Open-PAM working group                                         A.G. Morgan
Internet Draft:                                                Dec 8, 2001
Document: draft-morgan-pam-08.txt                                         
Expires: June 8, 2002                                                     
Obsoletes: draft-morgan-pam-07.txt                                        

                  Pluggable Authentication Modules (PAM)                  

  1  Status of this memo

  This document is a draft specification. Its contents are subject to
  change with revision. The latest version of this draft may be obtained
  from here:

    http://www.kernel.org/pub/linux/libs/pam/pre/doc/

  As

    Linux-PAM-'version'-docs.tar.gz

  It is also contained in the Linux-PAM tar ball.

  2  Abstract

  This document is concerned with the definition of a general
  infrastructure for module based authentication.  The infrastructure is
  named Pluggable Authentication Modules (PAM for short).

  3  Introduction

  Computers are tools.  They provide services to people and other
  computers (collectively we shall call these _users_ entities).  In
  order to provide convenient, reliable and individual service to
  different entities, it is common for entities to be labelled.  Having
  defined a label as referring to a some specific entity, the label is
  used for the purpose of protecting and allocating data resources.

  All modern operating systems have a notion of labelled entities and
  all modern operating systems face a common problem: how to
  authenticate the association of a predefined label with applicant
  entities.

  There are as many authentication methods as one might care to count.
  None of them are perfect and none of them are invulnerable.  In
  general, any given authentication method becomes weaker over time.  It
  is common then for new authentication methods to be developed in
  response to newly discovered weaknesses in the old authentication
  methods.

  The problem with inventing new authentication methods is the fact that
  old applications do not support them.  This contributes to an inertia
  that discourages the overhaul of weakly protected systems.  Another
  problem is that individuals (people) are frequently powerless to layer
  the protective authentication around their systems.  They are forced
  to rely on single (lowest common denominator) authentication schemes
  even in situations where this is far from appropriate.

  PAM, as discussed in this document, is a generalization of the
  approach first introduced in [1].  In short, it is a
  general framework of interfaces that abstract the process of
  authentication.  With PAM, a service provider can custom protect
  individual services to the level that they deem is appropriate.

  PAM has nothing explicit to say about transport layer encryption.
  Within the context of this document encryption and/or compression of
  data exchanges are application specific (strictly between client and
  server) and orthogonal to the process of authentication.

  4  Definitions

  Here we pose the authentication problem as one of configuring defined
  interfaces between two entities.

  4.1  Players in the authentication process

  PAM reserves the following words to specify unique entities in the
  authentication process:

    applicant
  	the entity (user) initiating an application for service
  	[PAM associates the PAM_RUSER _item_ with this requesting user].

    arbitrator
  	the entity (user) under whose identity the service application
  	is negotiated and with whose authority service is granted.

    user
  	the entity (user) whose identity is being authenticated
  	[PAM associates the PAM_USER _item_ with this identity].

    server
  	the application that provides service, or acts as an
  	authenticated gateway to the requested service.  This
  	application is completely responsible for the server end of
  	the transport layer connecting the server to the client.
  	PAM makes no assumptions about how data is encapsulated for
  	exchanges between the server and the client, only that full
  	octet sequences can be freely exchanged without corruption.

    client
  	application providing the direct/primary interface to
  	applicant.  This application is completely responsible
  	for the client end of the transport layer connecting the
  	server to the client.  PAM makes no assumptions about how data
  	is encapsulated for exchanges between the server and the
  	client, only that full octet sequences can be freely
  	exchanged without corruption.

    module
  	authentication binary that provides server-side support for
  	some (arbitrary) authentication method.

    agent
  	authentication binary that provides client-side support for
  	some (arbitrary) authentication method.

  Here is a diagram to help orient the reader:

                           +-------+              +--------+              
                  . . . . .| agent |             .| module |              
                  .        +-------+             .+--------+              
                  V            |                 .    |                   
                  .            |                 V    |                   
             +---------+   +-------+             . +------+               
             |         |   |libpamc|             . |libpam|               
             |         |   +-------+             . +------+               
             |applicant|       |                 .    |                   
             |         |   +--------+           +----------+              
             |         |---| client |-----------|  server  |              
             +---------+   +--------+           +----------+              

  Solid lines connecting the boxes represent two-way interaction.  The
  dotted-directed lines indicate an optional connection between the
  plugin module (agent) and the server (applicant). In the case of the
  module, this represents the module invoking the 'conversation'
  callback function provided to libpam by the server application when it
  initializes the libpam library. In the case of the agent, this may
  be some out-of-PAM API interaction (for example directly displaying a
  dialog box under X).

  4.2  Defined Data Types

  In this draft, we define two composite data types, the text string and
  the binary prompt. They are the data types used to communicate
  authentication requests and responses.

  4.2.1  text string

  The text string is a simple sequence of non-NUL (NUL = 0x00)
  octets. Terminated with a single NUL (0x00) octet. The character set
  employed in the octet sequence may be negotiated out of band, but
  defaults to utf-8.

                       ---------------------------                        
                       [  character data  |  NUL ]                        
                       [  octet sequence  | 0x00 ]                        
                       ---------------------------                        

  Within the rest of this text, PAM text strings are delimited with a
  pair of double quotes. Example, "this" = {'t';'h';'i';'s';0x00}.

  4.2.2  binary prompt

  A binary prompt consists of a stream of octets arranged as follows:

                 ----------------------------------------                 
                 [  u32   |   u8    | (length-5 octets) ]                 
                 [ length | control |       data        ]                 
                 ----------------------------------------                 

  That is, a 32-bit unsigned integer in network byte order, a single
  unsigned byte of control information and a sequence of octets of
  length (length-5). The composition of the _data_ is context dependent
  but is generally not a concern for either the server or the client. It
  is very much the concern of modules and agents.

  For purposes of interoperability, we define the following control
  characters as legal.

            value    symbol             description                       
            -------------------------------------------------             
            0x01     PAM_BPC_OK       - continuation packet               
            0x02     PAM_BPC_SELECT   - initialization packet             
            0x03     PAM_BPC_DONE     - termination packet                
            0x04     PAM_BPC_FAIL     - unable to execute                 

  The following control characters are only legal for exchanges between
  an agent and a client (it is the responsibility of the client to
  enforce this rule in the face of a rogue server):

            0x41     PAM_BPC_GETENV   - obtain client env.var             
            0x42     PAM_BPC_PUTENV   - set client env.var                
            0x43     PAM_BPC_TEXT     - display message                   
            0x44     PAM_BPC_ERROR    - display error message             
            0x45     PAM_BPC_PROMPT   - echo'd text prompt                
            0x46     PAM_BPC_PASS     - non-echo'd text prompt            
            0x46     PAM_BPC_STATUS   - ping all active clients           
            0x47     PAM_BPC_ABORT    - please abort session              

  Note, length is always equal to the total length of the binary
  prompt and represented by a network ordered unsigned 32 bit integer.

  4.2.2.1 PAM_BPC_SELECT binary prompts

  Binary prompts of control type PAM_BPC_SELECT have a defined
  data part. It is composed of three elements:

  	{agent_id;'/';data}

  The agent_id is a sequence of characters satisfying the following
  regexp:

  	/^[a-z0-9\_]+(@[a-z0-9\_.]+)?$/

  and has a specific form for each independent agent.

  o Agent_ids that do not contain an at-sign (@) are to be considered as
    representing some authentication mode that is a "public
    standard" see reference [2]. Registered names
    MUST NOT contain an at-sign (@).

  o Anyone can define additional agents by using names in the format
    name@domainname, e.g. "ouragent@example.com". The part following
    the at-sign MUST be a valid fully qualified internet domain name
    [RFC-1034] controlled by the person or organization defining the
    name. (Said another way, if you control the email address that
    your agent has as an identifier, they you are entitled to use
    this identifier.) It is up to each domain how it manages its local
    namespace.

  The '/' character is a mandatory delimiter, indicating the end of the
  agent_id. The trailing data is of a format specific to the agent with
  the given agent_id.


  4.3  Special cases

  In a previous section (4.1) we identified the most general
  selection of authentication participants.  In the case of network
  authentication, it is straightforward to ascribe identities to the
  defined participants.  However, there are also special (less general)
  cases that we recognize here.

  The primary authentication step, when a user is directly introduced
  into a computer system (log's on to a workstation) is a special case.
  In this situation, the client and the server are generally one
  application.  Before authenticating such a user, the applicant is
  formally unknown: PAM_RUSER is NULL.

  Some client-server implementations (telnet for example) provide
  effective full tty connections. In these cases, the four simple text
  string prompting cases (see below) can be handled as in the primary
  login step. In other words, the server absorbs most of the overhead of
  propagating authentication messages. In these cases, there needs to be
  special client/server support for handling binary prompts.

  In some circumstances, a legacy network transfer protocol can carry
  authentication information. In such cases, a desire to support legacy
  clients (with no client-side support for PAM) will neccessitate the
  'hardcoding' of an agent protocol into the server application. Whilst
  against the spirit of PAM, this special casing can be managed by the
  server's 'conversation function' (see below). The guiding principle
  when implementing such support is for the application developer to
  relegate the authentication process to the PAM module -- simply
  performing a transcription of data from binary-prompt to legacy
  network 'packet' and visa-versa for propagating replies back to the
  driving PAM module. A common case of this is with network protocols
  that define an initialization packet of "user+password". In such cases
  one should attempt to support the "userpass" agent-id and its defined
  protocol.

  5  Defined interfaces for information flow

  Here, we discuss the information exchange interfaces between the
  players in the authentication process. It should be understood that
  the server side is responsible for driving the authentication of the
  applicant. Notably, every request received by the client from the
  server must be matched with a single response from the client to the
  server.

  5.1  Applicant <-> client

  Once the client is invoked, requests to the applicant entity are
  initiated by the client application.  General clients are able to make
  the following requests directly to an applicant:

     echo text string
     echo error text string
     prompt with text string for echo'd text string input
     prompt with text string for concealed text string input

  the nature of the interface provided by the client for the benefit of
  the applicant entity is client specific and not defined by PAM.

  5.2  Client <-> agent

  In general, authentication schemes require more modes of exchange than
  the four defined in the previous section (5.1).  This
  provides a role for client-loadable agents.  The client and agent
  exchange binary-messages that can have one of the following forms:

     client -> agent
  	binary prompt agent expecting binary prompt reply to client

     agent -> client
  	binary prompt reply from agent to clients binary prompt

  Following the acceptance of a binary prompt by the agent, the agent
  may attempt to exchange information with the client before returning
  its binary prompt reply. Permitted exchanges are binary prompts of the
  following types:

     agent -> client
  	set environment variable (A)
  	get environment variable (B)
  	echo text string (C)
  	echo error text string (D)
  	prompt for echo'd text string input (E)
  	prompt for concealed text string input (F)

  In response to these prompts, the client must legitimately respond
  with a corresponding binary prompt reply. We list a complete set of
  example exchanges, including each type of legitimate response (passes
  and a single fail):

     Type | Agent request                  | Client response              
     ---------------------------------------------------------------      
     (A)  | {13;PAM_BPC_PUTENV;"FOO=BAR"}  | {5;PAM_BPC_OK;}              
          | {10;PAM_BPC_PUTENV;"FOO="}     | {5;PAM_BPC_OK;}              
          | {9;PAM_BPC_PUTENV;"FOO"}  (*)  | {5;PAM_BPC_OK;}              
          | {9;PAM_BPC_PUTENV;"BAR"}  (*)  | {5;PAM_BPC_FAIL;}            
     ---------------------------------------------------------------      
     (B)  | {10;PAM_BPC_GETENV;"TERM"}     | {11;PAM_BPC_OK;"vt100"}      
          | {9;PAM_BPC_GETENV;"FOO"}       | {5;PAM_BPC_FAIL;}            
     ---------------------------------------------------------------      
     (C)  | {12;PAM_BPC_TEXT;"hello!"}     | {5;PAM_BPC_OK;}              
          | {12;PAM_BPC_TEXT;"hello!"}     | {5;PAM_BPC_FAIL;}            
     ---------------------------------------------------------------      
     (D)  | {11;PAM_BPC_ERROR;"ouch!"}     | {5;PAM_BPC_OK;}              
          | {11;PAM_BPC_ERROR;"ouch!"}     | {5;PAM_BPC_FAIL;}            
     ---------------------------------------------------------------      
     (E)  | {13;PAM_BPC_PROMPT;"login: "}  | {9;PAM_BPC_OK;"joe"}         
          | {13;PAM_BPC_PROMPT;"login: "}  | {6;PAM_BPC_OK;""}            
          | {13;PAM_BPC_PROMPT;"login: "}  | {5;PAM_BPC_FAIL;}            
     ---------------------------------------------------------------      
     (F)  | {16;PAM_BPC_PASS;"password: "} | {9;PAM_BPC_OK;"XYZ"}         
          | {16;PAM_BPC_PASS;"password: "} | {6;PAM_BPC_OK;""}            
          | {16;PAM_BPC_PASS;"password: "} | {5;PAM_BPC_FAIL;}            

  (*) Used to attempt the removal of a pre-existing environment
  variable.

  5.3  Client <-> server

  Once the client has established a connection with the server (the
  nature of the transport protocol is not specified by PAM), the server
  is responsible for driving the authentication process.

  General servers can request the following from the client:

     (to be forwarded by the client to the applicant)
  	echo text string
  	echo error text string
  	prompt for echo'd text string response
  	prompt for concealed text string response

     (to be forwarded by the client to the appropriate agent)
  	binary prompt for a binary prompt response

  Client side agents are required to process binary prompts.  The
  agents' binary prompt responses are returned to the server.

  5.4  Server <-> module

  Modules drive the authentication process.  The server provides a
  conversation function with which it encapsulates module-generated
  requests and exchanges them with the client. Every message sent by a
  module should be acknowledged.

  General conversation functions can support the following five
  conversation requests:

     echo text string
     echo error string
     prompt for echo'd text string response
     prompt for concealed text string response
     binary prompt for binary prompt response

  The server is responsible for redirecting these requests to the
  client.

  6  C API for application interfaces (client and server)

  6.1  Applicant <-> client

  No API is defined for this interface.  The interface is considered to
  be specific to the client application.  Example applications include
  terminal login, (X)windows login, machine file transfer applications.

  All that is important is that the client application is able to
  present the applicant with textual output and to receive textual
  input from the applicant.  The forms of textual exchange are listed
  in an earlier section (5.1).  Other methods of
  data input/output are better suited to being handled via an
  authentication agent.

  6.2  Client <-> agent

  The client makes use of a general API for communicating with
  agents. The client is not required to communicate directly with
  available agents, instead a layer of abstraction (in the form of a
  library: libpamc) takes care of loading and maintaining communication
  with all requested agents. This layer of abstraction will choose which
  agents to interact with based on the content of binary prompts it
  receives that have the control type PAM_BPC_SELECT.

  6.2.1  Client <-> libpamc

  6.2.1.1  Compilation information

  The C-header file provided for client-agent abstraction is included
  with the following source line:

  	#include <security/pam_client.h>

  The library providing the corresponding client-agent abstraction
  functions is, libpamc.

  	cc .... -lpamc

  6.2.1.2  Initializing libpamc

  The libpamc library is initialized with a call to the following
  function:

  	pamc_handle_t pamc_start(void);

  This function is responsible for configuring the library and
  registering the location of available agents. The location of the
  available agents on the system is implementation specific.

  pamc_start() function returns NULL on failure. Otherwise, the return
  value is a pointer to an opaque data type which provides a handle to
  the libpamc library. On systems where threading is available, the
  libpamc library is thread safe provided a single (pamc_handler_t *)
  is used by each thread.

  6.2.1.3  Client (Applicant) selection of agents

  For the purpose of applicant and client review of available agents,
  the following function is provided.

  	char **pamc_list_agents(pamc_handle_t pch);

  This returns a list of pointers to the agent_id's of the agents which
  are available on the system. The list is terminated by a NULL pointer.
  It is the clients responsibility to free this memory area by calling
  free() on each agent id and the block of agent_id pointers in the
  result.

  PAM represents a server-driven authentication model, so by default
  any available agent may be invoked in the authentication process.

  6.2.1.3.1  Client demands agent

  If the client requires that a specific authentication agent is
  satisfied during the authentication process, then the client should
  call the following function, immediately after obtaining a
  pamc_handle_t from pamc_start().

  	int pamc_load(pamc_handle_t pch, const char *agent_id);

  agent_id is a PAM text string (see section 4.2.2.1) and is not
  suffixed with a '/' delimiter. The return value for this function is:

  	PAM_BPC_TRUE    - agent located and loaded.
  	PAM_BPC_FALSE   - agent is not available.

  Note, although the agent is loaded, no data is fed to it. The agent's
  opportunity to inform the client that it does not trust the server is
  when the agent is shutdown.

  6.2.1.3.2  Client marks agent as unusable

  The applicant might prefer that a named agent is marked as not
  available.  To do this, the client would invoke the following function
  immediately after obtaining a pamc_handle_t from pam_start().

  	int pamc_disable(pamc_handle_t pch, const char *agent_id);

  here agent_id is a PAM text string containing an agent_id (section
  4.2.2.1).

  The return value for this function is:

  	PAM_BPC_TRUE    - agent is disabled. This is the response
  	                  independent of whether the agent is locally
  	                  available.

  	PAM_BPC_FALSE   - agent cannot be disabled (this may be because
  	                  it has already been invoked).

  6.2.1.4  Allocating and manipulating binary prompts

  All conversation between an client and an agent takes place with
  respect to binary prompts. A binary prompt (see section 4.2.2), is
  obtained, resized and deleted via the following C-macro:

   CREATION of a binary prompt with control X1 and data length Y1:

  	pamc_bp_t prompt = NULL;
  	PAM_BP_RENEW(&prompt, X1, Y1);

   REPLACEMENT of a binary prompt with a control X2 and data length Y2:

  	PAM_BP_RENEW(&prompt, X2, Y2);

   DELETION of a binary prompt (the referenced prompt is scrubbed):

  	PAM_BP_RENEW(&prompt, 0, 0);

  Note, the PAM_BP_RENEW macro always overwrites any prompt that you
  call it with, deleting and liberating the old contents in a secure
  fashion. Also note that PAM_BP_RENEW, when returning a prompt of data
  size Y1>0, will always append a '\0' byte to the end of the prompt (at
  data offset Y1). It is thus, by definition, acceptable to treat the
  data contents of a binary packet as a text string (see 4.2.1).

   FILLING a binary prompt from a memory pointer U1 from offset O1 of
     length L1:

  	PAM_BP_FILL(prompt, O1, L1, U1);

   the CONTROL type for the packet can be obtained as follows:

  	control = PAM_PB_CONTROL(prompt);

   the LENGTH of a data within the prompt (_excluding_ its header
   information) can be obtained as follows:

  	length = PAM_BP_LENGTH(prompt);

   the total SIZE of the prompt (_including_ its header information)
   can be obtained as follows:

          size = PAM_BP_SIZE(prompt);

   EXTRACTING data from a binary prompt from offset O2 of length L2 to
     a memory pointer U2:

  	PAM_BP_EXTRACT(prompt, O2, L2, U2);

   If you require direct access to the raw prompt DATA, you should use
   the following macro:

  	__u8 *raw_data = PAM_BP_DATA(prompt);

  6.2.1.5  Client<->agent conversations

  All exchanges of binary prompts with agents are handled with the
  single function:

  	int pamc_converse(pamc_handle_t *pch, pamc_bp_t *prompt_p);

  The return value for pamc_converse(...) is PAM_BPC_TRUE when there is
  a response packet and PAM_BPC_FALSE when the client is unable to
  handle the request represented by the original prompt. In this latter
  case, *prompt_p is set to NULL.

  This function takes a binary prompt and returns a replacement binary
  prompt that is either a request from an agent to be acted upon by the
  client or the 'result' which should be forwarded to the server. In the
  former case, the following macro will return 1 (PAM_BPC_TRUE) and in
  all other cases, 0 (PAM_BPC_FALSE):

  	PAM_BPC_FOR_CLIENT(/* pamc_bp_t */ prompt)

  Note, all non-NULL binary prompts returned by pamc_converse(...), are
  terminated with a '\0', even when the full length of the prompt (as
  returned by the agent) does not contain this delimiter. This is a
  defined property of the PAM_BP_RENEW macro, and can be relied upon.

  Important security note: in certain implementations, agents are
  implemented by executable binaries, which are transparently loaded and
  managed by the PAM client library. To ensure there is never a leakage
  of elevated privilege to an unprivileged agent, the client application
  should go to some effort to lower its level of privilege. It remains
  the responsibility of the applicant and the client to ensure that it
  is not compromised by a rogue agent.

  6.2.1.6  Status of agents

  	int pamc_status(pamc_handle_t *pch, pamc_bp_t *prompt_p);

  At any time, the client may ping all active agents for their status
  (with a PAM_BPC_STATUS binary prompt). If any agent replies with
  PAM_BPC_ABORT, the client is responsible for terminating the
  connection to the server and then terminating all agents with a call
  to pamc_end(). In such cases, the return value of pamc_status() is
  PAM_BPC_FALSE.

  If the return status of pamc_status() is PAM_BPC_TRUE and *prompt_p is
  non-NULL, then an agent is requesting access to a server module.

  XXX - how this information gets propagated to the server, and
        ultimately to the server's module is yet to be determined.

  6.2.1.7  Termination of agents

  When closing the authentication session and severing the connection
  between a client and a selection of agents, the following function is
  used:

  	int pamc_end(pamc_handle_t *pch);

  Following a call to pamc_end, the pamc_handle_t will be invalid.

  The return value for this function is one of the following:

  	PAM_BPC_TRUE	- all invoked agents are content with
  			  authentication (the server is _not_ judged
  			  _un_trustworthy by any agent)

  	PAM_BPC_FALSE	- one or more agents were unsatisfied at
  			  being terminated.  In general, the client
  			  should terminate its connection to the
  			  server and indicate to the applicant that
  			  the server is untrusted.

  6.2.2 libpamc <-> agents

  The agents are manipulated from within libpamc. Each agent is an
  executable in its own right. This permits the agent to have access to
  sensitive data not accessible directly from the client. The mode of
  communication between libpamc and an agent is through a pair of
  pipes. The agent reads binary prompts (section 4.2.2)
  through its standard input file descriptor and writes response (to the
  server) binary prompts and instruction binary prompts (instructions
  for the client) through its standard output file descriptor.

  6.3 Client <-> server

  This interface is concerned with the exchange of text and binary
  prompts between the client application and the server application.  No
  API is provided for this as it is considered specific to the transport
  protocol shared by the client and the server.

  6.4 Server <-> modules

  The server makes use of a general API for communicating with
  modules. The client is not required to communicate directly with
  available modules. By abstracting the authentication interface, it
  becomes possible for the local administrator to make a run time
  decision about the authentication method adopted by the server.

  6.4.1 Functions and definitions available to servers and modules

  [This section will document the following functions

  	pam_set_item()
  	pam_get_item()
  	pam_fail_delay(pam_handle_t *pamh, unsigned int micro_sec)
  	pam_get_env(pam_handle_t *pamh, const char *varname)
  	pam_strerror(pam_handle_t *pamh, int pam_errno)

  Event driven support (XXX work in progress)

  	pam_register_event() - app or module associates an event poller/handler
  	pam_select_event()   - query for any outstanding event and act on any
  ]

  6.4.2 Server <-> libpam

  [This section will document the following pam_ calls:

  	pam_start
  	pam_end
  	pam_authenticate (*)
  	pam_setcred
  	pam_acct_mgmt
  	pam_open_session
  	pam_close_session
  	pam_chauthtok (*)

  The asterisked functions may return PAM_INCOMPLETE. In such cases, the
  application should be aware that the conversation function was called
  and that it returned PAM_CONV_AGAIN to a module. The correct action
  for the application to take in response to receiving PAM_INCOMPLETE,
  is to acquire the replies so that the next time the conversation
  function is called it will be able to provide the desired
  responses. And then recall pam_authenticate (pam_chauthtok) with the
  same arguments. Libpam will arrange that the module stack is resumed
  from the module that returned before. This functionality is required
  for programs whose user interface is maintained by an event loop. ]

  6.4.3 libpam <-> modules

  [This section will document the following pam_ and pam_sm_ calls:

  functions provided by libpam

  	pam_set_data
  	pam_get_data

  functions provided to libpam by each module

    groups:
  	AUTHENTICATION
  		pam_sm_authenticate
  		pam_sm_setcred
  	ACCOUNT
  		pam_sm_acct_mgmt
  	SESSION
  		pam_sm_open_session
  		pam_sm_close_session
  	AUTHENTICATION TOKEN MANAGEMENT
  		pam_sm_chauthtok
  ]

  6.4.4 The conversation function

  The server application, as part of its initialization of libpam,
  provides a conversation function for use by modules and libpam. The
  purpose of the conversation function is to enable direct communication
  to the applicant ultimately via the client and selected agents.

  [ this section will contain a definition for the conversation
    function, the conversation structure (appdata etc), and legitimate
    return codes for the application supplied function.

  	PAM_SUCCESS           - ok conversation completed
  	PAM_CONV_ERR          - conversation failed
  	PAM_CONV_AGAIN        - application needs control to complete conv
  	PAM_CONV_RECONSIDER   - application believes module should check if
  	                        it still needs to converse for this info
   ]

  7  Security considerations

  This document is devoted to standardizing authentication
  infrastructure: everything in this document has implications for
  security.

  8  Contact

  The email list for discussing issues related to this document is
  <pam-list@redhat.com>.

  9  References

  [1]  OSF RFC 86.0, "Unified Login with Pluggable Authentication
       Modules (PAM)", October 1995

  [2] Definitions for standard agents, "REGISTERED
       AGENTS AND THEIR AGENT-ID'S", to be found here:

    http://www.kernel.org/pub/linux/libs/pam/pre/doc/std-agent-ids.txt    

  10  Author's Address

  Andrew G. Morgan
  Email: morgan@kernel.org

                                   $Id$                                   
