From: Andy Brown <asb@nexor.co.uk>
Date: Thu, 27 Jul 95 02:44:49 PDT
To: cypherpunks@toad.com
Subject: Re: Encrypting block driver for Linux...need some advice
In-Reply-To: <Pine.LNX.3.91.950726091131.129A-100000@comet.aeinet.com>
Message-ID: <Pine.SOL.3.91.950727104038.17605B-100000@eagle.nexor.co.uk>
MIME-Version: 1.0
Content-Type: text/plain


On Wed, 26 Jul 1995, Johnathan Corgan wrote:
 
> Another, more crypto related question--how to deal with IV's?  Right now,
> I'm using 512 byte sectors with CBC.  For each sector, the IV is the
> sector number.  This frustrates the known plaintext attack issue, but I'm
> not sure if such a simple scheme is really effective.  Probably not.

Your scheme should be OK.  If you'd chosen the same IV for each sector 
then identical sectors would encrypt the same.  If I remember rightly 
then having a known IV only affects the security of the first block, 
after that the ciphertext chaining comes into effect.


- Andy

+-------------------------------------------------------------------------+
| Andrew Brown  Internet <asb@nexor.co.uk>  Telephone +44 115 952 0585    |
| PGP (2048/9611055D): 69 AA EF 72 80 7A 63 3A  C0 1F 9F 66 64 02 4C 88   |
+-------------------------------------------------------------------------+