Cryptology ePrint Archive: Report 2007/288

Secure Hybrid Encryption from Weakened Key Encapsulation

Dennis Hofheinz and Eike Kiltz

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.

Category / Keywords: public-key cryptography / Chosen-ciphertext security, weak security assumptions, hybrid encryption

Publication Info: A preliminary version of this paper appears in the proceedings of CRYPTO 2007. This is the full version.

Date: received 27 Jul 2007

Contact author: kiltz at cwi nl

Available format(s): Postscript (PS) | Compressed Postscript (PS.GZ) | PDF | BibTeX Citation

Version: 20070807:153308 (All versions of this report)

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