From Gnash Project Wiki
Obfuscated SWF movies are the ones in which the ActionScript bytecode is intentionally scrabled to make decompilation difficult. Such movies use constructs which are not documented and exploit implementation-specific behaviors of the proprietary player.
Given the antisocial pattern behind these kind of movies, nobody felt any urge to fix playback support for them, focusing instead on fixing support for free software SWF applications first.
Nevertheless, if anyone is interested in fixing this class of movies, don't hesitate to sponsor a Gnash developer!
Notes about reversing obfuscation
There will always be a number of pieces of software trying to encrypt, hide, obfuscate, and make impenetrable SWF files while at the same time there will be software for decrypting, unobfuscating (if only fuscating were a word...), and generally providing context to the same body of code. This is the way that it goes in any escalating conflict.
If someone is interested in dealing with obfuscated SWF code, it might make sense to start with a list of the top obfuscators/"encrypting tools" that are used on SWFs, as well as a list of the top "decrypting" tools. It's likely that few of these products are FOSS, but there might be a small handful that are available under an acceptable license. This page might be a good start.