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


int mcrypt_get_iv_size ( resource td)

int mcrypt_get_iv_size ( string cipher, string mode)

The first prototype is when linked against libmcrypt 2.2.x, the second when linked against libmcrypt 2.4.x or higher.

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.

td is the resource that is returned by mcrypt_module_open().

Example 1. mcrypt_create_iv() example

    $size = mcrypt_get_iv_size (MCRYPT_CAST_256, MCRYPT_MODE_CFB);

    $size = mcrypt_get_iv_size ('des', 'ecb');

See also mcrypt_get_block_size(), and mcrypt_create_iv().

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