From: Andrew Loewenstern <andrew@cubetech.com>
Date: Sun, 2 Jan 94 15:03:49 PST
To: cypherpunks@toad.com
Subject: Re: Anonymous Video on Demand
Message-ID: <9401022303.AA27235@valinor.cubetech.com>
MIME-Version: 1.0
Content-Type: text/plain


> It just occured to me that when this protocol is implemented with
> RSA, it is subject to a minor (and unlikely) failure that can
> allow Alice to determine which video Bob has selected (or at
> least eliminate some of them).  If each video keypair has a
> different modulus and the one Bob selects has a larger modulus
> than some of the "dummy" videos, then if the encryption of Bob's
> session key with his selected video public key results in a
> message that is close to the modulus itself, the keypairs with
> moduli that are smaller than Bob's message can be trivially
> eliminated as candidates.

This protocol also assumes that all of the movies (or pieces of  
information) cost the same amount.  Presumably in the video-on-demand  
business, a most movies would have the same cost or there would be a few  
'levels' of costs with many movies in each 'level.'  In that case you  
would only pick random 'padding' videos that have the same price.   
However, a video store could easily give all of the horror movies one  
price, all of the comedy ones another, all the pornos another, etc.... and  
at least be able to determine the general type of video the customer is  
purchasing.

In a general information market type setup, I would expect that the value  
of different pieces of information would vary greatly.


How would payment of the information be made?  In a general information  
market setup, where the bits of information have varrying values you could  
do something like the following.

in the oblivious transfer protocol, if the hardware used is implemented in  
tamper-proof chips, the price of each piece of information could be  
encoded with the information.  The chip would store a running total of the  
prices of information successfully decrypted by the customer.  At the end  
of the month, the box would send the total price to the vendor, which will  
bill the customer.  Depending on the number of pieces of information  
purchased, the vendor would be able to infer more or less information on  
the types of info bought by the customer...  Also, you could randomly  
purchase very cheap (or free and worthless) bits of information to make it  
more difficult for the vendor to figure out what you are interested in...


andrew