## Cryptology ePrint Archive: Report 2000/019

Threshold Cryptography Secure Against the Adaptive Adversary, Concurrently

Anna Lysyanskaya

Abstract: A threshold cryptosystem or signature scheme is a system with $n$ participants where an honest majority can successfully decrypt a message or issue a signature, but where the security and functionality properties of the system are retained even as the adversary corrupts up to $t$ players. We present the novel technique of a committed proof, which is a new general tool that enables security of threshold cryptosystems in the presence of the adaptive adversary. We also put forward a new measure of security for threshold schemes secure in the adaptive adversary model: security under concurrent composition. Using committed proofs, we construct concurrently and adaptively secure threshold protocols for a variety of cryptographic applications. In particular, based on the recent scheme by Cramer-Shoup, we construct adaptively secure threshold cryptosystems secure against adaptive chosen ciphertext attack under the DDH intractability assumption.

Category / Keywords: cryptographic protocols / threshold cryptography; adaptive adversary

Publication Info: Part of this paper will appear in Eurocrypt2000

Date: received 12 May 2000

Contact author: anna at theory lcs mit edu

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

Short URL: ia.cr/2000/019

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