From: Alan Barrett <apb@iafrica.com>
Date: Tue, 13 May 1997 16:28:24 +0800
To: cypherpunks@cyberpass.net
Subject: Re: key recovery vs data backup
In-Reply-To: <19970512230928.46454@bywater.songbird.com>
Message-ID: <Pine.NEB.3.95.970513093606.21416m-100000@apb.iafrica.com>
MIME-Version: 1.0
Content-Type: text/plain


On Mon, 12 May 1997, Kent Crispin wrote:
> > What I meant was, you can make n-of-m hardware stuff for both cases.
> > Surely you don't disagree with that?
>
> I don't disagree that you *could* do it.  I think it unlikely that you
> would do it for the multiple recipient case.  I believe that in the
> MR case the master key(s) (especially if there are multiple master
> keys) would use exactly the same encryption algorithm as the normal
> encryption case.  That is the obvious, straightforward way to do it.
> If you use another encryption algorithm for the master then you have a
> whole raft of other problems to deal with.

You can have the same algorithm (visible at the user side) whether or
not you have special hardware for protecting the private half of the key
pair from exposure.

> > Finding out which key to encrypt to in the MR case is analagous to
> > finding out which key safe to talk to in the KS case.  Securing and
> > authenticating the channel to the key safe in the KS case is an
> > extra issue that does not have a counterpart in the MR case.
>
> ?? How do you know that the channel through which you get the master
> key in the MR case is secure?  You surely don't just pull it off the
> net.  It's signed?  Then the problem just recurses -- how do you
> know the signature is good?  This is exactly the problem you have
> contacting the keysafe.

Knowing the right key in the MR case is static information, analagous
to knowing the network address of the key safe in the KS case.  For
example, in the MR case, the key could be distributed on a floppy disk
along with the special software, while in the KS case the location
of the key safe could be so distributed.  OK, you also need a way of
changing the (MR) key or of moving the (KS) key safe, but that doesn't
happen often and similar issues would appear to arise in either case.

But in the KS case, there would also need to be a mechanism to protect
the channel between the user and the key safe every time the channel is
used, and that extra mechanism does not appear to have a counterpart in
the MR case.  Not really a big deal, but it is a whole extra protocol to
be designed.

--apb (Alan Barrett)