Kerckhoff Principle / Cipher

[ghextreme]

I have a problem on how to encrypt a message or group of words. I have started the program and has been sourced by this link:

[http://www.microsofttranslator.com/bv.aspx?from=&to=en&a=http%3A%2F%2Fpetitcolas.net%2Ffabien%2Fkerckhoffs%2Fla_cryptographie_militaire_i.htm]

on that page, please find this type of cryptography [1 ° Systems simple key.]. That is the main definition of my program.

Can you help me on how to continue my program on encrypting group of words.

This are my started codes:

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

int main(void)
{
  char alpha[26]={'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
  int counter, x, y, z, a, b, c, d, ALS;
  x = 1;
  y = 1;
  z = 1;
 
  int key[10];
  
  printf("Note: Please make number [9] as the last number of the key.\n\n");

  do{
  printf("Enter [%d] Key : ", x);
  scanf("%d", &key[x]);
  x++;
  }while(x < 10);

  printf("\nYour key is : ");
  while(y < 10){
  printf("%d", key[y]);
  y++; 
  }

  printf("\n\n");
  
  
  printf("Plain Text Source:  ABCDEFGHIJKLMNOPQRSTUVWXYZ \n");
  printf("Cipher Text Source: ");
  
  
  //ciphering through the secret key..
  do{
  if(z <= 9)
  {
       a = key[z];
       b = a - 1;
       ALS = b;
       printf("%c", alpha[ALS]);
       z++;
       }else if(z <= 18){
             c = z - 9;
             a = key[c];
             b = a - 1;
             d = b + 9;
             ALS = d;
             printf("%c", alpha[ALS]);
             z++;
             }else{
                   c = z - 18;
                   a = key[c];
                   b = a - 1;
                   d = b + 18;
                   ALS = d;
                   printf("%c", alpha[ALS]);
                   z++;
                   } 
  
  }while(z<27);
  
  printf("\n\n");
  
  system("pause");
  
}

Thank You so much!