Since MCOP servers will listen on a TCP port, potentially everybody (if you are on the Internet) may try to connect MCOP services. Thus, it is important to authenticate clients. MCOP uses the md5-auth protocol.
The md5-auth protocol does the following to ensure that only selected (trusted) clients may connect to a server:
To give each client the secret cookie, MCOP will (normally) put it in the mcop directory (under /tmp/mcop-USER/secret-cookie). Of course, you can copy it to other computers. However, if you do so, use a secure transfer mechanism, such as scp (from ssh).
The authentication of clients uses the following steps:
[SERVER] generate a new (random) cookie R
[SERVER] send it to the client
[CLIENT] read the "secret cookie" S from a file
[CLIENT] mangle the cookies R and S to a mangled cookie M using the MD5 algorithm
[CLIENT] send M to the server
[SERVER] verify that mangling R and S gives just the same thing as the cookie M received from the client. If yes, authentication is successful.
This algorithm should be secure, given that
The MCOP protocol will start every new connection with an authentication process. Basically, it looks like this:
To see that the security actually works, we should look at how messages are processed on unauthenticated connections:
Before the authentication succeeds, the server will not receive other messages from the connection. Instead, if the server for instance expects a “ClientHello” message, and gets an mcopInvocation message, it will drop the connection.
If the client doesn't send a valid MCOP message at all (no MCOP magic in the message header) in the authentication phase, but something else, the connection is dropped.
If the client tries to send a very very large message (> 4096 bytes in the authentication phase, the message size is truncated to 0 bytes, which will cause that it isn't accepted for authentication) This is to prevent unauthenticated clients from sending e.g. 100 megabytes of message, which would be received and could cause the server to run out of memory.
If the client sends a corrupt ClientHello message (one, for which demarshalling fails), the connection is dropped.
If the client send nothing at all, then a timeout should occur (to be implemented).