Use the RSA Cipher Method - C Data Structure

C examples for Data Structure:Encrypt Decrypt

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Description

Use the RSA Cipher Method

Demo Code

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

long int i, j, p, q, n, t, flag;
long int e[100], d[100], temp[100], msgDecrypt[100], msgEncrypt[100];
char msgOriginal[100];
int prime(long int);
int findPrime(long int s);
void computeKeys();
long int cd(long int);
void encryptMsg();
void decryptMsg();

void main() {/* ww w  . j  av a 2  s  .  c o m*/
  long int s = 17;
  p = findPrime(s);
  printf("First prime number p is: %d \n", p) ;
  s = 19;
  q = findPrime(s);
  printf("\nEnter the Message to be Encrypted, Do Not Include Spaces:\n");
  fflush(stdin);
  scanf("%s",msgOriginal);
  for (i = 0; msgOriginal[i] != NULL; i++)
    msgDecrypt[i] = msgOriginal[i];
  n = p * q;
  t = (p - 1) * (q - 1);
  computeKeys();
  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]);
  printf("\nSample Public Key: (%ld,  %ld)", n, e[i-1]);
  printf("\nSample Private Key: (%ld,  %ld)", n, d[i-1]);
  encryptMsg();
  decryptMsg();
}

int findPrime(long int s)
{
  int f, d, tag;
  f = 2;
  i = 1;
  while(i <= s){
    tag = 1;
    for(d = 2 ; d <= f-1 ; d++){
      if(f % d == 0) {
        tag = 0;
        break ;
      }
    }
    if(tag == 1) {
      if (i == s)
        return(f);
      i++ ;
    }
    f++ ;
  }
  return(0);
}

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 computeKeys()
{
  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 encryptMsg()
{
  long int pt, ct, key = e[0], k, length;
  i = 0;
  length = strlen(msgOriginal);
  while(i != length) {
    pt = msgDecrypt[i];
    pt = pt-96;
    k = 1;
    for (j = 0; j < key; j++) {
      k = k * pt;
      k = k % n;
    }
    temp[i] = k;
    ct = k + 96;
    msgEncrypt[i] = ct;
    i++;
  }
  msgEncrypt[i] =- 1;
  printf("\nThe Encrypted Message:\n");
  for (i = 0; msgEncrypt[i] != -1; i++)
    printf("%c", msgEncrypt[i]);
}

void decryptMsg()
{
  long int pt, ct, key = d[0], k;
  i = 0;
  while(msgEncrypt[i] != -1) {
    ct = temp[i];
    k = 1;
    for (j = 0; j < key; j++) {
      k = k * ct;
      k = k % n;
    }
    pt = k + 96;
    msgDecrypt[i] = pt;
    i++;
  }
  msgDecrypt[i] =- 1;
  printf("\nThe Decrypted Message:\n");
  for (i = 0; msgDecrypt[i] != -1; i++)
    printf("%c", msgDecrypt[i]);
  printf("\nThank you. \n ");
}

Result


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