Creates an initialization vector (IV) from a random source

# mcrypt_create_iv

(PHP 4, PHP 5, PHP 7 < 7.2.0, PECL mcrypt >= 1.0.0)

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

警告

この関数は PHP 7.1.0 で 非推奨 となり、 PHP 7.2.0 で削除 されました。

この関数の代替として、これらが使えます。

- random_bytes()

### 説明

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

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.

### パラメータ

`size`

  The size of the IV.

`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 Windows.

  Note that the default value of this parameter was `MCRYPT_DEV_RANDOM` prior to PHP 5.6.0.

  > 注意: Note that `MCRYPT_DEV_RANDOM` may block until more entropy is available.

### 返り値

Returns the initialization vector, or `FALSE` on error.

### 変更履歴

バージョン | 説明
--- | ---
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() first. This is now done automatically.

### 例

例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);
?>`

### 参考

- » 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() - Generates cryptographically secure pseudo-random bytes