php-doc-en/reference/mcrypt/functions/mcrypt-get-iv-size.xml

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  <refentry id="function.mcrypt-get-iv-size">
   <refnamediv>
    <refname>mcrypt_get_iv_size</refname>
    <refpurpose>
     Returns the size of the IV belonging to a specific cipher/mode combination
    </refpurpose>
   </refnamediv>
   <refsect1>
    <title>Description</title>
     <methodsynopsis>
      <type>int</type><methodname>mcrypt_get_iv_size</methodname>
      <methodparam><type>string</type><parameter>cipher</parameter></methodparam>
      <methodparam><type>string</type><parameter>mode</parameter></methodparam>
     </methodsynopsis>
    <para>
     <function>mcrypt_get_iv_size</function> returns the size of
     the Initialisation Vector (IV) in bytes. On error the function
     returns &false;. If the IV is ignored in the specified cipher/mode
     combination zero is returned.
    </para>
    <para>
     <parameter>cipher</parameter> is one of the MCRYPT_ciphername
     constants of the name of the algorithm as string.
    </para>
    <para>
     <parameter>mode</parameter> is one of the MCRYPT_MODE_modename constants
     or one of "ecb", "cbc", "cfb", "ofb", "nofb" or "stream". The IV is
     ignored in ECB mode as this mode does not require it.  You will need to
     have the same IV (think: starting point) both at encryption and
     decryption stages, otherwise your encryption will fail.
    </para>
    <para>
     It is more useful to use the <function>mcrypt_enc_get_iv_size</function>
     function as this uses the resource returned by
     <function>mcrypt_module_open</function>.
    </para>
    <para>
     <example>
      <title><function>mcrypt_get_iv_size</function> example</title>
      <programlisting role="php">
<![CDATA[
<?php
    echo mcrypt_get_iv_size(MCRYPT_CAST_256, MCRYPT_MODE_CFB) . "\n";

    echo mcrypt_get_iv_size('des', 'ecb') . "\n";
?>
]]>
      </programlisting>
     </example>
    </para>
    <para>
     See also 
     <function>mcrypt_get_block_size</function>,
     <function>mcrypt_enc_get_iv_size</function> and
     <function>mcrypt_create_iv</function>.
    </para>
   </refsect1>
  </refentry>

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