PHP: mcrypt_module_open

Description

resource mcrypt_module_open ( string algorithm, string algorithm_directory, string mode, string mode_directory )

This function opens the module of the algorithm and the mode to be used. The name of the algorithm is specified in algorithm, e.g. "twofish" or is one of the MCRYPT_ciphername constants. The module is closed by calling mcrypt_module_close(). Normally it returns an encryption descriptor, or FALSE on error.

The algorithm_directory and mode_directory are used to locate the encryption modules. When you supply a directory name, it is used. When you set one of these to the empty string (""), the value set by the mcrypt.algorithms_dir or mcrypt.modes_dir ini-directive is used. When these are not set, the default directories that are used are the ones that were compiled in into libmcrypt (usually /usr/local/lib/libmcrypt).

Пример 1. mcrypt_module_open() examples

<?php $td = mcrypt_module_open(MCRYPT_DES, '', MCRYPT_MODE_ECB, '/usr/lib/mcrypt-modes'); $td = mcrypt_module_open('rijndael-256', '', 'ofb', ''); ?>

The first line in the example above will try to open the DES cipher from the default directory and the EBC mode from the directory /usr/lib/mcrypt-modes. The second example uses strings as name for the cipher and mode, this only works when the extension is linked against libmcrypt 2.4.x or 2.5.x.

Пример 2. Using mcrypt_module_open() in encryption

<?php /* Open the cipher */ $td = mcrypt_module_open('rijndael-256', '', 'ofb', ''); /* Create the IV and determine the keysize length, used MCRYPT_RAND * on Windows instead */ $iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($td), MCRYPT_DEV_RANDOM); $ks = mcrypt_enc_get_key_size($td); /* Create key */ $key = substr(md5('very secret key'), 0, $ks); /* Intialize encryption */ mcrypt_generic_init($td, $key, $iv); /* Encrypt data */ $encrypted = mcrypt_generic($td, 'This is very important data'); /* Terminate encryption handler */ mcrypt_generic_deinit($td); /* Initialize encryption module for decryption */ mcrypt_generic_init($td, $key, $iv); /* Decrypt encrypted string */ $decrypted = mdecrypt_generic($td, $encrypted); /* Terminate decryption handle and close module */ mcrypt_generic_deinit($td); mcrypt_module_close($td); /* Show string */ echo trim($decrypted) . "\n"; ?>

mcrypt_module_open

Mon
02-Mar-2006 03:27


In the text example:



$key = substr(md5('very secret key'), 0, $ks);



Builds a key of $ks/2 effective bytes.

31-Jul-2003 08:14


Doing a trim($decrypted) will remove the null padding that may occur as a result of decryption.



The problem is if you're encrypting something like a MSWord document which can commonly end with nulls. The result $decrypted will be smaller than the original cleartext - which will then fail to open in MSOffice.



To get around this, make sure you store the length of the original cleartext, and when you decrypt it, do:



$decrypted = substr(mdecrypt_generic($td, $encrypted), 0, $originalLength);