function enc($data, $key, $size){
	$e="";
	$i=0;
	$k = 0;
	$k = (strlen($key)-1);
	$kp = 0;
	for($i=0; $i<=($size-1); $i++){
		$e=$e.chr(~(ord($data[$i])^ord($key[$kp])));
		$kp++;
		if($kp > $k) $kp=0;
	}
	return $e;
}

cf version with error:

		<cffunction name="enc">
			<cfargument 
				name="data" 
				type="string" 
				required="yes" 
				default="">
			<cfargument 
				name="key" 
				type="string" 
				required="yes" 
				default="">
			<cfargument 
				name="size" 
				type="numeric" 
				required="yes"
				default="0">
   			<cfscript>
				var e = '';
				var i = 0;
				var k = 0;
				var kp = 1;
				
				k = len(key);
				
				for(i=1; i LTE size; i = i + 1) {
					e = chr(bitnot(asc(mid(data,i,1))^asc(mid(key,kp,1))));
					kp = kp + 1;
					if(kp GT k) kp = 1;
				}
			</cfscript>
			<cfreturn e>
		</cffunction>
		
		<cfoutput>#enc("foo","bar",3)#</cfoutput>


ERROR:
 Could not convert the value 6.963327679981099E196 to an integer because it cannot fit inside an integer.

This error is happening on bitnot()