pam_fail_delay(3)

NAME
     pam_fail_delay - request a delay on failure


SYNOPSIS
     #include <security/pam_appl.h>
     or,
     #include <security/pam_modules.h>

     int pam_fail_delay(pam_handle_t *pamh, unsigned int usec);



DESCRIPTION
     It is often possible to attack an authentication scheme by exploiting the
     time it takes the scheme to deny access to an applicant user.  In cases
     of short timeouts, it may prove possible to attempt a brute force
     dictionary attack -- with an automated process, the attacker tries all
     possible passwords to gain access to the system.  In other cases, where
     individual failures can take measurable amounts of time (indicating the
     nature of the failure), an attacker can obtain useful information about
     the authentication process.  These latter attacks make use of procedural
     delays that constitute a covert channel of useful information.

     To minimize the effectiveness of such attacks, it is desirable to
     introduce a random delay in a failed authentication process.  PAM
     provides such a facility.  The delay occurs upon failure of the
     pam_authenticate(3) and pam_chauthtok(3) functions.  It occurs after all
     authentication modules have been called, but before control is returned
     to the service application.

     The function, pam_fail_delay(3), is used to specify a required minimum
     for the length of the failure-delay; the usec argument.  This function
     can be called by the service application and/or the authentication
     modules, both may have an interest in delaying a reapplication for
     service by the user.  The length of the delay is computed at the time it
     is required.  Its length is pseudo-gausianly distributed about the
     maximum requested value; the resultant delay will differ by as much as
     25% of this maximum requested value (both up and down).

     On return from pam_authenticate(3) or pam_chauthtok(3), independent of
     success or failure, the new requested delay is reset to its default
     value: zero.


EXAMPLE
     For example, a login application may require a failure delay of roughly 3
     seconds. It will contain the following code:

          pam_fail_delay(pamh, 3000000 /* micro-seconds */ );
          pam_authenticate(pamh, 0);
     if the modules do not request a delay, the failure delay will be between
     2.25 and 3.75 seconds.

     However, the modules, invoked in the authentication process, may also
     request delays:

       (module #1)   pam_fail_delay(pamh, 2000000);

       (module #2)   pam_fail_delay(pamh, 4000000);

     in this case, it is the largest requested value that is used to compute
     the actual failed delay: here between 3 and 5 seconds.


RETURN VALUE
     Following a successful call to pam_fail_delay(3), PAM_SUCCESS is
     returned.  All other returns should be considered serious failures.


ERRORS
     May be translated to text with pam_strerror(3).


CONFORMING TO
     Under consideration by the X/Open group for future inclusion in the PAM
     RFC. 1996/1/10


BUGS
     none known.


SEE ALSO
     pam_start(3), pam_get_item(3) and pam_strerror(3).

     Also, see the three PAM Guides, for System administrators, module
     developers, and application developers.