Jiawei Wu

Weak coin flipping is a cryptographic primitive in which two mutually distrustful parties generate a shared random bit to agree on a winner via remote communication. While a stand-alone secure weak coin flipping protocol can be constructed from noiseless communication channels, its composability has not been explored. In this work, we demonstrate that no weak coin flipping protocol can be abstracted into a black box resource with composable security. Despite this, we also establish the overall stand-alone security of weak coin flipping protocols under sequential composition.

Rabin oblivious transfer is the cryptographic task where Alice wishes to receive a bit from Bob but it may get lost with probability 1/2. In this work, we provide protocol designs which yield quantum protocols with improved security. Moreover, we provide a constant lower bound on any Rabin oblivious transfer protocol. To quantify the security of this task with asymmetric cheating notions, we introduce the notion of cheating advantage which may be of independent interest in the study of other asymmetric cryptographic primitives as well.

Noisy channel is a valuable resource for cryptography. It can be used to build cryptographic primitives like bit commitment and oblivious transfer that are information-theoretically secure between two untrusting parties. Existing studies on this topic focus on the channel that does not change over successive uses. In this work, we study non-independent and identically distributed (non-i.i.d.) channels with constraint on min-entropy. The dishonest player is able to configure the channel at his will under the constraint. We devise a protocol that is complete, hiding, and binding, and give its commitment rate.

Dense coding is known as an attractive quantum information protocol. While the original study considers the noiseless setting, many subsequent studies extended this result to more general settings. However, all of them focused only on the communication speed in various noisy settings. While dense coding with the noiseless setting realizes twice communication speed, it also realizes quantum secure direct communication (QSDC) as follows.In dense coding, the sender, Alice, and the receiver, Bob, share perfect Bell states and Alice encodes her message by application of a unitary operation. Since Alice's local state is a completely mixed state, the eavesdropper, Eve, cannot obtain any information about the message even when Eve intercepts the transmitted quantum state. However, it is not easy to share a perfect Bell state. Hence, we need to consider secure communication under imperfect shared state. Specifically, we study secure direct communication by using a general preshared quantum state and a generalization of dense coding. In this scenario, Alice is allowed to apply a unitary operation on the preshared state to encode her message, and the set of allowed unitary operations forms a group. To decode the message, Bob is allowed to apply a measurement across his own system and the system he receives. In the worst scenario, we guarantee that Eve obtains no information for the message even when Eve access the joint system between the system that she intercepts and her original system of the preshared state. For a practical application, we construct a modular wiretap code by concatenating inverse universal hashing and an arbitrary error correcting code. Combining the wiretap code with error verification, we propose a concrete protocol for the private dense coding model and derive an upper bound of information leakage in the finite-length setting. We also discuss how to apply our scenario to the case with discrete Weyl-Heisenberg representation when the preshared state is unknown. In this case, Pauli encoding operation and Pauli channel are considered. Hence, our protocol can be applied many similar tasks.