You should define the cases where this would be neccessary, perhaps with some examples.
Much of this thought has come up because of work that I'm performing for a client. The client has a very large and rather complex application. I'm changing core parts of that application to improve the authentication and session management implementation, as well as a number of speed improvements, one of which includes caching.
One particular problem I've encountered is that the application receives a username and password, and then uses this combination to authenticate to a number of different services, snarfs up the relevant information returned by those services, and returns them to the user. As such, the username and password get passed around a lot. This is not my design, this is how the code was written before I arrived on the scene. I'm trying to move away from it, but that's a longer project.
Because passing around the password is so prevalent, I feel extremely uneasy. The code is so large that I'm unable to easily trace everywhere that it gets used. Something may have the password in a die or warn that simply hasn't been triggered yet. Something may pass it as an argument or in the environment to another program, which is a common mistake, but exposes the information to anyone using ps. Other things could be happening, but without auditing a very large amount of code, I simply cannot tell.
In this instance, I would be extremely pleased to be able to mark the password as restricted when it comes in. When the application then dies with an attempt to use restricted data, I can exaxmine the relevant section of code, ensure it's doing the right thing, and tweak it accordingly. I'd also feel a little bit better about there being a level of future-proofing -- I'd much rather have the logger throw a restricted exception than inadventantly log the password to a file due to poor coding somewhere in the application.
The idea of restricted data would have also been useful when debugging the caching mechanism this application is using. The user's password should not be cached, and while I explicitly remove the password before dropping the data structure into the cache, it ended up that two or three other data structures also stored the password, and these were being cached. They had to be tracked down and fixed manually. What's to say that there aren't any more hiding in the program?
Restricted data doesn't solve all the problems I've described above, but it does provide a helmet and safety belt which can help reduce the damage.
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