mcrypt_get_iv_size

(PHP 4 >= 4.0.2, PHP 5, PHP 7)

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

int mcrypt_get_iv_size ( string $cipher, string $mode )

Gets the size of the IV belonging to a specific cipher/mode combination.

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

Parameters:

cipher

One of the MCRYPT_ciphername constants, or the name of the algorithm as string.

mode

One of the MCRYPT_MODE_modename constants, or one of the following strings: "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.

Returns:

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

Examples:

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";
?>