   #PHP Manual Mcrypt Encryption Functions mcrypt_cfb mcrypt_decrypt

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                               mcrypt_create_iv

   (PHP 3>= 3.0.8, PHP 4 )
   mcrypt_create_iv --  Create an initialization vector (IV) from a
   random source

Description

   string mcrypt_create_iv ( int size, int source)

   mcrypt_create_iv() is used to create an IV.

   mcrypt_create_iv() takes two arguments, size determines the size of
   the IV, source specifies the source of the IV.

   The source can be MCRYPT_RAND (system random number generator),
   MCRYPT_DEV_RANDOM (read data from /dev/random) and MCRYPT_DEV_URANDOM
   (read data from /dev/urandom). If you use MCRYPT_RAND, make sure to
   call srand() before to initialize the random number generator.

   Example 1. mcrypt_create_iv() example
   <?php
       $size = mcrypt_get_iv_size(MCRYPT_CAST_256, MCRYPT_MODE_CFB);
       $iv = mcrypt_create_iv($size, MCRYPT_DEV_RANDOM);
   ?>

   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
   loosing security.

   More information can be found at
   http://www.ciphersbyritter.com/GLOSSARY.HTM#IV,
   http://fn2.freenet.edmonton.ab.ca/~jsavard/crypto/co0409.htm and in
   chapter 9.3 of Applied Cryptography by Schneier (ISBN 0-471-11709-9)
   for a discussion of this topic.
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