Cryptology ePrint Archive: Report 2021/653

Smooth Zero-Knowledge Hash Functions

Behzad Abdolmaleki and Hamidreza Khoshakhlagh and Helger Lipmaa

Abstract: We define smooth zero-knowledge hash functions (SZKHFs) as smooth projective hash functions (SPHFs) for which the completeness holds even when the language parameter lpar and the projection key HP were maliciously generated. We prove that blackbox SZKHF in the plain model is impossible even if lpar was honestly generated. We then define SZKHF in the registered public key (RPK) model, where both lpar and HP are possibly maliciously generated but accepted by an RPK server, and show that the CRS-model trapdoor SPHFs of Benhamouda et al. are also secure in the weaker RPK model. Then, we define and instantiate subversion-zero knowledge SZKHF in the plain model. In this case, both lpar and HP are completely untrusted, but one uses non-blackbox techniques in the security proof.

Category / Keywords: cryptographic protocols / plain model, RPK model, SPHF, trapdoor SPHF, subversion zero-knowledge

Date: received 19 May 2021

Contact author: hamidreza at cs au dk

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Version: 20210520:203043 (All versions of this report)

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