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I was asked to implement traffic encryption between the game server and clients. Currently, the traffic is not encrypted in any way, which lets players use cheats to listen to the traffic in order to obtain information about the location of the opponents.

Traffic packets are small (50-70 bytes) and the developers are not ready for a big overhead for encryption.

When the session is started, the client communicates with the server via https; and in this moment, we can send a secret key. We think that the player will not be able to get this key by extracting it from the memory or somehow else.

AES in CTR mode will be used for traffic encryption. A fast hash function with a key will be used for authentication. 4-6 bytes (not 16) will be used for the tag.

How safe is this implementation? Which hash function is better to use? Can we use the same key for both the hash function and AES?

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Your main problem is that you don't want security, only obfuscation. Any moderately talented reverser will be able to extract the AES key with standard AES implementations. –  CodesInChaos Dec 15 '13 at 13:27

2 Answers 2

SipHash would fit well for the low amount of data and small tag.

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We think that the player will not be able to get this key by extracting it from the memory or somehow else.

Forgive me, but I'm skeptical. If you really have figured out a way to do this --- and plenty of well-funded, intelligent people have tried and failed --- I'd recommend slapping a patent on it and making millions off of licensing fees.

How safe is this implementation?

Regardless of what you end up doing, the key will probably be the weak link. Or perhaps an attacker will try to read the decrypted packets from memory instead of the key.

I'm assuming that when you say "A fast hash function with a key will be used for authentication" what you mean is that you'll be using HMAC for authentication. If you mean something else, please clarify.

Using HMAC and CTR is a sound approach. Just make sure to authenticate the ciphertext ("encrypt-then-MAC"), rather than the plaintext ("MAC-then-encrypt" or "MAC and encrypt").

The real devil is in the details, though. You've given a broad overview of your approach, not a specific algorithm/protocol, and not an implementation. Each of these two transitions has the potential to go awry. For example, will your protocol prevent replay attacks? Will your implementation generate CTR nonces securely?

4-6 bytes (not 16) will be used for the tag.

I wouldn't use a tag this short to protect financial data, but for a game, it's probably fine.

Which hash function is better to use?

This depends on your definition of "better". SHA-2 will probably be the most secure. SHA-1 will probably be secure enough. HMAC-MD5 hasn't been broken, but using it is generally not recommended since MD5 has been broken. On the other hand, if your definition of "better" includes performance, than using MD5 may be worth the security risk (considering the relatively low stakes of the information you're protecting).

Can we use the same key for both the hash function and AES?

Do not do this. This may not lead directly to any obvious attacks, but key reuse in general will anger Crypto Gods because it violates a key assumption (heh) that cryptographers make when designing these algorithms. It's the crypto equivalent of not doing any bounds checking on your buffers. (And if you later switch to using something like CBC-MAC for authentication, key resuse will be come a definite problem.)

If transmitting two keys is problematic for whatever reason, you can securely derive keys from a master key by doing something like this:

encryption_key = HMAC-SHA1(master_key, "Encryption key")
authentication_key = HMAC-SHA1(master_key, "Authentication key")

A couple suggestions.

  1. This wasn't part of your questions, but I'd recommend using a dedicated authenticated encryption scheme such as GCM (galois counter mode) instead of separate MAC and encryption algorithms (such as HMAC-SHA1 and CTR-AES). Doing so will prevent some common implementation errors while quite possibly increasing performance.

  2. Regardless of whether you use HMAC or GCM, look into using "associated data" (sometimes called "additional authenticated data"). The idea here is that if there's data that doesn't need to be encrypted (i.e., kept secret from the players) but still needs to be authenticated (i.e., the players shouldn't be allowed to tamper with it), there's no need to eat the performance cost of encrypting it. A crypto library that offers GCM will likely take an optional argument containing a string of associated data.

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Another plus for AES-GCM is that if the architecture supports the AES-NI instruction set, encryption will be blazing fast. High-end PC CPUs, the Xbox One, PS4, and the iPhone 5S all support AES-NI, so AES-GCM is quite the attractive option for game development... it has all of the cryptographic niceties bundled with hardware support. –  Reid Dec 15 '13 at 8:56
    
Well, the task was to encrypt traffic. Client includes StarForce system, so encrypted packets should be adequately protected. I need maximum performance and minimum packet size overhead. This was a prototype: gist.github.com/Torikova/5236687ae9c45a4332ef I assumed that CTR mode prevents replay attacks and i don't need nonce, just counter. Can i use AES-GCM and still truncate mac to 4-5 bytes? –  Svetlana Torikova Dec 15 '13 at 12:34
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@Reid GCM needs 16 byte tags. If you truncate the tag, its security crumbles. –  CodesInChaos Dec 15 '13 at 13:28
    
@CodesInChaos: Oh, that's true. I didn't consider that part of the post. –  Reid Dec 15 '13 at 18:20
    
Regarding GCM-AES tag length, 8 bytes may be enough here. See eprint.iacr.org/2012/438.pdf, theorem 2. Assuming the 70-byte upper limit to plaintext length mentioned by the OP can be assured even against adversarial players, then $\ell_A = 5$. So the probability of a successful forgery after q forgery attempts would be at most ~ 6q/2^{64}. Which is, I think, perfectly reasonable for a game. –  Seth Dec 15 '13 at 21:01

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