  Computational privacy is a property of cryptographic system that ensures the
privacy of data being processed at an untrusted server. Fully Homomorphic
Encryption Schemes (FHE) promise to provide such property. Contemporary FHE
schemes are suited for applications that have single user and server. In
reality many of the cloud applications involve multiple users with various
degrees of trust and the server need not necessarily be aware of it too. We
present a Complementary Key Pairs technique and protocols based on that to
scale any generic FHE schemes to multi user scenarios. We also use such
technique along with FHE to show how attribute based access control can be
achieved while server being oblivious of the same. We analyze the protocols and
their security. Our protocols don't make any assumptions on how FHE scheme
itself works.
