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✏️ Editing: rfc86.0.txt

Open Software Foundation                              V. Samar (SunSoft)
   Request For Comments: 86.0                         R. Schemers (SunSoft)
   October 1995

UNIFIED LOGIN WITH
                    PLUGGABLE AUTHENTICATION MODULES (PAM)

1. INTRODUCTION

Since low-level authentication mechanisms constantly evolve, it is
      important to shield the high-level consumers of these mechanisms
      (system-entry services and users) from such low-level changes.  With
      the Pluggable Authentication Module (PAM) framework, we can provide
      pluggability for a variety of system-entry services -- not just
      system authentication _per se_, but also for account, session and
      password management.  PAM's ability to _stack_ authentication modules
      can be used to integrate `login' with different authentication
      mechanisms such as RSA, DCE, and Kerberos, and thus unify login
      mechanisms.  The PAM framework can also provide easy integration of
      smart cards into the system.

Modular design and pluggability have become important for users who
      want ease of use.  In the PC hardware arena, no one wants to set the
      interrupt vector numbers or resolve the addressing conflict between
      various devices.  In the software arena, people also want to be able
      to replace components easily for easy customization, maintenance, and
      upgrades.

Authentication software deserves special attention because
      authentication forms a very critical component of any secure computer
      system.  The authentication infrastructure and its components may
      have to be modified or replaced either because some deficiencies have
      been found in the current algorithms, or because sites want to
      enforce a different security policy than what was provided by the
      system vendor.  The replacement and modification should be done in
      such a way that the user is not affected by these changes.

The solution has to address not just how the applications use the new
      authentication mechanisms in a generic fashion, but also how the user
      will be authenticated to these mechanisms in a generic way.  The
      former is addressed by GSS-API [Linn 93], while this RFC addresses
      the later; these two efforts are complementary to each other.

Since most system-entry services (for example, `login', `dtlogin',
      `rlogin', `ftp', `rsh') may want to be independent of the specific
      authentication mechanisms used by the machine, it is important that
      there be a framework for _plugging_ in various mechanisms.  This
      requires that the system applications use a standard API to interact

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OSF-RFC 86.0                      PAM                       October 1995

with the authentication services.  If these system-entry services
      remain independent of the actual mechanism used on that machine, the
      system administrator can install suitable authentication modules
      without requiring changes to these applications.

For any security system to be successful, it has to be easy to use.
      In the case of authentication, the single most important ease-of-use
      characteristic is that the user should not be required to learn about
      various ways of authentication and remember multiple passwords.
      Ideally, there should be one all-encompassing authentication system
      where there is only one password, but for heterogeneous sites,
      multiple authentication mechanisms have to co-exist.  The problem of
      integrating multiple authentication mechanisms such as Kerberos
      [Steiner 88], RSA [Rivest 78], and Diffie-Hellman [Diffie 76, Taylor
      88], is also referred to as _integrated login_, or _unified login_
      problem.  Even if the user has to use multiple authentication
      mechanisms, the user should not be forced to type multiple passwords.
      Furthermore, the user should be able to use the new network identity
      without taking any further actions.  The key here is in modular
      integration of the network authentication technologies with `login'
      and other system-entry services.

In this RFC we discuss the architecture and design of pluggable
      authentication modules.  This design gives the capability to use
      field-replaceable authentication modules along with unified login
      capability.  It thus provides for both _pluggability_ and _ease-of-
      use_.

The RFC is organized as follows.  We first motivate the need for a
      generic way to authenticate the user by various system-entry services
      within the operating system.  We describe the goals and constraints
      of the design.  This leads to the architecture, description of the
      interfaces, and _stacking_ of modules to get unified login
      functionality.  We then describe our experience with the design, and
      end with a description of future work.

2. OVERVIEW OF IDENTIFICATION AND AUTHENTICATION MECHANISMS

An identification and authentication ("I&A") mechanism is used to
      establish a user's identity the system (i.e., to a local machine's
      operating system) and to other principals on the network.  On a
      typical UNIX system, there are various ports of entry into the
      system, such as `login', `dtlogin', `rlogin', `ftp', `rsh', `su', and
      `telnet'.  In all cases, the user has to be identified and
      authenticated before granting appropriate access rights to the user.
      The user identification and authentication for all these entry points
      needs to be coordinated to ensure a secure system.

In most of the current UNIX systems, the login mechanism is based
      upon verification of the password using the modified DES algorithm.

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OSF-RFC 86.0                      PAM                       October 1995

The security of the implementation assumes that the password cannot
      be guessed, and that the password does not go over the wire in the
      clear.  These assumptions, however, are not universally valid.
      Various programs are now available freely on the Internet that can
      run dictionary attack against the encrypted password.  Further, some
      of the network services (for example, `rlogin', `ftp', `telnet') send
      the password over in clear, and there are "sniffer" programs freely
      available to steal these passwords.  The classical assumptions may be
      acceptable on a trusted network, but in an open environment there is
      a need to use more restrictive and stronger authentication
      mechanisms.  Examples of such mechanisms include Kerberos, RSA,
      Diffie-Hellman, one-time password [Skey 94], and challenge-response
      based smart card authentication systems.  Since this list will
      continue to evolve, it is important that the system-entry services do
      not have hard-coded dependencies on any of these authentication
      mechanisms.

3. DESIGN GOALS

The goals of the PAM framework are as follows:

(a) The system administrator should be able to choose the default
            authentication mechanism for the machine.  This can range from
            a simple password-based mechanism to a biometric or a smart
            card based system.

(b) It should be possible to configure the user authentication
            mechanism on a per application basis.  For example, a site may
            require S/Key password authentication for `telnet' access,
            while allowing machine `login' sessions with just UNIX password
            authentication.

(c) The framework should support the display requirements of the
            applications.  For example, for a graphical login session such
            as `dtlogin', the user name and the password may have to be
            entered in a new window.  For networking system-entry
            applications such as `ftp' and `telnet', the user name and
            password has to be transmitted over the network to the client
            machine.

(d) It should be possible to configure multiple authentication
            protocols for each of those applications.  For example, one may
            want the users to get authenticated by both Kerberos and RSA
            authentication systems.

(e) The system administrator should be able to _stack_ multiple
            user authentication mechanisms such that the user is
            authenticated with all authentication protocols without
            retyping the password.

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OSF-RFC 86.0                      PAM                       October 1995

(f) The architecture should allow for multiple passwords if
            necessary to achieve higher security for users with specific
            security requirements.

(g) The system-entry services should not be required to change when
            the underlying mechanism changes.  This can be very useful for
            third-party developers because they often do not have the
            source code for these services.

(h) The architecture should provide for a _pluggable_ model for
            system authentication, as well as for other related tasks such
            as password, account, and session management.

(i) For backward-compatibility reasons, the PAM API should support
            the authentication requirements of the current system-entry
            services.

There are certain issues that the PAM framework does not specifically
      address:

(a) We focus only on providing a generic scheme through which users
            use passwords to establish their identities to the machine.
            Once the identity is established, how the identity is
            communicated to other interested parties is outside the scope
            of this design.  There are efforts underway at IETF [Linn 93]
            to develop a Generic Security Services Application Interface
            (GSSAPI) that can be used by applications for secure and
            authenticated communication without knowing the underlying
            mechanism.

(b) The _single-signon_ problem of securely transferring the
            identity of the caller to a remote site is not addressed.  For
            example, the problem of delegating credentials from the
            `rlogin' client to the other machine without typing the
            password is not addressed by our work.  We also do not address
            the problem of sending the passwords over the network in the
            clear.

(c) We do not address the source of information obtained from the
            "`getXbyY()'" family of calls (e.g., `getpwnam()').  Different
            operating systems address this problem differently.  For
            example, Solaris uses the name service switch (NSS) to
            determine the source of information for the "`getXbyY()'"
            calls.  It is expected that data which is stored in multiple
            sources (such as passwd entries in NIS+ and the DCE registry)
            is kept in sync using the appropriate commands (such as
            `passwd_export').

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OSF-RFC 86.0                      PAM                       October 1995

4. OVERVIEW OF THE PAM FRAMEWORK

We propose that the goals listed above can be met through a framework
      in which authentication modules can be _plugged_ independently of the
      application.  We call this the _Pluggable Authentication Modules_
      (PAM) framework.

The core components of the PAM framework are the authentication
      library API (the front end) and the authentication mechanism-specific
      modules (the back end), connected through the Service Provider
      Interface (SPI).  Applications write to the PAM API, while the
      authentication-system providers write to the PAM SPI and supply the
      back end modules that are independent of the application.

Figure 1: The Basic PAM Architecture

Figure 1 illustrates the relationship between the application, the
      PAM library, and the authentication modules.  Three applications
      (`login', `telnet' and `ftp') are shown which use the PAM
      authentication interfaces.  When an application makes a call to the
      PAM API, it loads the appropriate authentication module as determined
      by the configuration file, `pam.conf'.  The request is forwarded to
      the underlying authentication module (for example, UNIX password,
      Kerberos, smart cards) to perform the specified operation.  The PAM
      layer then returns the response from the authentication module to the
      application.

PAM unifies system authentication and access control for the system,
      and allows plugging of associated authentication modules through well
      defined interfaces.  The plugging can be defined through various
      means, one of which uses a configuration file, such as the one in
      Table 1.  For each of the system applications, the file specifies the
      authentication module that should be loaded.  In the example below,
      `login' uses the UNIX password module, while `ftp' and `telnet' use
      the S/Key module.

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OSF-RFC 86.0                      PAM                       October 1995

Table 1: A Simplified View of a Sample PAM Configuration File.

service    module_path
                               -------    -----------
                               login      pam_unix.so
                               ftp        pam_skey.so
                               telnet     pam_skey.so

Authentication configuration is only one aspect of this interface.
      Other critical components include account management, session
      management, and password management.  For example, the `login'
      program may want to verify not only the password but also whether the
      account has aged or expired.  Generic interfaces also need to be
      provided so that the password can be changed according to the
      requirements of the module.  Furthermore, the application may want to
      log information about the current session as determined by the
      module.

Not all applications or services may need all of the above
      components, and not each authentication module may need to provide
      support for all of the interfaces.  For example, while `login' may
      need access to all four components, `su' may need access to just the
      authentication component.  Some applications may use some specific
      authentication and password management modules but share the account
      and session management modules with others.

This reasoning leads to a partitioning of the entire set of
      interfaces into four areas of functionality: (1) authentication, (2)
      account, (3) session, and (4) password.  The concept of PAM was
      extended to these functional areas by implementing each of them as a
      separate pluggable module.

Breaking the functionality into four modules helps the module
      providers because they can use the system-provided libraries for the
      modules that they are not changing.  For example, if a supplier wants
      to provide a better version of Kerberos, they can just provide that
      new authentication and password module, and reuse the existing ones
      for account and session.

4.1. Module Description

More details on specific API's are described in Appendix A.  A brief
      description of four modules follows:

(a) Authentication management: This set includes the
            `pam_authenticate()' function to authenticate the user, and the
            `pam_setcred()' interface to set, refresh or destroy the user
            credentials.

(b) Account management: This set includes the `pam_acct_mgmt()'
            function to check whether the authenticated user should be

Samar, Schemers                                                   Page 6

OSF-RFC 86.0                      PAM                       October 1995

given access to his/her account.  This function can implement
            account expiration and access hour restrictions.

(c) Session management: This set includes the `pam_open_session()'
            and `pam_close_session()' functions for session management and
            accounting.  For example, the system may want to store the
            total time for the session.

(d) Password management: This set includes a function,
            `pam_chauthtok()', to change the password.

5. FRAMEWORK INTERFACES

The PAM framework further provides a set of administrative interfaces
      to support the above modules and to provide for application-module
      communication.  There is no corresponding service provider interface
      (SPI) for such functions.

5.1. Administrative Interfaces

Each set of PAM transactions starts with `pam_start()' and ends with
      the `pam_end()' function.  The interfaces `pam_get_item()' and
      `pam_set_item()' are used to read and write the state information
      associated with the PAM transaction.

If there is any error with any of the PAM interfaces, the error
      message can be printed with `pam_strerror()'.

5.2. Application-Module Communication

During application initialization, certain data such as the user name
      is saved in the PAM framework layer through `pam_start()' so that it
      can be used by the underlying modules.  The application can also pass
      opaque data to the module which the modules will pass back while
      communicating with the user.

5.3. User-Module Communication

The `pam_start()' function also passes conversation function that has
      to be used by the underlying modules to read and write module
      specific authentication information.  For example, these functions
      can be used to prompt the user for the password in a way determined
      by the application.  PAM can thus be used by graphical, non-
      graphical, or networked applications.

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OSF-RFC 86.0                      PAM                       October 1995

5.4. Inter-Module Communication

Though the modules are independent, they can share certain common
      information about the authentication session such as user name,
      service name, password, and conversation function through the
      `pam_get_item()' and `pam_set_item()' interfaces.  These API's can
      also be used by the application to change the state information after
      having called `pam_start()' once.

5.5. Module State Information

The PAM service modules may want to keep certain module-specific
      state information about the session.  The interfaces `pam_get_data()'
      and `pam_set_data()' can be used by the service modules to access and
      update module-specific information as needed from the PAM handle.
      The modules can also attach a cleanup function with the data.  The
      cleanup function is executed when `pam_end()' is called to indicate
      the end of the current authentication activity.

Since the PAM modules are loaded upon demand, there is no direct
      module initialization support in the PAM framework.  If there are
      certain initialization tasks that the PAM service modules have to do,
      they should be done upon the first invocation.  However, if there are
      certain clean-up tasks to be done when the authentication session
      ends, the modules should use `pam_set_data()' to specify the clean-up
      functions, which would be called when `pam_end()' is called by the
      application.

6. MODULE CONFIGURATION MANAGEMENT

Table 2 shows an example of a configuration file `pam.conf' with
      support for authentication, session, account, and password management
      modules.  `login' has three entries: one each for authentication
      processing, session management and account management.  Each entry
      specifies the module name that should be loaded for the given module
      type.  In this example, the `ftp' service uses the authentication and
      session modules.  Note that all services here share the same session
      management module, while having different authentication modules.

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OSF-RFC 86.0                      PAM                       October 1995

Table 2: Configuration File (pam.conf) with Different Modules
                     and Control Flow

service module_type control_flag module_path         options
            ------- ----------- ------------ -----------         -------
            login   auth        required     pam_unix_auth.so    nowarn
            login   session     required     pam_unix_session.so
            login   account     required     pam_unix_account.so
            ftp     auth        required     pam_skey_auth.so    debug
            ftp     session     required     pam_unix_session.so
            telnet  session     required     pam_unix_session.so
            login   password    required     pam_unix_passwd.so
            passwd  password    required     pam_unix_passwd.so
            OTHER   auth        required     pam_unix_auth.so
            OTHER   session     required     pam_unix_session.so
            OTHER   account     required     pam_unix_account.so

The first field, _service_, denotes the service (for example,
      `login', `passwd', `rlogin').  The name `OTHER' indicates the module
      used by all other applications that have not been specified in this
      file.  This name can also be used if all services have the same
      requirements.  In the example, since all the services use the same
      session module, we could have replaced those lines with a single
      `OTHER' line.

The second field, _module_type_, indicates the type of the PAM
      functional module.  It can be one of `auth', `account', `session', or
      `password' modules.

The third field, _control_flag_ determines the behavior of stacking
      multiple modules by specifying whether any particular module is
      _required_, _sufficient_, or _optional_.  The next section describes
      stacking in more detail.

The fourth field, _module_path_, specifies the location of the
      module.  The PAM framework loads this module upon demand to invoke
      the required function.

The fifth field, _options_, is used by the PAM framework layer to
      pass module specific options to the modules.  It is up to the module
      to parse and interpret the options.  This field can be used by the
      modules to turn on debugging or to pass any module specific
      parameters such as a timeout value.  It is also used to support
      unified login as described below.  The options field can be used by
      the system administrator to fine-tune the PAM modules.

If any of the fields are invalid, or if a module is not found, that
      line is ignored and the error is logged as a critical error via
      `syslog(3)'.  If no entries are found for the given module type, then
      the PAM framework returns an error to the application.

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OSF-RFC 86.0                      PAM                       October 1995

7. INTEGRATING MULTIPLE AUTHENTICATION SERVICES WITH STACKING

In the world of heterogeneous systems, the system administrator often
      has to deal with the problem of integrating multiple authentication
      mechanisms.  The user is often required to know about the
      authentication command of the new authentication module (for example,
      `kinit', `dce_login') after logging into the system.  This is not
      user-friendly because it forces people to remember to type the new
      command and enter the new password.  This functionality should be
      invisible instead of burdening the user with it.

There are two problems to be addressed here:

(a) Supporting multiple authentication mechanisms.

(b) Providing unified login in the presence of multiple mechanisms.

In the previous section, we described how one could replace the
      default authentication module with any other module of choice.  Now
      we demonstrate how the same model can be extended to provide support
      for multiple modules.

7.1. Design for Stacked Modules

One possibility was to provide hard-coded rules in `login' or other
      applications requiring authentication services [Adamson 95].  But
      this becomes very specific to the particular combination of
      authentication protocols, and also requires the source code of the
      application.  Digital's Security Integration Architecture [SIA 95]
      addresses this problem by specifying the same list of authentication
      modules for all applications.  Since requirements for various
      applications can vary, it is essential that the configuration be on a
      per-application basis.

To support multiple authentication mechanisms, the PAM framework was
      extended to support _stacking_.  When any API is called, the back
      ends for the stacked modules are invoked in the order listed, and the
      result returned to the caller.  In Figure 2, the authentication
      service of `login' is stacked and the user is authenticated by UNIX,
      Kerberos, and RSA authentication mechanisms.  Note that in this
      example, there is no stacking for session or account management
      modules.

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OSF-RFC 86.0                      PAM                       October 1995

login
                               |
                      +--------+--------+
                      |        |        |
                   session   auth    account
                      |        |        |
                   +--+--+  +--+--+  +--+--+
                   | PAM |  | PAM |  | PAM |
                   +--+--+  +--+--+  +--+--+
                      |        |        |
                    UNIX     UNIX     UNIX
                   session   auth    account
                               |
                            Kerberos
                              auth
                               |
                              RSA
                              auth

Figure 2: Stacking With the PAM Architecture

Stacking is specified through additional entries in the configuration
      file shown earlier.  As shown in Table 2, for each application (such
      as `login') the configuration file can specify multiple mechanisms
      that have to be invoked in the specified order.  When mechanisms
      fail, the _control_flag_ decides which error should be returned to
      the application.  Since the user should not know which authentication
      module failed when a bad password was typed, the PAM framework
      continues to call other authentication modules on the stack even on
      failure.  The semantics of the control flag are as follows:

(a) `required': With this flag, the module failure results in the
            PAM framework returning the error to the caller _after_
            executing all other modules on the stack.  For the function to
            be able to return success to the application all `required'
            modules have to report success.  This flag is normally set when
            authentication by this module is a _must_.

(b) `optional': With this flag, the PAM framework ignores the
            module failure and continues with the processing of the next
            module in sequence.  This flag is used when the user is allowed
            to login even if that particular module has failed.

(c) `sufficient': With this flag, if the module succeeds the PAM
            framework returns success to the application immediately
            without trying any other modules.  For failure cases, the
            _sufficient_ modules are treated as `optional'.

Table 3 shows a sample configuration file that stacks the `login'
      command.  Here the user is authenticated by UNIX, Kerberos, and RSA
      authentication services.  The `required' key word for _control_flag_

Samar, Schemers                                                  Page 11

OSF-RFC 86.0                      PAM                       October 1995

enforces that the user is allowed to login only if he/she is
      authenticated by _both_ UNIX and Kerberos services.  RSA
      authentication is optional by virtue of the `optional' key word in
      the _control_flag_ field.  The user can still log in even if RSA
      authentication fails.

Table 3: PAM Configuration File with Support for Stacking

service module_type control_flag module_path options
            ------- ----------- ------------ ----------- -------
            login   auth        required     pam_unix.so debug
            login   auth        required     pam_kerb.so use_mapped_pass
            login   auth        optional     pam_rsa.so  use_first_pass

Table 4 illustrates the use of the sufficient flag for the `rlogin'
      service.  The Berkeley `rlogin' protocol specifies that if the remote
      host is trusted (as specified in the `/etc/hosts.equiv' file or in
      the `.rhosts' file in the home directory of the user), then the
      `rlogin' daemon should not require the user to type the password.  If
      this is not the case, then the user is required to type the password.
      Instead of hard coding this policy in the `rlogin' daemon, this can
      be expressed with the `pam.conf' file in Table 4.  The PAM module
      `pam_rhosts_auth.so.1' implements the `.rhosts' policy described
      above.  If a site administrator wants to enable remote login with
      only passwords, then the first line should be deleted.

Table 4: PAM Configuration File for the rlogin service

service module_type control_flag module_path        options
            ------- ----------- ------------ -----------        -------
            rlogin  auth        sufficient   pam_rhosts_auth.so
            rlogin  auth        required     pam_unix.so

7.2. Password-Mapping

Multiple authentication mechanisms on a machine can lead to multiple
      passwords that users have to remember.  One attractive solution from
      the ease-of-use viewpoint is to use the same password for all
      mechanisms.  This, however, can also weaken the security because if
      that password were to be compromised in any of the multiple
      mechanisms, all mechanisms would be compromised at the same time.
      Furthermore, different authentication mechanisms may have their own
      distinctive password requirements in regards to its length, allowed
      characters, time interval between updates, aging, locking, and so
      forth.  These requirements make it problematic to use the same
      password for multiple authentication mechanisms.

The solution we propose, while not precluding use of the same
      password for every mechanism, allows for a different password for
      each mechanism through what we call _password-mapping_.  This
      basically means using the user's _primary_ password to encrypt the

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OSF-RFC 86.0                      PAM                       October 1995

user's other (_secondary_) passwords, and storing these encrypted
      passwords in a place where they are available to the user.  Once the
      primary password is verified, the authentication modules would obtain
      the other passwords for their own mechanisms by decrypting the
      mechanism-specific encrypted password with the primary password, and
      passing it to the authentication service.  The security of this
      design for password-mapping assumes that the primary password is the
      user's strongest password, in terms of its unguessability (length,
      type and mix of characters used, etc.).

If there is any error in password-mapping, or if the mapping does not
      exist, the user will be prompted for the password by each
      authentication module.

To support password-mapping, the PAM framework saves the primary
      password and provides it to stacked authentication modules.  The
      password is cleared out before the `pam_authenticate' function
      returns.

How the password is encrypted depends completely on the module
      implementation.  The encrypted secondary password (also called a
      "mapped password") can be stored in a trusted or untrusted place,
      such as a smart card, a local file, or a directory service.  If the
      encrypted passwords are stored in an untrusted publicly accessible
      place, this does provide an intruder with opportunities for potential
      dictionary attack.

Though password-mapping is voluntary, it is recommended that all
      module providers add support for the following four mapping options:

(a) `use_first_pass': Use the same password used by the first
            mechanism that asked for a password.  The module should not ask
            for the password if the user cannot be authenticated by the
            first password.  This option is normally used when the system
            administrator wants to enforce the same password across
            multiple modules.

(b) `try_first_pass': This is the same as `use_first_pass', except
            that if the primary password is not valid, it should prompt the
            user for the password.

(c) `use_mapped_pass': Use the password-mapping scheme to get the
            actual password for this module.  One possible implementation
            is to get the mapped-password using the XFN API [XFN 94], and
            decrypt it with the primary password to get the module-specific
            password.  The module should not ask for the password if the
            user cannot be authenticated by the first password.  The XFN
            API allows user-defined attributes (such as _mapped-password_)
            to be stored in the _user-context_.  Using the XFN API is
            particularly attractive because support for the XFN may be
            found on many systems in the future.

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OSF-RFC 86.0                      PAM                       October 1995

(d) `try_mapped_pass': This is the same as `use_mapped_pass',
            except that if the primary password is not valid, it should
            prompt the user for the password.

When passwords get updated, the PAM framework stores both the old as
      well as the new password to be able to inform other dependent
      authentication modules about the change.  Other modules can use this
      information to update the encrypted password without forcing the user
      to type the sequence of passwords again.  The PAM framework clears
      out the passwords before returning to the application.

Table 3 illustrates how the same password can be used by `login' for
      authenticating to the standard UNIX login, Kerberos and RSA services.
      Once the user has been authenticated to the primary authentication
      service (UNIX `login' in this example) with the primary password, the
      option `use_mapped_pass' indicates to the Kerberos module that it
      should use the primary password to decrypt the stored Kerberos
      password and then use the Kerberos password to get the ticket for the
      ticket-granting-service.  After that succeeds, the option
      `use_first_pass' indicates to the RSA module that instead of
      prompting the user for a password, it should use the primary password
      typed earlier for authenticating the user.  Note that in this
      scenario, the user has to enter the password just once.

Note that if a one-time password scheme (e.g., S/Key) is used,
      password mapping cannot apply.

7.3. Implications of Stacking on the PAM Design

Because of the stacking capability of PAM, we have designed the PAM
      API's to not return any data to the application, except status.  If
      this were not the case, it would be difficult for the PAM framework
      to decide which module should return data to the application.  When
      there is any error, the application does not know which of the
      modules failed.  This behavior enables (even requires) the
      application to be completely independent from the modules.

Another design decision we have made is that PAM gives only the user
      name to all the underlying PAM modules, hence it is the
      responsibility of the PAM modules to convert the name to their own
      internal format.  For example, the Kerberos module may have to
      convert the UNIX user name to a Kerberos principal name.

Stacking also forces the modules to be designed such that they can
      occur anywhere in the stack without any side-effects.

Since modules such as the authentication and the password module are
      very closely related, it is important they be configured in the same
      order and with compatible options.

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OSF-RFC 86.0                      PAM                       October 1995

8. INTEGRATION WITH SMART CARDS

Many networking authentication protocols require possession of a long
      key to establish the user identity.  For ease-of-use reasons, that
      long key is normally encrypted with the user's password so that the
      user is not required to memorize it.  However, weak passwords can be
      compromised through a dictionary attack and thus undermine the
      stronger network authentication mechanism.  Furthermore, the
      encrypted data is normally stored in a centrally accessible service
      whose availability depends upon the reliability of the associated
      service.  Solutions have been proposed to use a pass-phrase or one-
      time-password, but those are much longer than the regular eight
      character passwords traditionally used with UNIX `login'.  This makes
      the solution user-unfriendly because it requires longer strings to be
      remembered and typed.

For most authentication protocol implementations, the trust boundary
      is the local machine.  This assumption may not be valid in cases
      where the user is mobile and has to use publicly available networked
      computers.  In such cases, it is required that the clear text of the
      key or the password never be made available to the machine.

Smart cards solve the above problems by reducing password exposure by
      supporting a _two factor_ authentication mechanism: the first with
      the possession of the card, and the second with the knowledge of the
      PIN associated with the card.  Not only can the smart cards be a
      secure repository of multiple passwords, they can also provide the
      encryption and authentication functions such that the long (private)
      key is never exposed outside the card.

The PAM framework allows for integrating smart cards to the system by
      providing a smart card specific module for authentication.
      Furthermore, the unified login problem is simplified because the
      multiple passwords for various authentication mechanisms can be
      stored on the smart card itself.  This can be enabled by adding a
      suitable key-word such as `use_smart_card' in the _options_ field.

9. SECURITY ISSUES

It is important to understand the impact of PAM on the security of
      any system so that the site-administrator can make an informed
      decision.

(a) Sharing of passwords with multiple authentication mechanisms.

If there are multiple authentication modules, one possibility
            is to use the same password for all of them.  If the password
            for any of the multiple authentication system is compromised,
            the user's password in all systems would be compromised.  If
            this is a concern, then multiple passwords might be considered

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OSF-RFC 86.0                      PAM                       October 1995

at the cost of ease-of-use.

(b) Password-mapping.

This technique of encrypting all other passwords with the
            primary password assumes that it is lot more difficult to crack
            the primary password and that reasonable steps have been taken
            to ensure limited availability of the encrypted primary
            password.  If this is not done, an intruder could target the
            primary password as the first point of dictionary attack.  If
            one of the other modules provide stronger security than the
            password based security, the site would be negating the strong
            security by using password-mapping.  If this is a concern, then
            multiple passwords might be considered at the cost of ease-of-
            use.  If smart cards are used, they obviate the need for
            password-mapping completely.

Since the policy file dictates how the user is authenticated,
            this file should be protected from unauthorized modifications.

(d) Stacking various PAM modules.

The system administrator should fully understand the
            implications of stacking various modules that will be installed
            on the system and their respective orders and interactions.
            The composition of various authentication modules should be
            carefully examined.  The trusted computing base of the machine
            now includes the PAM modules.

10. EXPERIENCE WITH PAM

The PAM framework was first added in Solaris 2.3 release as a private
      internal interface.  PAM is currently being used by several system
      entry applications such as `login', `passwd', `su', `dtlogin',
      `rlogind', `rshd', `telnetd', `ftpd', `in.rexecd', `uucpd', `init',
      `sac', and `ttymon'.  We have found that PAM provides an excellent
      framework to encapsulate the authentication-related tasks for the
      entire system.  The Solaris 2.3 PAM API's were hence enhanced and
      simplified to support stacking.

PAM modules have been developed for UNIX, DCE, Kerberos, S/Key,
      remote user authentication, and dialpass authentication.  Other PAM
      modules are under development, and integration with smart cards is
      being planned.

Some third parties have used the PAM interface to extend the security
      mechanisms offered by the Solaris environment.

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OSF-RFC 86.0                      PAM                       October 1995

The PAM API has been accepted by Common Desktop Environment (CDE)
      vendors as the API to be used for integrating the graphical interface
      for login, `dtlogin' with multiple authentication mechanisms.

11. FUTURE WORK

Amongst the various components of PAM, the password component needs
      to be carefully examined to see whether the stacking semantics are
      particularly applicable, and how PAM should deal with partial
      failures when changing passwords.

The _control_flag_ of the configuration file can be extended to
      include other semantics.  For example, if the error is "name service
      not available", one may want to retry.  It is also possible to offer
      semantics of "return success if any of the modules return success".

In an earlier section, we had mentioned integration of smart cards
      with PAM.  Though we feel that integration should be straight forward
      from the PAM architecture point of view, there may be some issues
      with implementation because the interfaces to the smart cards have
      not yet been standardized.

One possible extension to PAM is to allow the passing of module-
      specific data between applications and PAM modules.  For example, the
      `login' program likes to build its new environment from a select list
      of variables, yet the DCE module needs the `KRB5CCNAME' variable to
      be exported to the child process.  For now we have modified the
      `login' program to explicitly export the `KRB5CCNAME' variable.

Administrative tools are needed to help system administrators modify
      `pam.conf', and perform sanity checks on it (i.e., a `pam_check'
      utility).

12. CONCLUSION

The PAM framework and the module interfaces provide pluggability for
      user authentication, as well as for account, session and password
      management.  The PAM architecture can be used by `login' and by all
      other system-entry services, and thus ensure that all entry points
      for the system have been secured.  This architecture enables
      replacement and modification of authentication modules in the field
      to secure the system against the newly found weaknesses without
      changing any of the system services.

The PAM framework can be used to integrate `login' and `dtlogin' with
      different authentication mechanisms such as RSA and Kerberos.
      Multiple authentication systems can be accessed with the same
      password.  The PAM framework also provides easy integration of smart
      cards into the system.

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OSF-RFC 86.0                      PAM                       October 1995

PAM provides complementary functionality to GSS-API, in that it
      provides mechanisms through which the user gets authenticated to any
      new system-level authentication service on the machine.  GSS-API then
      uses the credentials for authenticated and secure communications with
      other application-level service entities on the network.

13. ACKNOWLEDGEMENTS

PAM development has spanned several release cycles at SunSoft.
      Shau-Ping Lo, Chuck Hickey, and Alex Choy did the first design and
      implementation.  Bill Shannon and Don Stephenson helped with the PAM
      architecture.  Rocky Wu prototyped stacking of multiple modules.
      Paul Fronberg, Charlie Lai, and Roland Schemers made very significant
      enhancements to the PAM interfaces and took the project to completion
      within a very short time.  Kathy Slattery wrote the PAM
      documentation.  John Perry integrated PAM within the CDE framework.

APPENDIX A. PAM API'S

This appendix gives an informal description of the various interfaces
      of PAM.  Since the goal here is just for the reader to get a working
      knowledge about the PAM interfaces, not all flags and options have
      been fully defined and explained.  The API's described here are
      subject to change.

The PAM Service Provider Interface is very similar to the PAM API,
      except for one extra parameter to pass module-specific options to the
      underlying modules.

A.1. Framework Layer API's

int
            pam_start(
                char *service_name,
                char *user,
                struct pam_conv *pam_conversation,
                pam_handle_t **pamh
            );

`pam_start()' is called to initiate an authentication transaction.
      `pam_start()' takes as arguments the name of the service, the name of
      the user to be authenticated, the address of the conversation
      structure.  `pamh' is later used as a handle for subsequent calls to
      the PAM library.

The PAM modules do not communicate directly with the user; instead
      they rely on the application to perform all such interaction.  The
      application needs to provide the conversation functions, `conv()',
      and associated application data pointers through a `pam_conv'

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OSF-RFC 86.0                      PAM                       October 1995

structure when it initiates an authentication transaction.  The
      module uses the `conv()' function to prompt the user for data,
      display error messages, or text information.

int
            pam_end(
                pam_handle_t *pamh,
                int pam_status
            );

`pam_end()' is called to terminate the PAM transaction as specified
      by `pamh', and to free any storage area allocated by the PAM modules
      with `pam_set_item()'.

int
            pam_set_item(
                pam_handle_t *pamh,
                int item_type,
                void *item
            );

int
            pam_get_item(
                pam_handle_t *pamh,
                int item_type,
                void **item);

`pam_get_item()' and `pam_set_item()' allow the parameters specified
      in the initial call to `pam_start()' to be read and updated.  This is
      useful when a particular parameter is not available when
      `pam_start()' is called or must be modified after the initial call to
      `pam_start()'.  `pam_set_item()' is passed a pointer to the object,
      `item', and its type, `item_type'.  `pam_get_item()' is passed the
      address of the pointer, `item', which is assigned the address of the
      requested object.

The `item_type' is one of the following:

Table 5: Possible Values for Item_type

Item Name       Description
            ---------       -----------
            PAM_SERVICE     The service name
            PAM_USER        The user name
            PAM_TTY         The tty name
            PAM_RHOST       The remote host name
            PAM_CONV        The pam_conv structure
            PAM_AUTHTOK     The authentication token (password)
            PAM_OLDAUTHTOK  The old authentication token
            PAM_RUSER       The remote user name

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OSF-RFC 86.0                      PAM                       October 1995

Note that the values of `PAM_AUTHTOK' and `PAM_OLDAUTHTOK' are only
      available to PAM modules and not to the applications.  They are
      explicitly cleared out by the framework before returning to the
      application.

char *
            pam_strerror(
                int errnum
            );

`pam_strerror()' maps the error number to a PAM error message string,
      and returns a pointer to that string.

int
            pam_set_data(
                pam_handle_t *pamh,
                char *module_data_name,
                char *data,
                (*cleanup)(pam_handle_t *pamh, char *data,
                           int error_status)
            );

The `pam_set_data()' function stores module specific data within the
      PAM handle.  The `module_data_name' uniquely specifies the name to
      which some data and cleanup callback function can be attached.  The
      cleanup function is called when `pam_end()' is invoked.

int
            pam_get_data(
                pam_handle_t *pamh,
                char *module_data_name,
                void **datap
            );

The `pam_get_data()' function obtains module-specific data from the
      PAM handle stored previously by the `pam_get_data()' function.  The
      `module_data_name' uniquely specifies the name for which data has to
      be obtained.  This function is normally used to retrieve module
      specific state information.

A.2. Authentication API's

int
            pam_authenticate(
                pam_handle_t *pamh,
                int flags
            );

The `pam_authenticate()' function is called to verify the identity of
      the current user.  The user is usually required to enter a password
      or similar authentication token, depending upon the authentication

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OSF-RFC 86.0                      PAM                       October 1995

module configured with the system.  The user in question is specified
      by a prior call to `pam_start()', and is referenced by the
      authentication handle, `pamh'.

int
            pam_setcred(
                pam_handle_t *pamh,
                int flags
            );

The `pam_setcred()' function is called to set the credentials of the
      current process associated with the authentication handle, `pamh'.
      The actions that can be denoted through `flags' include credential
      initialization, refresh, reinitialization and deletion.

A.3. Account Management API

int
            pam_acct_mgmt(
                pam_handle_t *pamh,
                int flags
            );

The function `pam_acct_mgmt()' is called to determine whether the
      current user's account and password are valid.  This typically
      includes checking for password and account expiration, valid login
      times, etc.  The user in question is specified by a prior call to
      `pam_start()', and is referenced by the authentication handle,
      `pamh'.

A.4. Session Management API's

int
            pam_open_session(
                pam_handle_t *pamh,
                int flags
            );

`pam_open_session()' is called to inform the session modules that a
      new session has been initialized.  All programs which use PAM should
      invoke `pam_open_session()' when beginning a new session.

int
            pam_close_session(
                pam_handle_t *pamh,
                int flags
            );

Upon termination of this session, the `pam_close_session()' function
      should be invoked to inform the underlying modules that the session
      has terminated.

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OSF-RFC 86.0                      PAM                       October 1995

A.5. Password Management API's

int
            pam_chauthtok(
                pam_handle_t *pamh,
                int flags
            );

`pam_chauthtok()' is called to change the authentication token
      associated with the user referenced by the authentication handle
      `pamh'.  After the call, the authentication token of the user will be
      changed in accordance with the authentication module configured on
      the system.

APPENDIX B. SAMPLE PAM APPLICATION

This appendix shows a sample `login' application which uses the PAM
      API's.  It is not meant to be a fully functional login program, as
      some functionality has been left out in order to emphasize the use of
      PAM API's.

#include <security/pam_appl.h>

static int login_conv(int num_msg, struct pam_message **msg,
          struct pam_response **response, void *appdata_ptr);

static struct pam_conv pam_conv = {login_conv, NULL};

static pam_handle_t *pamh;    /* Authentication handle */

void
      main(int argc, char *argv[], char **renvp)
      {

/*
           * Call pam_start to initiate a PAM authentication operation
           */

if ((pam_start("login", user_name, &pam_conv, &pamh))
                      != PAM_SUCCESS)
                      login_exit(1);

pam_set_item(pamh, PAM_TTY, ttyn);
          pam_set_item(pamh, PAM_RHOST, remote_host);

while (!authenticated && retry < MAX_RETRIES) {
              status = pam_authenticate(pamh, 0);
              authenticated = (status == PAM_SUCCESS);
          }

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OSF-RFC 86.0                      PAM                       October 1995

if (status != PAM_SUCCESS) {
              fprintf(stderr,"error: %s\n", pam_strerror(status));
              login_exit(1);
          }

/* now check if the authenticated user is allowed to login. */

if ((status = pam_acct_mgmt(pamh, 0)) != PAM_SUCCESS) {
              if (status == PAM_AUTHTOK_EXPIRED) {
                   status = pam_chauthtok(pamh, 0);
                   if (status != PAM_SUCCESS)
                       login_exit(1);
              } else {
                   login_exit(1);
              }
          }

/*
           * call pam_open_session to open the authenticated session
           * pam_close_session gets called by the process that
           * cleans up the utmp entry (i.e., init)
           */
          if (status = pam_open_session(pamh, 0) != PAM_SUCCESS) {
              login_exit(status);
          }

/* set up the process credentials */
          setgid(pwd->pw_gid);

/*
           * Initialize the supplementary group access list.
           * This should be done before pam_setcred because
           * the PAM modules might add groups during the pam_setcred call
           */
          initgroups(user_name, pwd->pw_gid);

status = pam_setcred(pamh, PAM_ESTABLISH_CRED);
          if (status != PAM_SUCCESS) {
              login_exit(status);
          }

/* set the real (and effective) UID */
          setuid(pwd->pw_uid);

pam_end(pamh, PAM_SUCCESS);    /* Done using PAM */

/*
           * Add DCE/Kerberos cred name, if any.
           * XXX - The module specific stuff should be removed from login
           * program eventually.  This is better placed in DCE module and
           * will be once PAM has routines for "exporting" environment

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OSF-RFC 86.0                      PAM                       October 1995

* variables.
           */
          krb5p = getenv("KRB5CCNAME");
          if (krb5p != NULL) {
              ENVSTRNCAT(krb5ccname, krb5p);
              envinit[basicenv++] = krb5ccname;
          }
          environ = envinit; /* Switch to the new environment. */
          exec_the_shell();

/* All done */
      }

/*
       * login_exit        - Call exit()  and terminate.
       *              This function is here for PAM so cleanup can
       *              be done before the process exits.
       */
      static void
      login_exit(int exit_code)
      {
          if (pamh)
              pam_end(pamh, PAM_ABORT);
          exit(exit_code);
          /*NOTREACHED*/
      }

/*
       * login_conv():
       *    This is the conv (conversation) function called from
       *    a PAM authentication module to print error messages
       *    or garner information from the user.
       */

static int
      login_conv(int num_msg, struct pam_message **msg,
          struct pam_response **response, void *appdata_ptr)
      {

while (num_msg--) {
              switch (m->msg_style) {

case PAM_PROMPT_ECHO_OFF:
                  r->resp = strdup(getpass(m->msg));
                  break;

case PAM_PROMPT_ECHO_ON:
                  (void) fputs(m->msg, stdout);
                  r->resp = malloc(PAM_MAX_RESP_SIZE);
                  fgets(r->resp, PAM_MAX_RESP_SIZE, stdin);
                  /* add code here to remove \n from fputs */

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OSF-RFC 86.0                      PAM                       October 1995

break;

case PAM_ERROR_MSG:
                  (void) fputs(m->msg, stderr);
                  break;

case PAM_TEXT_INFO:
                  (void) fputs(m->msg, stdout);
                  break;

default:
                      /* add code here to log error message, etc */
                  break;
              }
          }
          return (PAM_SUCCESS);
      }

APPENDIX C. DCE MODULE

This appendix describes a sample implementation of a DCE PAM module.
      In order to simplify the description, we do not address the issues
      raised by password-mapping or stacking.  The intent is to show which
      DCE calls are being made by the DCE module.

The `pam_sm_*()' functions implement the PAM SPI functions which are
      called from the PAM API functions.

C.1. DCE Authentication Management

The algorithm for authenticating with DCE (not including error
      checking, prompting for passwords, etc.) is as follows:

pam_sm_authenticate()
            {
                sec_login_setup_identity(...);
                pam_set_data(...);
                sec_login_valid_and_cert_ident(...);
            }

pam_sm_setcred()
            {
                pam_get_data(...);
                sec_login_set_context(...);
            }

The `pam_sm_authenticate()' function for DCE uses the
      `pam_set_data()' and `pam_get_data()' functions to keep state (like
      the `sec_login_handle_t' context) between calls.  The following
      cleanup function is also registered and gets called when `pam_end()'

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OSF-RFC 86.0                      PAM                       October 1995

is called:

dce_cleanup()
            {
                if (/* PAM_SUCCESS and
                       sec_login_valid_and_cert_ident success */) {
                   sec_login_release_context(...);
                } else {
                   sec_login_purge_context(...);
                }
            }

If everything was successful we release the login context, but leave
      the credentials file intact.  If the status passed to `pam_end()' was
      not `PAM_SUCCESS' (i.e., a required module failed) we purge the login
      context which also removes the credentials file.

C.2. DCE Account Management

The algorithm for DCE account management is as follows:

pam_sm_acct_mgmt()
            {
                pam_get_data(...);
                sec_login_inquire_net_info(...);
                /* check for expired password and account */
                sec_login_free_net_info(...);
            }

The `sec_login_inquire_net_info()' function is called to obtain
      information about when the user's account and/or password are going
      to expire.  A warning message is displayed (using the conversation
      function) if the user's account or password is going to expire in the
      near future, or has expired.  These warning messages can be disabled
      using the `nowarn' option in the `pam.conf' file.

C.3. DCE Session Management

The DCE session management functions are currently empty.  They could
      be modified to optionally remove the DCE credentials file upon
      logout, etc.

C.4. DCE Password Management

The algorithm for DCE password management is as follows:

Samar, Schemers                                                  Page 26

OSF-RFC 86.0                      PAM                       October 1995

pam_sm_chauthtok
            {
                sec_rgy_site_open(...);
                sec_rgy_acct_lookup(...);
                sec_rgy_acct_passwd(...);
                sec_rgy_site_close(...);
            }

The `sec_rgy_acct_passwd()' function is called to change the user's
      password in the DCE registry.

REFERENCES

[Adamson 95]   W. A. Adamson, J. Rees, and P. Honeyman, "Joining
                     Security Realms: A Single Login for Netware and
                     Kerberos", CITI Technical Report 95-1, Center for
                     Information Technology Integration, University of
                     Michigan, Ann Arbor, MI, February 1995.

[Diffie 76]    W. Diffie and M. E. Hellman, "New Directions in
                     Cryptography", IEEE Transactions on Information
                     Theory, November 1976.

[Linn 93]      J. Linn, "Generic Security Service Application
                     Programming Interface", Internet RFC 1508, 1509, 1993.

[Rivest 78]    R. L. Rivest, A. Shamir, and L. Adleman., "A Method
                     for Obtaining Digital Signatures and Pubic-key
                     Cryptosystems", Communications of the ACM, 21(2),
                     1978.

[SIA 95]       "Digital UNIX Security", Digital Equipment
                     Corporation, Order Number AA-Q0R2C-TE, July 1995.

[Skey 94]      N. M. Haller, "The S/Key One-Time Password System",
                     ISOC Symposium on Network and Distributed Security,
                     1994.

[Steiner 88]   J.G. Steiner, B. C. Neuman, and J. I. Schiller,
                     "Kerberos, An Authentication Service for Open Network
                     Systems", in Proceedings of the Winter USENIX
                     Conference, Dallas, Jan 1988.

[Taylor 88]    B. Taylor and D. Goldberg, "Secure Networking in the
                     Sun Environment", Sun Microsystems Technical Paper,
                     1988.

[XFN 94]       "Federated Naming: the XFN Specifications", X/Open
                     Preliminary Specification, X/Open Document #P403,
                     ISBN:1-85912-045-8, X/Open Co. Ltd., July 1994.

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OSF-RFC 86.0                      PAM                       October 1995

AUTHOR'S ADDRESS

Vipin Samar                            Internet email: vipin@eng.sun.com
   SunSoft, Inc.                                 Telephone: +1-415-336-1002
   2550 Garcia Avenue
   Mountain View, CA 94043
   USA

Roland J. Schemers III              Internet email: schemers@eng.sun.com
   SunSoft, Inc.                                 Telephone: +1-415-336-1035
   2550 Garcia Avenue
   Mountain View, CA 94043
   USA

Samar, Schemers                                                  Page 28

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