Security of cryptographic protocols based on the trusted party

Ling Dong, Kefei Chen

Xianxian Li, Jinpeng Huai

In data communications, digital signature schemes have been used to ensure non repudiation of sending data and the integrality of data, and non repudiation of receiving data is also very important in the secure communication. In the past years, this kind of cryptographic protocols was mainly implemented by the intervention of the trusted third party in transmission and encryption of data, thus the dependability and security of the trusted third party was a bottleneck in these secure systems. To solve this problem, an undeniable cryptographic protocol for both sender and receiver is proposed. It is more efficient. Finally, its applications in electric mail are discussed.

Hemanta K. Maji, Manoj Prabhakaran, Mike Rosulek

It is well-known that most cryptographic tasks do not have universally composable (UC) secure protocols, if no trusted setup is available in the framework. On the other hand, if a task like fair coin-tossing is available as a trusted setup, then all cryptographic tasks have UC-secure protocols. What other trusted setups allow UC-secure protocols for all tasks? More generally, given a particular setup, what tasks have UC-secure protocols?We show that, surprisingly, every trusted setup is either useless (equivalent to having no trusted setup) or all-powerful (allows UC-secure protocols for all tasks). There are no “intermediate” trusted setups in the UC framework. We prove this zero-one law under a natural intractability assumption, and consider the class of deterministic, finite, 2-party functionalities as candidate trusted setups.One important technical contribution in this work is to initiate the comprehensive study of the cryptographic properties of reactive functionalities. We model these functionalities as finite automata and develop an automata-theoretic methodology for classifying and studying their cryptographic properties. Consequently, we completely characterize the reactive behaviors that lead to cryptographic non-triviality. Another contribution of independent interest is to optimize the hardness assumption used by Canetti et al. (STOC 2002) in showing that the common random string functionality is complete (a result independently obtained by Damgård et al. (TCC 2010)).

Shouhuai Xu, Gendu Zhang, Hong Zhu

Vipul Goyal

Adnan Memon

This paper presents a new scheme to distribute secret shares using two trusted third parties to increase security and eliminate the dependency on single trusted third party. This protocol for communication between a device and two trusted third parties uses the piggy bank cryptographic paradigm. We also present a protocol to give law enforcing agencies access to sensitive information present on a cell phone or a device using secret sharing scheme. The ideas for classical systems may also be applied to quantum schemes.