public static function base64Encode(str:String, charset:String = "GBK"):String{
			if(StringUtils.getInstance().isEmpty(str)){
				return "";
			}
			var base64:Base64Encoder = new Base64Encoder();
			base64.insertNewLines = false;//该值等于true时，输出的结果会自动换行，默认为true  
			var byte:ByteArray = new ByteArray();
			byte.writeMultiByte(str, charset);
			base64.encodeBytes(byte);
			return base64.toString();
		}
		
		public static function base64Decode(str:String, charset:String = "GBK"):String{
			if(StringUtils.getInstance().isEmpty(str)){
				return "";
			}
			var base64:Base64Decoder = new Base64Decoder();
			base64.decode(str);
			var byteArray:ByteArray = base64.toByteArray();
			return byteArray.readMultiByte(byteArray.length, charset);;
		}
	}

Flex/AS3 base64指定字符编码