Sunday, March 18, 2007

Protecting APIs - a different approach

I have been wondering lately if the current is the most optimal way of protecting APIs in the S60 (or Symbian in general) C++ SDKs. It must not come as a surprise to any programmers that there are far more features in these SDKs than what we can use with the publicly available functions. But they are usually hidden.

How? Before answering this question, some technical details. In order to use a certain feature in C++ the following two conditions must be met:

  • The header file including the function to be called must be publicly available in the SDK,
  • The component, typically a DLL and its associated LIB file, must be publicly available in the SDK.
The first condition ensures that the code compiles, the second that the link process will succeed. The most common practice that Nokia applies when they want to protect an API (i.e. not let 3rd party use it) is that they remove the header file in question. I would call it as a lazy protection as it does not protect against reverse engineering. For example, as soon as the API becomes publicly available there is nothing that can prevent a developer from making use of that functionality. Okay, Nokia clearly forbids reverse engineering in their license agreement, but there might be people who think it in a different way.

The second type of protection, let's call it hard protection, is to remove both the header and the LIB (that we import in our MMP) from the SDK so that neither compilation nor linking succeeds. Naturally, the DLL must remain in the SDK as usually it already has clients that use the provided functionality. This workaround is also applied by Nokia and Symbian alike. Sometimes they make it so that the LIB file is missing only under emulator platform (i.e. WINSCW), but not from the target platform (e.g. ARMv5). This makes it more difficult for developers to test their software - but not impossible. Since if you cannot link against a DLL statically, then you can still do it dynamically. You just need to use RLibrary class for that purpose and call the exported methods based on their ordinals. Okay, it's by far not trivial and now we're knee-deep in sensitive and confidential data, but it IS possible.

Let's just stop here for a minute to think about why Nokia (probably) follows the above-described practice! If I wanted to protect some data and not let others use it, then I would do everything in order to impede people to make use of it. Not just invent some hackish solutions that can be relatively easily bypassed, but make it really hard for others to hack. But it might not be in Nokia's interest to do so. You know, they have partners to whom they open up some - non-published, but available - APIs every now and then. And if they can choose from simply giving access to a header file OR sharing e.g. a LIB file, too, then they naturally vote for the first option. In my opinion it's much easier to do due to e.g. traceability reasons.

We have now finally reached the point where I can talk about my idea on a (probably) better way to protect APIs. You know, there's a rule of thumb in Symbian C++ programming as of the introduction of Platform Security: if you have sensitive information (I mean, data) to protect, then you're suggested to write a server and let it guard that data. This solution also enables you to smoothly control who and how can access the protected resource. I believe that Nokia has already re-factored their code according to this pattern so that all sensitive data is taken care of by various servers. These servers may then check against capabilities, secure and/or vendor IDs. Ideally, there is no sensitive data by now that is not protected by a server.

And this is the point where I ask: why not make every API public so that everyone can use it? Following from all above, security must not be a concern here, since access to sensitive data is already under control. On the other hand, developers could greatly benefit from having access to DLLs that have been hidden from them so far. You know, I'm talking about a big bunch of features that have been written by some talented developers at Nokia, Symbian, etc. and hidden unnecessarily. Why not let others make use of those features if it doesn't harm?

There's an exception, though. If the piece of information to be protected is not data, but an algorithm, for example. If it's not of big importance what data we work on, but how. Garnished possibly with some IPR issues. If this is an issue and the executing code is in a DLL (i.e. not behind a server), then we can't expect too much from Platform Security. We can assign some strong capability to the DLL so that using it would not be trivial for anyone, but we'd have no option for fine-tuned secure/vendor ID check, for example. Perhaps even capability constraints would not be sufficient, either. I think that we (err, I mean Nokia, Symbian and others) could re-use the existing approach in this case, that is, not publishing header and/or LIB files in the public SDKs. You know, I don't think we should fully get rid of the current solution, but at least suggest to use it moderately.

Wishing to read your comments!

Tote

5 comments:

jth said...

I believe the main reason for not publishing APIs in a platsec world is API comptibility, and very little to do with security (this is just obscurity, as you point out). Symbian & Nokia have run up a fair track-record of breaking API compatibility -- one of the tactics to reduce this going forward is only to publicly publish APIs which they believe to be well designed and suitable for maintaining compatibility with for the long-term.
This is a long way from saying all published APIs are well-designed. It could even be used as cover for shoddy workmanship [we can't publish this becuase as we may break this in the future, so it matters little if it's poorly designed and documented, viz. it will never get published]

Gábor Török said...

You're probably right! And indeed, it has nothing to do with PlatSec: Nokia and Symbian simply do not want the burden and responsibility of keeping their APIs backward compatible for ages. And I must admit that it's reasonable at some point. It's not necessarily about future-proof design, they just don't want to bother with it more than necessary.

Thanks for pointing it out!

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