The basic unit of “currency” in GSS-API is the token. Applications that use GSS-API communicate with each other by using tokens. Tokens are used for exchanging data and for making security arrangements. Tokens are declared as gss_buffer_t data types. Tokens are opaque to applications.
Two types of tokens are context-level tokens and per-message tokens. Context-level tokens are used primarily when a context is established, that is, initiated and accepted. Context-level tokens can also be passed afterward to manage a context.
Per-message tokens are used after a context has been established. Per-message tokens are used to provide protection services on data. For example, consider an application that wants to send a message to another application. That application might use GSS-API to generate a cryptographic identifier to go along with that message. The identifier would be stored in a token.
Per-message tokens can be considered with regard to messages as follows. A message is a piece of data that an application sends to a peer. For example, the ls command could be a message that is sent to an ftp server. A per-message token is an object generated by GSS-API for that message. A per-message token could be a cryptographic tag or the encrypted form of the message. Note that this last example is mildly inaccurate. An encrypted message is still a message and not a token. A token is only GSS-API-generated information. However, informally, message and per-message token are often used interchangeably.
An application is responsible for the following activities:
Sending and receiving tokens. The developer usually needs to write generalized read and write functions for performing these actions. The send_token() and recv_token() functions in Miscellaneous GSS-API Sample Functions.
Because tokens are opaque to applications, the application does not distinguish between one token and another. Without knowing a token's contents, an application must be able to distinguish the token's type to pass that token to an appropriate GSS-API function.
An application can distinguish token types through the following methods:
By state. Through the control-flow of a program. For example, an application that is waiting to accept a context might assume that any received tokens are related to context establishment. Peers are expected to wait until the context is fully established before sending message tokens, that is, data. After the context is established, the application assumes that new tokens are message tokens. This approach to handling tokens is a fairly common way to handle tokens. The sample programs in this book use this method.
By flags. For example, if an application has a function for sending tokens to peers, that application can include a flag to indicate the kind of token. Consider the following code:
gss_buffer_t token; /* declare the token */ OM_uint32 token_flag /* flag for describing the type of token */ <get token from a GSS-API function> token_flag = MIC_TOKEN; /* specify what kind of token it is */ send_a_token(&token, token_flag);
The receiving application would have a receiving function, for example, get_a_token(), that would check the token_flag argument.
Through explicit tagging. Applications can use meta-tokens. A meta-token is a user-defined structure that contain tokens that have been received from GSS-API functions. A meta-token includes user-defined fields that signal how the tokens that are provided by GSS-API are to be used.
GSS-API permits a security context to be passed from one process to another in a multiprocess application. Typically, a application has accepted a client's context. The application then shares the context among that application's processes. See Exporting and Importing Contexts in GSS-API for information on multiprocess applications.
The gss_export_context() function creates an interprocess token. This token contains information that enables the context to be reconstituted by a second process. The application is responsible for passing the interprocess token from one process to the other. This situation is similar to the application's responsibility for passing tokens to other applications.
The interprocess token might contain keys or other sensitive information. Not all GSS-API implementations cryptographically protect interprocess tokens. Therefore, the application must protect interprocess tokens before an exchange takes place. This protection might involve encrypting the tokens with gss_wrap(), if encryption is available.