Estimating spatial distributions is important in data analysis, such as traffic flow forecasting and epidemic prevention. To achieve accurate spatial distribution estimation, the analysis needs to collect sufficient user data. However, collecting data directly from individuals could compromise their privacy. Most previous works focused on private distribution estimation for one-dimensional data, which does not consider spatial data relation and leads to poor accuracy for spatial distribution estimation. In this paper, we address the problem of private spatial distribution estimation, where we collect spatial data from individuals and aim to minimize the distance between the actual distribution and estimated one under Local Differential Privacy (LDP). To leverage the numerical nature of the domain, we project spatial data and its relationships onto a one-dimensional distribution. We then use this projection to estimate the overall spatial distribution. Specifically, we propose a reporting mechanism called Disk Area Mechanism (DAM), which projects the spatial domain onto a line and optimizes the estimation using the sliced Wasserstein distance. Through extensive experiments, we show the effectiveness of our DAM approach on both real and synthetic data sets, compared with the state-of-the-art methods, such as Multi-dimensional Square Wave Mechanism (MDSW) and Subset Exponential Mechanism with Geo-I (SEM-Geo-I). Our results show that our DAM always performs better than MDSW and is better than SEM-Geo-I when the data granularity is fine enough.

Uncertain butterflies are fundamental and popular graphlet motifs within uncertain bipartite networks, serving as a crucial metric in structural analysis. Despite extensive research have studied butterflies sufficiently on deterministic networks, few of works explore uncertain butterflies. In this paper, we introduce the Most Probable Maximum Weighted Butterfly (MPMB), which holds the highest probability of becoming a maximum weighted butterfly on an uncertain bipartite network. Proved that searching MPMBs is NP-Hard, we then proposed two samplingbased methods, namely Ordering Sampling (OS), and Ordering-Listing Sampling (OLS). The OS method is suitable for singletrial sampling, while the OLS method is optimized for multiple trials, which first finds candidate butterflies in rough before searching MPMBs. Our experimental results indicate that our basic method (OS) performs 1000× faster than the baseline and the optimized method (OLS) achieves another 180× speedup.

Ridesharing services play an essential role in modern transportation, which significantly reduces traffic congestion and exhaust pollution. In the ridesharing problem, improving the sharing rate between riders can not only save the travel cost of drivers but also utilize vehicle resources more efficiently. The existing online-based and batch-based methods for the ridesharing problem lack the analysis of the sharing relationship among riders, leading to a compromise between efficiency and accuracy. In addition, the graph is a powerful tool to analyze the structure information between nodes. Therefore, in this paper, we propose a framework, namely StructRide, to utilize the structure information to improve the results for ridesharing problems. Specifically, we extract the sharing relationships between riders to construct a shareability graph. Then, we define a novel measurement, namely shareability loss, for vehicles to select groups of requests such that the unselected requests still have high probabilities of sharing with other requests. Our SARD algorithm can efficiently solve dynamic ridesharing problems to achieve dramatically improved results. Through extensive experiments, we demonstrate the efficiency and effectiveness of our SARD algorithm on two real datasets. Our SARD can run up to 72.68 times faster and serve up to 50% more requests than the state-of-the-art algorithms.

Performance predictability, ensuring low latency variability, is crucial for the reliability and efficiency of blockchain consensus. Byzantine Fault Tolerant Proof-of-Stake (BFT-PoS) consensus aims to achieve stable transaction processing latency by scheduling block generation at consistent intervals. However, BFT-PoS's incentive mechanisms grant all transaction tips to the block proposer, which can be exploited by delaying proposals to gain extra Maximal Extractable Value (MEV) rewards, thus undermining performance predictability. Existing solutions impose penalties for delays but lack a standard for measuring the extra rewards from delays or fail in malicious environments. This paper introduces InTime, a novel approach to safeguard performance predictability in BFT-PoS by economically motivating timely block proposals. We first introduce the untimely MEV ratio , a reliable metric to measure the extra rewards gained from proposal delays, facilitating our countermeasures against deliberate delays. Furthermore, we propose the arrival rate incentive (ARI), aligning rewards with transaction arrival timing among nodes to reduce potential MEV manipulation. To make ARI robust against malicious behaviors, we establish a committee time witness (CTW) workflow to accurately gather and verify transaction arrival times. Extensive experiments demonstrate that InTime can effectively reduce latency variability by up to 95.9%.

Blockchains have gained wide adoption for secure data processing. As blockchain data volumes grow, the demand for efficient data analysis, especially aggregate queries, becomes increasingly critical. However, current blockchains lack native support for efficient analytical query processing, forcing users to either maintain full replicas or rely on third-party services without integrity guarantees. In this paper, we propose an efficient framework, Merkle Bloom Filter Tree (MBFT), for authenticated aggregate queries that combine boolean keywords and range predicates on blockchains. At its core is a Bloom filter-based authenticated data structure that supports both types of predicates, constructed per block for efficient transaction indexing. For temporal predicates, we optimize time window queries through value pruning and block consolidation. We design a novel Merge Bloom Filter (MBF) for space-efficient handling of dynamic sets during query authentication. We provide a theoretical analysis of the storage overhead caused by the Bloom filter's false positive rates. Our framework employs data sketches to support various aggregate operations. Extensive experiments demonstrate that MBFT has improved the query speed by up to 286× compared to state-of-the-art authenticated query solutions.

Semantic caching significantly reduces computational costs and improves efficiency by storing and reusing large language model (LLM) responses. However, existing systems rely primarily on matching individual queries, lacking awareness of multi-turn dialogue contexts, which leads to incorrect cache hits when similar queries appear in different conversational settings. This demonstration introduces ContextCache, a context-aware semantic caching system for multi-turn dialogues. ContextCache employs a two-stage retrieval architecture that first executes vector-based retrieval on the current query to identify potential matches and then integrates current and historical dialogue representations through self-attention mechanisms for precise contextual matching. Evaluation of real-world conversations shows that ContextCache improves precision and recall compared to existing methods. Additionally, cached responses exhibit approximately 10 times lower latency than direct LLM invocation, enabling significant computational cost reductions for LLM conversational applications.

Ridesharing services, such as Uber or Didi, have attracted considerable attention in recent years due to their positive impact on environmental protection and the economy. Existing studies require quick responses to orders, which lack the flexibility to accommodate longer wait times for better grouping opportunities. In this paper, we address a NP-hard ridesharing problem, called Minimal Extra Time RideSharing (METRS), which balances waiting time and group quality (i.e., detour time) to improve riders' satisfaction. To tackle this problem, we propose a novel approach called WATTER (WAit To be fasTER), which leverages an order pooling management algorithm allowing orders to wait until they can be matched with suitable groups. The key challenge is to customize the extra time threshold for each order by reducing the original optimization objective into a convex function of threshold, thus offering a theoretical guarantee to be optimized efficiently. We model the dispatch process using a Markov Decision Process (MDP) with a carefully designed value function to learn the threshold. Through extensive experiments on three real datasets, we demonstrate the efficiency and effectiveness of our proposed approaches.

The popularity of decentralized finance has drawn attention to liquidity mining (LM). In LM, a user deposits her cryptocurrencies into liquidity pools to provide liquidity for exchanges and earn yields. Different liquidity pools offer varying yields and require different pairs of cryptocurrencies. A user can exchange a cryptocurrency for another with some exchange costs. Thus, an LM solution consists of exchange transactions and deposit transactions, guaranteeing (1) each exchange transaction must exchange one cryptocurrency for another at a specific rate (i.e., the exchange constraint); (2) the amounts of cryptocurrencies deposited in a liquidity pool must exceed the required threshold (i.e., the minimum constraint); (3) each deposit transaction must deposit a specific pair of cryptocurrencies at a certain rate in a liquidity pool (i.e., the deposit constraint); and (4) the cryptocurrencies used in the solution do not exceed the cryptocurrencies that the user has (i.e., the budget constraint). Selecting the most profitable LM solution is challenging due to the vast number of candidate solutions. To address this challenge, we define the yield maximization liquidity mining (YMLM) problem. Given a set of liquidity pools, a set of the user's cryptocurrencies, a set of exchange rates, and an evaluation function, YMLM aims to find an LM solution with maximal yields, satisfying the minimum, exchange, deposit, and budget constraints. We prove that YMLM is NP-hard and cannot be solved by algorithms with constant approximation ratios. To tackle YMLM, we propose two algorithms, namely YMLM\_GD and YMLM\_SK, with parameterized approximation ratios. Extensive experiments on both real and synthetic datasets show that our approaches outperform the baselines in yields.

Coin mixing methods are widely used to enhance anonymity in cryptocurrency transactions by obfuscating the linkages between recipients and senders. Specifically, coin mixing methods combine several users' transactions into a CoinJoin transaction and decompose the original transactions' outputs into a set of decomposed outputs with similar amounts. However, existing methods have two shortcomings. Firstly, CoinJoin transactions lack anonymity guarantees. Secondly, the number of decomposed outputs is not minimized. To tackle these two shortcomings, we develop a platform named CMixing for mixing transactions with anonymity guarantees and minimal fees. For a CoinJoin transaction obtained by CMixing, the probability of adversaries correctly guessing the original output of a decomposed output does not exceed c , where c is a privacy requirement. Thus, the first shortcoming is solved. Additionally, CMixing uses an approximation algorithm to decompose original outputs, which approximately minimizes the number of decomposed outputs. Thus, the second shortcoming is solved. Our demonstration will showcase how users can use CMixing to make CoinJoin transactions. We will also show the fees saved and the level of anonymity achieved using our algorithm.

Recently, transportation-as-a-service (TaaS) becomes an increasing trend, and online taxi platforms start to apply electric vehicles to serve passengers. Since the recharging time of an electric vehicle is long and non-negligible, it is necessary to smartly arrange the recharging schedules of electric vehicles in working schedules. In order to maximize the number of served taxi-calling tasks, online taxi platforms assign electric vehicles whose remaining electric power is enough to serve the dynamically arriving taxi-calling tasks and schedule suitable idle vehicles to recharging piles to recharge. We formally define the power-aware electric vehicle assignment (PAEVA) problem to serve as many taxi-calling tasks as possible under the constraints of remaining electric power and deadline. We prove that the PAEVA problem is NP-hard. To solve PAEVA, we design a novel strategy to help arrange the schedules of electric vehicles. Specifically, the strategy requires that, in a time slot and an area gird, the ratio of the number of electric vehicles whose remaining electric power is higher than a threshold$\alpha$to the number of predicted taxi-calling tasks should be higher than a threshold$\beta$. We propose two approximation approaches with theoretical guarantees to adaptively determine the values of the two thresholds of the strategy. We evaluate our solutions’ effectiveness and efficiency by comprehensive experiments on real datasets.

The payment channel network (PCN) is a promising solution to increase the throughput of blockchains. However, unidirectional transactions can deplete a user's deposits in a payment channel (PC), reducing the success ratio of transactions (SRoT). To address this depletion issue, rebalance protocols are used to shift tokens from well-deposited PCs to under-deposited PCs. To improve SRoT, it is beneficial to increase the balance of a PC with a lower balance and a higher weight (i.e., more transaction executions rely on the PC). In this paper, we define the utility of a transaction and the utility-aware rebalance (UAR) problem. The utility of a transaction is proportional to the weight of the PC and the amount of the transaction, and inversely proportional to the balance of the receiver. To maximize the effect of improving SRoT, UAR aims to find a set of transactions with maximized utilities, satisfying the budget and conservation constraints. The budget constraint limits the number of tokens shifted in a PC. The conservation constraint requires that the number of tokens each user sends equals the number of tokens received. We prove that UAR is NP-hard and cannot be approximately solved with a constant ratio. Thus, we propose two heuristic algorithms, namely Circuit Greedy and UAR_DC. Extensive experiments show that our approaches outperform the existing approach by at least 3.16 times in terms of utilities.

Recently, more and more higher education institutions have been using student feedback questionnaires (SFQ) to evaluate teaching. However, existing SFQ systems have two shortcomings. The first is that the respondent of an SFQ is not anonymous. The second is that the statistical report of SFQs can be manipulated. To tackle these two shortcomings, we develop a novel SFQ system, namely PSFQ. In PSFQ, the respondent of an SFQ is mixed with multiple users by a ring signature. PSFQ uses an advanced ring signature approach to minimize the size of a ring signature when anonymity satisfies the requirements. Thus, the first shortcoming has been overcome. Moreover, all answers are encrypted by homomorphic encryption and stored on the blockchain, enabling users to verify the correctness of the statistical reports. Our demonstration will showcase how PSFQ provides confidential SFQ responses while ensuring the correctness of statistical reports.

Due to the transparency of blockchain, adversaries can observe the details of a transaction, and then utilize the amount as a unique quasi-identifier to make deanonymization. Nowadays, to obscure the linkages between receivers and senders within a transaction on the blockchain, mixing services are widely applied in many real applications to enhance cryptocurrencies' anonymity. The basic idea of mixing services is to hide an output within several other outputs in a transaction such that adversaries cannot distinguish them by their amounts since they are purposely selected to have the same amount. For a set of original outputs with different amounts, mixing services need to decompose them into a set of decomposed outputs, where any decomposed output has some other decomposed outputs with the same amount. Since the transaction fee is related to the number of outputs, we are motivated to decompose original outputs into a minimal set of decomposed outputs, which is challenging to guarantee the privacy-preserving effect at the same time. In this paper, we formally define the anonymity-aware output decomposition (AA-OD) problem, which aims to find a c-decomposition with a minimum number of decomposed outputs for a given original output set. A c-decomposition guarantees that for any original output$o$, there are at most$c$of all decomposed outputs with an amount of$x$coming from$o$. We prove that the AA-OD problem is NP-hard. Thus, we propose an approximation algorithm, namely Boggart11Boggart is a magical creature in J. K. Rowling's Harry Potter series who can shift his shape and no one knows what it looks like., to solve the AA-OD problem with a (2/c + 3)-approximation bound on the number of decomposed outputs. We verify the efficiency and effectiveness of our approach through comprehensive experiments on both real and synthetic data sets.

To encourage users to buy more products, many online stores offer coupons. When a customer finds that the price of the products she/he wants to order is below the threshold of a coupon, she/he might want to place the order together with others to meet this threshold and enjoy more instant rebates. However, to conduct these orders and deliver products to receivers, users may need to pay extra delivery costs. When an order comprises several receivers' requests, the products in the order should first be delivered from stores to an assigned warehouse, packed into different packages, and delivered to the different receivers. It may be costly than directly delivering products from stores to receivers. For the benefits of buyers, we propose a platform, the Crowdrebate platform, which collects requests from users, groups requests into a set of orders to get more rebates, and relays products to different receivers in an order. The platform will make a profit by getting a proportion from the benefit of the receiver (defined as the rebate minus the extra cost) of orders as its revenue. In this paper, we define the Crowdrebate problem, which aims to maximize the benefit of receivers. We prove the NP-hardness of the Crowdrebate problem. Therefore, we propose a heuristic solution to address the problem. Moreover, we evaluate the effectiveness and efficiency of our algorithm via comprehensive experiments.

In privacy-preserving blockchain systems, to protect a sender's identity of a transaction in privacy-preserving blockchain systems, ring signature (RS) schemes have been widely implemented, which allow users to obscure consumed tokens via including "mixin'' (i.e., chaff tokens). However, recent works point out that existing RS schemes are vulnerable to the "chain-reaction'' analysis, where adversaries eliminate mixins of RSs by utilizing the fact that each token can only be consumed in a RS. By "chain-reaction'' analysis, adversaries can find some definite token-RS pair sets (DTRSs) to confirm the sender's identity of a RS. Besides, the existing RS schemes do not consider the diversity of mixins when generating a RS. Moreover, since the transaction fee is proportional to the number of mixins, a use is motivated to use a RS with the minimum number of mixins. In this paper, we formally define the diversity-aware mixins selection (DA-MS) problem, which aims to generate a RS with the minimum number of mixins satisfying the constraints of its diversity and the anonymity of other RSs. We prove the DA-MS problem is $\#P$ and propose a breadth-first search algorithm to get the optimal solution. Furthermore, to efficiently solve the DA-MS problem, we propose two practical configurations and two approximation algorithms with theoretic guarantees. Through comprehensive experiments on real data sets as well as synthetic data sets, we illustrate the effectiveness and the efficiency of our solutions.

Ubiquitous smart devices and high-quality wireless networks enable people to participate in spatial crowdsourcing tasks easily, which require workers to physically move to specific locations to conduct their assigned tasks. Spatial crowdsourcing has attracted much attention from both academia and industry. In this paper, we consider a spatial crowdsourcing scenario, where the tasks may have some dependencies among them. Specifically, one task can only be dispatched when its dependent tasks have already been assigned. In fact, task dependencies are quite common in many real-life applications, such as house repairing and holding sports games. We formally define the dependency-aware spatial crowdsourcing (DA-SC), which focuses on finding an optimal worker-and-task assignment under the constraints of dependencies, skills of workers, moving distances and deadlines to maximize the successfully assigned tasks. We prove that the DA-SC problem is NP-hard and thus intractable. Therefore, we propose two approximation algorithms, including a greedy approach and a game-theoretic approach, which can guarantee the approximate bounds of the results in each batch process. Through extensive experiments on both real and synthetic data sets, we demonstrate the efficiency and effectiveness of our DA-SC approaches.