Fair transaction ordering protocols that guarantee γ-Batch-Order-Fairness are a leading defense against Maximal Extractable Value (MEV) attacks in Byzantine consensus and blockchain systems. All existing such protocols build a tournament dependency graph among transactions, incurring Θ(|T|2) comparator queries per round on a transaction set T, which becomes the dominant performance bottleneck.

Distributed ledgers are enabling novel applications in traditional domains, such as finance, healthcare and supply chains, and in emerging domains such as metaverse. We observe that the current ecosystem is fragmented, with different blockchains operating in silos. Interledger applications that can access resources in different ledgers can tap into the billions of dollars worth of locked resources, but they require support for interledger communication. Existing interledger communications, however, are either insecure, inefficient, or application specific. Our goal is to design a system for interledger applications. To this end, we design and implement that achieves strong security guarantees and high performance while providing the general message passing abstraction to the applications. Our system leverages trusted hardware for performance and threshold signature schemes for strong security. It consists of multiple lightweight clients that verify transactions within their trusted execution environments before generating threshold signatures. We conduct extensive performance evaluations of and compare it against three state-of-the-art systems, namely WeCross, Cosmos IBC and LayerZero-V2. The results show that is efficient, achieving up to$42\times$higher throughput than the baselines.

Stream processing systems are important in modern applications in which data arrive continuously and need to be processed in real time. Because of their resource and scalability requirements, many of these systems run on the cloud, which is considered untrusted. Existing works on securing databases on the cloud focus on protecting the data, and most systems leverage trusted hardware for high performance. However, in stream processing systems, queries are as sensitive as the data because they contain the application logics. We demonstrate that it is practical to extract the queries from stream processing systems that use Intel SGX for securing the execution engine. The attack performed by a malicious cloud provider is based on timing side channels, and it works in two phases. In the offline phase, the attacker profiles the execution time of individual stream operators, based on synthetic data. This phase outputs a model that identifies individual stream operators. In the online phase, the attacker isolates the operators that make up the query, monitors its execution, and recovers the operators using the model in the previous phase. We implement the attack based on popular data stream benchmarks using SecureStream and NEXMark, and demonstrate attack success rates of up to 92%. We further discuss approaches that can harden streaming processing systems against our attacks without incurring high overhead.

Model inference systems are essential for implementing end-to-end data analytics pipelines that deliver the benefits of machine learning models to users. Existing cloud-based model inference systems are costly, not easy to scale, and must be trusted in handling the models and user request data. Serverless computing presents a new opportunity, as it provides elasticity and fine-grained npricing. Our goal is to design a serverless model inference system that protects models and user request data from untrusted cloud providers. It offers high performance and low cost, while requiring no intrusive changes to the current serverless platforms. To realize our goal, we leverage trusted hardware. We identify and address three challenges in using trusted hardware for serverless model inference. These challenges arise from the high-level abstraction of serverless computing, the performance overhead of trusted hardware, and the characteristics of model inference workloads. We present SeSeMI, a secure, efficient, and cost-effective serverless model inference system. It adds three novel features non-intrusively to the existing serverless infrastructure and nothing else. The first feature is a key service that establishes secure channels between the user and the serverless instances, which also provides access control to models and users' data. The second is an enclave runtime that allows one enclave to process multiple concurrent requests. The final feature is a model packer that allows multiple models to be executed by one serverless instance. We build SeSeMI on top of Apache Open Whisk, and conduct extensive experiments with three popular machine learning models. The results show that SeSeMI achieves low latency and low cost at scale for realistic workloads.

Decentralized Finance (DeFi), in which digital assets are exchanged without trusted intermediaries, has grown rapidly in value in recent years. Stablecoins , which are pegged to a non-volatile asset such as the US dollar, are a prominent feature of DeFi as they mitigate the risk associated with price fluctuations. However, existing stablecoin systems are tied to individual blockchain platforms, and trusted parties or complex protocols are needed to exchange stablecoin tokens between blockchains. Our goal is to design a practical stablecoin system for cross-chain commerce, and to do so we must overcome two main challenges. The first is to support a large and growing number of blockchains efficiently. The second is for the stablecoin to be resilient to blockchain platform failures and to price fluctuations that affect its collateral. We present CroCoDai to address these challenges. We demonstrate CroCoDai ’s efficiency by comparing the performance of a prototype implementation to related baselines, and its resilience through an empirical analysis of historical token price data.

Modern deep learning models capture the semantics of complex data by transforming them into high-dimensional embedding vectors. Emerging applications, such as retrieval-augmented generation, use approximate nearest neighbor (ANN) search in the embedding vector space to find similar data. Existing vector databases provide indexes for efficient ANN searches, with graph-based indexes being the most popular due to their low latency and high recall in real-world high-dimensional datasets. However, these indexes are costly to build, suffer from significant contention under concurrent read-write workloads, and scale poorly to multiple servers. Our goal is to build a vector database that achieves high throughput and high recall under concurrent read-write workloads. To this end, we first propose an ANN index with an explicit two-stage design combining a fast filter stage with highly compressed vectors and a refine stage to ensure recall, and we devise a novel lightweight machine learning technique to fine-tune the index parameters. We introduce an early termination check to dynamically adapt the search process for each query. Next, we add support for writes while maintaining search performance by decoupling the management of the learned parameters. Finally, we design HAKES, a distributed vector database that serves the new index in a disaggregated architecture. We evaluate our index and system against 12 state-of-the-art indexes and three distributed vector databases, using high-dimensional embedding datasets generated by deep learning models. The experimental results show that our index outperforms index baselines in the high recall region and under concurrent read-write workloads. Furthermore, HAKES is scalable and achieves up to 16x higher throughputs than the baselines.

While state-of-the-art permissioned blockchains can achieve thousands of transactions per second on commodity hardware with x86/64 architecture, their performance when running on different architectures, such as ARM, is not clear. The goal of this work is to characterize the performance and cost of permissioned blockchains on different hardware systems, which is important as diverse application domains are adopting blockchains. To this end, we conduct extensive cost and performance evaluation of two permissioned blockchains, namely Hyperledger Fabric and ConsenSys Quorum, on five different types of hardware covering both x86/64 and ARM architecture, as well as both cloud and edge computing. The hardware nodes include servers with Intel Xeon CPU, servers with ARM-based Amazon Graviton CPU, and edge devices with ARM-based CPU such as Nvidia Jetson TX2 and Raspberry Pi 4. Our results reveal a diverse profile of the two blockchains across different settings, demonstrating the impact of hardware choices on the overall performance and cost. We find that Graviton servers outperform Xeon servers in many settings due to their powerful CPU and high memory bandwidth. Edge devices with ARM architecture, on the other hand, exhibit low performance. When comparing the cloud with the edge, we show that the cost of the latter is much smaller in the long run if manpower cost is not considered.

Machine learning models are only as good as their training data. Simple models trained on well-chosen features extracted from the raw data often outperform complex models trained directly on the raw data. Data preparation pipelines, which clean and derive features from the data, are therefore important for machine learning applications. However, constructing such pipelines is a resource-intensive process that involves deep human expertise. Our goal is to design an efficient framework for automatically finding high-quality data preparation pipelines. The main challenge is how to explore a large search space of pipeline components with the objective of computing features that maximize the performance of the downstream models. Existing solutions are limited in terms of feature quality, which results in low accuracies of the downstream models, while incurring significant runtime overhead. We present CtxPipe, a novel framework that addresses the limitations of previous works by leveraging contextual information to improve the pipeline construction process. Specifically, it uses pre-trained embedding models to capture the data semantics, which are then used to guide the selection of pipeline components. We implement CtxPipe with deep reinforcement learning and evaluate it against state-of-the-art automated pipeline construction solutions. Our comprehensive experiments demonstrate that CtxPipe outperforms all of the baselines in both model performance and runtime cost.

A plethora of different blockchain platforms have emerged in recent years, but many of them operate in silos. As such, there is a need for reliable cross-chain communication to enable blockchain interoperability. Blockchain interoperability is challenging because transactions can typically not be reverted – as such, if one transaction is committed then the protocol must ensure that all related transactions are committed as well. Existing interoperability approaches, e.g., Cosmos and Polkadot, are limited in the sense that they only support interoperability between their own subchains, or require intrusive changes to existing blockchains. To overcome this limitation, we propose PIEChain, a general, Kafka-based cross-chain communication framework. We utilize PIEChain for a practical case study: a cross-chain auction in which users who hold tokens on multiple chains bid for a ticket sold on another chain. PIEChain is the first publicly available, practical implementation of a general framework for cross-chain communication.

Modern data analytics pipelines are highly dynamic, as they are constantly monitored and fine-tuned by both data engineers and scientists. Recent systems managing pipelines ease creating, deploying, and tracking their evolution. However, privacy concerns emerge as many of them are deployed on the public cloud with less or no trust. Unfortunately, the unique nature of pipelines prevents the adoption of existing confidential computing techniques with different computational patterns and large performance overhead. Being a potential approach, trusted execution environments (TEEs) are efficient in protecting the confidentiality and integrity of data and computation. However, fast-changing pipelines with latency requirements bring the challenge of reducing the cold start overhead --- the main bottleneck in the latest TEE. To support end-to-end private pipeline evolution, we present SecCask, a TEE-based data analytics pipeline management system. SecCask overcomes the problems of a naive design that isolates complete pipeline execution in one enclave by administering enclaves and runtimes. To reduce cold start overheads, our approach consists of reusing trusted runtimes for different pipeline components and caching them to avoid the cost of initialization. We leverage the latest Intel SGX to conduct experiments on representative workloads. The results demonstrate that SecCask reduces the total execution time by 68.4% compared to not reusing, is faster than running all components in one enclave, and incurs a modest average performance overhead of 29.9% over insecure baselines.

Federated learning (FL) enables multiple data owners to collaboratively train machine learning (ML) models without disclosing their raw data. In the vertical federated learning (VFL) setting, the collaborating parties have data from the same set of users but with disjoint attributes. After constructing the VFL models, the parties deploy the models in production systems to infer prediction requests. In practice, the prediction output itself may not be convincing for party users to make the decisions, especially in high-stakes applications. Model interpretability is therefore essential to provide meaningful insights and better comprehension on the prediction output. In this paper, we propose Falcon, a novel privacy-preserving and interpretable VFL system. First, Falcon supports VFL training and prediction with strong and efficient privacy protection for a wide range of ML models, including linear regression, logistic regression, and multi-layer perceptron. The protection is achieved by a hybrid strategy of threshold partially homomorphic encryption (PHE) and additive secret sharing scheme (SSS), ensuring no intermediate information disclosure. Second, Falcon facilitates understanding of VFL model predictions by a flexible and privacy-preserving interpretability framework, which enables the implementation of state-of-the-art interpretable methods in a decentralized setting. Third, Falcon supports efficient data parallelism of VFL tasks and optimizes the parallelism factors to reduce the overall execution time. Falcon is fully implemented, and on which, we conduct extensive experiments using six real-world and multiple synthetic datasets. The results demonstrate that Falcon achieves comparable accuracy to non-private algorithms and outperforms three secure baselines in terms of efficiency.

Verifiable ledger databases protect data history against malicious tampering. Existing systems, such as blockchains and certificate transparency, are based on transparency logs --- a simple abstraction allowing users to verify that a log maintained by an untrusted server is append-only. They expose a simple key-value interface without transactions. Building a practical database from transparency logs, on the other hand, remains a challenge. In this paper, we explore the design space of verifiable ledger databases along three dimensions: abstraction, threat model, and performance. We survey existing systems and identify their two limitations, namely, the lack of transaction support and the inferior efficiency. We then present GlassDB, a distributed database system that addresses these limitations under a practical threat model. GlassDB inherits the verifiability of transparency logs, but supports transactions and offers high performance. It extends a ledgerlike key-value store with a data structure for efficient proofs, and adds a concurrency control mechanism for transactions. GlassDB batches independent operations from concurrent transactions when updating the core data structures. In addition, we design a new benchmark for evaluating verifiable ledger databases, by extending YCSB and TPC-C benchmarks. Using this benchmark, we compare GlassDB against four baselines: reimplemented versions of three verifiable databases, and a verifiable map backed by a transparency log. Experimental results demonstrate that GlassDB is an efficient, transactional, and verifiable ledger database system.

Pooled mining has become the most popular mining approach in the Bitcoin system, which can effectively reduce the variance of the block generation reward of participants. The security of pooled mining depends on whether it is incentive compatible, that is, an honest participant will get a reward proportional to his work. Recent attacks on mining pools, for example, Block Withholding, Fork After Withholding, and Power Adjusting Withholding (PAW) attacks, show that malicious participants may undermine the revenue of the honest pools and receive an unfair share of the mining reward. This paper shows that the security of Bitcoin is even worse than what the recent attacks demonstrated. We describe an attack called Fork Withholding Attack under a Protection Racket (FWAP), in which the mining pool pays the attacker for withholding a fork. Our insight is that the mining pools under forking attacks have incentives to pay in exchange for not being forked. The attacker and the paying pool negotiate how much to be paid, and we show that it is possible for both the attacker and the paying pool to earn higher rewards at the expense of the other pools. In particular, our formal analysis and simulation demonstrate that the payer and the FWAP attacker can get up to 1.8 × and 3.8 × of extra reward as in PAW, respectively. Furthermore, FWAP can escape from the “miners’ dilemma’’ when two FWAP attackers attack each other under some circumstances. We also propose simple approaches that serve as the first step towards preventing the FWAP attack.

Smart grids leverage the data collected from smart meters to make important operational decisions. However, they are vulnerable to False Data Injection (FDI) attacks in which an attacker manipulates meter data to disrupt the grid operations. Existing works on FDI are based on a simple threat model in which a single grid operator has access to all the data, and only some meters can be compromised.

Smart cities deploy large numbers of sensors and collect a tremendous amount of data from them. For example, Advanced Metering Infrastructures (AMIs), which consist of physical meters that collect usage data about public utilities such as power and water, are an important building block in a smart city. In a typical sensor network, the measurement devices are connected through a computer network, which exposes them to cyber attacks. Furthermore, the data is centrally managed at the operator’s servers, making it vulnerable to insider threats.Our goal is to protect the integrity of data collected by large-scale sensor networks and the firmware in measurement devices from cyber attacks and insider threats. To this end, we first develop a comprehensive threat model for attacks against data and firmware integrity, which can target any of the stakeholders in the operation of the sensor network. Next, we use our threat model to analyze existing defense mechanisms, including signature checks, remote firmware attestation, anomaly detection, and blockchain-based secure logs. However, the large size of the Trusted Computing Base and a lack of scalability limit the applicability of these existing mechanisms. We propose the Feather-Light Blockchain Infrastructure (FLBI) framework to address these limitations. Our framework leverages a two-layer architecture and cryptographic threshold signature chains to support large networks of low-capacity devices such as meters and data aggregators. We have fully implemented the FLBI’s end-to-end functionality on the Hyperledger Fabric and private Ethereum blockchain platforms. Our experiments show that the FLBI is able to support millions of end devices.

Vehicle-to-everything (V2X) communication is the key enabler for emerging intelligent transportation systems. Applications built on top of V2X require both authentication and privacy protection for the vehicles. The common approach to meet both requirements is to use pseudonyms which are short-term identities. However, both industrial standards and state-of-the-art research are not designed for resource-constrained environments. In addition, they make a strong assumption about the security of the vehicle's on-board computation units. In this paper, we propose a lightweight auto-refreshing pseudonym protocol (LARP) for V2X. LARP supports efficient operations for resource-constrained devices, and provides security even when parts of the vehicle are compromised. We provide formal security proof showing that the protocol is secure. We conduct experiments on a Raspberry Pi 4. The results demonstrate that LARP is feasible and practical.

Machine learning (ML) is an important part of modern data science applications. Data scientists today have to manage the end-to-end ML life cycle that includes both model training and model serving, the latter of which is essential, as it makes their works available to end-users. Systems of model serving require high performance, low cost, and ease of management. Cloud providers are already offering model serving choices, including managed services and self-rented servers. Recently, serverless computing, whose advantages include high elasticity and a fine-grained cost model, brings another option for model serving.

Time-based One-Time Password (TOTP) provides a strong second factor for user authentication. In TOTP, a prover authenticates to a verifier by using the current time and a secret key to generate an authentication token (or password) which is valid for a short time period. Our goal is to extend TOTP to the group setting, and to provide both authentication and privacy. To this end, we introduce a new authentication scheme, called Group TOTP (GTOTP), that allows the prover to prove that it is a member of an authenticated group without revealing its identity. We propose a novel construction that transforms any asymmetric TOTP scheme into a GTOTP scheme. Our approach combines Merkle tree and Bloom filter to reduce the verifier’s states to constant sizes.

Blockchain has come a long way - a system that was initially proposed specifically for cryptocurrencies is now being adapted and adopted as a general-purpose transactional system. As blockchain evolves into another data management system, the natural question is how it compares against distributed database systems. Existing works on this comparison focus on high-level properties, such as security and throughput. They stop short of showing how the underlying design choices contribute to the overall differences. Our work fills this important gap.

Consensus protocols are currently the bottlenecks that prevent blockchain systems from scaling. However, we argue that transaction execution is also important to the performance and security of blockchains. In other words, there are ample opportunities to speed up and further secure blockchains by reducing the cost of transaction execution. Our goal is to understand how much we can speed up blockchains by exploiting transaction concurrency available in blockchain workloads. To this end, we first analyze historical data of seven major public blockchains, namely Bitcoin, Bitcoin Cash, Litecoin, Dogecoin, Ethereum, Ethereum Classic, and Zilliqa. We consider two metrics for concurrency, namely the single-transaction conflict rate per block, and the group conflict rate per block. We find that there is more concurrency in UTXO-based blockchains than in account-based ones, although the amount of concurrency in the former is lower than expected. Another interesting finding is that some blockchains with larger blocks have more concurrency than blockchains with smaller blocks. Next, we propose an analytical model for estimating the transaction execution speed-up given an amount of concurrency. Using results from our empirical analysis, the model estimates that 6× speed-ups in Ethereum can be achieved if all available concurrency is exploited.

Data collaboration activities typically require systematic or protocol-based coordination to be scalable. Git, an effective enabler for collaborative coding, has been attested for its success in countless projects around the world. Hence, applying the Git philosophy to general data collaboration beyond coding is motivating. We call it Git for data. However, the original Git design handles data at the file granule, which is considered too coarse-grained for many database applications. We argue that Git for data should be co-designed with database systems. To this end, we developed ForkBase to make Git for data practical. ForkBase is a distributed, immutable storage system designed for data version management and data collaborative operation. In this demonstration, we show how ForkBase can greatly facilitate collaborative data management and how its novel data deduplication technique can improve storage efficiency for archiving massive data versions.

With 5G on the verge of being adopted as the next mobile network, there is a need to analyze its impact on the landscape of computing and data management. In this paper, we analyze the impact of 5G on both traditional and emerging technologies and project our view on future research challenges and opportunities. With a predicted increase of 10-100× in bandwidth and 5-10x decrease in latency, 5G is expected to be the main enabler for smart cities, smart IoT and efficient healthcare, where machine learning is conducted at the edge. In this context, we investigate how 5G can help the development of federated learning. Network slicing, another key feature of 5G, allows running multiple isolated networks on the same physical infrastructure. However, security remains the main concern in the context of virtualization, multi-tenancy and high device density. Formal verification of 5G networks can be applied to detect security issues in massive virtualized environments. In summary, 5G will make the world even more densely and closely connected. What we have experienced in 4G connectivity will pale in comparison to the vast amounts of possibilities engendered by 5G.

Existing blockchain systems scale poorly because of their distributed consensus protocols. Current attempts at improving blockchain scalability are limited to cryptocurrency. Scaling blockchain systems under general workloads (i.e., non-cryptocurrency applications) remains an open question. This work takes a principled approach to apply sharding to blockchain systems in order to improve their transaction throughput at scale. This is challenging, however, due to the fundamental difference in failure models between databases and blockchain. To achieve our goal, we first enhance the performance of Byzantine consensus protocols, improving individual shards' throughput. Next, we design an efficient shard formation protocol that securely assigns nodes into shards. We rely on trusted hardware, namely Intel SGX, to achieve high performance for both consensus and shard formation protocol. Third, we design a general distributed transaction protocol that ensures safety and liveness even when transaction coordinators are malicious. Finally, we conduct an extensive evaluation of our design both on a local cluster and on Google Cloud Platform. The results show that our consensus and shard formation protocols outperform state-of-the-art solutions at scale. More importantly, our sharded blockchain reaches a high throughput that can handle Visa-level workloads, and is the largest ever reported in a realistic environment.

The success of Bitcoin and other cryptocurrencies bring enormous interest to blockchains. A blockchain system implements a tamper-evident ledger for recording transactions that modify some global states. The system captures entire evolution history of the states. The management of that history, also known as data provenance or lineage, has been studied extensively in database systems. However, querying data history in existing blockchains can only be done by replaying all transactions. This approach is applicable to large-scale, offline analysis, but is not suitable for online transaction processing. We present LineageChain , a fine-grained, secure and efficient provenance system for blockchains. LineageChain exposes provenance information to smart contracts via simple and elegant interfaces, thereby enabling a new class of blockchain applications whose execution logics depend on provenance information at runtime. LineageChain captures provenance during contract execution, and efficiently stores it in a Merkle tree. LineageChain provides a novel skip list index designed for supporting efficient provenance query processing. We have implemented LineageChain on top of Hyperledger and a blockchain-optimized storage system called ForkBase. Our extensive evaluation of LineageChain demonstrates its benefits to the new class of blockchain applications, its efficient query, and its small storage overhead.

Existing data storage systems offer a wide range of functionalities to accommodate an equally diverse range of applications. However, new classes of applications have emerged, e.g., blockchain and collaborative analytics, featuring data versioning, fork semantics, tamper-evidence or any combination thereof. They present new opportunities for storage systems to efficiently support such applications by embedding the above requirements into the storage. In this paper, we present ForkBase , a storage engine designed for blockchain and forkable applications. By integrating core application properties into the storage, ForkBase not only delivers high performance but also reduces development effort. The storage manages multiversion data and supports two variants of fork semantics which enable different fork worklflows. ForkBase is fast and space efficient, due to a novel index class that supports efficient queries as well as effective detection of duplicate content across data objects, branches and versions. We demonstrate ForkBase 's performance using three applications: a blockchain platform, a wiki engine and a collaborative analytics application. We conduct extensive experimental evaluation against respective state-of-the-art solutions. The results show that ForkBase achieves superior performance while significantly lowering the development effort.

Blockchain technologies are gaining massive momentum in the last few years. Blockchains are distributed ledgers that enable parties who do not fully trust each other to maintain a set of global states. The parties agree on the existence, values, and histories of the states. As the technology landscape is expanding rapidly, it is both important and challenging to have a firm grasp of what the core technologies have to offer, especially with respect to their data processing capabilities. In this paper, we first survey the state of the art, focusing on private blockchains (in which parties are authenticated). We analyze both in-production and research systems in four dimensions: distributed ledger, cryptography, consensus protocol, and smart contract. We then present BLOCKBENCH, a benchmarking framework for understanding performance of private blockchains against data processing workloads. We conduct a comprehensive evaluation of three major blockchain systems based on BLOCKBENCH, namely Ethereum, Parity, and Hyperledger Fabric. The results demonstrate several trade-offs in the design space, as well as big performance gaps between blockchain and database systems. Drawing from design principles of database systems, we discuss several research directions for bringing blockchain performance closer to the realm of databases.

Blockchain technologies are taking the world by storm. Public blockchains, such as Bitcoin and Ethereum, enable secure peer-to-peer applications like crypto-currency or smart contracts. Their security and performance are well studied. This paper concerns recent private blockchain systems designed with stronger security (trust) assumption and performance requirement. These systems target and aim to disrupt applications which have so far been implemented on top of database systems, for example banking, finance and trading applications. Multiple platforms for private blockchains are being actively developed and fine tuned. However, there is a clear lack of a systematic framework with which different systems can be analyzed and compared against each other. Such a framework can be used to assess blockchains' viability as another distributed data processing platform, while helping developers to identify bottlenecks and accordingly improve their platforms.

Abstract We consider privacy-preserving computation of big data using trusted computing primitives with limited private memory. Simply ensuring that the data remains encrypted outside the trusted computing environment is insufficient to preserve data privacy, for data movement observed during computation could leak information. While it is possible to thwart such leakage using generic solution such as ORAM [42], designing efficient privacy-preserving algorithms is challenging. Besides computation efficiency, it is critical to keep trusted code bases lean, for large ones are unwieldy to vet and verify. In this paper, we advocate a simple approach wherein many basic algorithms (e.g., sorting) can be made privacy-preserving by adding a step that securely scrambles the data before feeding it to the original algorithms. We call this approachScramble-then-Compute(StC), and give a sufficient condition whereby existing external memory algorithms can be made privacy-preserving via StC. This approach facilitates code-reuse, and its simplicity contributes to a smaller trusted code base. It is also general, allowing algorithm designers to leverage an extensive body of known efficient algorithms for better performance. Our experiments show that StC could offer up to 4.1× speedups over known, application-specific alternatives.

Recently, deep learning techniques have enjoyed success in various multimedia applications, such as image classification and multimodal data analysis. Large deep learning models are developed for learning rich representations of complex data. There are two challenges to overcome before deep learning can be widely adopted in multimedia and other applications. One is usability, namely the implementation of different models and training algorithms must be done by nonexperts without much effort, especially when the model is large and complex. The other is scalability, namely the deep learning system must be able to provision for a huge demand of computing resources for training large models with massive datasets. To address these two challenges, in this article we design a distributed deep learning platform called SINGA , which has an intuitive programming model based on the common layer abstraction of deep learning models. Good scalability is achieved through flexible distributed training architecture and specific optimization techniques. SINGA runs on both GPUs and CPUs, and we show that it outperforms many other state-of-the-art deep learning systems. Our experience with developing and training deep learning models for real-life multimedia applications in SINGA shows that the platform is both usable and scalable.

Recently, deep learning techniques have enjoyed success in various multimedia applications, such as image classification and multi-modal data analysis. Two key factors behind deep learning's remarkable achievement are the immense computing power and the availability of massive training datasets, which enable us to train large models to capture complex regularities of the data. There are two challenges to overcome before deep learning can be widely adopted in multimedia and other applications. One is usability, namely the implementation of different models and training algorithms must be done by non-experts without much effort. The other is scalability, that is the deep learning system must be able to provision for a huge demand of computing resources for training large models with massive datasets. To address these two challenges, in this paper, we design a distributed deep learning platform called SINGA which has an intuitive programming model and good scalability. Our experience with developing and training deep learning models for real-life multimedia applications in SINGA shows that the platform is both usable and scalable.

In this paper, we focus on the problem of data privacy on the cloud, particularly on access controls over stream data. The nature of stream data and the complexity of sharing data make access control a more challenging issue than in traditional archival databases. We present Streamforce -- a system allowing data owners to securely outsource their data to an untrusted (curious-but-honest) cloud. The owner specifies fine-grained policies which are enforced by the cloud. The latter performs most of the heavy computations, while learning nothing about the data content. To this end, we employ a number of encryption schemes, including deterministic encryption, proxy-based attribute based encryption and sliding-window encryption. In Streamforce, access control policies are modeled as secure continuous queries, which entails minimal changes to existing stream processing engines, and allows for easy expression of a wide-range of policies. In particular, Streamforce comes with a number of secure query operators including Map, Filter, Join and Aggregate. Finally, we implement Streamforce over an open-source stream processing engine (Esper) and evaluate its performance on a cloud platform. The results demonstrate practical performance for many real-world applications, and although the security overhead is visible, Streamforce is highly scalable.

Peer-to-peer (P2P) based marketplaces have a number of advantages over traditional centralized systems (such as eBay). Peers form a distributed hash table and store sale offers for other peers. A key problem in such a system is ensuring that the peers store and report all sale offers fairly, and do not for instance favor their own offers. We give a solution to this problem based on trusted computing, but unlike other approaches we do not measure and restrict all firmware and software running on a peer. Instead, we tie offers to monotonic counters in such a way that any attempt to not report an offer, or report it falsely, will be detected.

P2P systems have witnessed phenomenal development in recent years. Evaluating and analyzing new and existing algorithms and techniques is a key issue for developers of P2P systems. In this context, simulation is an important tool for P2P developers. However, such systems are often very large and few existing simulators offer the ability to execute simulations with an Internet scale. In this paper we utilize parallel discrete event simulation simulation techniques for executing large scale simulation of P2P systems which scale effectively, only limited by the amount of computational resource available (memory and CPU). We show results from a number of P2P protocols, indicating good scalability both in terms of size (memory) and execution time (CPU). The results demonstrate how the differences of these protocols and which of the underlying factors affect the performance of the distributed simulation infrastructure.

P2P systems have witnessed phenomenal development in recent years, evaluating and analysing new and existing new algorithms and techniques is a key issue for developers of p2p systems. In this context Simulation is an important tool for p2p developers. However, such systems are often very large and few existing simulators offer the ability to execute systems of real world size. In this paper we present a tool for executing large scale simulation of p2p systems which scale effectively, only limited by the amount of computational resource available (memory and CPU). This is achieved through the application of parallel discrete event simulation techniques to an existing, already scalable simulator, peersim. We show results from a case study using the chord p2p protocol, indicating good scalability both in terms of size (memory) and execution time (CPU).