Previous Work
Gennaro-Shoup: under the assumption that Random Oracles exist and the DDH intractability assumption, show a Threshold PKC which is t-secure and t-robust for t< n/2 against CCA. (No interaction is necessary.)
Dolev-Dwork-Naor: under the assumption trapdoor functions exist show single server PKC secure against CCA. Use NIZK for construction. ( Prior [NY] LTA )
Cramer-Shoup: under the DDH intractability assumption
show a single server PKC secure against CCA. Quite Efficient.