Try using a different block cipher mode. When encrypting with ECB, as you
said, your plaintext must have a length that is a multiple of the blocksize
for the cipher. If it's anything less, you're going to get some garbage at
the end of the decrypted ciphertext. I believe that mcrypt uses zeroes
padding for ECB mode.
You might want to try one of the other block cipher modes, like CBC or CFB.
This will require that you use random IVs for each encryption, but it's
much more secure than using ECB mode. Even though you create an IV in your
code below, ECB mode ignores it -- if you encrypt the same plaintext
multiple times using the same key, you'll end up with the same ciphertext.
With random IVs and the proper mode, this wouldn't happen. This will
require that you store the IV for each encryption along with the
ciphertext, but that's fine, as storing the IV along with the ciphertext is
not a security problem.
I'm not sure what mcrypt uses for padding on it's various block cipher
modes, but it might help. It'll be more secure than ECB, at any rate.
J
Steve Yates wrote:
> Hello,
>
> I recently implemented a database using MySQL that is storing selected
> fields encrypted. However on a very small number of records the decrypted
> result is not correct for some fields, for example for this credit card
> number:
>
> 9999-999999-9999Fhx
>
> It appears in fact the same way as the problem I first experienced, when
> the database field was not big enough to store the encrypted text (which I
> discovered takes a multiple of the blocksize, so it is usually bigger than
> the original string). However the blocksize is 8 and to provide a safety
> margin all the fields to be encrypted have 10 extra characters in them
> (varchar fields).
>
> So far this happens on at most one field in a record, perhaps on less than
> 5% of the records. At first I was thinking maybe the addslashes() was
> adding text but MySQL should be stripping that out before entering it into
> the database, right? Also I can't seem to duplicate this by entering the
> same values in the form again.
>
> Any suggestions? Here is my encryption code:
>
> $hrkey = '$R^a$nd()M%'; // changed text
> $td = mcrypt_module_open(MCRYPT_TRIPLEDES,'', MCRYPT_MODE_ECB, '');
> $iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($td), 99999999999999);
> //changed the number
> $ks = mcrypt_enc_get_key_size ($td);
> $key = substr(md5($hrkey), 0, $ks);
> mcrypt_generic_init($td, $key, $iv);
> $CreditCardNumber = addslashes(mcrypt_generic($td,
> $_POST['Credit_Card_Number']));
> (...post to database here...)
> mcrypt_module_close($td);
>
> Decryption code:
>
> function mydecrypt($enc) {
> global $td;
> return rtrim(mdecrypt_generic($td, $enc), "\0");
> }
>
> Thanks for any insight!
>
> - Steve Yates
> - ASCII stupid question, get a stupid ANSI.
>
> ~ Taglines by Taglinator - www.srtware.com ~
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