Date: Fri, 30 Jul 2004 11:43:31 +0200
From: Daniel Struck <struck.d@retrovirology.lu>
To: Bruno Wolff III <bruno@wolff.to>
Cc: Greg Stark <gsstark@mit.edu>,
	Mitch Pirtle <mitchy@spacemonkeylabs.com>, pgsql-php@postgresql.org
Subject: Re: [ADMIN] Secure DB Systems - How to
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In-Reply-To: <20040729180010.GA23523@wolff.to>
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	<20040727134133.676ab9cf@localhost>
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> IVs act to make the key appear longer. This is especially useful when hum=
ans
> are picking passphrases that are used to generate the key. If you control
> what the actual keys are, then you can make dictionary attacks impractica=
l.
> However, there still would be the problem that identical items in the
> database would be identical. Which, depnding on your application, might
> be a problem because of information leakage.

I don't think this is true for CBC mode. Here the first block of plaintext =
is XORed with the IV then encrypted with a block ciper like AES for example=
. The next plaintext block is than XORed with the previous cyphertext and s=
o on.

So IV in CBC is used to make the encrypted cyphertext unique.

You can find some info in chapter 9.3 of "Appplied Cryptopgraphy" from Bruc=
e Schneier.

In my application I do use a different IV for every encrypted plaintext:
$td =3D mcrypt_module_open('rijndael-128', '', 'cbc', '');
$iv =3D base64_encode(mcrypt_create_iv(mcrypt_enc_get_iv_size($td), MCRYPT_=
DEV_URANDOM));
I do use base64 as it is easier to store it in the database.

The plaintext is then encrypted using:
$ciphertext =3D "encrypt_iv('" . $plaintext . "','" . $password] . "',decod=
e('$iv','base64'),'aes')";

The IV is then stored together with the ciphertext in the database.


Daniel


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