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The CAPTCHA package includes a component to display the CAPTCHA security token as a visual CAPTCHA (CaptchaVisual) and a component that can read out each letter of the token (CaptchaAudio). By using both the visual and audio CAPTCHA components, a robust CAPTCHA can be built.
Creating the CAPTCHA
CaptchaVisual extends SkinnableTextBase to inherit text styles and can be added to any container that implements IVisualElementContainer. The CaptchaAudio component extends EventDispatcher and is not a visual component. To use it, either create an instance in the <fx:Declarations> tag of an MXML document or create an instance in Actionscript and store a reference in your application.
Using the CAPTCHA
By default, no method to validate user input is provided. The components will generate the CAPTCHA or read the CAPTCHA but it is up to user to create the actual test. See the CaptchaDemo application for an example of an integrated visual and audio CAPTCHA with validation.
Set the token to be displayed via the "token" property. To use a randomized token, set the desired length via the "randomTokenLength" property and set the token property to null. The allowable token characters are not restricted, but if you plan on using an audio CAPTCHA, limiting the token to letters and numbers is advisable. By default, random tokens only use letters and numbers. To manually retrieve a randomized string, use the getRandomString() method and set it to the token property.
Due to extending the SkinnableTextBase, all text styles are accepted and will be pushed to the label skin part. By default the text is black and compressed via the trackingLeft/Right styles. See the defaults.css file for all of the set styles.
The text distortion is created by a custom version of the DisplacementMapFilter class from the Flex SDK.
To add a perlin noise background and/or a polyline, include the required skin parts and set the "drawNoise" and "drawPolyLine" properties to true, respectively.
Extracting BitmapData from the Visual CAPTCHA
The visual CAPTCHA is not an image. It is at most a perlin noise bitmap, with a label above the noise, and a polyline above everything else. It may be desirable to extract the entire component as a bitmap. To do so, use the BitmapData.draw() method or the ImageSnapshot class on the CaptchaVisual.
Simple class used to read a CAPTCHA string.
Note: sound assets are not embedded because of the size and the download time implications. Instead, each individual character is downloaded as an mp3 file. Asset URLs are fetched according to the "assetURLFunction" property. Supply a function to the "assetURLFunction" function that will return the location of the mp3 file for the passed character.
English (female Midwestern American accent) mp3 files are provided for the characters A-Z, 1-9. They are located in the demo application and they are released under the Apache License version 2.0.
This visual CAPTCHA is not very secure. Any spam bot that could inspect swf properties during runtime could easily figure out the token and pass the CAPTCHA test. The token would be available to the bot via both the "token" property of the CaptchaVisual and via the "text" property of the label display skin part.
Improve security by using the CaptchaVisual component to generate the CAPTCHA, then extract the bitmap data to create a bitmap. Next, store the hash of the CAPTCHA token, remove the CaptchaVisual from the display list and remove all references to the token, expect for its hash. This way, all references to the token have been removed, which would make it more difficult for a swf inspector to extract the token value. To test if the user passes the CAPTCHA, compare the hash of the user provided string with the stored hash of the token.
The CaptchaVisual component dispatches a single custom event:
Dispatched when the token changes.
The CaptchaAudio component dispatches five custom events:
Dispatched when the speech begins playing.
Dispatched when the speech ends.
Dispatched when all of the audio assets have begun loading.
Dispatched when the audio assets are fully loaded.
Dispatched when loading the audio assets has thrown an error.
Themes & Skinning
A skin for the CaptchaVisual component is included for the Spark theme. See the application demo for a rollup example of using the CaptchaVisual and the CaptchaAudio components to create a working CAPTCHA.
See the CaptchaDemo application for example code.Back To Top