php-doc-en/reference/mcrypt/functions/mdecrypt-generic.xml

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  <refentry id="function.mdecrypt-generic">
   <refnamediv>
    <refname>mdecrypt_generic</refname>
    <refpurpose>This function decrypts data</refpurpose>
   </refnamediv>
   <refsect1>
    <title>Description</title>
     <methodsynopsis>
      <type>string</type><methodname>mdecrypt_generic</methodname>
      <methodparam><type>resource</type><parameter>td</parameter></methodparam>
      <methodparam><type>string</type><parameter>data</parameter></methodparam>
     </methodsynopsis>
    <para>
     This function decrypts data. Note that the length of the returned string
     can in fact be longer then the unencrypted string, due to the padding of
     the data.
    </para>
    <para>
     <example>
      <title><function>mdecrypt_generic</function> Example</title>
      <programlisting role="php">
<![CDATA[
<?php
    /* Data */
    $key = 'this is a very long key, even too long for the cipher';
    $plain_text = 'very important data';
   
    /* Open module, and create IV */ 
    $td = mcrypt_module_open ('des', '', 'ecb', '');
    $key = substr ($key, 0, mcrypt_enc_get_key_size ($td));
    $iv_size = mcrypt_enc_get_iv_size ($td);
    $iv = mcrypt_create_iv ($iv_size, MCRYPT_RAND);

    /* Initialize encryption handle */
    if (mcrypt_generic_init ($td, $key, $iv) != -1) {

        /* Encrypt data */
        $c_t = mcrypt_generic ($td, $plain_text);
        mcrypt_generic_deinit ($td);

        /* Reinitialize buffers for decryption */
        mcrypt_generic_init ($td, $key, $iv);
        $p_t = mdecrypt_generic ($td, $c_t);

        /* Clean up */
        mcrypt_generic_deinit ($td);
        mcrypt_module_close ($td);
    }

    if (strncmp ($p_t, $plain_text, strlen($plain_text)) == 0) {
        echo "ok\n";
    } else {
        echo "error\n";
    }
?>
]]>
      </programlisting>
     </example>
    </para>
    <para>
     The above example shows how to check if the data before the encryption is
     the same as the data after the decryption. It is very important to
     reinitialize the encryption buffer with
     <function>mcrypt_generic_init</function> before you try to decrypt the
     data.
    </para>
    <para>
     The decryption handle should alwayws be initialized with
     <function>mcrypt_generic_init</function> with a key and an IV before
     calling this function. Where the encryption is done, you should free the
     encryption buffers by calling <function>mcrypt_generic_deinit</function>.
     See <function>mcrypt_module_open</function> for an example.
    </para>
    <para>
     See also <function>mcrypt_generic</function>,
     <function>mcrypt_generic_init</function> and
     <function>mcrypt_generic_deinit</function>.
    </para>
   </refsect1>
  </refentry>

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