Filtres de chiffrement

mcrypt.* et mdecrypt.* fournissent un système de chiffrement symétrique basé sur libmcrypt. Les deux filtres supportent les mêmes algorithmes disponibles avec la bibliothèque mcrypt, sous la forme de la directive mcrypt.ciphernameciphername est le nom du chiffrement qui sera passé à mcrypt_module_open(). Les cinq paramètres suivants sont aussi disponibles :

Paramètres du filtre mcrypt
Paramètre Obligatoire Par défaut Valeurs possibles
mode Optionnel cbc cbc, cfb, ecb, nofb, ofb, stream
algorithms_dir Optionnel ini_get('mcrypt.algorithms_dir') Chemin vers les algorithmes de modules
modes_dir Optionnel ini_get('mcrypt.modes_dir') Chemin vers les modes de modules
iv Obligatoire N/A Généralement 8, 16 ou 32 octets de données binaires. Dépend du chiffrement
key Obligatoire N/A Généralement 8, 16 ou 32 octets de données binaires. Dépend du chiffrement

Exemple #1 Chiffrement d'un fichier avec 3DES

<?php
$passphrase 
'My secret';

/* Transforme un mot de passe humain
 * en une paire iv/clé
 */
$iv substr(md5('iv'.$passphrasetrue), 08);
$key substr(md5('pass1'.$passphrasetrue) . 
               
md5('pass2'.$passphrasetrue), 024);
$opts = array('iv'=>$iv'key'=>$key);

$fp fopen('secret-file.enc''wb');
stream_filter_append($fp'mcrypt.tripledes'STREAM_FILTER_WRITE$opts);
fwrite($fp'Secret secret secret data');
fclose($fp);
?>

Exemple #2 Lecture d'un fichier chiffré

<?php
$passphrase 
'My secret';

/* Transforme un mot de passe humain
 * en une paire iv/clé
 */
$iv substr(md5('iv'.$passphrasetrue), 08);
$key substr(md5('pass1'.$passphrasetrue) . 
               
md5('pass2'.$passphrasetrue), 024);
$opts = array('iv'=>$iv'key'=>$key);

$fp fopen('secret-file.enc''rb');
stream_filter_append($fp'mdecrypt.tripledes'STREAM_FILTER_READ$opts);
$data rtrim(stream_get_contents($fp));
fclose($fp);

echo 
$data;
?>
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User Contributed Notes 2 notes

up
5
Uwe Ohse
6 months ago
1) The mcrypt module has been deprecated with PHP 7.1.
What does that mean for the encryption filters?

2) deriving the IV from the secret passphrase seems wrong. This leaks information about the passphrase without need.

3) using md5() for this? Really? hash() has been available since PHP 5.1.2.

4) hashing the passphrase twice does not add any security.

5) using substr() on binary data (why, oh why, is true passed to md5()?) might lead to headaches if mbstring.func_overload is used.
up
0
Maarten Bodewes
1 month ago
The examples on this page should be destroyed with utmost urgency.

Strangely most people will fall over the use of generating the IV and 3DES key using MD5, a weak hash function, e.g. the previous note and CryptoFails blog.

http://www.cryptofails.com/post/70059608390/php-documentation-woes

A password based key derivation function should be used (bcrypt, PBKDF2).

However the use of MD-5 for key derivation however isn't that bad and if the password is strong enough (it usually isn't) then the generated DES ABC key is strong enough even now.

Using an identical IV for each password means that this function directly leaks information about the encrypted file. If the start of two encrypted files is identical then this function will directly leak information. For instance, if the routine encrypts multiple images then the JPEG header would be easily distinguished.

All in all these examples use deprecated routines (mcrypt), deprecated cryptographic functions (MD5 / DES) and then incorrectly perform the actual encryption. Enough reason to destroy them in the first place.
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