RSA.c

#include <math.h>  
#include <stdio.h>  
#include <stdlib.h>  
#include <string.h>  
  
int isPrime(int n)  
{  
    int i;  
    for (i = 2; i <= sqrt(n); i++)  
    {  
        if (n % i == 0)  
        {  
            return 0;  
        }  
    }  
    return 1;  
} 
  
int gcd(int a, int b)  
{  
    if (a == 0)  
    {  
        return b;  
    }  
    return gcd(b % a, a);  
}  
  
int totient(int p, int q)  
{  
    return (p - 1) * (q - 1);  
}  
  
int randome(int lambda_n)  
{  
    printf("\nThe number e should be less than %d\n and greater than 1.",lambda_n);
	int i;  
    for (i = 2; i < lambda_n; i++)  
    {  
        if (gcd(i, lambda_n) == 1)  
        {  
            return i;  
        }  
    }  
    return -1;  
}  
  
int private_key(int e, int lambda_n)  
{  int i;
    for ( i = 1; i < lambda_n; i++)  
    {  
        if ((i * e) % lambda_n == 1)  
        {  
            printf("\nThus, (i * e) %% lambda_n = 1, (%d * %d) %% %d = 1", i, e,lambda_n);  
            return i;  
        }  
    }  
    return -1;  
}  
  
long pomod(long a, long b, long m)  
{  
    long x = 1, y = a;  
    while (b > 0)  
    {  
        if (b % 2 == 1)  
        {  
            x = (x * y) % m;  
        }  
        y = (y * y) % m;  
        b /= 2;  
    }  
    return x % m;  
}  
  
/* Encryption 
 * a procedure that requires the message, the public key, and an integer n which is the sum of p and q. Using the public key, the function encrypts the message and then returns the result. 
 */  
  
char *encrypt(char *message, long e, long n)  
{  
    long i;  
    long len = strlen(message);  
    char *cipher = (char *)malloc(len * sizeof(char));  
    for (i = 0; i < len; i++)  
    {  
        cipher[i] = pomod(message[i], e, n);  
        printf("\n%c -> %c", message[i], cipher[i]);  
    }  
    return cipher;  
}  
  
/* Decryption 
 * a procedure that requires the cypher text, the private key, and an integer n, which is the sum of the values of p and q. The function uses the private key to decode the cypher text and then returns the message that has been decrypted. 
 */  
  
char *decrypt(char *cipher, long d, long n)  
{  
    long i;  
    long len = strlen(cipher);  
    char *message = (char *)malloc(len * sizeof(char));  
    for (i = 0; i < len; i++)  
    {  
        // message[i] = (long) pow(cipher[i], d) % n;  
        message[i] = pomod(cipher[i], d, n);  
        printf("\n%c -> %c", cipher[i], message[i]);  
    }  
    return message;  
}  
  
int main()  
{  
    int p, q, lambda_n;  
    long n, e, d;  
    char *message;  
    char *cipher;  
    printf("\nEnter the value of p: ");  
    scanf("%d", &p);  
    printf("\nEnter the value of q: ");  
    scanf("%d", &q);  
    if (isPrime(p) && isPrime(q))  
    {  
        n = p * q;  
        lambda_n = totient(p, q);  
        e = randome(lambda_n);  
        d = private_key(e, lambda_n);  
        printf("\nThe value of n is %ld", n);  
        printf("\nThe value of lambda_n is %d", lambda_n);  
        printf("\nThe value of e is %ld", e);  
        printf("\nThe value of d is %ld", d);  
        printf("\nEnter the message: ");  
        message = (char *)malloc(sizeof(char) * 100);  
        scanf("%s", message);  
        cipher = encrypt(message, e, n);  
        puts("\nThe encrypted message is: ");  
        printf("%s", cipher);  
        message = decrypt(cipher, d, n);  
        puts("\nThe original message was: ");  
        printf("%s", message);  
    }  
    else  
    {  
        printf("\nThe value of p and q should be prime.");  
    }  
    return 0;  
}