Implement Playfair substitution cipher encryption and decryption technique

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

char mssg[100]; 
char dmssg[200];
int ind[200];

char table[][5] = { {'M','O','N','A','R'}, {'C','H','Y','B','D'}, 
                    {'E','F','G','I','K'}, {'L','P','Q','S','T'}, 
                    {'U','V','W','X','Z'}
                  };
struct pos
{
  int i;
  int j;      
}a1,a2;
 
void get_mssg()
{
   printf("\n Enter the Plaintext (in caps): ");
   gets(mssg);
}

struct pos get_pos(char ch)
{
  struct pos t;
  for(t.i = 0; t.i < 5; t.i++)
  {
   for(t.j = 0; t.j < 5; t.j++)
   {
     if(table[t.i][t.j] == ch)
      return t;      
   }     
  }      
}

void set_pos(struct pos a1, struct pos a2, int & l)
{
  if(a1.i == a2.i)
  {
    dmssg[l] = table[a1.i][(a1.j+1)%5];    dmssg[l+1] = table[a2.i][(a2.j+1)%5];        
  }
  else if(a1.j == a2.j)
  {
    dmssg[l] = table[(a1.i+1)%5][a1.j];    dmssg[l+1] = table[(a2.i+1)%5][a2.j];        
  }
  else
  {
    dmssg[l] = table[a1.i][a2.j];          dmssg[l+1] = table[a2.i][a1.j]; 
  }     
}

void set_pos2(struct pos a1, struct pos a2, int & l)
{
  if(a1.i == a2.i)
  {
    mssg[l] = table[a1.i][a1.j == 0 ? 4 : a1.j-1];
    mssg[l+1] = table[a2.i][a2.j == 0 ? 4 : a2.j-1];        
  }

  else if(a1.j == a2.j)
  {
    mssg[l] = table[a1.i == 0 ? 4 : a1.i-1][a1.j];
    mssg[l+1] = table[a2.i == 0 ? 4 : a2.i-1][a2.j];        
  }
  else
  {
    mssg[l] = table[a1.i][a2.j];
    mssg[l+1] = table[a2.i][a1.j]; 
  }     
}

void en_mssg()
{
  int k =0,l = 0;
  while(k < strlen(mssg))
  {
    a1 = get_pos(mssg[k]);

    if(mssg[k+1] == '\0' || mssg[k+1] == mssg[k])
     a2 = get_pos( ((mssg[k] - 65 + 12) % 26)+65 );
    else
     a2 = get_pos(mssg[k+1]);
      
    set_pos(a1,a2,l);
   
    ind[l] = 0;
    if(mssg[k+1] == '\0' || mssg[k+1] == mssg[k])
    {
      ind[l+1] = 1;
      k++;           
    }
    else
    {
      ind[l+1] = 0;      
      k += 2; 
    }
    l += 2;         
  }  
  dmssg[l] = '\0';
  printf("\n The Ciphertext is : ");
  puts(dmssg);
}

void de_mssg()
{
  int k = 0;
  int l = 0;

  while(k < strlen(dmssg))
  {
    a1 = get_pos(dmssg[k]);   
    a2 = get_pos(dmssg[k+1]);
       
    set_pos2(a1,a2,l);
   
    if(ind[k+1] == 1)
     l += 1;   
    else
     l += 2;   
    k += 2;         
  }  
  mssg[l] = '\0';
 
  printf("\n The Decrypted Plaintext is : ");
  puts(mssg);
}

int main()
{
  get_mssg();
  en_mssg();
  de_mssg(); 
  getch();
  return 0; 
}