Subject: [Cfrg] Redistribution in Proactive Secret Sharing schemes

From: Henrick Hellström <[email-masked]>
Date: Sun, 04 May 2003 21:32:58 +0200

I have a problem with the Pedersen-VSS Discrete Logarithm based secret 
sharing protocol, I hope someone here could help me with. Forgive me and 
blame my ignorance if the problem I describe has already been solved, 
but as far as I could tell it appears to have been overlooked.

Background:

In Pedersen-VSS the shared secret x is never calculated. Instead, each 
of the n share holders Pi, 1 <= i <= n, generates an independent secret 
zi such that the sum of all zi equals x. Each share holder Pi splits zi 
into n shares sij, 1 <= j <= n, and transmits sij secretly to Pj. There 
are various methods for redistribution of the shares sij that leaves the 
values zi unchanged.

The problem:

I am looking for a protocol for "redistribution" of the secrets zi that 
would leave x unchanged. Let z'i denote the secret owned by Pi after the 
redistribution. One requirement is that the redistribution must be done 
in such way that knowledge of t-1 value pairs (zi,z'i) and the value zj, 
will not reveal the value z'j.

The reason for this is that I need a method for dealing with the 
possibility that a share holder Pi might be compromised during the 
reconstruction of zi, and that the adversary thereby obtains the value 
zi and not just the set of sub shares sji given to Pi.

Practically, I am thinking of software implementations that are swapped 
to the hard drive at the critical moment etc. There are also protocols 
that reveals zi by having the share holders broadcasting the secret 
shares sij in the event Pi gets corrupted, such as the DKG protocol 
proposed by Gennaro et al (1999). An adversary that is able to block the 
communication of any share holder and intercept the communication of at 
least two share holders, would only need to launch DoS attacks against 
each of the share holders in order, to obtain all of the values zi.

Citeseer context:
     http://citeseer.nj.nec.com/context/7536/0
     http://citeseer.nj.nec.com/context/1945661/0
     http://citeseer.nj.nec.com/wong01verifiable.html

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