Implement Ceaser Substitution Cipher encryption and decryption technique

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

char mssg[100];
int k;
 
void get_mssg()
{
   printf("\n Enter the Plaintext (in caps): ");
   gets(mssg);
   printf("\n Enter the key (less than 26): ");
   scanf("%d",&k);
}

void en_mssg()
{
  int i = 0;
  while(mssg[i] != '\0')
  { 
    if(mssg[i] >= 65 && mssg[i] <= 90)          
     mssg[i] = ((mssg[i] - 65 + k) % 26)+65;
    i++;

  } 
  printf("\n The Ciphertext is : ");
  puts(mssg);
}

void de_mssg()
{
  int i = 0;
  while(mssg[i] != '\0')
  { 
    if(mssg[i] >= 65 && mssg[i] <= 90)    
    {    
      if((mssg[i]-65) >= k)
       mssg[i] = mssg[i] - k;
      else
       mssg[i] = 90-(k-(mssg[i]-65))+1;
    }     
    i++;
  } 
  printf("\n The Decrypted Plaintext is : ");
  puts(mssg);
}


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