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phpencryptionaesrijndaelcbc-mode

Is it safe to trim a decrypted string?

I am encrypting and decrypting a string using:

$key = 'my key';
$data = 'my string';
$ivSize = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC);
$iv = mcrypt_create_iv($ivSize, MCRYPT_DEV_URANDOM);
$encrypted = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key, $data, MCRYPT_MODE_CBC, $iv);
$decrypted = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $encrypted, MCRYPT_MODE_CBC, $iv);
$data = trim($decrypted, chr(0));

http://codepad.viper-7.com/1JgCRs

Is it safe to just trim off the padding added by the encryption algorithm, or is it necessary to store the length of the data before encrypting?

Solution

  • Padding is added on the right normally, so consider rtrim():

    $data = rtrim($decrypted, chr(0));

    However this is still not yet perfectly safe because in PHP strings can contain NUL-bytes. If for some reason the plain did had NUL-bytes at the end, the rtrim will remove the padding and those previous NUL-bytes.