Sending an mcrypt-encrypted string via a URL parameter - decoded text is mangled

Question

I'm messing with a simple authorization scheme. I think the easiest way to do it without SSL or other HTTP auth would be a shared key encryption. Adapting a simple example from the PHP manual, I came up with the following:

$text = "boggles the invisible monkey will rule the world";
$key = "This is a very secret key";

$iv_size = mcrypt_get_iv_size(MCRYPT_BLOWFISH, MCRYPT_MODE_ECB);
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);

$enc = mcrypt_encrypt(MCRYPT_BLOWFISH, $key, $text, MCRYPT_MODE_ECB, $iv);

$iv = base64_encode($iv);
$enc = base64_encode($enc);

echo '<a href="temp2.php?iv='.$iv.'&text='.$enc.'">link</a><br />';

The page that receives this request (temp2.php) looks like this:

$key = "This is a very secret key";

$iv = base64_decode($_GET["iv"]);
$enc = base64_decode($_GET["text"]);

$crypttext = mcrypt_decrypt(MCRYPT_BLOWFISH, $key, $enc, MCRYPT_MODE_ECB, $iv);

echo "$crypttext<br>";

This gets very close, but it doesn't decode properly-- it echoes

boggles the invisible monkey will rule t—;eôügJë

I'm not sure what the hangup is, I tried urlencode/urldecode and htmlentities, thinking maybe a character was mangled in the request, but no difference.

Is there something else I'm missing? Perhaps padding?

Thanks

Answer 1

It happens that your encrypted text will end up looking something like:

Z5DlBqT8yEB4HKLkAlvfJoWaHgnNVLFh7jls6L85sLriedc82uFQxm+M62I41oB7

See that plus sign there? Plus signs in URLs get turned into spaces.

When manually converting that plus sign to a space and decrypting the result, the output is the corrupted junk that you've witnessed.

You should run both the IV and the encrypted text through rawurlencode (not urlencode) before sticking it into the link. This will properly encode the plus sign, which will preserve it through the process. You don't need to (and should not) urldecode the string on the other side -- PHP will have done this for you.