PHP Manual: mdecrypt

Description

string mdecrypt_generic ( resource td, string data )

This function decrypts data. Note that the length of the returned string can in fact be longer then the unencrypted string, due to the padding of the data.

Example 1. mdecrypt_generic() example

<?php
    /* Data */
    $key = 'this is a very long key, even too long for the cipher';
    $plain_text = 'very important data';
   
    /* Open module, and create IV */ 
    $td = mcrypt_module_open('des', '', 'ecb', '');
    $key = substr($key, 0, mcrypt_enc_get_key_size($td));
    $iv_size = mcrypt_enc_get_iv_size($td);
    $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);

    /* Initialize encryption handle */
    if (mcrypt_generic_init($td, $key, $iv) != -1) {

        /* Encrypt data */
        $c_t = mcrypt_generic($td, $plain_text);
        mcrypt_generic_deinit($td);

        /* Reinitialize buffers for decryption */
        mcrypt_generic_init($td, $key, $iv);
        $p_t = mdecrypt_generic($td, $c_t);

        /* Clean up */
        mcrypt_generic_deinit($td);
        mcrypt_module_close($td);
    }

    if (strncmp($p_t, $plain_text, strlen($plain_text)) == 0) {
        echo "ok\n";
    } else {
        echo "error\n";
    }
?>

The above example shows how to check if the data before the encryption is the same as the data after the decryption. It is very important to reinitialize the encryption buffer with mcrypt_generic_init() before you try to decrypt the data.

The decryption handle should always be initialized with mcrypt_generic_init() with a key and an IV before calling this function. Where the encryption is done, you should free the encryption buffers by calling mcrypt_generic_deinit(). See mcrypt_module_open() for an example.