I have an issue when communicating encrypted between iOS and PHP. I have an app that encrypts a string and sends it to a PHP server that decrypts it. That part works just fine. Now the PHP server needs to send an encrypted response back to the app, which seems to be causing a bit more gray hair.
The issue is, that when I encrypt a string in PHP it looks different from the same string encrypted in iOS and even .NET - obviously all places use the same algorithm, key and IV.
I use Rijndael 128 in CBC mode with an IV consisting of empty bytes (so far).
The PHP encryption looks so:
$encrypted = mcrypt_encrypt( MCRYPT_RIJNDAEL_128, $this->secret_key, $str, MCRYPT_MODE_CBC, $this->iv );
$encrypted = base64_encode( $encrypted );
The iOS encryption is attached in this file:
StringEncryption.m: http://pastie.org/1365766
I hope someone can help me spot where I'm missing something or have some different parameters of values. I have looked at this for several hours, and can't find anything else to try.
Most likely it's a padding issue... Please see here or here for more information.
EDIT after OP comment:
PHP has no built-in support for other padding modes than the NULL-padding. At least .Net allows you to specify NULL-padding (I think), the other option would be to implement PKCS#7-padding in PHP which is not that difficult to do.
Pad the input with a padding string of between 1 and 8 bytes to make the total length an exact multiple of 8 bytes. The value of each byte of the padding string is set to the number of bytes added - i.e. 8 bytes of value 0x08, 7 bytes of value 0x07, ..., 2 bytes of 0x02, or one byte of value 0x01.
$blockSize = mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
$padding = $blockSize - (strlen($data) % $blockSize);
$data .= str_repeat(chr($padding), $padding);