What I really need is the strongest encryption possible in Flash/PHP, and a way to prevent people calling the PHP page other than through my Flash file. I have tried some simple methods in the past of making multiple calls for a single score and completing a checksum / fibonacci sequence etc, and also obfuscating the SWF with Amayeta SWF Encrypt, but they were all hacked eventually.
Now before the game posts the high score, it decrypts the high-score-encrypting-session key, which it can do because you hardcoded the high-score-encrypting-session-key-decrypting-key into the Flash binary. You encrypt the high score with this decrypted key, along with the SHA1 hash of the high score:
amayeta swf encrypt 7.0 crack
The PHP code on the server checks the token to make sure the request came from a valid game instance, then decrypts the encrypted high score, checking to make sure the high-score matches the SHA1 of the high-score (if you skip this step, decryption will simply produce random, likely very high, high scores).
So now the attacker decompiles your Flash code and quickly finds the AES code, which sticks out like a sore thumb, although even if it didn't it'd be tracked down in 15 minutes with a memory search and a tracer ("I know my score for this game is 666, so let's find 666 in memory, then catch any operation that touches that value --- oh look, the high score encryption code!"). With the session key, the attacker doesn't even have to run the Flash code; she grabs a game launch token and a session key and can send back an arbitrary high score.
I made kind of workaround... I had a gave where scores incremented ( you always get +1 score ). First, I started to count from random num (let's say 14 ) and when I display the scores, just showed the scores var minus 14. This was so if the crackers are looking for example for 20, they won't find it ( it will be 34 in the memory ). Second, since I know what the next point should be... I used adobe crypto library, to create the hash of what the next point should be. When I have to increment the scores, I check if the hash of the incremented scores is equal to the hash is should be. If the cracker have changed the points in the memory, the hashes are not equal. I perform some server-side verification and when I got different points from game and from the PHP, I know that cheating were involved. Here is snippet ot my code ( I'm using Adobe Crypto libraty MD5 class and random cryptography salt. callPhp() is my server side validation )
Using this technique + SWF obfustication, I was able to stop the crackers. Also, when I'm sending the scores to the server-side, I use my own small, encryption / decryption function. Something like this (server side code not included, but you can see the algorithm and write it in PHP) :
There is no way to make it completely unhackable, as it is easy to decompile SWFs, and a skilled developer hacker could then trace through your code and work out how to bypass any encrypted system you might employ.
If you simply want to stop kids cheating via the use of simple tools like TamperData though, then you could generate an encryption key that you pass to the SWF at startup. Then use something like to encrypt the high score before passing it back to the PHP code. Then decrypt it at the server end before storing it in the database.
Whenever your highscore system is based on the fact that the Flash application sends unencrpyted/unsigned highscore data via the network, that can be intercepted and manipulated/replayed. The answer follows from that: encrypt (decently!) or cryptographically sign highscore data. This, at least, makes it harder for people to crack your highscore system because they'll need to extract the secret key from your SWF file. Many people will probably give up right there. On the other hand, all it takes is a singly person to extract the key and post it somewhere.
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