## A New Protocol for Conditional Disclosure of Secrets And Its Applications

Sven Laur and Helger Lipmaa. A New Protocol for Conditional Disclosure of Secrets And Its Applications. In Jonathan Katz and Moti Yung, editors, ACNS 2007, volume 4521 of Lecture Notes in Computer Science, pages 207--225, Zhuhai, China, June 5--8, 2007. Springer, Heidelberg.

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Abstract:

Many protocols that are based on homomorphic encryption are private only if a client submits inputs from a limited range $S$. Conditional disclosure of secrets (CDS) helps to overcome this restriction. In a CDS protocol for a set $S$, the client obtains server's secret if and only if the client's inputs belong to $S$ and thus the server can guard itself against malformed queries. We extend the existing CDS protocols to work over additively homomorphic cryptosystems for every set from $\mathbf{NP/poly}$. The new construction is modular and easy to apply. As an example, we derive a new oblivious transfer protocol with log-squared communication and a millionaire's protocol with logarithmic communication. We also implement private, universally verifiable and robust multi-candidate electronic voting so that all voters only transmit an encryption of their vote. The only hardness assumption in all these protocols is that the underlying public-key cryptosystem is IND-CPA secure and the plaintext order does not have small factors..

Keywords: Conditional disclosure of secrets, crypto-computing, homomorphic encryption, oblivious transfer, two-party computation.

Authors:

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