Extended notions of security for multicast public key cryptosystems

Abstract

In this paper we introduce two notions of security: multi-user indistinguishability and multi-user non-malleability. We believe that they encompass the correct requirements for public key encryption schemes in the context of multicast communications. A precise and non-trivial analysis proves that they are equivalent to the former single-user notions, provided the number of participants is polynomial. We also introduce a new definition for non-malleability which is simpler than those currently in use. We believe that our results are of practical significance: especially they support the use of PKCS#1 v.2 based on OAEP in the multicast setting.