Dynamic access policies for unconditionally secure secret sharing schemes

Secret sharing schemes are cryptographic primitives that can he employed in any situation where it is desirable to distribute information relating to secret data amongst a number of entities. Following a secure initialization process, a secret sharing scheme normally has a fixed access policy (specified by an access structure). However in dynamic environments it is likely that access policies will be subject to change: for example entities may wish to join the scheme, entities may be expelled from the scheme or security thresholds may change. Rather than undergo a costly re-initialization process, there have been several different proposals for designing secret sharing schemes that permit dynamic access policies. We will review various existing models and proposals, and identify several open problems.