URI.MungeSecretKey
TYPE: string/null
VERSION: 3.1.1
DEFAULT: NULL
--DESCRIPTION--
<p>
    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:
</p>

<pre>$checksum === hash_hmac("sha256", $url, $secret_key)</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>
    Remember this has no effect if %URI.Munge is not on.
</p>
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