I have a client-server game, where the client connects to a server and stays connected during the game (approx 5-60 min).
I want new clients to be able to register securely, as well as allowing existing clients to authenticate with no worries that the login credentials are exposed.
The thing is that for performance reasons it would be best to stick with a simple and cheap encryption like RC4 for the game session, but a symmetric key does not make it easy to secure the registration procedure.
Since I will want to keep a separate login server anyway, my idea is like this:
Possible security concerns:
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. This could be alleviated somewhat by rotating these keys, so that every day or month has a new key. Both game and login servers could get this from a third server that manages their keys. It might be overkill since: a) in case of a break-in where source code is exposed on the servers, they can be restarted with a new key b) a good enough key + encryption should make brute force attacks hard (suggestions on algorithm?)
RC4 isn't the most secure algorithm, but I make sure to throw away the first 512 bytes or so and each key is only valid for a limited time, e.g. 24h.
Doesn't seem susceptible to man-in-the middle from what I can see: SSL secures the RC4 session key, in (5) the RC4 session key sent to the game server is encrypted as well. All that is possible is DoS and to cause the user request a key again. If the data in (2) is cached until it expires, this should not create a new packet.
The encryption in (3) could be improved by adding random bits to the key. Those random bits are sent together with the encrypted packet, and presented to the game server in (5). In (6) the game server adds those random bits to his key and uses the result to decrypt the data. This way and attacker cannot see when the packed data changes.
Are there any vulnerabilities I'm overlooking here?
A summary of payloads created:
A summary of encryption keys:
I ended up also posting on sci.crypt and I'll try to summarize the suggested changes (as far as I understand them) below in case it might be of interest.
Step 1: Client sends a HTTPS request to the login server with credentials
Assuming that the credentials take the form of a login token, also add a self-assigned unique id.
Step 3: The user information + RC4 session + timestamp + digest
Use an encryption algorithm that ensures integrity, instead of using a digest explicitly. E.g. AES-GCM or AES-CCM. Add the extra id field in step 1. Add the ip to the game server as well.
Step 4: The packaged data + RC4 session encryption key + ip address to the game server is sent as a reply.
Giving the timestamp to the client will allow the client to know when the session has expired. This avoids unnecessary connects to the game server with expired credentials.
Step 5: The client opens a connection to the game server, sends an initial unencrypted hello message with the encrypted user information as a payload.
Add the self-assigned id in step 1 unencrypted to the payload.
Step 6: The game server unpacks the data packaged in (3). It now knows the user and the RC4 encryption key it is supposed to use.
The game server matches both its own ip with the encrypted ip, as well as the encrypted id with the id given by the client. The first prevents the user from going to a different server with the same credentials.
Step 8: If everything checks ok, the server sends an unencrypted LOGIN_OK, and the RC4 encrypted communication starts.
At this point the game server cannot be sure of the client's identity. Use the session key and encrypt nonce + strictly increasing session id + login success state using AES-GCM/CCM and send it to the client.
The client decrypts and checks the login success state. If this is true, then the client knows that the game server knows the session key (GCM/CCM verifies that the packet has not been tampered with). The client returns sid + nonce.
The server verifies sid + nonce is the same as the values sent.
Finally the client and server creates new session keys by hash-ing the session key with sid + nonce + salt to create the key for the consequent communication, to prevent a possible replay attack.
Regarding RC4
There are vulnerabilities in RC4, but it probably would suffice for this scheme because of the rather aggressive key rescheduling. However, there are modern ciphers which are more secure and faster, such as Snow 2.0 or Trivium.