Implement RSA Algorithm Encryption and Decryption in C

• RSA (Rivest–Shamir–Adleman) is an algorithm used by modern computers to encrypt and decrypt messages. 
• It is an asymmetric cryptographic algorithm. 
• Asymmetric means that there are two different keys. 
• This is also called public key cryptography, because one of the keys can be given to anyone.

EXAMPLE :-

#include <stdio.h>

#include <math.h>

#include <stdlib.h>

typedef struct

{

int d;

int x;

int y;

}EE;

EE extended_euclid(int a, int b) {

EE e1, e2, e3;

if (b == 0)

{

e1.d = a;

e1.x = 1;

e1.y = 0;

return e1;

}

else

{

e2 = extended_euclid(b, a % b);

e3.d = e2.d;

e3.x = e2.y;

e3.y = e2.x - floor(a / b) * e2.y;

return e3;

}

}

int modulo(int x, int N){

return (x % N + N) % N;

}

void decimal_to_binary(int op1, int pq[]){

int result, i = 0;

do

{

result = op1 % 2;

op1 /= 2;

pq[i] = result;

i++;

}

while(op1 > 0);

}

int modular_exponentiation(int a, int b, int n){

int *bb;

int count = 0, c = 0, d = 1, i;

count = (int) (log(b)/log(2)) + 1;

bb = (int *) malloc(sizeof(int*) * count);

decimal_to_binary(b, bb);

for (i = count - 1; i >= 0; i--)

{

c = 2 * c;

d = (d*d) % n;

if (bb[i] == 1)

{

c = c + 1;

d = (d*a) % n;

}

}

return d;

}

int get_d(int e, int phi1){

EE Ee;

Ee = extended_euclid(e, phi1);

return modulo(Ee.x, phi1);

}

int main(int argc, char* argv[])

{

int p1, q1, phi1, n1, e1, d1, m1, c1;

printf("Enter the value of p1: ");

scanf("%d", &p1);

printf("Enter the valeu of q1: ");

scanf("%d", &q1);

n1 = p1*q1;

phi1 = (p1 - 1) * (q1 - 1);

printf("Enter the value of e: ");

scanf("%d", &e1);

d1 = get_d(e1, phi1);

printf("Public Key: (n = %d, e = %d)\n", n1, e1);

printf("Private Key: (n = %d, d = %d)\n", n1, d1);

printf("Enter message to encrypt: ");

scanf("%d", &m1);

c1 = modular_exponentiation(m1, e1, n1);

printf("Encrypted message is: %d\n", c1);

m1 = modular_exponentiation(c1, d1, n1);

printf("Message is decrypted to %d\n", m1);

return 0;

}

OUTPUT