Description:
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I work with mcrypt to encrypt the string by Triple DES. But I meet with the key length problem. 
I use a 24BYTE key, the key is(HEX)3FD3A3DABD10B0FF6EAFB5A0103D386EAF6E3F8CAED6CD93 

After executing a instance, 
the system reports the following error: 
mcrypt_generic_init(): Key size too large; supplied length: 48, max: 24 in 
and cut off the half of the original key to calculate the 3des result. 
And I find there exists the same problem in php.net' online help-- the 2nd example provided in http://www.php.net/manual/en/ref.mcrypt.php.

Reproduce code:
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$strSource = "http://www.php.net/test.php";
$strKey = "3FD3A3DABD10B0FF6EAFB5A0103D386EAF6E3F8CAED6CD93"; 

$td = mcrypt_module_open (MCRYPT_3DES, '', 'ecb', ''); 
$iv = mcrypt_create_iv (mcrypt_enc_get_iv_size ($td), MCRYPT_RAND); 
mcrypt_generic_init ($td, $strKey, $iv); 
$strCode = mcrypt_generic ($td, $strSource); 
mcrypt_generic_end ($td); 
echo base64_encode($strCode);

Expected result:
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w6e8c9Tp0/PejfYYvgJJu3cHUXYg29CQAthGmi480Ng=

Actual result:
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LMke4PuG37H9vP5gvRoVwQkX0hZrtfE9NB/az+lSLcc=