Key-dependent message security: Generic amplification and completeness

Benny Applebaum*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

Key-dependent message (KDM) secure encryption schemes provide secrecy even when the attacker sees encryptions of messages related to the secret-key sk. Namely, the scheme should remain secure even when messages of the form f(sk) are encrypted, where f is taken from some function class F. A KDM amplification procedure takes an encryption scheme which satisfies F -KDM security, and boosts it into a G -KDM secure scheme, where the function class G should be richer than F. It was recently shown by Brakerski et al. (TCC 2011) and Barak et al. (EUROCRYPT 2010) that a strong form of amplification is possible, provided that the underlying encryption scheme satisfies some special additional properties. In this work, we prove the first generic KDM amplification theorem which relies solely on the KDM security of the underlying scheme without making any other assumptions. Specifically, we show that an elementary form of KDM security against functions in which each output bit either copies or flips a single bit of the key (a.k.a. projections) can be amplified into KDM security with respect to any function family that can be computed in arbitrary fixed polynomial-time. Furthermore, our amplification theorem and its proof are insensitive to the exact setting of KDM security, and they hold in the presence of multiple-keys and in the symmetric-key/public-key and the CPA/CCA cases. As a result, we can amplify the security of most known KDM constructions, including ones that could not be amplified before. Finally, we study the minimal conditions under which full-KDM security (with respect to all functions) can be achieved. We show that under strong notion of KDM security, the existence of fully homomorphic encryption which allows to encrypt the secret-key (i.e., "cyclic- secure") is not only sufficient for full-KDM security, as shown by Barak et al., but also necessary. On the other hand, we observe that for standard KDM security, this condition can be relaxed by adopting Gentry's bootstrapping technique (STOC 2009) to the KDM setting.

Original languageEnglish
Pages (from-to)429-451
Number of pages23
JournalJournal of Cryptology
Volume27
Issue number3
DOIs
StatePublished - Jul 2014

Funding

FundersFunder number
Check Point Institute for Information Security
Koshland and Alon Fellowships
Weizmann Institute of Science
Weizmann Institute of Science
Israel Science Foundation1155/11

    Keywords

    • Cyclic-security
    • Garbled circuits
    • Key-dependent message
    • Randomized encoding

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