PHP: Cryptage et décryptage

gistfile1.php

function crypter($maCleDeCryptage="", $maChaineACrypter){
	if($maCleDeCryptage==""){
		$maCleDeCryptage=$GLOBALS['PHPSESSID'];
	}
	$maCleDeCryptage = md5($maCleDeCryptage);
	$letter = -1;
	$newstr = '';
	$strlen = strlen($maChaineACrypter);
	
	for($i = 0; $i < $strlen; $i++ ){
		$letter++;
		if ( $letter > 31 ) {
			$letter = 0;
		}
		$neword = ord($maChaineACrypter{$i}) + ord($maCleDeCryptage{$letter});

		if ( $neword > 255 ){
			$neword -= 256;
		}
		$newstr .= chr($neword);
	}

	return base64_encode($newstr);
}

function decrypter($maCleDeCryptage="", $maChaineCrypter){
	if($maCleDeCryptage==""){
		$maCleDeCryptage=$GLOBALS['PHPSESSID'];
	}
	$maCleDeCryptage = md5($maCleDeCryptage);
	$letter = -1;
	$newstr = '';
	$maChaineCrypter = base64_decode($maChaineCrypter);
	$strlen = strlen($maChaineCrypter);

	for ( $i = 0; $i < $strlen; $i++ ){
		$letter++;
		if ( $letter > 31 ){
			$letter = 0;
		}
		$neword = ord($maChaineCrypter{$i}) - ord($maCleDeCryptage{$letter});
		if ( $neword < 1 ){
			$neword += 256;
		}
		$newstr .= chr($neword);
	}
	return $newstr;
}