  Virtual organizations are dynamic, inter-organizational collaborations that
involve systems and services belonging to different security domains. Several
solutions have been proposed to guarantee the enforcement of the access control
policies protecting the information exchanged in a distributed system, but none
of them addresses the dynamicity characterizing virtual organizations. In this
paper we propose a dynamic hierarchical attribute-based encryption (D-HABE)
scheme that allows the institutions in a virtual organization to encrypt
information according to an attribute-based policy in such a way that only
users with the appropriate attributes can decrypt it. In addition, we introduce
a key management scheme that determines which user is entitled to receive which
attribute key from which domain authority.
