International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Secure Hybrid Encryption from Weakened Key Encapsulation

Dennis Hofheinz
Eike Kiltz
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Abstract: 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. CCCA has less demanding security requirements than standard chosen-ciphertext (CCA) security (since it requires the adversary to have a certain plaintext-knowledge when making a decapsulation query) yet we can prove that CCCA is 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.
  title={Secure Hybrid Encryption from Weakened Key Encapsulation},
  booktitle={IACR Eprint archive},
  keywords={public-key cryptography / Chosen-ciphertext security, weak security assumptions, hybrid encryption},
  note={A preliminary version of this paper appears in the proceedings of CRYPTO 2007. This is the full version. 13721 received 27 Jul 2007},
  author={Dennis Hofheinz and Eike Kiltz},