A New and Improved Paradigm for Hybrid Encryption Secure Against Chosen-Ciphertext Attack

We present a new encryption scheme which is secure against adaptive chosenciphertext attack (or CCA2-secure) in the standard model (i.e. without the use of random oracle). Our scheme is a hybrid one: it first uses a public-key step (the Key Encapsulation Module or KEM) to encrypt a random key, which is then used to encrypt the actual message using a symmetric encryption algorithm (the Data Encapsulation Module or DEM). Our scheme is a modification of the hybrid scheme presented by Shoup in [18] (based on the Cramer-Shoup scheme in [4]). Its major practical advantage is that it saves the computation of one exponentiation and produces shorter ciphertexts. This effciency improvement is the result of a surprising observation: previous hybrid schemes were proven secure by proving that both the KEM and the DEM were CCA2secure. On the other hand, our KEM is not CCA2-secure, yet the whole scheme is, assuming the Decisional Diffie-Hellman (DDH) Assumption. Finally we generalize our new sche...

• Type: Journal
• Year: 2010
• Where: JOC
• Authors: Yvo Desmedt, Rosario Gennaro, Kaoru Kurosawa, Victor Shoup
• Keywords: Encapsulation Module, Hybrid Schemes, JOC 2010, Random Oracle