RSA Algorithm in C

rsa.c

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

int checkPrime(int n) {
	int i;
	int m = n / 2;
	
	for (i = 2; i <= m; i++) {
		if (n % i == 0) {
			return 0; // Not Prime
		}
	}

	return 1; // Prime
}

int findGCD(int n1, int n2) {
	int i, gcd;

	for(i = 1; i <= n1 && i <= n2; ++i) {
		if(n1 % i == 0 && n2 % i == 0)
			gcd = i;
	}

	return gcd;
}

int powMod(int a, int b, int n) {
	long long x = 1, y = a;

	while (b > 0) {
		if (b % 2 == 1)
			x = (x * y) % n;
		y = (y * y) % n; // Squaring the base
		b /= 2;
	}

	return x % n;
}

int main(int argc, char* argv[]) {
	int p, q;
	int n, phin;

	int data, cipher, decrypt;

	while (1) {
		printf("Enter any two prime numbers: ");
		scanf("%d %d", &p, &q);

		if (!(checkPrime(p) && checkPrime(q)))
			printf("Both numbers are not prime. Please enter prime numbers only...\n");
		else if (!checkPrime(p))
			printf("The first prime number you entered is not prime, please try again...\n");
		else if (!checkPrime(q))
			printf("The second prime number you entered is not prime, please try again...\n");
		else
			break;
	}
	
	n = p * q;

	phin = (p - 1) * (q - 1);

	int e = 0;
	for (e = 5; e <= 100; e++) {
		if (findGCD(phin, e) == 1)
			break;
	}
	
	int d = 0;
	for (d = e + 1; d <= 100; d++) {
		if ( ((d * e) % phin) == 1)
			break;
	}

	printf("Value of e: %d\nValue of d: %d\n", e, d);

	printf("Enter some numerical data: ");
	scanf("%d", &data);

	cipher = powMod(data, e, n);
	printf("The cipher text is: %d\n", cipher);

	decrypt = powMod(cipher, d, n);
	printf("The decrypted text is: %d\n", decrypt);
	return 0;
}

Hey can you please explain the functioning of powMod() function?

Check Stallings' "Cryptography and Network Security: Principles and Practice" page 299