NAME
   CGI::Application::Plugin::LinkIntegrity - Make tamper-resisistent links
   in CGI::Application

VERSION
   Version 0.02

SYNOPSIS
   In your application:

       use base 'CGI::Application';
       use CGI::Application::Plugin::LinkIntegrity;

       sub setup {
           my $self = shift;
           $self->link_integrity_config(
               secret => 'some secret string known only to you and me',
           );
       }

       sub account_info {
           my $self = shift;

           my $account_id = get_user_account_id();

           my $template = $self->load_tmpl('account.html');

           $template->param(
               'balance'    => $self->make_link("/account.cgi?rm=balance&acct_id=$account_id");
               'transfer'   => $self->make_link("/account.cgi?rm=transfer&acct_id=$account_id");
               'withdrawal' => $self->make_link("/account.cgi?rm=withdrawl&acct_id=$account_id");
           );
       }

   In your template:

       <h1>Welcome to The Faceless Banking Corp.</h1>
       <h3>Actions:</h3>
       <br /><a href="<TMPL_VAR NAME="balance">">Show Balance</a>
       <br /><a href="<TMPL_VAR NAME="transfer">">Make a Transfer</a>
       <br /><a href="<TMPL_VAR NAME="withdrawal">">Get Cash</a>

   This will send the following HTML to the browser:

       <h1>Welcome to The Faceless Banking Corp.</h1>
       <h3>Actions:</h3>
       <br /><a href="/account.cgi?rm=balance&acct_id=73&_checksum=1d7c4b82d075785de04fa6b98b572691">Show Balance</a>
       <br /><a href="/account.cgi?rm=transfer&acct_id=73&_checksum=d41d8cd98f00b204e9800998ecf8427e">Make a Transfer</a>
       <br /><a href="/account.cgi?rm=withdrawl&acct_id=73&_checksum=3c5ad17bdeef3c4281abd39c6386cfd6">Get Cash</a>

   The URLs created are now tamper-resistent. If the user changes "acct_id"
   from 73 to 74, the "_checksum" will not match, and the system will treat
   it as an intrusion attempt.

DESCRIPTION
   "CGI::Application::Plugin::LinkIntegrity" lets you create
   tamper-resistent links within your CGI::Application project. When you
   create an URL with "make_link", a "_checksum" is added to the URL:

       my $link = $self->make_link("/account.cgi?rm=balance&acct_id=73");
       print $link; # /account.cgi?rm=balance&acct_id=73&_checksum=1d7c4b82d075785de04fa6b98b572691

   The checksum is a (cryptographic) hash of the URL, plus a secret string
   known only to the server.

   If the user attempts to change part of the URL (e.g. a query string
   parameter, or the PATH_INFO), then the checksum will not match. The run
   mode will be changed to "link_tampered", and the "invalid_checksum" hook
   will be called.

   You can define the "link_tampered" run mode yourself, or you can use the
   default "link_tampered" run mode built into
   CGI::Application::Plugin::LinkIntegrity.

   You can disable link checking during development by passing a true value
   to the "disable" parameter of "$self->link_integrity_config".

METHODS
 link_integrity_config
   Configure the CGI::Application::Plugin::LinkIntegrity. Usually, it makes
   sense to configure this in the "setup" method of your application's base
   class:

       use CGI::Application::Plugin::LinkIntegrity;
       use base 'CGI::Application';
       package My::Project;

       sub setup {
           my $self = shift;

           $self->run_modes(['bad_user_no_biscuit']);
           $self->link_integrity_config(
               secret                 => 'some secret string known only to you and me',
               link_tampered_run_mode => 'bad_user_no_biscuit',
               digest_module          => 'Digest::MD5',
               disable                => 1,
           );
       }

   Or you can pull in this configuration info from a config file. For
   instance, with using CGI::Application::Config::Context:

       use CGI::Application::Plugin::LinkIntegrity;
       use CGI::Application::Plugin::Config::Context;

       use base 'CGI::Application';
       package My::Project;

       sub setup {
           my $self = shift;

           $self->conf->init(
               file   => 'app.conf',
               driver => 'ConfigGeneral',
           );

           my $config = $self->conf->context;

           $self->link_integrity_config(
               $config->{'LinkIntegrity'},
               additional_data => sub {
                   my $self = shift;
                   return $self->session->id;
               },
           );

           my $link_tampered_rm = $config->{'LinkIntegrity'}{'link_tampered_run_mode'} || 'link_tampered';

           $self->run_modes([$link_tampered_rm]);
       }

   Then in your configuration file:

       <LinkIntegrity>
           secret                 = some REALLY secret string
           link_tampered_run_mode = bad_user_no_biscuit
           hash_algorithm         = SHA1
           disable                = 1
       </LinkIntegrity>

   This strategy allows you to enable and disable link checking on the fly
   by changing the value of "disable" in the config file.

   The following configuration parameters are available:

   secret
       A string known only to your application. At a commandline, you can
       generate a secret string with md5:

        $ perl -MDigest::MD5 -le"print Digest::MD5::md5_hex($$, time, rand(42));"

       Or you can use Data::UUID:

        $ perl -MData::UUID -le"$ug = new Data::UUID; $uuid = $ug->create; print $ug->to_string($uuid)"

       If someone knows your secret string, then they can generate their
       own checksums on arbitrary data that will always pass the integrity
       check in your application. That's a Bad Thing, so don't let other
       people know your secret string, and don't use the default secret
       string if you can help it.

   additional_data
       You can pass constant additional data to the checksum generator for
       every link.

           $self->link_integrity_config(
               secret          => 'really secret',
               additional_data => 'some other secret data',
           }

       For instance, to stop one user from following a second user's link,
       you can add a user-specific component to the session, such as the
       user's session id:

           $self->link_integrity_config(
               secret          => 'really secret',
               additional_data => sub {
                   my $self = shift;
                   return $self->session->id;
               }
           }

       You can pass a string instead of a subroutine. But in the case of
       the user's session, a subroutine is useful so that you get the value
       of the user's session at the time when the checksum is generated,
       not at the time when the link integrity system is configured.

   checksum_param
       The name of the checksum parameter, which is added to the query
       string and contains the cryptographic checksum of link. By default,
       this parameter is named "_checksum".

   link_tampered_run_mode
       The run mode to go to when it has been detected that the user has
       tampered with the link. The default is "link_tampered".

       See "The link_tampered Run Mode", below.

   digest_module
       Which digest module to use to create the checksum. Typically, this
       will be either "Digest::MD5" or "Digest::SHA1". However any module
       supported by "Digest::HMAC" will work.

       The default "digest_module" is "Digest::MD5".

   checksum_generator
       If you want to provide a custom subroutine to make your own
       checksums, you can define your own subroutine do it via the
       "make_checksum" param. Here's an example of one that uses
       Digest::SHA2:

               $self->link_integrity_config(
                   checksum_generator => sub {
                       my ($url, $secret) = @_;
                       require Digest::SHA2;

                       my $ctx = Digest::SHA2->new();
                       $ctx->add($url . $secret);

                       return $ctx->hexdigest;
                   },
               );

   disable
       You can disable link checking entirely by setting "disable" to a
       true value. This can be useful when you are developing or debugging
       the application and you want the ability to tweak URL params without
       getting busted.

 make_link
   Create a link, and add a checksum to it.

   You can add parameters to the link directly in the URL:

       my $link = $self->make_link('/cgi-bin/app.cgi?var=value&var2=value2');

   Or you can add them as a hashref of parameters after the URL:

       my $link = $self->make_link(
           '/cgi-bin/app.cgi',
           {
               var  => 'value',
               var2 => 'value2',
           }
       );

 make_self_link
   Make a link to the current application, with optional parameters, and
   add a checksum to the URL.

       my $link = $self->make_self_link(
           params => {
               var1 => 'value1',
               var1 => 'value2',
           },
       );

   If you want to keep the existing "PATH_INFO" that was passed to the
   current application, use the "keep_path_info" param:

       my $link = $self->make_self_link(
           keep_path_info => 1,
           params         => {
               var1 => 'value1',
               var1 => 'value2',
           },
       );

   If you want to use a different "PATH_INFO" than the one that was used in
   calling the current application use the "path_info" param:

       my $link = $self->make_self_link(
           path_info      => '/some/path',
           params         => {
               var1 => 'value1',
               var1 => 'value2',
           },
       );

RUN MODES
 The link_tampered Run Mode
   Your application is redirected to this run mode when it has been
   detected that the user has tampered with the link. You can change the
   name of this run mode by changing the value of the
   "link_tampered_runmode" param to "link_integrity_config".

   CGI::Application::Plugin::LinkIntegrity provides a default
   "link_tampered" run mode, which just displays a page with some stern
   warning text.

   You can define your own as follows:

       sub link_tampered {
           my $self = shift;
           my $template = $self->load_template('stern_talking_to');
           return $template->output;
       }

HOOKS
   When a link is followed that doesn't match the checksum, the
   "invalid_checksum" hook is called. You can add a callback to this hook
   to do some cleanup such as deleting the user's session. For instance:

       sub setup {
           my $self = shift;
           $self->add_callback('invalid_checksum' => \&bad_user);
       }

       sub bad_user {
           my $self = shift;

           # The user has been messing with the URLs, possibly trying to
           # break into the system.  We don't tolerate this behaviour.
           # So we delete the user's session:

           $self->session->delete;
       }

AUTHOR
   Michael Graham, "<[email protected]>"

ACKNOWLEDGEMENTS
   This module was based on the checksum feature originally built into
   Richard Dice's CGI::Application::Framework.

BUGS
   Please report any bugs or feature requests to
   "[email protected]", or through the
   web interface at <http://rt.cpan.org>. I will be notified, and then
   you'll automatically be notified of progress on your bug as I make
   changes.

COPYRIGHT & LICENSE
   Copyright 2005 Michael Graham, All Rights Reserved.

   This program is free software; you can redistribute it and/or modify it
   under the same terms as Perl itself.

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