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PHP Manual

# mcrypt_create_iv

(PHP 4, PHP 5)

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.

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 #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 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.

mcrypt_cfb
mcrypt_decrypt
Mcrypt Functions
PHP Manual