Simple RSA implementation in C

rsa.c

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

/* Refer to http://www.coders-hub.com/2013/04/c-code-to-encrypt-and-decrypt-message.html#.Vhs81MryNC1 */

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()
{
  printf("\nENTER FIRST PRIME NUMBER\n");
  scanf("%d",&p);
  flag=prime(p);
  if(flag==0)
  {
      printf("\nWRONG INPUT\n");
      exit(1);
  }
  printf("\nENTER ANOTHER PRIME NUMBER\n");
  scanf("%d",&q);
  flag=prime(q);
  if(flag==0||p==q)
  {
      printf("\nWRONG INPUT\n");
      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();
}

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]);
  printf("\n");
}