For a flash game platform a friend and i were searching for a security solution.
The problem with flash is that a possible hacker could decompile swf-files and looking for the URL where the score of the game is uploaded. For sure obfuscater is a possibility, but we are searching for a method which is not decompileable in any ways.
In flash there are some properties of movieclips which are not readable with the common AS Decompiler (Sothing, Burak, etc.). And there are undocumented methods and AS 1.0 hacks which made it nearly impossible to understand the code.
With this primitive possibilities we have tried to build a secure checksum. This checksum will be send to server where a PHP script check if it is valid or not. The important thing was to generate a key with all these parameters which is not readable threw a Decompiler. For a better camouflage we use some hacks with __resolve, static variables, pseudo functions and a little bit we obfuscate the code.
As a summary we can say, a pretty secure solution, but we know the disadvantages. If there is the chance to win a big price, maybe 99% of possible attacks will be a failure, but the 1% percent left is enough anger. Never use only flash based solutions for things wich require security standards and obfuscate what you can.
For an increase of security we build-in some other features, like Sessions, SSL(Secure Socket Layer) and a random based interval where we check the validation. Another possibility would be given threw a XML-Socket, but maybe this is oversized for 99% of all flash games.