From: "Josh M. Osborne" <stripes@va.pubnix.com>
Date: Tue, 15 Aug 95 06:52:43 PDT
To: cypherpunks@toad.com
Subject: Use of the IV in DES & stuffing the first block w/ random stuff
Message-ID: <JAA01935.199508151352@garotte.va.pubnix.com>
MIME-Version: 1.0
Content-Type: text/plain


I have recently started writing a small pair of encription and
decription programs.  I was planning of gennerating the key by
taking the MD5 of the text password supplied by the user.

Seeing that I have 64 bits left over (MD5 gives me 128 bits, single
DES needs 56, tripple DES needs 168 - so I have a bit over 64 bits
left in both cases), is there anything useful that I could do with
them?

Does setting the IV (normally left at zero) buy me anything?

Does craming it into the first data block help protect me from
known plaintext attacks?  (I was going to use CBC so unless they
know the first block they can't use a known plaintext attack,
right?)

Or am I better off putting the extra 64 bits of "key" into the IV,
and gennerating a strong random number to stuff in the first block
- since the decoder can just ignore that block anyway.

(or should I ignore the IV, and stuffing random crud in the first
block?)

I don't recall Applyed Crypto. addressing these issues, but if I
just managed to forget can someone remind me what chapter I need
to re-read?