mcrypt messing with data

Question

Im trying to make a crypt/decrypt routine using mcrypt but it seems to mess the data im trying to encrypt.

Heres the code:

 $data = 'Some important data'; $key = "mycryptKey"; $cipher = "rijndael-128"; $encryptedData = mcrypt_encrypt($cipher, $key, $data, MCRYPT_MODE_ECB); $decryptedData = mcrypt_decrypt($cipher, $key, $encryptedData, MCRYPT_MODE_ECB); var_dump($data); //string 'Some important data' (length=19) var_dump($encryptedData); //string '™ì{*¾xv-&n5’Œü½Ýëc®n)mSƒ7]' (length=32) var_dump($decryptedData); //string 'Some important data�������������' (length=32)

It keeps adding those characters at the end of the original string. I saw an example at How do you Encrypt and Decrypt a PHP String? but it didn't work

Thats the actual test. The key and data I'm using are the same posted here

Edit

I realized, after @jonhopkins comment, that mcrypt was padding some "\0" characters after $data content, so i clean it up after decryption using 'strtok':

$decryptedData = \strtok( mcrypt_decrypt($cipher, $key, $encryptedData, MCRYPT_MODE_ECB), "\0" ); var_dump($decryptedData); //string 'Some important data' (length=19)

If I had to guess, I would guess that the encryption method is automatically padding the string. IIRC rijndael-128 requires its input to be of length that is a multiple of 16, so mcrypt_encrypt is padding with 0s up to 32 bytes, but I suppose mcrypt_decrypt is unable to determine the original length so it is leaving those extra bytes. — jonhopkins
– jonhopkins, Commented Feb 6, 2015 at 3:34
You are a genius @jonhopkins! But heres another question: how im suposed to return the same data if i have to modify it before passing to mcrypt? — CarlosCarucce
– CarlosCarucce, Commented Feb 6, 2015 at 3:50
That is a great question that I wish I knew the answer to... I'm not even 100% sure that's what's happening. But if I can find anything I'll let you know. edit: stackoverflow.com/q/20507050 this might help — jonhopkins
– jonhopkins, Commented Feb 6, 2015 at 3:54
Got the answer: leaseweblabs.com/2014/02/aes-php-mcrypt-key-padding and updated my question. Thanks again @jonhopkins — CarlosCarucce
– CarlosCarucce, Commented Feb 6, 2015 at 4:14

Community · Accepted Answer · 2017-05-23 12:12:51Z

What you're missing here is the padding; the data to be encrypted must be padded first:

$data = pkcs7_pad($data, 16);

Likewise, after decryption you need to reverse the padding:

$decryptedData = pkcs7_unpad($decryptedData, 16);

Functions used:

function pkcs7_unpad($data) { return substr($data, 0, -ord($data[strlen($data) - 1])); } function pkcs7_pad($data, $size) { $length = $size - strlen($data) % $size; return $data . str_repeat(chr($length), $length); }

Also, it's important to note that the mcrypt extension is rather old and not well maintained; I would recommend switching to OpenSSL instead. See my earlier answer for a full example.

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