I found this lying around. It might work
package com.one2one.cliex.util{
import mx.utils.Base64Encoder;
import mx.utils.Base64Decoder;
import flash.utils.ByteArray;
public class B64String {
public static function
encodeString(plainString:String):String{
var b64encoder:Base64Encoder = new Base64Encoder();
var bArray:ByteArray = new ByteArray;
bArray.writeUTFBytes(plainString);
b64encoder.encodeBytes(bArray);
return(b64encoder.flush());
}//encodeString
public static function decodeString(eString : String):
String{
var b64decoder:Base64Decoder = new Base64Decoder();
var bArray:ByteArray;
b64decoder.decode(eString);
bArray = b64decoder.drain();
var temp : String = bArray.toString();
return(temp);
}//decodeString
}//class
}//package
Regards, John Luke Mills