Caesar Cipher using PHP

[HFD]

Hi, I'm currently in the midst of coding some simple cryptography applications for my portfolio in University. I've decided to create a simple Caesar cipher encrypter/decrypter, and I'm struggling with the underlying code. Of course, I just want to get it working on a predefined string and shift before I add in code to let the user decide.

 

The part I'm struggling with is converting each character in the string to it's ASCII value, then incrementing it. Can anyone offer any tips on how to do this? Here's my code so far:

 

<?php
$string = "Hello World";
$stringlength = strlen($string);

for ($counter = 0; $counter < $stringlength; $counter++)
{
$string[$counter]++;
echo $string[$counter];
}

?>

 

Thanks

[Yesideez]

Might be an idea to explain what a string looks like before and after so we have more of an idea - never heard of Caesar encryption before.

[HFD]

Thanks for your reply.

 

What a Caesar cipher does is increment every letter by a certain amount, so if by incrementing by two HELLO becomes JGNNQ

[Yesideez]

I think I know it now.

 

You'll need to make an array containing all the offsets of the new characters.

$key=array('A' => 'Q', 'B' => 'W', 'C' => 'E'...);

 

Then a loop to read through your string replacing the characters.

$str='HELLO WORLD';
$length=strlen($str);
$newstr='';
for ($i=0;$i<$length;++$i) {
  if (in_array($str[$i],$key[$i])) {
    $newstr.=$key[$i];
  }
}

 

Something like that.

 

EDIT: Replaced count() with strlen()

[HFD]

Thanks only problem is in that scenario I'd have to make 25 arrays with the offsets for the different shifts - that's of course entirely possible and fine but just wondering if there's a more efficient way? Thanks

[Yesideez]

I don't think so - you're always going to need an array defining what each letter will become. Plus to decrypt you'll need another array with the letters reversed...

$key=array('Q' => 'A', 'W' => 'B', 'E' => 'C'..);

 

You can't make it from random either because then you won't be able to decrypt anything unless you keep the key.

[HFD]

Ah ok thanks, the arrays should do fine

[adam291086]

hey

 

I have been looking at the cipher with uni too.

 

do an array

 

$key=array('A' => '1', 'B' => '2', 'C' => '3'...);

 

Then split a string down into the characters and compare it to the array and that will give you the letters as numbers you can then add a N amount to each number for the shift.

 

 

 

[lonewolf217]

this works, thought it might not be the most optimized.  currently it is designed for letters, though the concept can be used for any section of ascii values

 

<?php

$string = "hello";
$newstring = "hello";

for ($i=0;$i<strlen($string);$i++) {

$ascii = ord($string[$i]);
if($ascii == 90) { //uppercase bound
	$ascii = 65; //reset back to 'A' 
	} 
else if($ascii == 122) { //lowercase bound
	$ascii = 97; //reset back to 'a' 
	} 
else {
	$ascii++;
}
$newstring[$i] = chr($ascii);
echo $newstring."<br>";
}

?>

[Yesideez]

Just what I have said but adam is only going to be able to use 10 letters otherwise the encrypted string will be much longer and decrypting it will be a nightmare!