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From: viavant@ernie.Berkeley.EDU (Steven Viavant)
Newsgroups: sci.crypt
Subject: Re: Cheap more-or-less one-time-pads
Message-ID: <18860@ucbvax.BERKELEY.EDU>
Date: Tue, 12-May-87 17:12:04 EDT
Article-I.D.: ucbvax.18860
Posted: Tue May 12 17:12:04 1987
Date-Received: Fri, 15-May-87 03:15:36 EDT
References: <57@decvax.UUCP> <182@vianet.UUCP> <573@faline.bellcore.com>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: viavant@ernie.Berkeley.EDU.UUCP (Steven Viavant)
Organization: University of California, Berkeley
Lines: 28
Keywords: DES
Summary: might be made to work...


	Regarding using CDs for one time pads: Phil Karn (I think?)
states that the available key length is too small, based on ~20,000 titles 
 ~= 15 bits, plus ~5e9 bits per title => ~32 bits to use as an index into the
position to begin the X-or pad operation (with wrap-around). I like his 
analysis, however: Couldn't the effective key-length be made (arbitrarily?)
large by including extra information (bits) in the key, to be interpreted
by the receiver according to a predefined protocol. For example, the key
might specify CD-title (15 bits), index into starting position (32 bits),
and "x" bits used as a seed for a psuedo-random number generator. The
first bit of the decoding pad would be specified by the title and index;
subsequent bits in the pad would be determined by "skipping" over a number
of bits in the CD, that number determined by (say) the middle eight bits of
the succesive iterations of the psuedo-random number generator. This
would both extend the effective key-space, and diminish the effect of the
undesirably large correlation that succesive bits on a CD tend to exhibit.
	I doubt that one would achieve the full potential secrecy of the
length of key employed, but we should still get an effective key space
which is exponential in the length of the key (though perhaps somewhat
less than 2^(key-length)). One could also decide to cycle through several 
different CD titles before wrapping around (as specified in the key), if 
there were difficulty in coming up with a (fast) "random" number generator with
a large enough period in its middle "x" bits.

		Steven Viavant


Sign me: just curious, not a cat.	Steven viavant@ernie.berkeley.edu