This directive enables secure checksum generation along with %URI.Munge.
It should be set to a secure key that is not shared with anyone else.
The checksum can be placed in the URI using %t. Use of this checksum
affords an additional level of protection by allowing a redirector
to check if a URI has passed through HTML Purifier with this line:
<pre>$checksum === hash_hmac("sha256", $url, $secret_key)</pre>
If the output is TRUE, the redirector script should accept the URI.
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.
Remember this has no effect if %URI.Munge is not on.
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