# mcrypt_get_iv_size

(PHP 4 >= 4.0.2, PHP 5)

mcrypt_get_iv_size — Returns the size of the IV belonging to a specific cipher/mode combination

### Description

int mcrypt_get_iv_size ( string `$cipher` , string `$mode` )

mcrypt_get_iv_size() returns the size of the Initialisation Vector (IV) in bytes. On error the function returns `FALSE`. If the IV is ignored in the specified cipher/mode combination zero is returned.

`cipher` is one of the MCRYPT_ciphername constants of the name of the algorithm as string.

`mode` is one of the MCRYPT_MODE_modename constants or one of "ecb", "cbc", "cfb", "ofb", "nofb" or "stream". The IV is ignored in ECB mode as this mode does not require it. You will need to have the same IV (think: starting point) both at encryption and decryption stages, otherwise your encryption will fail.

It is more useful to use the mcrypt_enc_get_iv_size() function as this uses the resource returned by mcrypt_module_open().

Example#1 mcrypt_get_iv_size() example

`<?php
echo mcrypt_get_iv_size(MCRYPT_CAST_256, MCRYPT_MODE_CFB) . "\n";
echo mcrypt_get_iv_size('des', 'ecb') . "\n";
?>`

See also mcrypt_get_block_size(), mcrypt_enc_get_iv_size() and mcrypt_create_iv().