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mcrypt_create_iv

Create an initialization vector (IV) from a random source (PHP 4, PHP 5)
string mcrypt_create_iv ( int size [, int source] )

mcrypt_create_iv() is used to create an IV.

Parameter size determines the size of the IV, parameter source (defaults to random value) 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). MCRYPT_RAND is the only one supported on Windows because Windows (of course) doesn't have /dev/random or /dev/urandom.

Note:

When using MCRYPT_RAND, remember to call srand() before mcrypt_create_iv() to initialize the random number generator; it is not seeded automatically like rand() is.

Example 1290. 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 losing 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.