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>mcrypt_create_iv</H1
><DIV
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><A
NAME="AEN115349"
></A
><P
>    (PHP 3 &#62;= 3.0.8, PHP 4, PHP 5)</P
>mcrypt_create_iv&nbsp;--&nbsp;
     Create an initialization vector (IV) from a random source
    </DIV
><DIV
CLASS="refsect1"
><A
NAME="AEN115352"
></A
><H2
>Description</H2
>string <B
CLASS="methodname"
>mcrypt_create_iv</B
> ( int size [, int source] )<BR
></BR
><P
>&#13;     <B
CLASS="function"
>mcrypt_create_iv()</B
> is used to create an IV.
    </P
><P
>&#13;     Parameter
     <CODE
CLASS="parameter"
>size</CODE
> determines the size of the IV,
     parameter <CODE
CLASS="parameter"
>source</CODE
> (defaults to random value)
     specifies the source of the IV.
    </P
><P
>&#13;     The source can be <TT
CLASS="constant"
><B
>MCRYPT_RAND</B
></TT
> (system random
     number generator), <TT
CLASS="constant"
><B
>MCRYPT_DEV_RANDOM</B
></TT
> (read
     data from <TT
CLASS="filename"
>/dev/random</TT
>) and
     <TT
CLASS="constant"
><B
>MCRYPT_DEV_URANDOM</B
></TT
> (read data from
     <TT
CLASS="filename"
>/dev/urandom</TT
>). <TT
CLASS="constant"
><B
>MCRYPT_RAND</B
></TT
>
     is the only one supported on Windows because Windows (of course)
     doesn't have <TT
CLASS="filename"
>/dev/random</TT
> or
     <TT
CLASS="filename"
>/dev/urandom</TT
>.
    </P
><DIV
CLASS="note"
><BLOCKQUOTE
CLASS="note"
><P
><B
>注意: </B
>
      When using <TT
CLASS="constant"
><B
>MCRYPT_RAND</B
></TT
>, remember to call
      <A
HREF="function.srand.html"
><B
CLASS="function"
>srand()</B
></A
> before
      <B
CLASS="function"
>mcrypt_create_iv()</B
> to initialize the random
      number generator; it is not seeded automatically like
      <A
HREF="function.rand.html"
><B
CLASS="function"
>rand()</B
></A
> is.
     </P
></BLOCKQUOTE
></DIV
><P
>&#13;     <TABLE
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><DIV
CLASS="example"
><A
NAME="AEN115384"
></A
><P
><B
>例 1. <B
CLASS="function"
>mcrypt_create_iv()</B
> example</B
></P
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><TR
><TD
><code><font color="#000000">
<font color="#0000BB">&lt;?php<br />&nbsp;&nbsp;&nbsp;&nbsp;$size </font><font color="#007700">= </font><font color="#0000BB">mcrypt_get_iv_size</font><font color="#007700">(</font><font color="#0000BB">MCRYPT_CAST_256</font><font color="#007700">, </font><font color="#0000BB">MCRYPT_MODE_CFB</font><font color="#007700">);<br />&nbsp;&nbsp;&nbsp;&nbsp;</font><font color="#0000BB">$iv </font><font color="#007700">= </font><font color="#0000BB">mcrypt_create_iv</font><font color="#007700">(</font><font color="#0000BB">$size</font><font color="#007700">, </font><font color="#0000BB">MCRYPT_DEV_RANDOM</font><font color="#007700">);<br /></font><font color="#0000BB">?&gt;</font>
</font>
</code></TD
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></TD
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>
    </P
><P
>&#13;     The IV is only meant to give an alternative seed to the encryption
     routines. This IV does not need to be secret at all, though it can be
     desirable. You even can send it along with your ciphertext without
     losing security.
    </P
><P
>&#13;     More information can be found at <A
HREF="http://www.ciphersbyritter.com/GLOSSARY.HTM#IV"
TARGET="_top"
>http://www.ciphersbyritter.com/GLOSSARY.HTM#IV</A
>, <A
HREF="http://fn2.freenet.edmonton.ab.ca/~jsavard/crypto/co0409.htm"
TARGET="_top"
>http://fn2.freenet.edmonton.ab.ca/~jsavard/crypto/co0409.htm</A
> and in chapter 9.3 of
     Applied Cryptography by Schneier (ISBN 0-471-11709-9) for a discussion of
     this topic.
    </P
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