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If you are not distributing your ActionScript source code with your Flash application, then you have to use secureSWF to protect your SWF files before uploading them. Anyone can download and decompile your SWF files in seconds and unconditionally view your source code.
ActionScript obfuscation makes decompiling a very time and effort consuming task by removing the information Flash decompilers use to generate useful ActionScript source code. secureSWF renames anything you have assigned a name to into meaningless and confusing names.» More About ActionScript Obfuscation
ActionScript encryption prevents Flash decompilers from generating any ActionScript code that is relevant, in any way, to the original code. It's not real encryption, but in the context of code, it might be better. ActionScript encryption (or byte-code obfuscation) implemented by secureSWF does not need to keep a decryption-key. It is actually designed not to be reversed either by the Flash player and, of course, by Flash decompilers.
secureSWF implements five highly effective ActionScript encryption techniques to protect SWF files against Flash decompilers that can be applied by several orders of magnitude.» More About ActionScript Encryption