help with XOR - symmetrical encryption

[sfullman]

hi.

this WORKS but the decrypt is NOT every elegant - and I know some of you smart guys understand XOR and flipping bits etc. for this type of application. here is the function:

function symmetrical_encrypt($options=array()){
	global $MASTER_PASSWORD;
	extract($options);
	if(!$action)$action='byte_encrypt';
	if(!$key)$key=md5($MASTER_PASSWORD);
	if(!function_exists('byte_encrypt')){
		function byte_encrypt($base,$key){
			return $base ^ $key;
		}
		function byte_decrypt($result, $key){
			/*
			!!!    this is the part I would like to improve..
			!!!
			*/
			$a=str_pad(base_convert(ord($result),10,2),8,0,STR_PAD_LEFT);
			$b=str_pad(base_convert(ord($key),10,2),8,0,STR_PAD_LEFT);
			for($i=0; $i<8; $i++){
				$s.=($a{$i} xor $b{$i}) ? 1 : 0;
			}
			return chr(base_convert($s,2,10));
		}
	}
	if($action=='byte_decrypt' && !$suppressBase64)$string=base64_decode($string);
	for($i=0;$i<strlen($string);$i++){
		$out.=$action($string{$i}, substr($key,fmod($i,strlen($key)),1));
	}
	if($action=='byte_encrypt' && !$suppressBase64)$out=base64_encode($out);
	return $out;
}
//example
$encrypted=symmetrical_encrypt( array( 
   'string'=>'how are you doing bob',
   'key'=> 'G48COX4E',
));

echo $decrypted=symmetrical_encrypt( array(
   'string'=>$encrypted,
   'key'=>'G48COX4E',
   'action'=>'byte_decrypt',
));

theoretically byte_decrypt() is probably a simple ^ operation to remove the overlay key's effect but I couldn't make it work. Please help if you know the correct way to decrypt in strict logical expressions

[laserlight]

This works for me:

<?php
function symmetrical_encrypt($string, $key) {
    $out = '';
    for ($i = 0; $i < strlen($string); $i++) {
        $out .= $string[$i] ^ $key[$i % strlen($key)];
    }
    return $out;
}

//example
$plaintext = 'how are you doing bob';
$key = 'G48COX4E';
$encrypted = symmetrical_encrypt($plaintext, $key);
$decrypted = symmetrical_encrypt($encrypted, $key);

printf('"%s" (%s)', $decrypted, (($plaintext == $decrypted) ? 'pass' : 'fail'));

[johanafm]

I believe that operating on strlen($key) long chunks of the input string at a time should be a lot faster, and it will produce the exact same result.

function symmetrical_crypt($string, $key)
{
	$tmp = str_split($string, strlen($key);
	foreach ($tmp as &$t)
		$t ^= $key;
	return implode('', $tmp);
}

$enrypted = symmetrical_crypt($input, $key);
$decrypted = symmetrical_crypt($encrypted, $key);

[bradgrafelman]

Well if speed is an issue, then you could always try iterating through the string only once:

function symmetrical_crypt($string, $key)
{
    return $string ^ str_repeat($key, ceil(strlen($string) / strlen($key)));
}

//example 
$plaintext = 'how are you doing bob'; 
$key = 'G48COX4E'; 
$encrypted = symmetrical_crypt($plaintext, $key); 
$decrypted = symmetrical_crypt($encrypted, $key); 

printf('"%s" (%s)', $decrypted, (($plaintext == $decrypted) ? 'pass' : 'fail'));

(example section shamelessly stolen from laserlight above).