
    n = p*q;
    f = (p-1)*(q-1);
    printf("E: ");
    while (e < f) {
        if (gcd(e, f) == 1) break;
        else scanf("%ld", &e);
    }
    d = (1+(2*f))/e;
    printf("%ld:%ld\n", e, n);
}
void encrypt() {
    printf("Encryption\n");
    printf("e: ");
    scanf("%ld", &e);
    printf("n: ");
    scanf("%ld", &n);
    printf("msg: ");
    scanf("%ld", &msg);
    c = binPow(msg,e)%n;
    printf("Encrypted: %ld", c);
}
void decrypt() {
    printf("Decryption\n");
    creatKeyPairs();
    printf("Give a c: ");
    scanf("%ld", &c);
    msg = binPow(c,d)%n;
    printf("Original msg: %ld", msg);
}
int main()
{
    printf("e = 1 : d = any ");
    int decision = 0;
    scanf("%d", &decision);
    if (decision == 1) encrypt();
    else decrypt();
    return 0;
}