NOTE: the Sudo auth API is subject to change

Purpose: to provide a simple API for authentication methods that
         encapsulates things nicely without turning into a maze
	 of #ifdef's

The sudo_auth struct looks like this:

typedef struct sudo_auth {
    int flags;                  /* various flags, see below */
    int status;                 /* status from verify routine */
    char *name;			/* name of the method in string form */
    void *data;                 /* method-specific data pointer */

    int (*init)(struct passwd *pw, sudo_auth *auth);
    int (*setup)(struct passwd *pw, char **prompt, sudo_auth *auth);
    int (*verify)(struct passwd *pw, char *p, sudo_auth *auth);
    int (*cleanup)(struct passwd *pw, sudo_auth *auth);
    int (*begin_session)(struct passwd *pw, sudo_auth *auth);
    int (*end_session)(sudo_auth *auth);
} sudo_auth;

The variables in the struct are as follows:
    flags	Bitwise binary flags, see below.

    status	Contains the return value from the last run of
		the "verify" function.  Starts out as AUTH_FAILURE.

    name	The name of the authentication method as a C string.

    data	A pointer to method-specific data.  This is passed to
		all the functions of an auth method and is usually
		initialized in the "init" or "setup" routines.

Possible values of sudo_auth.flags:
    FLAG_DISABLED	Set if an "init" or "setup" function fails.

    FLAG_STANDALONE	If set, this indicates that the method must
			be the only auth method configured, and that
			it will prompt for the password itself.

    FLAG_ONEANDONLY	If set, this indicates that the method is the
			only one in use.  Can be used by auth functions
			to determine whether to return a fatal or nonfatal
			error.

The member functions can return the following values:
    AUTH_SUCCESS	Function succeeded.  For a ``verify'' function
			this means the user correctly authenticated.

    AUTH_FAILURE	Function failed.  If this is an ``init'' or
			``setup'' routine, the auth method will be
			marked as !configured.

    AUTH_FATAL		A fatal error occurred.  The routine should have
			written an error message to stderr and optionally
			sent mail to the administrator.
			When verify_user() gets AUTH_FATAL from an auth
			function it does an exit(1).

The functions in the struct are as follows:

    int init(struct passwd *pw, sudo_auth *auth)
        Function to do any one-time initialization for the auth
        method.  All of the "init" functions are run before anything
        else.

    int setup(struct passwd *pw, char **prompt, sudo_auth *auth)
        Function to do method-specific setup.  All the "setup"
        routines are run before any of the "verify" routines.  A
        pointer to the prompt string may be used to add method-specific
        info to the prompt.

    int verify(struct passwd *pw, char *p, sudo_auth *auth)
        Function to do user verification for this auth method.  For
        standalone auth methods ``p'' is the prompt string.  For
        normal auth methods, ``p'' is the password the user entered.
        Note that standalone auth methods are responsible for
        rerading the password themselves.

    int cleanup(struct passwd *pw, sudo_auth *auth)
        Function to do per-auth method cleanup.  This is only run
        at the end of the authentication process, after the user
        has completely failed or succeeded to authenticate.
	The ``auth->status'' variable contains the result of the
	last authentication attempt which may be interesting.

A note about standalone methods.  Some authentication methods can't
coexist with any others.  This may be because they encapsulate other
methods (pam, sia) or because they have a special way of interacting
with the user (securid).

Adding a new authentication method:

Each method should live in its own file.  Add prototypes for the functions
in sudo_auth.h.

Add the method to the ``auth_switch'' in sudo_auth.c.  Note that
standalone methods must go first.  If ``fooauth'' is a normal auth
method, its entry would look like:

#ifdef HAVE_FOOAUTH
AUTH_ENTRY("foo", 0, foo_init, foo_setup, foo_verify,
    foo_cleanup, foo_begin_session, foo_end_session)
#endif

If this is a standalone method, it would be:

#ifdef HAVE_FOOAUTH
AUTH_ENTRY("foo", FLAG_STANDALONE, foo_init, foo_setup, foo_verify,
    foo_cleanup, foo_begin_session, foo_end_session)
#endif

If the method needs to run as the user, not root, add FLAG_USER to
the second argument in the  AUTH_ENTRY line.  If you don't have an
init/setup/cleanup/begin/end routine, just use a NULL for that
field.
