From: "William H. Geiger III" <whgiii@invweb.net>
Date: Wed, 26 Nov 1997 09:10:01 +0800
To: cypherpunks@Algebra.COM
Subject: Re: Quoting Portions of a Signed Document
In-Reply-To: <199711260002.BAA28863@basement.replay.com>
Message-ID: <199711260102.UAA25089@users.invweb.net>
MIME-Version: 1.0
Content-Type: text/plain



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In <199711260002.BAA28863@basement.replay.com>, on 11/25/97 
   at 07:02 PM, nobody@REPLAY.COM (Anonymous) said:

>It would be nifty if there was a way to show that any continuous set of
>bits were signed given only one signature on a whole document.
>Intuitively, it seems to me that this might be provably inconsistent with
>a secure hash.  Still, crypto results are full of surprises, so I could
>imagine there is a way to do this.

Well you can do it. Wether you want to do it is another matter.

For the level of granularity you are sugesting a hash is not pratical.

You could just use RSA encryption to encrypt the message in the following
manner:

The user encrypts the message with his *private* key. Rather than
encrypting the entire document in one operation he would encrypt each
[insert you level of granularity here] and then concantinate the results.
Say we wanted a level of granularity of a word:

word1 word2 word3 word4

the resulting cypher text would be:

cypher1 cypher2 cypher3 cypher4

Now if someone wished to verifiably quote words 1,3,4 they would include
cypher1 cypher3 cypher4 in their document.

Since cypher 1,3,4 could only be generated by original author it can be
verified that he actually wrote those words.

At a bare minimum this would have to be done on a level of granularity of
a sentance to have any meaning at all and even then it's relavance would
be questioned.


- -- 
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William H. Geiger III  http://users.invweb.net/~whgiii
Geiger Consulting    Cooking With Warp 4.0

Author of E-Secure - PGP Front End for MR/2 Ice
PGP & MR/2 the only way for secure e-mail.
OS/2 PGP 2.6.3a at: http://users.invweb.net/~whgiii/pgpmr2.html                        
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