php man page for mcrypt_create_iv

MCRYPT_CREATE_IV(3)							 1						       MCRYPT_CREATE_IV(3)

mcrypt_create_iv - Creates an initialization vector (IV) from a random source

SYNOPSIS
       string mcrypt_create_iv (int  $size, [int  $source = MCRYPT_DEV_URANDOM])

DESCRIPTION
	Creates an initialization vector (IV) from a random source.

	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.

PARAMETERS
	      o $size
		- The size of the IV.

	      o $source
		- 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). Prior to 5.3.0, MCRYPT_RAND was the only one supported on Win-
		dows.  Note that the default value of this parameter was MCRYPT_DEV_RANDOM prior to PHP 5.6.0.

	      Note

		      Note that MCRYPT_DEV_RANDOM may block until more entropy is available.

RETURN VALUES
	Returns the initialization vector, or FALSE on error.

CHANGELOG
       +--------+---------------------------------------------------+
       |Version |						    |
       |	|						    |
       |	|		     Description		    |
       |	|						    |
       +--------+---------------------------------------------------+
       | 5.6.0	|						    |
       |	|						    |
       |	|						    |
       |	| MCRYPT_DEV_URANDOM is now the  default  value  of |
       |	| $source.					    |
       |	|						    |
       | 5.3.0	|						    |
       |	|						    |
       |	|						    |
       |	| MCRYPT_DEV_RANDOM  and  MCRYPT_DEV_URANDOM became |
       |	| available on Windows platforms.		    |
       |	|						    |
       | 5.3.0	|						    |
       |	|						    |
       |	|  It is no longer required to call srand(3) first. |
       |	| This is now done automatically.		    |
       |	|						    |
       +--------+---------------------------------------------------+
EXAMPLES
       Example #1

	      mcrypt_create_iv(3) Example

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

SEE ALSO
       http://www.ciphersbyritter.com/GLOSSARY.HTM#IV, http://www.quadibloc.com/crypto/co0409.htm, Chapter 9.3 of Applied Cryptography by Schneier
       (ISBN 0-471-11709-9), random_bytes(3).

PHP Documentation Group 												       MCRYPT_CREATE_IV(3)