---
-- The brute library is an attempt to create a common framework for performing
-- password guessing against remote services.
--
-- The library currently attempts to parallelize the guessing by starting
-- a number of working threads. The number of threads can be defined using
-- the brute.threads argument, it defaults to 10.
--
-- The library contains the following classes:
-- * <code>Engine</code>
-- ** The actual engine doing the brute-forcing .
-- * <code>Error</code>
-- ** Class used to return errors back to the engine.
-- * <code>Options</code>
-- ** Stores any options that should be used during brute-forcing.
--
-- In order to make use of the framework a script needs to implement a Driver
-- class. The Driver class is then to be passed as a parameter to the Engine
-- constructor, which creates a new instance for each guess. The Driver class
-- SHOULD implement the following four methods:
--
-- <code>
-- Driver:login = function( self, username, password )
-- Driver:check = function( self )
-- Driver:connect = function( self )
-- Driver:disconnect = function( self )
-- </code>
--
-- The <code>login</code> method does not need a lot of explanation. The login
-- function should return two parameters. If the login was successful it should
-- return true and a <code>creds.Account</code>. If the login was a failure it
-- should return false and an <code>Error</code>. The driver can signal the
-- Engine to retry a set of credentials by calling the Error objects
-- <code>setRetry</code> method. It may also signal the Engine to abort all
-- password guessing by calling the Error objects <code>setAbort</code> method.
--
-- The following example code demonstrates how the Error object can be used.
--
-- <code>
-- -- After a number of incorrect attempts VNC blocks us, so we abort
-- if ( not(status) and x:match("Too many authentication failures") ) then
--   local err = brute.Error:new( data )
--   -- signal the engine to abort
--   err:setAbort( true )
--   return false, err
-- elseif ( not(status) ) then
--   local err = brute.Error:new( "VNC handshake failed" )
--   -- This might be temporary, signal the engine to retry
--   err:setRetry( true )
--   return false, err
-- end
-- .
-- .
-- .
-- -- Return a simple error, no retry needed
-- return false, brute.Error:new( "Incorrect password" )
-- </code>
--
-- The purpose of the <code>check</code> method is to be able to determine
-- whether the script has all the information it needs, before starting the
-- brute force. It's the method where you should check, e.g., if the correct
-- database or repository URL was specified or not. On success, the
-- <code>check</code> method returns true, on failure it returns false and the
-- brute force engine aborts.
--
-- NOTE: The <code>check</code> method is deprecated and will be removed from
-- all scripts in the future. Scripts should do this check in the action
-- function instead.
--
-- The <code>connect</code> method provides the framework with the ability to
-- ensure that the thread can run once it has been dispatched a set of
-- credentials. As the sockets in NSE are limited we want to limit the risk of
-- a thread blocking, due to insufficient free sockets, after it has acquired a
-- username and password pair.
--
-- The following sample code illustrates how to implement a sample
-- <code>Driver</code> that sends each username and password over a socket.
--
-- <code>
-- Driver = {
--   new = function(self, host, port, options)
--     local o = {}
--     setmetatable(o, self)
--     self.__index = self
--     o.host = host
--     o.port = port
--     o.options = options
--     return o
--   end,
--   connect = function( self )
--     self.socket = nmap.new_socket()
--     return self.socket:connect( self.host, self.port )
--   end,
--   disconnect = function( self )
--     return self.socket:close()
--   end,
--   check = function( self )
--     return true
--   end,
--   login = function( self, username, password )
--     local status, err, data
--     status, err = self.socket:send( username .. ":" .. password)
--     status, data = self.socket:receive_bytes(1)
--
--     if ( data:match("SUCCESS") ) then
--       return true, creds.Account:new(username, password, creds.State.VALID)
--     end
--     return false, brute.Error:new( "login failed" )
--   end,
-- }
-- </code>
--
-- The following sample code illustrates how to pass the <code>Driver</code>
-- off to the brute engine.
--
-- <code>
-- action = function(host, port)
--   local options = { key1 = val1, key2 = val2 }
--   local status, accounts = brute.Engine:new(Driver, host, port, options):start()
--   if( not(status) ) then
--     return accounts
--   end
--   return stdnse.format_output( true, accounts )
-- end
-- </code>
--
-- For a complete example of a brute implementation consult the
-- <code>svn-brute.nse</code> or <code>vnc-brute.nse</code> scripts
--
-- @args brute.useraspass guess the username as password for each user
--       (default: true)
-- @args brute.emptypass guess an empty password for each user
--       (default: false)
-- @args brute.unique make sure that each password is only guessed once
--       (default: true)
-- @args brute.firstonly stop guessing after first password is found
--       (default: false)
-- @args brute.passonly iterate over passwords only for services that provide
--       only a password for authentication. (default: false)
-- @args brute.retries the number of times to retry if recoverable failures
--       occur. (default: 3)
-- @args brute.delay the number of seconds to wait between guesses (default: 0)
-- @args brute.threads the number of initial worker threads, the number of
--       active threads will be automatically adjusted.
-- @args brute.mode can be user, pass or creds and determines what mode to run
--       the engine in.
--       * user - the unpwdb library is used to guess passwords, every password
--                password is tried for each user. (The user iterator is in the
--                outer loop)
--       * pass - the unpwdb library is used to guess passwords, each password
--                is tried for every user. (The password iterator is in the
--                outer loop)
--       * creds- a set of credentials (username and password pairs) are
--                guessed against the service. This allows for lists of known
--                or common username and password combinations to be tested.
--       If no mode is specified and the script has not added any custom
--       iterator the pass mode will be enabled.
-- @args brute.credfile a file containing username and password pairs delimited
--       by '/'
-- @args brute.guesses the number of guesses to perform against each account.
--       (default: 0 (unlimited)). The argument can be used to prevent account
--       lockouts.
--
-- @author "Patrik Karlsson <patrik@cqure.net>"
-- @copyright Same as Nmap--See https://nmap.org/book/man-legal.html

--
-- Version 0.73
-- Created 06/12/2010 - v0.1 - created by Patrik Karlsson <patrik@cqure.net>
-- Revised 07/13/2010 - v0.2 - added connect, disconnect methods to Driver
--                             <patrik@cqure.net>
-- Revised 07/21/2010 - v0.3 - documented missing argument brute.mode
-- Revised 07/23/2010 - v0.4 - fixed incorrect statistics and changed output to
--                             include statistics, and to display "no accounts
--                             found" message.
-- Revised 08/14/2010 - v0.5 - added some documentation and smaller changes per
--                             David's request.
-- Revised 08/30/2010 - v0.6 - added support for custom iterators and did some
--                             needed cleanup.
-- Revised 06/19/2011 - v0.7 - added support for creds library [Patrik]
-- Revised 07/07/2011 - v0.71- fixed some minor bugs, and changed credential
--                             iterator to use a file handle instead of table
-- Revised 07/21/2011 - v0.72- added code to allow script reporting invalid
--                             (non existing) accounts using setInvalidAccount
-- Revised 11/12/2011 - v0.73- added support for max guesses per account to
--                             prevent account lockouts.
--                             bugfix: added support for guessing the username
--                             as password per default, as suggested by the
--                             documentation.

local coroutine = require "coroutine"
local creds = require "creds"
local io = require "io"
local nmap = require "nmap"
local os = require "os"
local stdnse = require "stdnse"
local table = require "table"
local unpwdb = require "unpwdb"
_ENV = stdnse.module("brute", stdnse.seeall)

-- Engine options that can be set by scripts
-- Supported options are:
--   * firstonly   - stop after finding the first correct password
--                   (can be set using script-arg brute.firstonly)
--   * passonly    - guess passwords only, don't supply a username
--                   (can be set using script-arg brute.passonly)
--   * max_retries - the amount of retries to do before aborting
--                   (can be set using script-arg brute.retries)
--   * delay       - sets the delay between attempts
--                   (can be set using script-arg brute.delay)
--   * mode        - can be set to either cred, user or pass and controls
--                   whether the engine should iterate over users, passwords
--                   or fetch a list of credentials from a single file.
--                   (can be set using script-arg brute.mode)
--   * title       - changes the title of the result table where the
--                   passwords are returned.
--   * nostore     - don't store the results in the credential library
--   * max_guesses - the maximum amount of guesses to perform for each
--                   account.
--   * useraspass  - guesses the username as password (default: true)
--   * emptypass   - guesses an empty string as password (default: false)
--
Options = {

  new = function(self)
    local o = {}
    setmetatable(o, self)
    self.__index = self

    o.emptypass = self.checkBoolArg("brute.emptypass", false)
    o.useraspass = self.checkBoolArg("brute.useraspass", true)
    o.firstonly = self.checkBoolArg("brute.firstonly", false)
    o.passonly = self.checkBoolArg("brute.passonly", false)
    o.max_retries = tonumber( nmap.registry.args["brute.retries"] ) or 3
    o.delay = tonumber( nmap.registry.args["brute.delay"] ) or 0
    o.max_guesses = tonumber( nmap.registry.args["brute.guesses"] ) or 0

    return o
  end,

  --- Checks if a script argument is boolean true or false
  --
  -- @param arg string containing the name of the argument to check
  -- @param default boolean containing the default value
  -- @return boolean, true if argument evaluates to 1 or true, else false
  checkBoolArg = function( arg, default )
    local val = stdnse.get_script_args(arg) or default
    return (val == "true" or val==true or tonumber(val)==1)
  end,

  --- Sets the brute mode to either iterate over users or passwords
  -- @see description for more information.
  --
  -- @param mode string containing either "user" or "password"
  -- @return status true on success else false
  -- @return err string containing the error message on failure
  setMode = function( self, mode )
    local modes = { "password", "user", "creds" }
    local supported = false

    for _, m in ipairs(modes) do
      if ( mode == m ) then supported = true end
    end

    if ( not(supported) ) then
      stdnse.debug1("ERROR: brute.options.setMode: mode %s not supported", mode)
      return false, "Unsupported mode"
    else
      self.mode = mode
    end
    return true
  end,

  --- Sets an option parameter
  --
  -- @param param string containing the parameter name
  -- @param value string containing the parameter value
  setOption = function( self, param, value ) self[param] = value end,

  --- Set an alternate title for the result output (default: Accounts)
  --
  -- @param title string containing the title value
  setTitle = function(self, title) self.title = title end,

}

-- The account object which is to be reported back from each driver
-- The Error class, is currently only used to flag for retries
-- It also contains the error message, if one was returned from the driver.
Error =
{
  retry = false,

  new = function(self, msg)
    local o = { msg = msg, done = false }
    setmetatable(o, self)
    self.__index = self
    return o
  end,

  --- Is the error recoverable?
  --
  -- @return status true if the error is recoverable, false if not
  isRetry = function( self ) return self.retry end,

  --- Set the error as recoverable
  --
  -- @param r boolean true if the engine should attempt to retry the
  --        credentials, unset or false if not
  setRetry = function( self, r ) self.retry = r end,

  --- Set the error as abort all threads
  --
  -- @param b boolean true if the engine should abort guessing on all threads
  setAbort = function( self, b ) self.abort = b end,

  --- Was the error abortable
  --
  -- @return status true if the driver flagged the engine to abort
  isAbort = function( self ) return self.abort end,

  --- Get the error message reported
  --
  -- @return msg string containing the error message
  getMessage = function( self ) return self.msg end,

  --- Is the thread done?
  --
  -- @return status true if done, false if not
  isDone = function( self ) return self.done end,

  --- Signals the engine that the thread is done and should be terminated
  --
  -- @param b boolean true if done, unset or false if not
  setDone = function( self, b ) self.done = b end,

  -- Marks the username as invalid, aborting further guessing.
  -- @param username
  setInvalidAccount = function(self, username)
    self.invalid_account = username
  end,

  -- Checks if the error reported the account as invalid.
  -- @return username string containing the invalid account
  isInvalidAccount = function(self)
    return self.invalid_account
  end,

}

-- The brute engine, doing all the nasty work
Engine =
{
  STAT_INTERVAL = 20,

  --- Creates a new Engine instance
  --
  -- @param driver, the driver class that should be instantiated
  -- @param host table as passed to the action method of the script
  -- @param port table as passed to the action method of the script
  -- @param options table containing any script specific options
  -- @return o new Engine instance
  new = function(self, driver, host, port, options)
    local o = {
      driver = driver,
      host = host,
      port = port,
      driver_options = options,
      terminate_all = false,
      error = nil,
      counter = 0,
      threads = {},
      tps = {},
      iterator = nil ,
      usernames = usernames_iterator(),
      passwords = passwords_iterator(),
      found_accounts = {},
      account_guesses = {},
      options = Options:new(),
    }
    setmetatable(o, self)
    self.__index = self
    o.max_threads = stdnse.get_script_args("brute.threads") or 10
    return o
  end,

  --- Sets the username iterator
  --
  -- @param usernameIterator function to set as a username iterator
  setUsernameIterator = function(self,usernameIterator)
    self.usernames = usernameIterator
  end,

  --- Sets the password iterator
  --
  -- @param passwordIterator function to set as a password iterator
  setPasswordIterator = function(self,passwordIterator)
    self.passwords = passwordIterator
  end,

  --- Limit the number of worker threads
  --
  -- @param max number containing the maximum number of allowed threads
  setMaxThreads = function( self, max ) self.max_threads = max end,

  --- Returns the number of non-dead threads
  --
  -- @return count number of non-dead threads
  threadCount = function( self )
    local count = 0

    for thread in pairs(self.threads) do
      if ( coroutine.status(thread) == "dead" ) then
        self.threads[thread] = nil
      else
        count = count + 1
      end
    end
    return count
  end,

  --- Calculates the number of threads that are actually doing any work
  --
  -- @return count number of threads performing activity
  activeThreads = function( self )
    local count = 0
    for thread, v in pairs(self.threads) do
      if ( v.guesses ~= nil ) then count = count + 1 end
    end
    return count
  end,

  --- Iterator wrapper used to iterate over all registered iterators
  --
  -- @return iterator function
  get_next_credential = function( self )
    local function next_credential ()
      for user, pass in self.iterator do
        -- makes sure the credentials have not been tested before
        self.used_creds = self.used_creds or {}
        pass = pass or "nil"
        if ( not(self.used_creds[user..pass]) ) then
          self.used_creds[user..pass] = true
          coroutine.yield( user, pass )
        end
      end
      while true do coroutine.yield(nil, nil) end
    end
    return coroutine.wrap( next_credential )
  end,

  --- Does the actual authentication request
  --
  -- @return true on success, false on failure
  -- @return response Account on success, Error on failure
  doAuthenticate = function( self )

    local status, response
    local next_credential = self:get_next_credential()
    local retries = self.options.max_retries
    local username, password

    repeat
      local driver = self.driver:new( self.host, self.port, self.driver_options )
      status = driver:connect()

      -- Did we successfully connect?
      if ( status ) then
        if ( not(username) and not(password) ) then
          repeat
            username, password = next_credential()
            if ( not(username) and not(password) ) then
              driver:disconnect()
              self.threads[coroutine.running()].terminate = true
              return false
            end
          until ( ( not(self.found_accounts) or not(self.found_accounts[username]) ) and
            ( self.options.max_guesses == 0 or not(self.account_guesses[username]) or
            self.options.max_guesses > self.account_guesses[username] ) )

          -- increases the number of guesses for an account
          self.account_guesses[username] = self.account_guesses[username] and self.account_guesses[username] + 1 or 1
        end

        -- make sure that all threads locked in connect stat terminate quickly
        if ( Engine.terminate_all ) then
          driver:disconnect()
          return false
        end

        local c
        -- Do we have a username or not?
        if ( username and #username > 0 ) then
          c = ("%s/%s"):format(username, #password > 0 and password or "<empty>")
        else
          c = ("%s"):format(#password > 0 and password or "<empty>")
        end

        local msg = ( retries ~= self.options.max_retries ) and "Re-trying" or "Trying"
        stdnse.debug2("%s %s against %s:%d", msg, c, self.host.ip, self.port.number )
        status, response = driver:login( username, password )

        driver:disconnect()
        driver = nil
      end

      retries = retries - 1

      -- End if:
      -- * The guess was successful
      -- * The response was not set to retry
      -- * We've reached the maximum retry attempts
    until( status or ( response and not( response:isRetry() ) ) or retries == 0)

    -- Increase the amount of total guesses
    self.counter = self.counter + 1

    -- did we exhaust all retries, terminate and report?
    if ( retries == 0 ) then
      Engine.terminate_all = true
      self.error = "Too many retries, aborted ..."
      response = Error:new("Too many retries, aborted ...")
      response.abort = true
    end
    return status, response
  end,

  login = function(self, cvar )
    local condvar = nmap.condvar( cvar )
    local thread_data = self.threads[coroutine.running()]
    local interval_start = os.time()

    while( true ) do
      -- Should we terminate all threads?
      if ( self.terminate_all or thread_data.terminate ) then break end

      local status, response = self:doAuthenticate()

      if ( status ) then
        -- Prevent locked accounts from appearing several times
        if ( not(self.found_accounts) or self.found_accounts[response.username] == nil ) then
          if ( not(self.options.nostore) ) then
            creds.Credentials:new( self.options.script_name, self.host, self.port ):add(response.username, response.password, response.state )
          else
            self.credstore = self.credstore or {}
            table.insert(self.credstore, tostring(response) )
          end

          stdnse.debug1("Discovered account: %s", tostring(response))

          -- if we're running in passonly mode, and want to continue guessing
          -- we will have a problem as the username is always the same.
          -- in this case we don't log the account as found.
          if ( not(self.options.passonly) ) then
            self.found_accounts[response.username] = true
          end

          -- Check if firstonly option was set, if so abort all threads
          if ( self.options.firstonly ) then self.terminate_all = true end
        end
      else
        if ( response and response:isAbort() ) then
          self.terminate_all = true
          self.error = response:getMessage()
          break
        elseif( response and response:isDone() ) then
          break
        elseif ( response and response:isInvalidAccount() ) then
          self.found_accounts[response:isInvalidAccount()] = true
        end
      end

      local timediff = (os.time() - interval_start)

      -- This thread made another guess
      thread_data.guesses = ( thread_data.guesses and thread_data.guesses + 1 or 1 )

      -- Dump statistics at regular intervals
      if ( timediff > Engine.STAT_INTERVAL ) then
        interval_start = os.time()
        local tps = self.counter / ( os.time() - self.starttime )
        table.insert(self.tps, tps )
        stdnse.debug2("threads=%d,tps=%d", self:activeThreads(), tps )
      end

      -- if delay was specified, do sleep
      if ( self.options.delay > 0 ) then stdnse.sleep( self.options.delay ) end
    end
    condvar "signal"
  end,

  --- Starts the brute-force
  --
  -- @return status true on success, false on failure
  -- @return err string containing error message on failure
  start = function(self)

    local cvar = {}
    local condvar = nmap.condvar( cvar )

    assert(self.options.script_name, "SCRIPT_NAME was not set in options.script_name")
    assert(self.port.number and self.port.protocol, "Invalid port table detected")
    self.port.service = self.port.service or "unknown"

    -- Only run the check method if it exist. We should phase this out
    -- in favor of a check in the action function of the script
    if ( self.driver:new( self.host, self.port, self.driver_options ).check ) then
      -- check if the driver is ready!
      local status, response = self.driver:new( self.host, self.port, self.driver_options ):check()
      if( not(status) ) then return false, response end
    end

    local usernames = self.usernames
    local passwords = self.passwords

    if ( "function" ~= type(usernames) ) then
      return false, "Invalid usernames iterator"
    end
    if ( "function" ~= type(passwords) ) then
      return false, "Invalid passwords iterator"
    end

    local mode = self.options.mode or stdnse.get_script_args("brute.mode")

    -- if no mode was given, but a credfile is present, assume creds mode
    if ( not(mode) and stdnse.get_script_args("brute.credfile") ) then
      if ( stdnse.get_script_args("userdb") or
        stdnse.get_script_args("passdb") ) then
        return false, "\n  ERROR: brute.credfile can't be used in combination with userdb/passdb"
      end
      mode = 'creds'
    end

    -- Are we guessing against a service that has no username (eg. VNC)
    if ( self.options.passonly ) then
      local function single_user_iter(next)
        local function next_user() coroutine.yield( "" ) end
        return coroutine.wrap(next_user)
      end
      -- only add this iterator if no other iterator was specified
      if self.iterator == nil  then
        self.iterator = Iterators.user_pw_iterator( single_user_iter(), passwords )
      end
    elseif ( mode == 'creds' ) then
      local credfile = stdnse.get_script_args("brute.credfile")
      if ( not(credfile) ) then
        return false, "No credential file specified (see brute.credfile)"
      end

      local f = io.open( credfile, "r" )
      if ( not(f) ) then
        return false, ("Failed to open credfile (%s)"):format(credfile)
      end

      self.iterator = Iterators.credential_iterator( f )
    elseif ( mode and mode == 'user' ) then
      self.iterator = self.iterator or Iterators.user_pw_iterator( usernames, passwords )
    elseif( mode and mode == 'pass' ) then
      self.iterator = self.iterator or Iterators.pw_user_iterator( usernames, passwords )
    elseif ( mode ) then
      return false, ("Unsupported mode: %s"):format(mode)
      -- Default to the pw_user_iterator in case no iterator was specified
    elseif ( self.iterator == nil ) then
      self.iterator = Iterators.pw_user_iterator( usernames, passwords )
    end

    if ( ( not(mode) or mode == 'user' or mode == 'pass' ) and self.options.useraspass ) then
      -- if we're only guessing passwords, this doesn't make sense
      if ( not(self.options.passonly) ) then
        self.iterator = unpwdb.concat_iterators(Iterators.pw_same_as_user_iterator(usernames, "lower"),self.iterator)
      end
    end

    if ( ( not(mode) or mode == 'user' or mode == 'pass' ) and self.options.emptypass ) then
      local function empty_pass_iter()
        local function next_pass()
          coroutine.yield( "" )
        end
        return coroutine.wrap(next_pass)
      end
      self.iterator = Iterators.account_iterator(usernames, empty_pass_iter(), mode or "pass")
    end


    self.starttime = os.time()

    -- Startup all worker threads
    for i=1, self.max_threads do
      local co = stdnse.new_thread( self.login, self, cvar )
      self.threads[co] = {}
      self.threads[co].running = true
    end

    -- wait for all threads to finish running
    while self:threadCount()>0 do condvar "wait" end

    local valid_accounts

    if ( not(self.options.nostore) ) then
      valid_accounts = creds.Credentials:new(self.options.script_name, self.host, self.port):getTable()
    else
      valid_accounts = self.credstore
    end

    local result = stdnse.output_table()
    -- Did we find any accounts, if so, do formatting
    if ( valid_accounts and #valid_accounts > 0 ) then
      result[self.options.title or "Accounts"] = valid_accounts
    else
      result.Accounts = "No valid accounts found"
    end

    -- calculate the average tps
    local sum = 0
    for _, v in ipairs( self.tps ) do sum = sum + v end
    local time_diff = ( os.time() - self.starttime )
    time_diff = ( time_diff == 0 ) and 1 or time_diff
    local tps = ( sum == 0 ) and ( self.counter / time_diff ) or ( sum / #self.tps )

    -- Add the statistics to the result
    result.Statistics = ("Performed %d guesses in %d seconds, average tps: %d"):format( self.counter, time_diff, tps )

    if ( self.options.max_guesses > 0 ) then
      -- we only display a warning if the guesses are equal to max_guesses
      for user, guesses in pairs(self.account_guesses) do
        if ( guesses == self.options.max_guesses ) then
          result.Information = ("Guesses restricted to %d tries per account to avoid lockout"):format(self.options.max_guesses)
          break
        end
      end
    end

    -- Did any error occur? If so add this to the result.
    if ( self.error ) then
      result.ERROR = self.error
      return false, result
    end
    return true, result
  end,

}

--- Default username iterator that uses unpwdb
--
usernames_iterator = function()
  local status, usernames = unpwdb.usernames()
  if ( not(status) ) then return "Failed to load usernames" end
  return usernames
end

--- Default password iterator that uses unpwdb
--
passwords_iterator = function()
  local status, passwords = unpwdb.passwords()
  if ( not(status) ) then return "Failed to load passwords" end
  return passwords
end

Iterators = {

  --- Iterates over each user and password
  --
  -- @param users table/function containing list of users
  -- @param pass table/function containing list of passwords
  -- @param mode string, should be either 'user' or 'pass' and controls
  --        whether the users or passwords are in the 'outer' loop
  -- @return function iterator
  account_iterator = function(users, pass, mode)
    local function next_credential ()
      local outer, inner
      if "table" == type(users) then
        users = unpwdb.table_iterator(users)
      end
      if "table" == type(pass) then
        pass = unpwdb.table_iterator(pass)
      end

      if ( mode == 'pass' ) then
        outer, inner = pass, users
      elseif ( mode == 'user' ) then
        outer, inner = users, pass
      else
        return
      end

      for o in outer do
        for i in inner do
          if ( mode == 'pass' ) then
            coroutine.yield( i, o )
          else
            coroutine.yield( o, i )
          end
        end
        inner("reset")
      end
      while true do coroutine.yield(nil, nil) end
    end
    return coroutine.wrap( next_credential )
  end,


  --- Try each password for each user (user in outer loop)
  --
  -- @param users table/function containing list of users
  -- @param pass table/function containing list of passwords
  -- @return function iterator
  user_pw_iterator = function( users, pass )
    return Iterators.account_iterator( users, pass, "user" )
  end,

  --- Try each user for each password (password in outer loop)
  --
  -- @param users table/function containing list of users
  -- @param pass table/function containing list of passwords
  -- @return function iterator
  pw_user_iterator = function( users, pass )
    return Iterators.account_iterator( users, pass, "pass" )
  end,

  --- An iterator that returns the username as password
  --
  -- @param users function returning the next user
  -- @param case string [optional] 'upper' or 'lower', specifies if user
  --        and password pairs should be case converted.
  -- @return function iterator
  pw_same_as_user_iterator = function( users, case )
    local function next_credential ()
      for user in users do
        if ( case == 'upper' ) then
          coroutine.yield(user, user:upper())
        elseif( case == 'lower' ) then
          coroutine.yield(user, user:lower())
        else
          coroutine.yield(user, user)
        end
      end
      users("reset")
      while true do coroutine.yield(nil, nil) end
    end
    return coroutine.wrap( next_credential )
  end,

  --- An iterator that returns the username and uppercase password
  --
  -- @param users table containing list of users
  -- @param pass table containing list of passwords
  -- @param mode string, should be either 'user' or 'pass' and controls
  --        whether the users or passwords are in the 'outer' loop
  -- @return function iterator
  pw_ucase_iterator = function( users, passwords, mode )
    local function next_credential ()
      for user, pass in Iterators.account_iterator(users, passwords, mode) do
        coroutine.yield( user, pass:upper() )
      end
      while true do coroutine.yield(nil, nil) end
    end
    return coroutine.wrap( next_credential )
  end,

  --- Credential iterator (for default or known user/pass combinations)
  --
  -- @param f file handle to file containing credentials separated by '/'
  -- @return function iterator
  credential_iterator = function( f )
    local function next_credential ()
      local c = {}
      for line in f:lines() do
        if ( not(line:match("^#!comment:")) ) then
          local trim = function(s) return s:match('^()%s*$') and '' or s:match('^%s*(.*%S)') end
          line = trim(line)
          local user, pass = line:match("^([^%/]*)%/(.*)$")
          coroutine.yield( user, pass )
        end
      end
      f:close()
      while true do coroutine.yield( nil, nil ) end
    end
    return coroutine.wrap( next_credential )
  end,

  unpwdb_iterator = function( mode )
    local status, users, passwords

    status, users = unpwdb.usernames()
    if ( not(status) ) then return end

    status, passwords = unpwdb.passwords()
    if ( not(status) ) then return end

    return Iterators.account_iterator( users, passwords, mode )
  end,

}

return _ENV;