From: paul@fatmans.demon.co.uk
Date: Tue, 26 Nov 1996 09:55:30 -0800 (PST)
To: cypherpunks@toad.com
Subject: Re: IPG Algorith Broken!
Message-ID: <849030305.93656.0@fatmans.demon.co.uk>
MIME-Version: 1.0
Content-Type: text/plain



> > Ahh... an OTP isn't unbreakable.  Its just so encredibly breakable that
> > you never know which break was the correct one ;)
> 
> Note that Schneier says "perfect", not "unbreakable".

yes,

Perfect is a better term. Strictly speaking it is because there is no 
finite unicity distance (the amount of ciphertext with which the 
cipher can theoretically be broken). So, stricly speaking, for a given 
message C and a prospective pad, P, out of a set of N pads which may 
or may not be correct:

P(P|C) = N^-1

The length of C and the amount of ciphertexts given have no effect in 
determining the key, nor is there any prospect of a know plaintext 
attack as the pad is true random and the next bits are totally 
independent of any others before them.
Of course the reason it is perfect is because there are many 
different pads which give valid decryptions and there is no way of 
knowing which one is correct.

IPG`s algorithm is definitely NOT an OTP and Don Wood is a snakeoil 
merchant.


 

  Datacomms Technologies web authoring and data security
       Paul Bradley, Paul@fatmans.demon.co.uk
  Paul@crypto.uk.eu.org, Paul@cryptography.uk.eu.org    
       Http://www.cryptography.home.ml.org/
      Email for PGP public key, ID: 5BBFAEB1
     "Don`t forget to mount a scratch monkey"