RSA Encryption in C: Step-by-Step Guide

Here is a simple example code in C language implementing the RSA encryption algorithm.

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

int gcd(int a, int b) {
    if (b == 0)
        return a;
    else
        return gcd(b, a % b);
}

int is_prime(int n) {
    if (n <= 1)
        return 0;
    for (int i = 2; i <= sqrt(n); i++) {
        if (n % i == 0)
            return 0;
    }
    return 1;
}

int generate_prime() {
    int p;
    do {
        p = rand() % 100 + 1;
    } while (!is_prime(p));
    return p;
}

int mod_exp(int base, int exp, int mod) {
    int result = 1;
    while (exp > 0) {
        if (exp % 2 == 1)
            result = (result * base) % mod;
        base = (base * base) % mod;
        exp /= 2;
    }
    return result;
}

int main() {
    int p = generate_prime();
    int q = generate_prime();

    int n = p * q;
    int phi = (p - 1) * (q - 1);

    int e = 2;
    while (gcd(e, phi) != 1) {
        e++;
    }

    int d = 1;
    while ((d * e) % phi != 1) {
        d++;
    }

    printf("Public key: (%d, %d)\n", e, n);
    printf("Private key: (%d, %d)\n", d, n);

    int plaintext = 123;
    int ciphertext = mod_exp(plaintext, e, n);
    printf("Encrypted message: %d\n", ciphertext);

    int decrypted_text = mod_exp(ciphertext, d, n);
    printf("Decrypted message: %d\n", decrypted_text);

    return 0;
}

This code generates two random prime numbers, p and q, calculates the values of n and phi, selects a public key e and private key d, then encrypts plaintext using the public key and decrypts ciphertext using the private key. Please note that the encryption and decryption here are only for demonstration purposes and in real applications, larger prime numbers and more complex operations should be used.