PAM working group                 ##              A.G. Morgan
Internet Draft:                   ##          October 6, 1999
Document: draft-morgan-pam-07.txt ##
Expires: June 13, 2000            ##
Obsoletes: draft-morgan-pam-06.txt##

## Pluggable Authentication Modules ##

#$  Status of this memo

This document is an draft specification. The latest version of this
draft may be obtained from here:

  http://linux.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.

#$  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).

#$  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 [#$R#{OSF_RFC_PAM}].  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.

#$  Definitions

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

#$$#{players}  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 beteween 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
inititializes 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).

#$$  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.

#$$$#{text_string}  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}.

#$$$#{binary_prompt}  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##

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

#$$$$#{agent_ids} 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 reserved to be
  assigned by IANA (Internet Assigned Numbers Authority). Names of
  this format MUST NOT be used without first registering with IANA.
  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.


#$$  Special cases

In a previous section (#{players}) 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 is special
client/server support for handling binary prompts.

#$  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.

#$$#{applicant_client}  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.

#$$#{client_agent}  Client <-> agent

In general, authentication schemes require more modes of exchange than
the four defined in the previous section (#{applicant_client}).  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_TEXT;"ouch!"}      | {5;PAM_BPC_OK;}         ##
##      | {11;PAM_BPC_TEXT;"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.

#$$  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.

#$$  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.

#$  C API for application interfaces (client and server)

#$$  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 (#{applicant_client}).  Other methods of
data input/output are better suited to being handled via an
authentication agent.

#$$  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.

#$$$  Client <-> libpamc

#$$$$  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

#$$$$  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 libraray is thread safe provided a single (pamc_handler_t *)
is used by each thread.

#$$$$  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.

#$$$$$  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 #{agent_ids}) 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.

#$$$$$  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
#{agent_ids}).

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).

#$$$$  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 #{binary_prompt}), 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 #{text_string}).

 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);

#$$$$  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.

#$$$$  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.

#$$$ 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 #{binary_prompt})
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.

#$$ 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.

#$$ 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.

#$$$ 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)
]

#$$$ 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. ]

#$$$ 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
]

#$  Security considerations

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

#$  Contact

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

#$  References

[#{OSF_RFC_PAM}]  OSF RFC 86.0, "Unified Login with Pluggable Authentication
     Modules (PAM)", October 1995

#$  Author's Address

Andrew G. Morgan
Email: morgan@ftp.kernel.org

## $Id$ ##