php-doc-en/appendices/filters.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<appendix xml:id="filters" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <title>List of Available Filters</title>
 <para>
  The following is a list of a few built-in stream filters for
  use with <function>stream_filter_append</function>.
  Your version of PHP may have more filters (or fewer) than those
  listed here.
 </para>

 <para>
  It is worth noting a slight asymmetry between 
  <function>stream_filter_append</function>
  and <function>stream_filter_prepend</function>.  
  Every PHP stream contains a small <emphasis>read buffer</emphasis>
  where it stores blocks of data retrieved from the
  filesystem or other resource in order to process data
  in the most efficient manner.  As soon as data is pulled
  from the resource into the stream's internal buffer, it
  is immediately processed through any attached filters whether
  the PHP application is actually ready for the data or not.
  If data is sitting in the read buffer when a filter is 
  <emphasis>appended</emphasis>, this data will be immediately
  processed through that filter making the fact that it was
  sitting in the buffer seem transparent.  However, if data is
  sitting in the read buffer when a filter is 
  <emphasis>prepended</emphasis>, this data will <emphasis>NOT</emphasis>
  be processed through that filter.  It will instead wait until
  the next block of data is retrieved from the resource.
 </para>

 <para>
  For a list of filters installed in your version of
  PHP use <function>stream_get_filters</function>.
 </para>

 <section xml:id="filters.string">
  <title>String Filters</title>

  <simpara>
   Each of these filters does precisely what their name implies and
   correspond to the behavior of a built-in php string handling function.
   For more information on a given filter, refer to the manual page for
   the corresponding function.
  </simpara>

  <section xml:id="filters.string.rot13">
   <title>string.rot13</title>
   <simpara>
    Use of this filter is equivalent to processing all stream data through
    the <function>str_rot13</function> function.
   </simpara>
   <example>
    <title>string.rot13</title>
    <programlisting role="php">
<![CDATA[
<?php
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'string.rot13');
fwrite($fp, "This is a test.\n");
/* Outputs:  Guvf vf n grfg.   */
?>
]]>
    </programlisting>
   </example>
  </section>

  <section xml:id="filters.string.toupper">
   <title>string.toupper</title>
   <simpara>
    Use of this filter is equivalent to processing all stream data through
    the <function>strtoupper</function> function.
   </simpara>
   <example>
    <title>string.toupper</title>
    <programlisting role="php">
<![CDATA[
<?php
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'string.toupper');
fwrite($fp, "This is a test.\n");
/* Outputs:  THIS IS A TEST.   */
?>
]]>
    </programlisting>
   </example>
  </section>

  <section xml:id="filters.string.tolower">
   <title>string.tolower</title>
   <simpara>
    Use of this filter is equivalent to processing all stream data through
    the <function>strtolower</function> function.
   </simpara>
   <example>
    <title>string.tolower</title>
    <programlisting role="php">
<![CDATA[
<?php
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'string.tolower');
fwrite($fp, "This is a test.\n");
/* Outputs:  this is a test.   */
?>
]]>
    </programlisting>
   </example>
  </section>

  <section xml:id="filters.string.strip_tags">
   <title>string.strip_tags</title>
   <simpara>
    Use of this filter is equivalent to processing all stream data through
    the <function>strip_tags</function> function.
    It accepts parameters in one of two forms:
    Either as a string containing a list of tags similar to the
    second parameter of the <function>strip_tags</function> function,
    or as an array of tag names.
   </simpara>
   <para>
    &warn.deprecated.feature-7-3-0;
   </para>
   <example>
    <title>string.strip_tags</title>
    <programlisting role="php">
<![CDATA[
<?php
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'string.strip_tags', STREAM_FILTER_WRITE, "<b><i><u>");
fwrite($fp, "<b>bolded text</b> enlarged to a <h1>level 1 heading</h1>\n");
fclose($fp);
/* Outputs:  <b>bolded text</b> enlarged to a level 1 heading   */

$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'string.strip_tags', STREAM_FILTER_WRITE, array('b','i','u'));
fwrite($fp, "<b>bolded text</b> enlarged to a <h1>level 1 heading</h1>\n");
fclose($fp);
/* Outputs:  <b>bolded text</b> enlarged to a level 1 heading   */
?>
]]>
    </programlisting>
   </example>
  </section>
 </section>

 <section xml:id="filters.convert">
  <title>Conversion Filters</title>

  <simpara>
   Like the string.* filters, the convert.* filters perform actions
   similar to their names.
   For more information on a given filter, refer to the manual page for
   the corresponding function.
  </simpara>

  <section xml:id="filters.convert.base64">
   <title>convert.base64-encode and convert.base64-decode</title>
   <simpara>
    Use of these filters are equivalent to processing all stream data through
    the <function>base64_encode</function> and <function>base64_decode</function>
    functions respectively.
    <literal>convert.base64-encode</literal> supports parameters given as
    an associative array.  If <parameter>line-length</parameter> is given, the
    base64 output will be split into chunks of <parameter>line-length</parameter>
    characters each.  If <parameter>line-break-chars</parameter> is given, each
    chunk will be delimited by the characters given.  These parameters give the
    same effect as using <function>base64_encode</function> with
    <function>chunk_split</function>.
   </simpara>
   <example>
    <title>
     convert.base64-encode &amp;
     convert.base64-decode
    </title>
    <programlisting role="php">
<![CDATA[
<?php
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'convert.base64-encode');
fwrite($fp, "This is a test.\n");
fclose($fp);
/* Outputs:  VGhpcyBpcyBhIHRlc3QuCg==  */

$param = array('line-length' => 8, 'line-break-chars' => "\r\n");
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'convert.base64-encode', STREAM_FILTER_WRITE, $param);
fwrite($fp, "This is a test.\n");
fclose($fp);
/* Outputs:  VGhpcyBp
          :  cyBhIHRl
          :  c3QuCg==  */

$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'convert.base64-decode');
fwrite($fp, "VGhpcyBpcyBhIHRlc3QuCg==");
fclose($fp);
/* Outputs:  This is a test.  */
?>
]]>
    </programlisting>
   </example>
  </section>

  <section xml:id="filters.covert.quoted-printable">
   <title>convert.quoted-printable-encode and convert.quoted-printable-decode</title>
   <simpara>
    Use of the decode version of this filter is equivalent to processing all stream 
    data through the <function>quoted_printable_decode</function> functions.
    There is no function equivalent to <literal>convert.quoted-printable-encode</literal>.
    <literal>convert.quoted-printable-encode</literal> supports parameters given as
    an associative array.  In addition to the parameters supported by 
    <literal>convert.base64-encode</literal>, <literal>convert.quoted-printable-encode</literal>
    also supports boolean arguments <parameter>binary</parameter> and
    <parameter>force-encode-first</parameter>.
    <literal>convert.base64-decode</literal> only supports the
    <parameter>line-break-chars</parameter> parameter as a type-hint
    for stripping from the encoded payload.
   </simpara>
   <example>
    <title>
     convert.quoted-printable-encode &amp;
     convert.quoted-printable-decode
    </title>
    <programlisting role="php">
<![CDATA[
<?php
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'convert.quoted-printable-encode');
fwrite($fp, "This is a test.\n");
/* Outputs:  =This is a test.=0A  */
?>
]]>
    </programlisting>
   </example>
  </section>

  <section xml:id="filters.convert.iconv">
   <title>convert.iconv.*</title>
   <simpara>
    The <literal>convert.iconv.*</literal> filters are available, if
    <link linkend="book.iconv">iconv</link> support is enabled, and their use is
    equivalent to processing all stream data with <function>iconv</function>.
    These filters do not support parameters, but instead expect the input and
    output encodings to be given as part of the filter name, i.e. either as
    <literal>convert.iconv.&lt;input-encoding&gt;.&lt;output-encoding&gt;</literal> or
    <literal>convert.iconv.&lt;input-encoding&gt;/&lt;output-encoding&gt;</literal>
    (both notations are semantically equivalent).
   </simpara>

   <example>
    <title>convert.iconv.*</title>
    <programlisting role="php">
<![CDATA[
<?php
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'convert.iconv.utf-16le.utf-8');
fwrite($fp, "T\0h\0i\0s\0 \0i\0s\0 \0a\0 \0t\0e\0s\0t\0.\0\n\0");
fclose($fp);
/* Outputs: This is a test. */
?>
]]>
    </programlisting>
   </example>
  </section>
 </section>

 <section xml:id="filters.compression">
  <title>Compression Filters</title>

  <simpara>
   While the <link linkend="wrappers.compression">Compression Wrappers</link>
   provide a way of creating
   gzip and bz2 compatible files on the local filesystem, they do not provide a 
   means for generalized compression over network streams, nor do they provide a 
   means to begin with a non-compressed stream and transition to a compressed one.  
   For this, a compression filter may be applied to any stream resource at any time.
  </simpara>

  <note>
   <simpara>
    Compression filters do <emphasis>not</emphasis> generate headers and trailers
    used by command line utilities such as <literal>gzip</literal>.  They only compress
    and decompress the payload portions of compressed data streams.
   </simpara>
  </note>

  <section xml:id="filters.compression.zlib">
   <title>zlib.deflate and zlib.inflate</title>
   <simpara>
    <literal>zlib.deflate</literal> (compression) and
    <literal>zlib.inflate</literal> (decompression) are implementations of
    the compression methods described in <link xlink:href="&url.rfc;1951">RFC 1951</link>.
    The <literal>deflate</literal> filter takes up to three parameters passed as
    an associative array.  

    <parameter>level</parameter> describes the compression
    strength to use (1-9).  Higher numbers will generally yield smaller payloads at
    the cost of additional processing time.  Two special compression levels also exist:
    0 (for no compression at all), and -1 (zlib internal default -- currently 6).

    <parameter>window</parameter> is the base-2 log of the compression loopback window size.
    Higher values (up to 15 -- 32768 bytes) yield better compression at a cost of memory,
    while lower values (down to 9 -- 512 bytes) yield worse compression in a smaller memory footprint.
    Default <parameter>window</parameter> size is currently <constant>15</constant>.

    <parameter>memory</parameter> is a scale indicating how much work memory should be allocated.
    Valid values range from 1 (minimal allocation) to 9 (maximum allocation).  This memory allocation
    affects speed only and does not impact the size of the generated payload.
   </simpara>

   <note>
    <simpara>
     Because compression level is the most commonly used parameter, it may be alternatively
     provided as a simple integer value (rather than an array element).
    </simpara>
   </note>

   <simpara>
    zlib.* compression filters are available with PHP as of version <literal>5.1.0</literal> if
    <link linkend="ref.zlib">zlib</link> support is enabled.  They are also available as a backport in version
    <literal>5.0.x</literal> by installing the <link xlink:href="&url.pecl.package;zlib_filter">zlib_filter</link>
    package from <link xlink:href="&url.pecl;">PECL</link>.
   </simpara>

   <example>
    <title>
     <literal>zlib.deflate</literal> and
     <literal>zlib.inflate</literal>
    </title>
    <programlisting role="php">
<![CDATA[
<?php
$params = array('level' => 6, 'window' => 15, 'memory' => 9);

$original_text = "This is a test.\nThis is only a test.\nThis is not an important string.\n";
echo "The original text is " . strlen($original_text) . " characters long.\n";

$fp = fopen('test.deflated', 'w');
stream_filter_append($fp, 'zlib.deflate', STREAM_FILTER_WRITE, $params);
fwrite($fp, $original_text);
fclose($fp);

echo "The compressed file is " . filesize('test.deflated') . " bytes long.\n";
echo "The original text was:\n";
/* Use readfile and zlib.inflate to decompress on the fly */
readfile('php://filter/zlib.inflate/resource=test.deflated');

/* Generates output:

The original text is 70 characters long.
The compressed file is 56 bytes long.
The original text was:
This is a test.
This is only a test.
This is not an important string.

 */
?>
]]>
    </programlisting>
   </example>

   <example>
    <title>
     <literal>zlib.deflate</literal> simple
    </title>
    <programlisting role="php">
<![CDATA[
<?php
$original_text = "This is a test.\nThis is only a test.\nThis is not an important string.\n";
echo "The original text is " . strlen($original_text) . " characters long.\n";

$fp = fopen('test.deflated', 'w');
/* Here "6" indicates compression level 6 */
stream_filter_append($fp, 'zlib.deflate', STREAM_FILTER_WRITE, 6);
fwrite($fp, $original_text);
fclose($fp);

echo "The compressed file is " . filesize('test.deflated') . " bytes long.\n";

/* Generates output:

The original text is 70 characters long.
The compressed file is 56 bytes long.

 */
?>
]]>
    </programlisting>
   </example>
  </section>

  <section xml:id="filters.compression.bzip2">
   <title>bzip2.compress and bzip2.decompress</title>
   <simpara>
    <literal>bzip2.compress</literal> and
    <literal>bzip2.decompress</literal>
    work in the same manner as the zlib filters described above.

    The <literal>bzip2.compress</literal> filter accepts up to two parameters given as 
    elements of an associative array: 

    <parameter>blocks</parameter> is an integer value
    from 1 to 9 specifying the number of 100kbyte blocks of memory to allocate for
    workspace. 

    <parameter>work</parameter> is also an integer value ranging from
    0 to 250 indicating how much effort to expend using the normal compression method
    before falling back on a slower, but more reliable method.  Tuning this parameter
    effects only compression speed.  Neither size of compressed output nor memory usage
    are changed by this setting.  A work factor of 0 instructs the bzip library to use
    an internal default.   

    The <literal>bzip2.decompress</literal> filter only accepts one parameter,
    which can be passed as either an ordinary boolean value, or as the 
    <parameter>small</parameter> element of an associative array.

    <parameter>small</parameter>, when set to a &true; value, instructs the bzip library
    to perform decompression in a minimal memory footprint at the cost of speed.
   </simpara>

   <simpara>
     bzip2.* compression filters are available with PHP as of version <literal>5.1.0</literal> if
     <link linkend="ref.bzip2">bz2</link> support is enabled.  They are also available as a backport in version
     <literal>5.0.x</literal> by installing the <link xlink:href="&url.pecl.package;bz2_filter">bz2_filter</link>
     package from <link xlink:href="&url.pecl;">PECL</link>.
   </simpara>

   <example>
    <title>
     <literal>bzip2.compress</literal> and
     <literal>bzip2.decompress</literal>
    </title>
    <programlisting role="php">
<![CDATA[
<?php
$param = array('blocks' => 9, 'work' => 0);

echo "The original file is " . filesize('LICENSE') . " bytes long.\n";

$fp = fopen('LICENSE.compressed', 'w');
stream_filter_append($fp, 'bzip2.compress', STREAM_FILTER_WRITE, $param);
fwrite($fp, file_get_contents('LICENSE'));
fclose($fp);

echo "The compressed file is " . filesize('LICENSE.compressed') . " bytes long.\n";

/* Generates output:

The original text is 3288 characters long.
The compressed file is 1488 bytes long.

 */
?>
]]>
    </programlisting>
   </example>
  </section>
  </section>

 <section xml:id="filters.encryption">
  <title>Encryption Filters</title>

  <para>
    Encryption filters are especially useful for file/stream encryption.
  </para>

  <section xml:id="filters.encryption.mcrypt">
   <title>mcrypt.* and mdecrypt.*</title>
   <para>
     &warn.deprecated.feature-7-1-0;
   </para>

   <simpara>
    <literal>mcrypt.*</literal> and <literal>mdecrypt.*</literal>
    provide symmetric encryption and decryption using libmcrypt.
    Both sets of filters support the same algorithms available to
    <link linkend="ref.mcrypt">mcrypt extension</link> in the form of
    <literal>mcrypt.ciphername</literal> where <parameter>ciphername</parameter>
    is the name of the cipher as it would be passed to
    <function>mcrypt_module_open</function>.
    The following five filter parameters are also available:
   </simpara>

   <para>
    <table>
     <title>mcrypt filter parameters</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Parameter</entry>
        <entry>Required?</entry>
        <entry>Default</entry>
        <entry>Sample Values</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>mode</entry>
        <entry>Optional</entry>
        <entry>cbc</entry>
        <entry>cbc, cfb, ecb, nofb, ofb, stream</entry>
       </row>
       <row>
        <entry>algorithms_dir</entry>
        <entry>Optional</entry>
        <entry>ini_get('mcrypt.algorithms_dir')</entry>
        <entry>Location of algorithms modules</entry>
       </row>
       <row>
        <entry>modes_dir</entry>
        <entry>Optional</entry>
        <entry>ini_get('mcrypt.modes_dir')</entry>
        <entry>Location of modes modules</entry>
       </row>
       <row>
        <entry>iv</entry>
        <entry>Required</entry>
        <entry>N/A</entry>
        <entry>Typically 8, 16, or 32 bytes of binary data.  Depends on cipher</entry>
       </row>
       <row>
        <entry>key</entry>
        <entry>Required</entry>
        <entry>N/A</entry>
        <entry>Typically 8, 16, or 32 bytes of binary data.  Depends on cipher</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>

   <example>
    <title>Encrypt/Decrypt with Blowfish</title>
    <programlisting role="php">
<![CDATA[
<?php
//$key assumed to be previously generated
$iv_size = mcrypt_get_iv_size(MCRYPT_BLOWFISH, MCRYPT_MODE_CBC);
$iv = mcrypt_create_iv($iv_size, MCRYPT_DEV_URANDOM);
$fp = fopen('encrypted-file.enc', 'wb');
fwrite($fp, $iv);
$opts = array('mode'=>'cbc','iv'=>$iv, 'key'=>$key);
stream_filter_append($fp, 'mcrypt.blowfish', STREAM_FILTER_WRITE, $opts);
fwrite($fp, 'message to encrypt');
fclose($fp);

//decrypt...
$fp = fopen('encrypted-file.enc', 'rb');
$iv = fread($fp, $iv_size = mcrypt_get_iv_size(MCRYPT_BLOWFISH, MCRYPT_MODE_CBC));
$opts = array('mode'=>'cbc','iv'=>$iv, 'key'=>$key)
stream_filter_append($fp, 'mdecrypt.blowfish', STREAM_FILTER_READ, $opts);
$data = rtrim(stream_get_contents($fp));//trims off null padding
fclose($fp);
echo $data;
?>
]]>
    </programlisting>
   </example>
   <example>
    <title>Encrypt file using AES-128 CBC with SHA256 HMAC in PHP 5.5+</title>
    <programlisting role="php">
<![CDATA[
<?php
AES_CBC::encryptFile($password, "plaintext.txt", "encrypted.enc");
AES_CBC::decryptFile($password, "encrypted.enc", "decrypted.txt");

class AES_CBC
{
   protected static $KEY_SIZES = array('AES-128'=>16,'AES-192'=>24,'AES-256'=>32);
   protected static function key_size() { return self::$KEY_SIZES['AES-128']; } //default AES-128
   public static function encryptFile($password, $input_stream, $aes_filename){
      $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
      $fin = fopen($input_stream, "rb");
      $fc = fopen($aes_filename, "wb+");
      if (!empty($fin) && !empty($fc)) {
         fwrite($fc, str_repeat("_", 32) );//placeholder, SHA256 HMAC will go here later
         fwrite($fc, $hmac_salt = mcrypt_create_iv($iv_size, MCRYPT_DEV_URANDOM));
         fwrite($fc, $esalt = mcrypt_create_iv($iv_size, MCRYPT_DEV_URANDOM));
         fwrite($fc, $iv = mcrypt_create_iv($iv_size, MCRYPT_DEV_URANDOM));
         $ekey = hash_pbkdf2("sha256", $password, $esalt, $it=1000, self::key_size(), $raw=true);
         $opts = array('mode'=>'cbc', 'iv'=>$iv, 'key'=>$ekey);
         stream_filter_append($fc, 'mcrypt.rijndael-128', STREAM_FILTER_WRITE, $opts);
         $infilesize = 0;
         while (!feof($fin)) {
            $block = fread($fin, 8192);
            $infilesize+=strlen($block);
            fwrite($fc, $block);
         }
         $block_size = mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
         $padding = $block_size - ($infilesize % $block_size);//$padding is a number from 1-16
         fwrite($fc, str_repeat(chr($padding), $padding) );//perform PKCS7 padding
         fclose($fin);
         fclose($fc);
         $hmac_raw = self::calculate_hmac_after_32bytes($password, $hmac_salt, $aes_filename);
         $fc = fopen($aes_filename, "rb+");
         fwrite($fc, $hmac_raw);//overwrite placeholder
         fclose($fc);
      }
   }
   public static function decryptFile($password, $aes_filename, $out_stream) {
      $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
      $hmac_raw = file_get_contents($aes_filename, false, NULL,  0, 32);
      $hmac_salt = file_get_contents($aes_filename, false, NULL, 32, $iv_size);
      $hmac_calc = self::calculate_hmac_after_32bytes($password, $hmac_salt, $aes_filename);
      $fc = fopen($aes_filename, "rb");
      $fout = fopen($out_stream, 'wb');
      if (!empty($fout) && !empty($fc) && self::hash_equals($hmac_raw,$hmac_calc)) {
         fread($fc, 32+$iv_size);//skip sha256 hmac and salt
         $esalt = fread($fc, $iv_size);
         $iv    = fread($fc, $iv_size);
         $ekey = hash_pbkdf2("sha256", $password, $esalt, $it=1000, self::key_size(), $raw=true);
         $opts = array('mode'=>'cbc', 'iv'=>$iv, 'key'=>$ekey);
         stream_filter_append($fc, 'mdecrypt.rijndael-128', STREAM_FILTER_READ, $opts);
         while (!feof($fc)) {
            $block = fread($fc, 8192);
            if (feof($fc)) {
               $padding = ord($block[strlen($block) - 1]);//assume PKCS7 padding
               $block = substr($block, 0, 0-$padding);
            }
            fwrite($fout, $block);
         }
         fclose($fout);
         fclose($fc);
      }
   }
   private static function hash_equals($str1, $str2) {
      if(strlen($str1) == strlen($str2)) {
         $res = $str1 ^ $str2;
         for($ret=0,$i = strlen($res) - 1; $i >= 0; $i--) $ret |= ord($res[$i]);
         return !$ret;
      }
      return false;
   }
   private static function calculate_hmac_after_32bytes($password, $hsalt, $filename) {
      static $init=0;
      $init or $init = stream_filter_register("user-filter.skipfirst32bytes", "FileSkip32Bytes");
      $stream = 'php://filter/read=user-filter.skipfirst32bytes/resource=' . $filename;
      $hkey = hash_pbkdf2("sha256", $password, $hsalt, $iterations=1000, 24, $raw=true);
      return hash_hmac_file('sha256', $stream, $hkey, $raw=true);
   }
}
class FileSkip32Bytes extends php_user_filter
{
   private $skipped=0;
   function filter($in, $out, &$consumed, $closing)  {
      while ($bucket = stream_bucket_make_writeable($in)) {
         $outlen = $bucket->datalen;
         if ($this->skipped<32){
            $outlen = min($bucket->datalen,32-$this->skipped);
            $bucket->data = substr($bucket->data, $outlen);
            $bucket->datalen = $bucket->datalen-$outlen;
            $this->skipped+=$outlen;
         }
         $consumed += $outlen;
         stream_bucket_append($out, $bucket);
      }
      return PSFS_PASS_ON;
   }
}
class AES_128_CBC extends AES_CBC {
   protected static function key_size() { return self::$KEY_SIZES['AES-128']; }
}
class AES_192_CBC extends AES_CBC {
   protected static function key_size() { return self::$KEY_SIZES['AES-192']; }
}
class AES_256_CBC extends AES_CBC {
   protected static function key_size() { return self::$KEY_SIZES['AES-256']; }
}
]]>
    </programlisting>
   </example>
  </section>
 </section>

</appendix>

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