RSA Algorithm (Encryption and Decryption) implementation in C programming.

main.c

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

long int p, q, n, t, flag, e[100], d[100], temp[100], j, m[100], en[100], i;
char msg[100];
int prime(long int);
void ce();
long int cd(long int);
void encrypt();
void decrypt();
void main() {
  clrscr();
  printf("\nENTER FIRST PRIME NUMBER\n");
  scanf("%d", & p);
  flag = prime(p);
  if (flag == 0) {
    printf("\nWRONG INPUT\n");
    getch();
    exit(1);
  }
  printf("\nENTER ANOTHER PRIME NUMBER\n");
  scanf("%d", & q);
  flag = prime(q);
  if (flag == 0 || p == q) {
    printf("\nWRONG INPUT\n");
    getch();
    exit(1);
  }
  printf("\nENTER MESSAGE\n");
  fflush(stdin);
  scanf("%s", msg);
  for (i = 0; msg[i] != NULL; i++)
    m[i] = msg[i];
  n = p * q;
  t = (p - 1) * (q - 1);
  ce();
  printf("\nPOSSIBLE VALUES OF e AND d ARE\n");
  for (i = 0; i < j - 1; i++)
    printf("\n%ld\t%ld", e[i], d[i]);
  encrypt();
  decrypt();
  getch();
}
int prime(long int pr) {
  int i;
  j = sqrt(pr);
  for (i = 2; i <= j; i++) {
    if (pr % i == 0)
      return 0;
  }
  return 1;
}
void ce() {
  int k;
  k = 0;
  for (i = 2; i < t; i++) {
    if (t % i == 0)
      continue;
    flag = prime(i);
    if (flag == 1 && i != p && i != q) {
      e[k] = i;
      flag = cd(e[k]);
      if (flag > 0) {
        d[k] = flag;
        k++;
      }
      if (k == 99)
        break;
    }
  }
}
long int cd(long int x) {
  long int k = 1;
  while (1) {
    k = k + t;
    if (k % x == 0)
      return (k / x);
  }
}
void encrypt() {
  long int pt, ct, key = e[0], k, len;
  i = 0;
  len = strlen(msg);
  while (i != len) {
    pt = m[i];
    pt = pt - 96;
    k = 1;
    for (j = 0; j < key; j++) {
      k = k * pt;
      k = k % n;
    }
    temp[i] = k;
    ct = k + 96;
    en[i] = ct;
    i++;
  }
  en[i] = -1;
  printf("\nTHE ENCRYPTED MESSAGE IS\n");
  for (i = 0; en[i] != -1; i++)
    printf("%c", en[i]);
}
void decrypt() {
  long int pt, ct, key = d[0], k;
  i = 0;
  while (en[i] != -1) {
    ct = temp[i];
    k = 1;
    for (j = 0; j < key; j++) {
      k = k * ct;
      k = k % n;
    }
    pt = k + 96;
    m[i] = pt;
    i++;
  }
  m[i] = -1;
  printf("\nTHE DECRYPTED MESSAGE IS\n");
  for (i = 0; m[i] != -1; i++)
    printf("%c", m[i]);
}

The RSA algorithm is widely used for secure data encryption and digital signature.