From: chen@intuit.com (Mark Chen)
Date: Fri, 27 Oct 1995 12:07:55 +0800
To: corby@pela.intuit.com (Corby Anderson)
Subject: Re: One Time Pad encryption over the Internet, securely? (fwd)
In-Reply-To: <9510262248.AA19876@doom>
Message-ID: <9510270345.AA21918@doom.intuit.com>
MIME-Version: 1.0
Content-Type: text/plain


Corby,

> I *had* to forward this to you.  Can you give him the answer, or are you
> bound by an NDA?  I can't remember.
> 
> Corby

Unfortunately, I am bound by an NDA.

However, I am not violating the NDA by saying that Craig is correct.
There is no way to do a one-time pad except by doing a one-time pad.
In other words, it is not possible to generate synchronized, truly
random key streams at remote locations non-algorithmically.

One-time pads have a property that no other encryption scheme has.  If
I am an attacker and I somehow gain access to all of the key material
and all of the plaintext that two communicating parties have so far
used, I am still unable to decipher the next transmission.  *Only*
one-time pads have this property, and there is no way to achieve it
without negotiating a key stream out of band.

OTPs also have some other properties that are more mathematical in
nature.  For instance, the probability of getting any particular bit
of plaintext from a given ciphertext is equal to the raw probability
of getting that plaintext by itself.  In other words:

   p_P(x_i | c) = p_P(x_i)

where P is the plaintext space, x_i is the plaintext, and c is the
ciphertext.

Furthermore,

   p_K(K) = 1/|K|

which means that all keys are used with equal probability,
irrespective of plaintext and ciphertext.

Hope this helps!

   - Mark -

> Forwarded message:
> > From firewalls-owner@GreatCircle.COM Thu Oct 26 03:39:33 1995
> > X-Delivered: at request of corby on doom
> > X-Authentication-Warning: miles.greatcircle.com: majordom set sender to firewalls-owner using -f
> > From: Craig Bishop <csb@connect.com.au>
> > Message-Id: <199510260745.RAA16385@koromiko.off.connect.com.au>
> > Subject: One Time Pad encryption over the Internet, securely?
> > To: firewalls@greatcircle.com
> > Date: Thu, 26 Oct 1995 17:45:13 +1000 (EST)
> > X-Mailer: ELM [version 2.4 PL21]
> > Mime-Version: 1.0
> > Content-Type: text/plain; charset=US-ASCII
> > Content-Transfer-Encoding: 7bit
> > Sender: firewalls-owner@GreatCircle.COM
> > Precedence: bulk
> > 
> > I was contacted and asked whether I was interested in software which
> > which used a one time pad for encrytion.
> > 
> > It comes from Elementrix an Isreali company is offering encrypted email,
> > ftp and what they call "personal firewall" software.
> > 
> > This software uses a One Time Pad via a patent pending method. I am no
> > encryption expert and the information available is limited but it would
> > seem to me that there is no way to do this over the internet securely.
> > 
> > See, http://www.elementrix.co.il/
> > 
> > Cheers, Craig
> > 
> > -- 
> > Craig Bishop - Internet Security Analyst
> > csb@connect.com.au
> > http://www.connect.com.au/people/csb/
> > 
> 



--
Mark Chen 
chen@intuit.com
415/329-6913
finger for PGP public key
D4 99 54 2A 98 B1 48 0C  CF 95 A5 B0 6E E0 1E 1D