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These are the ramblings of Matthijs Kooijman, concerning the software he hacks on, hobbies he has and occasionally his personal life.

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URL-encoding in Flash: Be careful of plus signs!

Adobe Flash PHP

Recently I have been doing some Flash debugging for my work at Brevidius. In a video player we have been developing (based on work done by Jeroen Wijering) we needed to escape some url parameter, since our flash code could not be certain what would be in the value (and characters like & and = could cause problems). The obvious way to do this is of course the escape function in ActionScript. This function promises to escape all "non-alphanumeric characters", which would solve all our problems.

However, afters implementing this, we find that there are spaces magically appearing in our GET parameters. Upon investigation, it turns out that there are plus signs in our actual values (it's Base64 encoded data, which uses the plus sign). However, the escape function apparently thinks a plus sign is alphanumeric, since it does not escape it (note that the flash 10 documentation documents this fact). Which shouldn't be a problem, since a plus sign isn't special in an url according to RFC1738:

Thus, only alphanumerics, the special characters "$-_.+!*'(),", and reserved characters used for their reserved purposes may be used unencoded within a URL.

(Note that RFC3986 does recommend escaping plus signs, since they might be used to separate variables, but that's not the case here).

However, the urls we generate in flash point to PHP scripts and thus pass their variables to PHP. Unfortunately, PHP does not adhere to the RFC's strictly: It interprets plus signs in an url as spaces. Historically, spaces in an url were replaced by plus signs, while spaces should really be encoded as %20 nowadays. There is of course a simple way get Flash (or any other url-generating piece of code) work properly with PHP: Simply encode plus signs in your data as %2B (which is the "official" way). This makes sure you get a real plus in your $_GET array in PHP, and the problem is resolved.

After some searching, and asking around in ##swf on Freenode, I found the encodeURIComponent function, which is similar to escape, but does encode the plus sign. If we use this function, we can again send data with spaces to PHP! And since encoding more than needed is still fine according to the specs, there are no downsides (except that you need Flash >= 9.0).

So, if you're developing in Flash, please stop using escape, and use encodeURIComponent instead.

 
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