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From: hanafee@cory.Berkeley.EDU.UUCP
Newsgroups: sci.crypt
Subject: Re: generating RSA keys
Message-ID: <2837@zen.berkeley.edu>
Date: Fri, 5-Jun-87 23:42:25 EDT
Article-I.D.: zen.2837
Posted: Fri Jun  5 23:42:25 1987
Date-Received: Sun, 7-Jun-87 00:45:18 EDT
References: <537@ucdavis.UUCP>
Sender: news@zen.berkeley.edu
Reply-To: hanafee@cory.Berkeley.EDU.UUCP (Brian Hanafee)
Distribution: world
Organization: University of California, Berkeley
Lines: 42
Keywords: Keys, RSA


>I have been trying to come up with a scheme to implement an algorithm
>that would allow the user to pick any key (presumably text), and map it 
>to a _good_ key (as defined above).

[goes on to describe a proposed method for obtaining "good" keys]

>I have been trying to find any weaknesses in this scheme (such as dramatically
>reducing the number of possible keys, etc.).  While the scheme does reduce
>the number of keys, it also will guarentee the generation of a good key.
>My feeling is that if the key text that is gotten from the user is long
>enough (say 10 characters, 6 bits per char -- around 60 unique bits
>or from 1 to 1E19), then the results should be fairly good.

	I think your primary weakness is exactly the one you suggested;
this reduces the number of possible keys.  The RSA encryption scheme is
well known, its strength lies in making decryption computationally 
infeasible.  Given 60 unique bits, you have only 2**60 possible unique keys.
I don't know how well this compares to just using DES, but I suspect
it's not much better.  
	A second, possibly more serious problem lies in the distribution
of the "good" keys.  Since you do a linear search from a "bad" key, looking
for a "good" key, you will arrive at less than a one-to-one mapping of
text to keys (e.g. if "aaa", "aab" and "aac" are bad, but "aad" is good, 
and you use a linear search, then all four text strings will yield the
same key).  You should *very* carefully study how rich your domain is
in good keys before implementing your mapping.
	Finally, there is a well-known problem with any encryption based
on user-entered keys.  They tend to be anything but random, often with
a strong correlation with the English/native language.  System managers
today have serious problems with people using words found in the dictionary
as passwords, since they are found easily with brute force search.

-----------------------------------------------------------------------------
My opinions are mine, and I take full responsibility.  So there.
				(signed) Brian Hanafee
UUCP:		!ucbvax!cory!hanafee
ARPAnet:	hanafee@cory.Berkeley.EDU
My opinions are mine, and I take full responsibility.  So there.
				(signed) Brian Hanafee
UUCP:		!ucbvax!cory!hanafee
ARPAnet:	hanafee@cory.Berkeley.EDU