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# mcrypt_create_iv

(PHP 3 >= 3.0.8, 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.

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