Secure Hybrid Encryption from Weakened Key Encapsulation

We put forward a new paradigm for building hybrid encryption schemes from constrained chosen-ciphertext secure (CCCA) key-encapsulation mechanisms (KEMs) plus authenticated symmetric encryption. Constrained chosen-ciphertext security is a new security notion for KEMs that we propose. It has less demanding security requirements than standard CCCA security (since it requires the adversary to have a certain plaintext-knowledge when making a decapsulation query) yet we can prove that it is CCCA sufficient for secure hybrid encryption.

Our notion is not only useful to express the Kurosawa-Desmedt public-key encryption scheme and its generalizations to hash-proof systems in an abstract KEM/DEM security framework. It also has a very constructive appeal, which we demonstrate with a new encryption scheme whose security relies on a class of intractability assumptions that we show (in the generic group model) strictly weaker than the Decision Diffie-Hellman (DDH) assumption. This appears to be the first practical public-key encryption scheme in the literature from an algebraic assumption strictly weaker than DDH.