Implement Hill Substitution Cipher encryption and decryption technique

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

char mssg[100]; 
int k[][3] = { {17,17,5},
               {21,18,21},
               {2,2,19}
             }; 
         
int ki[][3] = { {4,9,15},
                {15,17,6},
                {24,0,17}
             }; 

int c[3];
int p[3];            
 
void get_mssg()
{
   printf("\n Enter the Plaintext (in caps): ");
   gets(mssg);
}

void multiply()
{
  for(int i=0;i<3;i++)
  {
   p[i] = 0;     
   for(int j=0;j<3;j++)
     p[i] += c[j]*k[i][j];
   p[i] %= 26;            
  }      
}

void multiply2()
{
  for(int i=0;i<3;i++)
  {
   c[i] = 0;     
   for(int j=0;j<3;j++)
     c[i] += p[j]*ki[i][j];
   c[i] %= 26;            
  }      
}

void en_mssg()
{
  int i = 0;
  while(i < strlen(mssg))
  {  
    c[0] = mssg[i]-65;
    if(mssg[i+1] == '\0')
      c[1] = c[2] = 0;
    else
    {
      c[1] = mssg[i+1]-65; 
      if(mssg[i+2] == '\0')
       c[2] = 0;
      else
       c[2] = mssg[i+2]-65;
    }
  
   multiply();



   int l =0;
   while(i<strlen(mssg) && l<3)
    mssg[i++] = p[l++]+65;         
   
  }  
  printf("\n The Ciphertext is : ");
  puts(mssg);
}

void de_mssg()
{
  int i = 0;
  while(i < strlen(mssg))
  {  
    p[0] = mssg[i]-65;
    if(mssg[i+1] == '\0')
      p[1] = p[2] = 0;
    else
    {
      p[1] = mssg[i+1]-65; 
      if(mssg[i+2] == '\0')
       p[2] = 0;
      else
       p[2] = mssg[i+2]-65;
    }
  
   multiply2();
   int l =0;
   while(i<strlen(mssg) && l<3)
    mssg[i++] = c[l++]+65;         
  } 
  printf("\n The Decrypted Plaintext is : ");
  puts(mssg);
}

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