Using the network for distributing an application is challenging not only because of the physical limitations of bandwidth and latency. It also raises new issues related to security between and among clients and components. Since many operations are now physically accessible by anyone with access to the network, access to these operations has to be restricted at a higher level.
Without security support from the distributed development platform, each application would be forced to implement its own security mechanisms. A typical mechanism would involve passing some kind of username and password (or a public key)—usually encrypted—to some kind of logon method. The application would validate these credentials against a user database or directory and return some dynamic identifier for use in future method calls. On each subsequent call to a secure method, the clients would have to pass this security identifier. Each application would have to store and manage a list of usernames and passwords, protect the user directory against unauthorized access, and manage changes to passwords, as well as dealing with the security hazard of sending passwords over the network.
A distributed platform must thus provide a security framework to safely distinguish different clients or different groups of clients so that the system or the application has a way of knowing who is trying to perform an operation on a component. DCOM uses the extensible security framework provided by Windows NT. Windows NT provides a solid set of built-in security providers that support multiple identification and authentication mechanisms, from traditional trusted-domain security models to noncentrally managed, massively scaling public-key security mechanisms. A central part of the security framework is a user directory, which stores the necessary information to validate a user's credentials (user name, password, public key). Most DCOM implementations on non-Windows NT platforms provide a similar or identical extensibility mechanism to use whatever kind of security providers is available on that platform. Most UNIX-implementations of DCOM will include a Windows NT-compatible security provider.
Before looking more closely at these Windows NT security and directory providers, let's take a look at how DCOM uses this general security framework to make building secure applications easier.