On Mon, Jun 07, 2010 at 11:59:14PM -0400, Adam Richardson wrote:
> On Mon, Jun 7, 2010 at 11:20 PM, Paul M Foster <paulf@xxxxxxxxxxxxxxxxx> wrote:
>
> I've got a file of passwords I'd like to encrypt/decrypt using blowfish.
> I'd
> like to be able to do so with PHP and via the command line. I have a
> Linux utility call "bcrypt" which encrypts/decrypts files using
> blowfish. And I'm using the following code under PHP to do
> encryption/decryption:
>
> $raw_data = file('junk');
> $input = implode('', $raw_data);
>
> $td = mcrypt_module_open(MCRYPT_BLOWFISH, '', MCRYPT_MODE_ECB, '');
> $iv = mcrypt_create_iv (mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
> mcrypt_generic_init($td, $key, $iv);
> $encrypted_data = mcrypt_generic($td, $input);
> file_put_contents('encjunk', $encrypted_data);
> mcrypt_generic_deinit($td);
> mcrypt_module_close($td);
>
>
> Now, here's the problem. I'm using bcrypt to encrypt my junk file and
> dump it out to an encrypted file. And I'm using the above PHP code to
> encrypt the same file out to a different file. Using the same keys in
> both cases, I get different encrypted files.
>
> My logic: using the same encryption method and the same key, two
> different implementations should produce equivalent files.
>
> Yet they don't. I'm guessing that the "initialization vector" is
> different between the two implementations, resulting in the difference
> between the encrypted files.
>
> For those who know more about encryption than I do, does that sound
> right?
>
> Paul
>
> --
> Paul M. Foster
>
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>
>
>
> ECB (Electronic Code Book) mode works without an initialization vector.
> Imagine having a big, thick code book, and every possible 8-letter combination
> in the book. To encrypt the message, you thumb through the book and find the
> plain text "THE KIDS", and write down its cipher text representation,
> "JKWSCTFI." Easy to implement, and easy to maintain (you don't have to make
> sure an IV is shared between the exchanging parties.)
Good explanation. Thanks.
>
> If one has multiple samples of encrypted emails, it's likely that the several
> of the samples will end using the same cipher text, as many people end their
> emails with a consistent signature. This repeated cipher text improves the
> ability of those trying to attack (decrypt the message.) Hence, most
> professionals recommend avoiding ECB mode.
Well, bcrypt mentions CBC in the (brief) documentation, but the C code
contains a couple of tables, one containing probably 1024 32-bit long
integer values. Based on your description, that sounds like ECB, right?
Bcrypt doesn't all the specification of what mode enc/dec is done in.
That is, I can't specify to the program ECB, CBC or other.
Also, according to the docs for bcrypt, it hashes your password out to
he maximum size for the cipher (448 bytes?). This sounds like an
implementation-specific decision which may not be echoed by PHP's mcrypt
functions. Does that sound reasonable?
>
> Now, looking at your PHP code, I see that it appears your mixing and matching
> some of the families of calls in ways that might lead to unexpected results.
> Try the below:
>
> $ciphertext = mcrypt_encrypt(
> $cipher = MCRYPT_BLOWFISH,
> $key,
> $plaintext,
> $mode = 'cbc', // I just tossed this in as an example, but you should match
> the mode bcrypt is using
> $iv = 'use only once, sometimes a count, or a date' // needed for
> decryption, too, although it doesn't have to remain a secret.
> );
Another point: my code above is actually from a post by someone else on
this list. Now, the iv above is based on a random number. If I encrypt
the file on Monday, and then attempt to decrypt it on Tuesday using a
different (random-number-based) iv, will the file decrypt properly?
Paul
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Paul M. Foster
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