Wai Hong Chan

Linear Suffix Array Construction by Almost Pure Induced-Sorting

We present a linear time and space suffix array (SA) construction algorithm called the SA-IS algorithm.The SA-IS algorithm is novel because of the LMS-substrings used for the problem reduction and the pure induced-sorting (specially coined for this algorithm)used to propagate the order of suffixes as well as that of LMS-substrings, which makes the algorithm almost purely relying on induced sorting at both its crucial steps.The pure induced-sorting renders the algorithm an elegant design and in turn a surprisingly compact implementation which consists of less than 100 lines of C code.The experimental results demonstrate that this newly proposed algorithm yields noticeably better time and space efficiencies than all the currently published linear time algorithms for SA construction.

Two Efficient Algorithms for Linear Time Suffix Array Construction

We present, in this paper, two efficient algorithms for linear time suffix array construction. These two algorithms achieve their linear time complexities, using the techniques of divide-and-conquer, and recursion. What distinguish the proposed algorithms from other linear time suffix array construction algorithms (SACAs) are the variable-length leftmost S-type (LMS) substrings and the fixed-length d-critical substrings sampled for problem reduction, and the simple algorithms for sorting these sampled substrings: the induced sorting algorithm for the variable-length LMS substrings and the radix sorting algorithm for the fixed-length d-critical substrings. The very simple sorting mechanisms render our algorithms an elegant design framework, and, in turn, the surprisingly succinct implementations. The fully functional sample implementations of our proposed algorithms require only around 100 lines of C code for each, which is only 1/10 of the implementation of the KA algorithm and comparable to that of the KS algorithm. The experimental results demonstrate that these two newly proposed algorithms yield the best time and space efficiencies among all the existing linear time SACAs.

Semiquantum secret sharing using entangled states

Secret sharing is a procedure for sharing a secret among a number of participants such that only the qualified subsets of participants have the ability to reconstruct the secret. Even in the presence of eavesdropping, secret sharing can be achieved when all the members are quantum. So what happens if not all the members are quantum? In this paper, we propose two semiquantum secret sharing protocols by using maximally entangled Greenberger-Horne-Zeilinger-type states in which quantum Alice shares a secret with two classical parties, Bob and Charlie, in a way that both parties are sufficient to obtain the secret, but one of them cannot. The presented protocols are also shown to be secure against eavesdropping.

Suffix Array Construction in External Memory Using D-Critical Substrings

We present a new suffix array construction algorithm that aims to build, in external memory, the suffix array for an input string of length n measured in the magnitude of tens of Giga characters over a constant or integer alphabet. The core of this algorithm is adapted from the framework of the original internal memory SA-DS algorithm that samples fixed-size d-critical substrings. This new external-memory algorithm, called EM-SA-DS, uses novel cache data structures to construct a suffix array in a sequential scanning manner with good data spatial locality: data is read from or written to disk sequentially. On the assumed external-memory model with RAM capacity Ω((nB)0.5), disk capacity O(n), and size of each I/O block B, all measured in log n-bit words, the I/O complexity of EM-SA-DS is O(n/B). This work provides a general cache-based solution that could be further exploited to develop external-memory solutions for other suffix-array-related problems, for example, computing the longest-common-prefix array, using a modern personal computer with a typical memory configuration of 4GB RAM and a single disk.

Sharing a quantum secret without a trusted party

In a conventional quantum (k, n) threshold scheme, a trusted party shares a secret quantum state with n participants such that any k of those participants can cooperate to recover the original secret, while fewer than k participants obtain no information about the secret. In this paper we show how to construct a quantum (k, n) threshold scheme without the assistance of a trusted party, who generates and distributes shares among the participants. Instead, each participant chooses his private state and contributes the same to the determination of the final secret quantum state.

Linear Time Suffix Array Construction Using D-Critical Substrings

In this paper we present in detail a new efficient linear time and space suffix array construction algorithm(SACA), called the D-Critical-Substring algorithm. The algorithm is built upon a novel concept called fixed-size D-Critical-Substrings, which allow us to compute suffix arrays through a balanced combination of the bucket-sort and the induction sort. The D-Critical-Substring algorithm is very simple, a fully-functioning sample implementation of which in C++ is embodied in only about 100 effective lines. The results of the experiment that we conducted on the data from the Canterbury and Manzini-Ferragina corpora indicate that our algorithm outperforms the two previously best-known linear time algorithms: the Karkkainen-Sanders (KS) and the Ko-Aluru (KA) algorithms.

Semiquantum key distribution with secure delegated quantum computation

Semiquantum key distribution allows a quantum party to share a random key with a “classical” party who only can prepare and measure qubits in the computational basis or reorder some qubits when he has access to a quantum channel. In this work, we present a protocol where a secret key can be established between a quantum user and an almost classical user who only needs the quantum ability to access quantum channels, by securely delegating quantum computation to a quantum server. We show the proposed protocol is robust even when the delegated quantum server is a powerful adversary, and is experimentally feasible with current technology. As one party of our protocol is the most quantum-resource efficient, it can be more practical and significantly widen the applicability scope of quantum key distribution.

Induced Sorting Suffixes in External Memory

We present in this article an external memory algorithm, called disk SA-IS (DSA-IS), to exactly emulate the induced sorting algorithm SA-IS previously proposed for sorting suffixes in RAM. DSA-IS is a new disk-friendly method for sequentially retrieving the preceding character of a sorted suffix to induce the order of the preceding suffix. For a sizen string of a constant or integer alphabet, given the RAM capacity Ω ((nW)0.5), where W is the size of each I/O buffer that is large enough to amortize the overhead of each access to disk, both the CPU time and peak disk use of DSA-IS are O(n). Our experimental study shows that on average, DSA-IS achieves the best time and space results of all of the existing external memory algorithms based on the induced sorting principle.

Efficient arbitrated quantum signature and its proof of security

In this paper, an efficient arbitrated quantum signature scheme is proposed by combining quantum cryptographic techniques and some ideas in classical cryptography. In the presented scheme, the signatory and the receiver can share a long-term secret key with the arbitrator by utilizing the key together with a random number. While in previous quantum signature schemes, the key shared between the signatory and the arbitrator or between the receiver and the arbitrator could be used only once, and thus each time when a signatory needs to sign, the signatory and the receiver have to obtain a new key shared with the arbitrator through a quantum key distribution protocol. Detailed theoretical analysis shows that the proposed scheme is efficient and provably secure.

On the security of arbitrated quantum signature schemes

Due to the potential capability of providing unconditional security, arbitrated quantum signature (AQS) schemes, whose implementation depends on the participation of a trusted third party, received intense attention in the past decade. Recently, some typical AQS schemes were cryptanalyzed and improved. In this paper, we analyze the security property of some AQS schemes and show that all the previous AQS schemes, no matter whether original or improved, are still insecure in the sense that the messages and the corresponding signatures can be exchanged among different receivers, allowing the receivers to deny having accepted the signature of an appointed message. Some further improved methods on the AQS schemes are also discussed.