Example: c rsa encryption
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <math.h>
int e, d, n;
int gcd(int a, int b)
{
int q, r1, r2, r;
if (a > b) {
r1 = a;
r2 = b;
}
else {
r1 = b;
r2 = a;
}
while (r2 > 0) {
q = r1 / r2;
r = r1 - q * r2;
r1 = r2;
r2 = r;
}
return r1;
}
int PrimarityTest(int a, int i)
{
int n = i - 1;
int k = 0;
int j, m, T;
while (n % 2 == 0) {
k++;
n = n / 2;
}
m = n;
T = FindT(a, m, i);
if (T == 1 || T == i - 1)
return 1;
for (j = 0; j < k; j++) {
T = FindT(T, 2, i);
if (T == 1)
return 0;
if (T == i - 1)
return 1;
}
return 0;
}
int FindT(int a, int m, int n)
{
int r;
int y = 1;
while (m > 0) {
r = m % 2;
FastExponention(r, n, &y, &a);
m = m / 2;
}
return y;
}
int FastExponention(int bit, int n, int* y, int* a)
{
if (bit == 1)
*y = (*y * (*a)) % n;
*a = (*a) * (*a) % n;
}
int inverse(int a, int b)
{
int inv;
int q, r, r1 = a, r2 = b, t, t1 = 0, t2 = 1;
while (r2 > 0) {
q = r1 / r2;
r = r1 - q * r2;
r1 = r2;
r2 = r;
t = t1 - q * t2;
t1 = t2;
t2 = t;
}
if (r1 == 1)
inv = t1;
if (inv < 0)
inv = inv + a;
return inv;
}
int KeyGeneration()
{
int p, q;
int phi_n;
do {
do
p = rand();
while (p % 2 == 0);
} while (!PrimarityTest(2, p));
do {
do
q = rand();
while (q % 2 == 0);
} while (!PrimarityTest(2, q));
n = p * q;
phi_n = (p - 1) * (q - 1);
do
e = rand() % (phi_n - 2) + 2;
while (gcd(e, phi_n) != 1);
d = inverse(phi_n, e);
}
int Encryption(int value, FILE* out)
{
int cipher;
cipher = FindT(value, e, n);
fprintf(out, "%d ", cipher);
}
int Decryption(int value, FILE* out)
{
int decipher;
decipher = FindT(value, d, n);
fprintf(out, "%c", decipher);
}
int main(void)
{
FILE *inp, *out;
out = fopen("cipher.txt", "w+");
fclose(out);
out = fopen("decipher.txt", "w+");
fclose(out);
KeyGeneration();
inp = fopen("plain.txt", "r+");
if (inp == NULL) {
printf("Error opening Source File.\n");
exit(1);
}
out = fopen("cipher.txt", "w+");
if (out == NULL) {
printf("Error opening Destination File.\n");
exit(1);
}
while (1) {
char ch = getc(inp);
if (ch == -1)
break;
int value = toascii(ch);
Encryption(value, out);
}
fclose(inp);
fclose(out);
inp = fopen("cipher.txt", "r");
if (inp == NULL) {
printf("Error opening Cipher Text.\n");
exit(1);
}
out = fopen("decipher.txt", "w+");
if (out == NULL) {
printf("Error opening File.\n");
exit(1);
}
while (1) {
int cip;
if (fscanf(inp, "%d", &cip) == -1)
break;
Decryption(cip, out);
}
fclose(out);
return 0;
}