11 Jun 2013 joolean   » (Journeyer)

Like a lot of programmers, I think, I developed a mode of thinking about and designing a software system as a set of mostly independent components, each with a limited, discrete function, working in concert to produce a complex epiphenomenal behavior. Until relatively recently, though, I didn't think of these systems as potentially spanning multiple processes or machines. It may seem like a trivial observation, but I've come to find it useful to think of complex systems as appliances that use some set of computing hardware to host one or more processes whose combined behavior forms the behavior of the whole system. The benefit of this kind of thinking is that you no longer need to figure out a reasonable way to wedge a web server into, say, your spreadsheet application process code. Instead, you've got your web server, and you've got your spreadsheet. The difficulty is that you may need to launch and coordinate several processes -- or machines -- to get the complete appliance into the right state, such that its different parts are relaying data back and forth and responding to requests properly.

krb5

...Which brings me to my plodding, ongoing experiments in writing an online game. I'd invested quite a bit of time attempting to model the concept of downloadable assets of different types from within my gzochi application code before ultimately deciding that the game server had no business manipulating asset data. That kind of thing, I figure, is the rightful purview of some kind of independent asset management system that's aware of user authorization but not necessarily game state. So I set about figuring out how to manage authorization across processes, and, naturally, Kerberos came to mind. Everything you read about Kerberos steers you in the direction of using it via GSS, the Generic Security Services API. A lot of what you read about GSS suggests that perhaps you ought to consider using SASL, the Simple Authentication & Security Layer. So I did. On first glance, SASL looked like a bad fit -- your SASL-ized applications get to enter into negotiations over which of a set of mutually-supported authentication mechanisms will be used to initiate a session. I guess the idea is that you want secure authentication and you don't care how it happens. But I did care how it happened. So I dropped down to GSS, and found that at first it sort of made sense: Everything is a principal and has credentials, and two principals can create a security context with each other through which they can securely exchange information. But the GSS API designers seemed desperate to avoid explicit representation of anything that might remotely suggest that it's a wrapper around any particular security implementation, much less Kerberos -- no ticket-granting tickets, no service tickets, no distinction between user and service principals. I spent weeks trying to figure out how to model the authentication and authorization flow I had in mind: A client application would obtain a TGT for user with a password, and then use it to obtain tickets to authenticate with the asset server and game server.

When, out of frustration, I dug into the verboten krb5 API, I found it easy to understand -- in the course of trying to get GSS to work I'd figured out the details of key tables and credential caches -- and had something approximating my desired architecture working in an evening. And it's, like, ten lines of code. So I'm kind of on board with what Simon Josefsson says in the appendix of the GNU GSS manual:

...GSS may not be the simplest solution available to solve actual problems, since otherwise more projects would have chosen to take advantage of the work that went into GSS instead of using another framework (or designing their own solution).
I'm with him right up until he says the only circumstance under which you should use GSS is when you're sure you want a Kerberos 5 implementation. Bzzzht!

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