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Players in MMO games can usually send messages over different channels (private, public, guilds, et cetera).

How would I transmit and store this data so as to prevent outside users from being able to access somebody's private chat messages? Should I store the data in a temporary game log, or a database?

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Asking "how do MMOs do it" is too broad, and requires inside knowledge to answer with conviction. I adjusted your question to focus more on how you should implement chat for your project, with an aim on preserving security as much as possible. –  Josh Petrie Jul 3 '13 at 15:56
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You're never going to really remove the ability for an outside party to intercept chat packets from clients, and you shouldn't worry too much, because when you're building an MMO you're building a game, not a industrial-strength crypographically secure chat platform.

You should implement chat messages as follows:

  • The client transmits the chat data, which at a minimum includes the chat message but probably also includes channel or group information (such as public chat, guild chat, player-to-player whisper, et cetera).
  • The server gets the chat data. First it logs it, probably somewhere offline because there's no need to keep it resident in memory for longer than necessary. Logging is important and often overlooked, but it's incredibly useful for GM arbitration and forensic diagnostics. When you run an MMO, you want to log everything.
  • Once the data's been logged, the server determines who should get the chat message and broadcasts it appropriately. The server can then drop the remaining data on the floor; it doesn't need it any longer.

The most-vulnerable point here is the initial client-to-server transmission. If somebody snooped those packets, they could see messages not technically intended for them. You could encrypt the data, but that's a lot of effort for only minimal gain. The client must be able to decrypt chat packets eventually, and intercepting the packets as they leave the client would be the best place to do the aforementioned snooping, so the key is already available to the snooper, just a little harder to find.

It's very important that you pass your chat through a server that you are in control of, though. It need not be the game server, it can be a dedicated server for funneling chat, but you want to control the routing. It lets you have oversight for GM disputes, it lets you enforce user requests to block or mute other users, it solves the issues you might have with NAT punching or other networking shenanigans you can have trying to do peer-to-peer chat, and it provides enough of a barrier to casual hacking (which peer-to-peer chat is prone to) to solve 90% of the security concerns. Which is about as good as you'll get.

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Would using HTTPS work? Presumably chat messages are not as latency critical as the rest of the game and so could be sent and recieved via HTTPS (e.g. using WebSockets) using a separate mechanism from other game related packets. –  Justin Jul 3 '13 at 16:32
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You could make it work, but I don't see why it would be worthwhile to implement that additional side channel, especially when you may still want some chat messages to processed by the game server for other reasons (perhaps they are actually slash-commands, perhaps they contain some kind of item linking markup, et cetera). It's an option though. –  Josh Petrie Jul 3 '13 at 16:47
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I am honestly not 100% sure what you are asking. But hopefully the following is helpful.

If you look into planeshift, an open-source MMO, you can look at their code and see how they decided to architect their system. This will not give you exact insight into all MMO's but most likely implementations are similar across different games.

Here are some directions for downloading their source. You will need to check it out via SVN

I have taken a look and if you go into src/client/gui/ you will see a ChatWindow class. It takes care of sending messages back and fourth to the server for chatting and it also takes care of input history. It looks like they package up information and send it off to the server for it to deliver. So I imagine this chat information is stored on their server and thus not reachable by just anyone. That is unless you are able to get into the servers database to access chat records. I cant imagine them being able to keep everything either. If they do keep chat logs its probably only for a certain amount of time (A few days perhaps?)

Anyways, a broad answer for a broad question. I cannot really do much better than that without a more focused question. Feel free to comment if you have more narrow questions and I can try answering them.

Edit1: Also note that in the client folder, you can find and authentclient class. This handles client authentication, etc. I imagine it will also handle chat-related security in terms of authenticating itself into a chat server. They may not have dedicated servers for chat. So it could just be the game server that is also running chat services for all its clients. This is something you could figure out by really studying the code.

For bigger MMO's I would put money on them having dedicated chat servers just for taking care of chatting between players. (Just like they do for instances).

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Most MMO's I played do not store messages for a longer period of time. Because that would most likely turn them into a GameCompany with a negative badge like the NSA. Unless, the players had direct access to their database.

Runescape has the messages stored in cache, and it only is avaiable for a small ammount of time. I have never heard of anybody getting banned due to the fact that such data has been stored. If one should ever report another play for what he said. He has to be quick, until his 500~ line chat limit gets used up. After that time all his messages will be deleted. In the most crowdy place, where players shout out each other it is up to 10 seconds.

The messages are always sent via server, and storing takes place at server not client. Otherwise, Player A who hacked into the client, would be able to manipulate the array responsible for chat and literally write that PLAYER B wrote something else. After a short while he could report that player for writing something and the other players cache might no longer hold such info.

If you store the messages on the CLIENT, which has done by a chat portal GaduGadu.pl the users will lose the ability to check their chats on different PC's. The data generated by the user would be vulnerable to third persons. Mostly in public internet Cafes. Or at friends house who might seek vengeance on his former friend.

If you store it on your server, it is impossible for Player B to access data of Player A and vice versa. Keep in mind that saying "can player B access that data" is a broad question. You can never tell if the data is safe, unless your server is offline! Here is an example.

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Storing messages is only needed for delivery when you want to deliver them at a later time. For example when you want to allow people to send messages to players which are offline which are delivered the next time they log in or when you want to allow an admin to read chat at a later time.

When you don't need this, chat handling can and should be done completely in-memory.

Each chat message a client sends has to include some information about the recipient (public, guild, private message to player X). The server should then determine the characters which are allowed to read the message:

  • For public messages, these are the characters in a specific radius around the sending character
  • For guild messages, these are the characters who have the same guild as the sender who are currently online
  • For private messages, it's the character of the same name, when it is online

The message should then be sent to the clients which control these characters.

Sending a message to a client which is not supposed to show it to the user is a waste of traffic and a violation of the "never trust the client" principle.

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Such an approach removes the ability for the game administrators to review chat logs at a later date to arbitrate disputes and handle abuse complaints. So it might be efficient, but as an MMO administrator, it's probably not what you want. All chat should be logged and held securely, at least for some predefined period of time before it is archived / purged. –  MrCranky Jul 3 '13 at 13:37
    
I agree with what you say about not sending it to clients that shouldn't be allowed to read it though; filtering should be done server-side. –  MrCranky Jul 3 '13 at 13:44
    
@MrCranky When you really want to log private chat - which might violate privacy laws in some countries under some conditions - you can still log it to some file or database, but that's unrelated to who you send it to. Answer updated, though. –  Philipp Jul 3 '13 at 14:28
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It's not private if you're sending it to the game server, and the EULA should always make that clear. I don't think you'll find any MMOs out there that pretend otherwise. There are no private interactions when the player is using the game as an intermediary; this isn't a peer-to-peer chat experience, they are interacting with the game. –  MrCranky Jul 3 '13 at 15:01
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There is no expectation of privacy in an online game, laws don't work that way. –  Patrick Hughes Jul 3 '13 at 17:12
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