The passwordcheck module checks users' passwords
  whenever they are set with
  CREATE ROLE or
  ALTER ROLE.
  If a password is considered too weak, it will be rejected and
  the command will terminate with an error.
 
  To enable this module, add '$libdir/passwordcheck'
  to shared_preload_libraries in
  postgresql.conf, then restart the server.
 
  You can adapt this module to your needs by changing the source code.
  For example, you can use
  CrackLib
  to check passwords — this only requires uncommenting
  two lines in the Makefile and rebuilding the
  module.  (We cannot include CrackLib
  by default for license reasons.)
  Without CrackLib, the module enforces a few
  simple rules for password strength, which you can modify or extend
  as you see fit.
 
To prevent unencrypted passwords from being sent across the network, written to the server log or otherwise stolen by a database administrator, PostgreSQL allows the user to supply pre-encrypted passwords. Many client programs make use of this functionality and encrypt the password before sending it to the server.
   This limits the usefulness of the passwordcheck
   module, because in that case it can only try to guess the password.
   For this reason, passwordcheck is not
   recommended if your security requirements are high.
   It is more secure to use an external authentication method such as GSSAPI
   (see Chapter 20) than to rely on
   passwords within the database.
  
   Alternatively, you could modify passwordcheck
   to reject pre-encrypted passwords, but forcing users to set their
   passwords in clear text carries its own security risks.