how to properly create an encryption in c++? - c++

Question

I have finished my program about encryption. The problem is, I need a shortcut. The program I have coded is too long and not very suitable. I'm just new to c++ programming. Can someone help me out? thanks! :) Here's my program:
#include <iostream.h>
#include <stdlib.h>
#include <stdio.h>
#include <conio.h>
int main()
{
c:char crypt[25], cons;
int ctr;
cout<<"Input your 26-character cipherstring below.\n\n";
cout<<"ABCDEFGHIJKLMNOPQRSTUVWXYZ\n";cin>>crypt;
e:char string[65535];
cout<<"\nInput your string (input your spaces as any non-alphabet character).\n";cin>>string;
cout<<"\n\nEncrypted string: "; //std::string
for(ctr=0;ctr<=strlen(string);ctr++)
{
switch(string[ctr])
{
case('a'): cout<<crypt[0]; break;
case('b'): cout<<crypt[1]; break;
case('c'): cout<<crypt[2]; break;
case('d'): cout<<crypt[3]; break;
case('e'): cout<<crypt[4]; break;
case('f'): cout<<crypt[5]; break;
case('g'): cout<<crypt[6]; break;
case('h'): cout<<crypt[7]; break;
case('i'): cout<<crypt[8]; break;
case('j'): cout<<crypt[9]; break;
case('k'): cout<<crypt[10]; break;
case('l'): cout<<crypt[11]; break;
case('m'): cout<<crypt[12]; break;
case('n'): cout<<crypt[13]; break;
case('o'): cout<<crypt[14]; break;
case('p'): cout<<crypt[15]; break;
case('q'): cout<<crypt[16]; break;
case('r'): cout<<crypt[17]; break;
case('s'): cout<<crypt[18]; break;
case('t'): cout<<crypt[19]; break;
case('u'): cout<<crypt[20]; break;
case('v'): cout<<crypt[21]; break;
case('w'): cout<<crypt[22]; break;
case('x'): cout<<crypt[23]; break;
case('y'): cout<<crypt[24]; break;
case('z'): cout<<crypt[25]; break;
case('A'): cout<<crypt[0]; break;
case('B'): cout<<crypt[1]; break;
case('C'): cout<<crypt[2]; break;
case('D'): cout<<crypt[3]; break;
case('E'): cout<<crypt[4]; break;
case('F'): cout<<crypt[5]; break;
case('G'): cout<<crypt[6]; break;
case('H'): cout<<crypt[7]; break;
case('I'): cout<<crypt[8]; break;
case('J'): cout<<crypt[9]; break;
case('K'): cout<<crypt[10]; break;
case('L'): cout<<crypt[11]; break;
case('M'): cout<<crypt[12]; break;
case('N'): cout<<crypt[13]; break;
case('O'): cout<<crypt[14]; break;
case('P'): cout<<crypt[15]; break;
case('Q'): cout<<crypt[16]; break;
case('R'): cout<<crypt[17]; break;
case('S'): cout<<crypt[18]; break;
case('T'): cout<<crypt[19]; break;
case('U'): cout<<crypt[20]; break;
case('V'): cout<<crypt[21]; break;
case('W'): cout<<crypt[22]; break;
case('X'): cout<<crypt[23]; break;
case('Y'): cout<<crypt[24]; break;
case('Z'): cout<<crypt[25]; break;
default: cout<<" "; break;
}
}
cout<<"\n\n";
/*cout<<"Input 'c' to re-input your cipherstring.\n 'e' to reuse your cipherstring.\n 'q' to quit. ";
comm:cout<<"\nCommand: "; cin>>cons;
switch (cons)
{
case('c'): cout<<endl; goto c; break;
case('C'): goto c; break;
case('e'): goto e; break;
case('E'): goto e; break;
case('q'): break;
case('Q'): break;
default: cout<<"Invalid command. Please refer to the command list above.\n";goto comm;
}*/
system("PAUSE"); return 0;
}

Answer 1

You can replace that huge switch statement with this:
if (isalpha(string[ctr]))
{
int index = toupper(string[ctr]) - 'A';
cout << crypt[index];
}
else
cout << " ";
Also, do not call strlen for every iteration of the for loop, just do it once:
for(int ctr=0, len=strlen(string); ctr<len; ctr++)
{
....
}
A few other things of note:
crypt is too small, it should have size 27
If you use getline instead of operator>>, you can have the user enter spaces properly
Your code is prone to buffer overflows. Use std::string and you can pretty much forget about those.
Note, that as Kerrek says, this assumes that in the character set you are using, the uppercase letters A-Z are in sequence. But I think that the cases where this is not true are so exceedingly rare, that you can safely ignore them.