[3.1.1] Implement %URI.SecureMunge and %URI.SecureMungeSecretKey, thanks Chris!

- URIFilter->prepare can return false in order to abort loading of the filter
- Implemented post URI filtering. Set member variable $post to true to set a URIFilter as such.

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@1772 48356398-32a2-884e-a903-53898d9a118a

library/HTMLPurifier/ConfigSchema/schema/URI.SecureMunge.txt

@@ -0,0 +1,33 @@
+URI.SecureMunge
+TYPE: string/null
+VERSION: 3.1.1
+DEFAULT: NULL
+--DESCRIPTION--
+<p>
+    Like %URI.Munge, this directive munges browsable external resources
+    into another URI redirection service. %URI.SecureMunge accepts a URI
+    with a %s located where the original URI should be substituted in,
+    and %t located where the secure checksum should be provided.
+    However, this directive affords
+    an additional level of protection by generating a secure checksum from
+    the URI as well as a secret key provided by %URI.SecureMungeSecretKey.
+    Any redirector script can check this key by using:
+</p>
+
+<pre>$checksum === sha1($secret_key . ':' . $url)</pre>
+
+<p>
+    If the output is TRUE, the redirector script should accept the URI.
+</p>
+
+<p>
+    Please note that it would still be possible for an attacker to procure
+    secure hashes en-mass by abusing your website's Preview feature or the
+    like, but this service affords an additional level of protection
+    that should be combined with website blacklisting.
+</p>
+
+<p>
+    <strong>This is a post-filter.</strong> This filter may conflict with other
+    post-filters that deal with external links.
+</p>

library/HTMLPurifier/ConfigSchema/schema/URI.SecureMungeSecretKey.txt

@@ -0,0 +1,11 @@
+URI.SecureMungeSecretKey
+TYPE: string/null
+VERSION: 3.1.1
+DEFAULT: NULL
+--DESCRIPTION--
+<p>
+    This is the secret key used in conjunction with %URI.SecureMunge. Your
+    redirector script needs to know about this key, and no one else should
+    know about this key. Please see the above
+    directive for more details.
+</p>