From: Kent Crispin <kent@songbird.com>
Date: Wed, 26 Nov 1997 09:17:30 +0800
To: cypherpunks@Algebra.COM
Subject: Re: Quoting Portions of a Signed Document
In-Reply-To: <199711260002.BAA28863@basement.replay.com>
Message-ID: <19971125170517.14061@songbird.com>
MIME-Version: 1.0
Content-Type: text/plain



On Tue, Nov 25, 1997 at 06:49:17PM -0600, William H. Geiger III wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> 
> 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.

The interesting case is when you do it at the granularity of the bit....

-- 
Kent Crispin				"No reason to get excited",
kent@songbird.com			the thief he kindly spoke...
PGP fingerprint:   B1 8B 72 ED 55 21 5E 44  61 F4 58 0F 72 10 65 55
http://songbird.com/kent/pgp_key.html