On the Relation between Redactable and Sanitizable Signature Schemes

Malleable signature schemes (MSS) enable a third party to alter signed data in a controlled way, maintaining a valid signature after an authorized change. Most well studied cryptographic constructions are (1) redactable signatures (RSS), and (2) sanitizable signatures (SSS). RSSs allow the removal of blocks from a signed document, while SSSs allow changing blocks to arbitrary strings. We rigorously prove that RSSs are less expressive than SSSs: no unforgeable RSS can be transformed into an SSS. For the opposite direction we give a black-box transformation of a single SSS, with tightened security, into an RSS.