people using phpmyadmin are redirected to this manual if they don't have mcrypt installed. If you want to install mcrypt on debian, first check your php version:

yourserver# php --version

Then install the appropriate version of mcrypt (php5-mcrypt if your php version is 5.x)

yourserver# apt-get install php4-mcrypt
yourserver# apt-get install php5-mcrypt
2010-04-11 08:38:36
If you want a quick way to see what ciphers, modes, key, block and iv sizes are supported on your server, try something like the following. 

Note: I used this simple bash: `locate libmcrypt` from terminal on Mac OS X to determine the install paths to the algorithms and modes directories. Lots of function calls generate warnings for certain ciphers, hence the use of error suppression.


$algorithms mcrypt_list_algorithms();
$algorithms as $cipher)
"<h1 style=\"border-top:1px solid black;\">".$cipher."</h1>\n";
$modes as $mode)
$td mcrypt_module_open(
$key_size mcrypt_enc_get_key_size($td);
$block_size mcrypt_get_block_size($cipher,$mode);
$iv_size mcrypt_get_iv_size($cipher$mode);
            key_size: "
. ($key_size?$key_size:'n/a'
"    block_size: ". ($block_size?$block_size:'n/a'
"    iv_size: ". ($iv_size?$iv_size:'n/a'
"  </pre>\n";

2010-11-23 15:23:41
These are two simple functions I built for 256-bit encryption/decryption with mcrypt.  I've decided to use MCRYPT_RIJNDAEL_128 because it's AES-compliant, and MCRYPT_MODE_CBC.  (ECB mode is inadequate for many purposes because it does not use an IV.)

This function stores a hash of the data to verify that the data was decrypted successfully, but this could be easily removed if necessary.

function encrypt($decrypted$password$salt='!kQm*fF3pXe1Kbm%9') { 
// Build a 256-bit $key which is a SHA256 hash of $salt and $password.
$key hash('SHA256'$salt $passwordtrue);
// Build $iv and $iv_base64.  We use a block size of 128 bits (AES compliant) and CBC mode.  (Note: ECB mode is inadequate as IV is not used.)
srand(); $iv mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128MCRYPT_MODE_CBC), MCRYPT_RAND);
 if (
strlen($iv_base64 rtrim(base64_encode($iv), '=')) != 22) return false;
// Encrypt $decrypted and an MD5 of $decrypted using $key.  MD5 is fine to use here because it's just to verify successful decryption.
$encrypted base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_128$key$decrypted md5($decrypted), MCRYPT_MODE_CBC$iv));
// We're done!
return $iv_base64 $encrypted;

decrypt($encrypted$password$salt='!kQm*fF3pXe1Kbm%9') {
// Build a 256-bit $key which is a SHA256 hash of $salt and $password.
$key hash('SHA256'$salt $passwordtrue);
// Retrieve $iv which is the first 22 characters plus ==, base64_decoded.
$iv base64_decode(substr($encrypted022) . '==');
// Remove $iv from $encrypted.
$encrypted substr($encrypted22);
// Decrypt the data.  rtrim won't corrupt the data because the last 32 characters are the md5 hash; thus any \0 character has to be padding.
$decrypted rtrim(mcrypt_decrypt(MCRYPT_RIJNDAEL_128$keybase64_decode($encrypted), MCRYPT_MODE_CBC$iv), "\0\4");
// Retrieve $hash which is the last 32 characters of $decrypted.
$hash substr($decrypted, -32);
// Remove the last 32 characters from $decrypted.
$decrypted substr($decrypted0, -32);
// Integrity check.  If this fails, either the data is corrupted, or the password/salt was incorrect.
if (md5($decrypted) != $hash) return false;
// Yay!
return $decrypted;
2012-02-13 00:35:15
Note that there are severe performance problems with PHP mcrypt on many CentOS versions. Please see this CentOS bug:
2015-06-24 02:54:42
The Mcrypt library has been declared DEPRECATED since PHP 7.1, to use in its OpenSSL
2016-12-28 13:06:49
This was posted before by another user but has been downvoted. I just wanted to confirm that we suffered massive performance issues related to mcrypt on CentOS (PHP 5.6.32) that are not present in other flavors of Linux. 

A sampling of 25,000 encrypts/decrypts takes 4-5x longer when running mcrypt on Centos 7 as compared to Ubuntu. Switching out mcrypt for OpenSSL on Centos will result in a massive increase in performance. 

For lower traffic website it can be negligible, but when you start seeing significant traffic/load it will quickly bring down a server.
2017-11-14 23:41:27
Issue Solved when installing php7.2-mcrypt

I was also facing the same issue. Check this link to get the right solution for installing it in PHP
2018-04-05 13:33:39
If you're wanting to use mcrypt on a newer version of PHP where it's been deprecated try the shim for it instead:
2019-07-26 00:02:48

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