Encryption Filters

mcrypt.* and mdecrypt.* provide symmetric encryption and decryption using libmcrypt. Both sets of filters support the same algorithms available to mcrypt extension in the form of mcrypt.ciphername where ciphername is the name of the cipher as it would be passed to mcrypt_module_open(). The following five filter parameters are also available:

mcrypt filter parameters
Parameter Required? Default Sample Values
mode Optional cbc cbc, cfb, ecb, nofb, ofb, stream
algorithms_dir Optional ini_get('mcrypt.algorithms_dir') Location of algorithms modules
modes_dir Optional ini_get('mcrypt.modes_dir') Location of modes modules
iv Required N/A Typically 8, 16, or 32 bytes of binary data. Depends on cipher
key Required N/A Typically 8, 16, or 32 bytes of binary data. Depends on cipher

Przykład #1 Encrypting file output using 3DES

<?php
$passphrase 
'My secret';

/* Turn a human readable passphrase
 * into a reproducable iv/key pair
 */
$iv substr(md5('iv'.$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);
?>

Przykład #2 Reading an encrypted file

<?php
$passphrase 
'My secret';

/* Turn a human readable passphrase
 * into a reproducable iv/key pair
 */
$iv substr(md5('iv'.$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
7 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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