Should I Trim the Decrypted String after mcrypt_decrypt?

Question

I have a code that goes something like:

$cipher_alg = MCRYPT_RIJNDAEL_128;
$decrypted_string = mcrypt_decrypt($cipher_alg, $key, 
$encrypted_string , MCRYPT_MODE         


        

Answer 1

In my implementation of TripleDES, I found the decrypted string was padded with \5 or \6 characters. This wasn't the expected \0 or \4 characters mentioned above or in the PHP.net examples. To determine the ASCII value of the padding character use the ord() function. ord() works on a single character so use str_split() to break up a string or access the character directly with array notation - $string[5].

Final trim result - trim($decrypt, "\0..\32");

Final code result -

    $key        = "encryption key";
    $encrypt    = base64_decode($encrypt);
    $iv_size    = mcrypt_get_iv_size(MCRYPT_3DES, MCRYPT_MODE_ECB);
    $iv         = mcrypt_create_iv($iv_size, MCRYPT_RAND);
    $decrypt    = mcrypt_decrypt(MCRYPT_3DES, $key, $encrypt, MCRYPT_MODE_ECB, $iv);
    $final      = trim($decrypt, "\0..\32"); // removes potential null padding