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+Client preauthentication interface (clpreauth)
+==============================================
+
+During an initial ticket request, a KDC may ask a client to prove its
+knowledge of the password before issuing an encrypted ticket, or to
+use credentials other than a password.  This process is called
+preauthentication, and is described in :rfc:`4120` and :rfc:`6113`.
+The clpreauth interface allows the addition of client support for
+preauthentication mechanisms beyond those included in the core MIT
+krb5 code base.  For a detailed description of the clpreauth
+interface, see the header file ``<krb5/preauth_plugin.h>``.
+
+A clpreauth module is generally responsible for:
+
+* Supplying a list of preauth type numbers used by the module in the
+  **pa_type_list** field of the vtable structure.
+
+* Indicating what kind of preauthentication mechanism it implements,
+  with the **flags** method.  In the most common case, this method
+  just returns ``PA_REAL``, indicating that it implements a normal
+  preauthentication type.
+
+* Examining the padata information included in the preauth_required
+  error and producing padata values for the next AS request.  This is
+  done with the **process** method.
+
+* Examining the padata information included in a successful ticket
+  reply, possibly verifying the KDC identity and computing a reply
+  key.  This is also done with the **process** method.
+
+* For preauthentication types which support it, recovering from errors
+  by examining the error data from the KDC and producing a padata
+  value for another AS request.  This is done with the **tryagain**
+  method.
+
+* Receiving option information (supplied by ``kinit -X`` or by an
+  application), with the **gic_opts** method.
+
+A clpreauth module can create and destroy per-library-context and
+per-request state objects by implementing the **init**, **fini**,
+**request_init**, and **request_fini** methods.  Per-context state
+objects have the type krb5_clpreauth_moddata, and per-request state
+objects have the type krb5_clpreauth_modreq.  These are abstract
+pointer types; a module should typically cast these to internal
+types for the state objects.
+
+The **process** and **tryagain** methods have access to a callback
+function and handle (called a "rock") which can be used to get
+additional information about the current request, including the
+expected enctype of the AS reply, the FAST armor key, and the client
+long-term key (prompting for the user password if necessary).  A
+callback can also be used to replace the AS reply key if the
+preauthentication mechanism computes one.