Fast RSA encryption in AS3 - performance

Question

I have a Flex 4 application that uses a 2048 bits private RSA key to encrypt/sign some string values that are around 30 characters long.
I'm currently using the com.hurlant.crypto.RSAKey implementation to do so but I'm facing the following limitations:
It takes way too long (around 4 seconds for signing a 30 chars
string)
The UI freezes while the class is processing (implementation obviously doesn't
use events or chunks processing)
I have been looking for other libraries but I haven't found so far any other with the same level of functionalities (read RSA key from PEM string, allow RSA.sign(), RSA.encrypt and decrypt), that is free to use in commercial applications and that is faster than the one I currently use.
So my questions are:
Does anyone know a great/fast AS3 library that do that?
If there are none and I decide to write one myself from scratch will I be
facing the same problems because the low performances are somehow induced by the
flash platform ?
EDIT2: Below code uses a PEM encoded private key stored in a file. If you don't have one you can create one using the following piece of code:
var exp:String = "10001";
var bits:int = 2048;
rsa = RSAKey.generate(bits, exp);
Don't forget to sit down and take a coffee as it takes nearly one minute to generate.
EDIT: Here is piece of code showing the limitations.
Simply click on the start button and see how the progress bar and the application completely freezes.
<?xml version="1.0" encoding="utf-8"?>
<s:WindowedApplication xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/mx">
<fx:Script>
<![CDATA[
import com.hurlant.crypto.rsa.RSAKey;
import com.hurlant.util.der.PEM;
import mx.utils.Base64Encoder;
var pkfilePath:String = "/Users/david/Desktop/private_key.pem";
var stringToSign:String = "Hello this is a string to be signed by an efficient AS3 library";
private function readPKContent():String {
var f:File = new File(pkfilePath);
var fs:FileStream = new FileStream();
fs.open(f,FileMode.READ);
var rawKey:String = fs.readUTFBytes(fs.bytesAvailable);
fs.close();
return rawKey;
}
private function logTime(msg:String, start:Date, end:Date):void {
resultTA.text = msg + " " + (end.time-start.time) + " ms\n" + resultTA.text;
}
protected function button1_clickHandler(event:MouseEvent):void
{
cryptWithHurlant();
}
private function cryptWithHurlant():void {
var start:Date = new Date();
//Load key and use it to sign something
var rawPK:String = readPKContent();
var time:Date = new Date();
var rsa:RSAKey = PEM.readRSAPrivateKey(rawPK);
logTime("Hurlant:ReadRSA", time, new Date());
//Compute a signature of the string
var srcBA:ByteArray = new ByteArray();
srcBA.writeUTFBytes(stringToSign);
//Now sign inside the second BA
var desBA:ByteArray = new ByteArray();
time = new Date();
rsa.sign(srcBA, desBA, srcBA.length);
logTime("Hurlant:Encrypt", time, new Date());
//desBA.position = 0;
//Recover as a Base64 response
//var b64encoder:Base64Encoder = new Base64Encoder();
//time = new Date();
//b64encoder.encodeBytes(desBA);
//logTime("Base64:Encoded "+b64encoder.toString(),time, new Date());
logTime("Hurlant:Total",start,new Date());
}
]]>
</fx:Script>
<s:VGroup width="100%" height="100%" horizontalAlign="center">
<s:Button click="button1_clickHandler(event)" label="Start"/>
<mx:ProgressBar indeterminate="true"/>
<s:TextArea width="100%" height="100%" editable="false" id="resultTA"/>
</s:VGroup>
</s:WindowedApplication>

Answer 1

As noted above:
You may want to look into this timer based "threading" implementation. This won't speed anything up, but it will get rid of the UI lock. I have code using this working on mobile.
Based on all that has been said in comments and the absence of answers, I think that my best option for now will be to edit the RSA implementation to make it able to run with the pseudo-threading model proposed by f-a. I will move to workers once they are supported by mobile platforms.
References
Threads in ActionScript 3
flash.system.Worker