CODEWITHSUNDEEP
androidipcandroid-securityandroid-binder
Vasiliy
Vasiliy

Reputation: 16268

Android: secure IPC alternative to AIDL?

What I'm trying to do: implement lightweight, secure IPC protocol between two applications installed on the device. Client app should be able to send commands and queries to Service running in a service app, and receive back results of computations.

Apps relationship: source code of both apps is under my control, but the apps will have different signatures (non-negotiable).

Security requirements: service app should provide its services to a single client. Client's application ID (package name) is known and constant.

What I tried: I tried to implement IPC with two-way Messenger communication scheme (similar to this blog post). This approach works fine, but I encountered one major issue - I can't find a way to obtain UID of the client, therefore I can't satisfy security requirements.

Consider this code found in Service of the service application:

// This messenger will be used by the clients of this service in order to send commands
private Messenger inboxMessenger = new Messenger(new Handler() {

    @Override
    public void handleMessage(Message msg)

        // TODO: verify the identity of the client

        switch (msg.what) {
            case MSG_GET_DATA:
                returnDataToClient(msg.replyTo);
                break;
        }
    }
});

The idea here is that when the client app sends a message to this Service, it puts its local "callback" Messenger into replyTo member of the sent Message. Documentation for Messenger states that:

Note: the implementation underneath is just a simple wrapper around a Binder that is used to perform the communication.

so I thought that I could somehow map the Binder returned by Messenger#getBinder() to client's UID, but I'm now having troubles:

  1. Messenger#getBinder() returns IBinder which can't be cast to Binder
  2. Even if I do manage to get a reference to client's Binder, the method Binder#getCallingUid() is static and does not accept arguments...

So, in order to make this particular implementation work securely, I need to find a way to obtain caller's UID based on the contents of Message or based on a particular Messenger which was created by the client. Since Android's security architecture is built around Binders, it seems strange that there is no straightforward approach to mapping Binders to UIDs (or package names)... So, how can I do this?

Bonus question: except for AIDL, is there any other IPC technique on Android that fulfills the requirements stated above?

Upvotes: 3

Views: 1744

Answers (2)

Swing
Swing

Reputation: 926

You can check the client's signature in server side, to make sure it's qualified.

Upvotes: -1

Vasiliy
Vasiliy

Reputation: 16268

After some research and experimentation, I came to conclusion that Android does not provide an alternative to AIDL when it comes to secured, signature protected communication.

The good news are that AIDL is not difficult to implement and official tutorials on this particular subject are not bad at all.

Upvotes: 3

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