From: mpd@netcom.com (Mike Duvos)
Date: Fri, 29 Jul 94 14:17:43 PDT
To: cypherpunks@toad.com
Subject: Re: Q: Chaum style blind signatures?
In-Reply-To: <9407291929.AA21163@burgess.Eng.Sun.COM>
Message-ID: <199407292058.NAA01975@netcom11.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


Jacob.Levy@Eng.Sun.COM (Jacob Levy) writes:

 > Could someone please explain what is a "Chaum style blind
 > signature" and how it can be used? I looked in all the FAQs
 > on rtfm.mit.edu and could not find anything about this.

 > P.S. I've seen the term used in a document claiming these
 > can be used for untraceable e-cash

Given a pair of RSA keys (e,n) and (d,n), the owner may sign a
number x by computing x^d mod n using his private key.  In real
life, x usually consists of a message digest and a small amount
of constant information.  This prevents the product of two
signatures from also being a valid signature.  Anyone may verify
a signature by performing a similar operation using the public
key and recovering x.

Blind signatures allow you to obtain a signature from someone
without disclosing to them what they are signing.  You pick a
random number r and ask the signer to sign x*r^e mod n.  Since r
is arbitrary, this tells the signer nothing about the value of x.

When the signer gives you back r*x^d mod n, you simply multiply
by the multiplicative inverse of r mod n to obtain x^d mod n, the
signed message.  The signer still has no idea what he has signed
and cannot recognize it later if he sees it.

This allows untraceable digital cash, since the bank can sign new
notes for customers that it cannot later recognize.  It has other
interesting uses as well.

-- 
     Mike Duvos         $    PGP 2.6 Public Key available     $
     mpd@netcom.com     $    via Finger.                      $