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    <p>First of all I wouldn't use RC4. There are both faster and more secure stream ciphers around so if you control both client and server then you can probably do better than RC4. Discarding only 512 bytes may not be enough for the Fluhrer, Mantin and Shamir attack, but even if you discard more bytes there's also the Klein's attack etc. I don't know if it's worth the trouble.</p> <p>Second, make sure that the keys you generate are random. I know it seems obvious but for an example see: <a href="http://www.debian.org/security/2008/dsa-1571" rel="nofollow">http://www.debian.org/security/2008/dsa-1571</a></p> <p>But the real problem is this part: "The game server 100% trusts the user info it has decrypted. This makes the servers completely decoupled which is nice, but if the key is compromised, users could completely fake their user info."</p> <p>You have to assume that the user knows the key. His game client has to know the key so it can communicate with the server. If the user can use his real data to log in via ssl, get a key for stream cipher and then send whatever info he wants to the game server then all the attacker has to do is just get an account and do whatever he wants.</p> <p>It doesn't matter how often you change the key because every time you change it you have to still give it to the client so you might as well change it after every byte and it still wouldn't matter here.</p> <p>This is more important than the cipher used or the key generation because no one will brute force the key if he just gets it.</p> <p>You should never trust the client. You should store the client data on the server side and match it with the key or sign the data and verify it or use HMAC etc. because if the game server 100% trusts the user info then you will have problems sooner o later. There is pretty much no way around it.</p>
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    1. POSecurity: How vulnerable is this authentication/encryption scheme?
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    1. CO"You have to assume that the user knows the key. His game client has to know the key so it can communicate with the server." Ummm, the client only passes on the encrypted user info from the login to the game server. Since the client never decrypts it, the client does not have the key. The client is given a session key for encrypted communication with the game server, but this is quite a different key.
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    2. COFor generating keys in Java (and in this case both game server and login server is in java) I usually go with SecureRandom and in particular SHA1PRNG. Good enough?
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    3. COWhat would you suggest as alternatives to RC4?
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