The Distributed Hash Table (DHT) architecture is known to be a very efficient way to implement peer-to-peer (P2P) computer networks. However, the scientific literature also proved that DHT functioning in P2P networks can be easily disrupted by a single entity controlling many peers, known as the Sybil Attack. Various defensive mechanisms are known to prevent such attacks, or at least hinder them. The current study evaluates the resiliency of the InterPlanetary File System (IPFS) P2P network to a legacy Sybil Attack. We show that, surprisingly, IPFS does not implement basic defense mechanisms, allowing the most simple attack from a single computer to easily take the control of any DHT entry. A practical use of this attack is to almost entirely deny access to a given content on the network. Thus we provide some recommendations to quickly remediate this vulnerability.

A key feature for most collaborative applications is their ability to distinguish access rights to shared documents in a dynamic group of collaborators. To achieve high availability and avoid single points of failure, systems can replicate this access control policy across sites. Increasingly, Conflict-free Replicated Data Types (CRDTs) are used to this end. They embed conflict resolution strategies that take into account concurrent modifications to the policy but also to the document that it manages. However, few systems in the literature support multiple administrators, and those which do fall short of considering distributed applications where no node or group of nodes is central to its operation. To allow multiple concurrent edits of the policy in massively collaborative settings with no central server, we devise a specialized causal model that minimizes concurrency. Crucially our model allows to correct the document state in case operations have become unauthorized following a policy change. We apply our model to a CRDT replicating an access control policy with read and write rights on a collaborative document and admin rights on the policy. We end by studying flexible conflict resolution strategies, providing our underlying algorithms.

Summary Wikis are one of the most important tools of Web 2.0 allowing users to easily edit shared data. However, wikis offer limited support for merging concurrent contributions on the same pages. Users have to manually merge concurrent changes, and there is no support for an automatic merging. Real‐time collaborative editing reduces conflicts as the time frame for concurrent work is very short. In this paper, we propose extending wiki systems with real‐time collaboration. We propose an automatic merging solution adapted for rich content wikis. Our solution relies on an operational transformation algorithm defined for high level operations that capture user actions such as move, merge, and split.

CRDTs (Conflict-free Replicated Data Types) have properties desirable for large-scale distributed systems with variable network latency or transient partitions. With CRDT, data are always available for local updates and data states converge when the replicas have incorporated the same updates. Undo is useful for correcting human mistakes and for restoring system-wide invariant violated due to long delays or network partitions. There is currently no generally applicable undo support for CRDTs. There are at least two reasons for this. First, there is currently no abstraction that we can practically use to capture the relations between undo and normal operations with respect to concurrency and causality. Second, using inverse operations as the existing partial solutions, the CRDT designer has to hard-code certain rules and design a new CRDT for almost every operation that needs undo support. In this paper, we present an approach to generic support of undo for CRDTs. The approach consists of two major parts. We first work out an abstraction that captures the semantics of concurrent undo and redo operations through equivalence classes. The abstraction is a natural extension of undo and redo in sequential applications and is straightforward to implement in practice. By using this abstraction, we then device a mechanism to augment existing CRDTs. The mechanism provides an "out of the box" support for undo without the involvement of the CRDT designers. We also present a practical application of the approach in collaborative editing.

The assessment of trust between users is essential for collaboration. General reputation and ID mechanisms may support users’ trust assessment. However, these mechanisms lack sensitivity to pairwise interactions and specific experience such as betrayal over time. Moreover, they place an interpretation burden that does not scale to dynamic, large-scale systems. While several pairwise trust mechanisms have been proposed, no empirical research examines trust score influence on participant behavior. We study the influence of showing a partner trust score and/or ID on participants’ behavior in a small-group collaborative laboratory experiment based on the trust game. We show that trust score availability has the same effect as an ID to improve cooperation as measured by sending behavior and receiver response. Excellent models based on the trust score predict sender behavior and document participant sensitivity to the provision of partner information. Models based on the trust score for recipient behavior have some predictive ability regarding trustworthiness, but suggest the need for more complex functions relating experience to participant response. We conclude that the parameters of a trust score, including pairwise interactions and betrayal, influence the different roles of participants in the trust game differently, but complement traditional ID and have the advantage of scalability.

Wikipedia is considered as the largest knowledge repository in the history of humanity and plays a crucial role in modern daily life. Assigning the correct quality class to Wikipedia articles is an important task in order to provide guidance for both authors and readers of Wikipedia. The manual review cannot cope with the editing speed of Wikipedia. An automatic classification is required to classify the quality of Wikipedia articles. Most existing approaches rely on traditional machine learning with manual feature engineering, which requires a lot of expertise and effort. Furthermore, it is known that there is no general perfect feature set because information leak always occurs in feature extraction phase. Also, for each language of Wikipedia, a new feature set is required.

Since the Web 2.0 era, the Internet is a huge content editing place in which users contribute to the content they browse. Users do not just edit the content but they collaborate on this content. Such shared content can be edited by thousands of people. However, current consistency maintenance algor

Within last years multi-synchronous collaborative editing systems became widely used. Multi-synchronous collaboration maintains multiple, simultaneous streams of activity which continually diverge and synchronized. These streams of activity are represented by means of logs of operations, i.e. user modifications. A malicious user might tamper his log of operations. At the moment of synchronization with other streams, the tampered log might generate wrong results. In this paper, we propose a solution relying on hash-chain based authenticators for authenticating logs that ensure the authenticity, the integrity of logs, and the user accountability. We present algorithms to construct authenticators and verify logs. We prove their correctness and provide theoretical and practical evaluations.

In multi-synchronous collaboration users replicate shared data, modify it and redistribute modified versions of this data without the need of a central authority. However, in this model, no usage restriction mechanism was proposed to control what users can do with the data after it has been released to them. In this paper, we extend the multi-synchronous collaboration model with contracts that express usage restrictions and that are checked a posteriori by users when they receive the modified data. We propose a merging algorithm that deals not only with changes on data but also with contracts. A log auditing protocol is used to detect users who do not respect contracts and to adjust user trust levels. Our contract-based model was implemented and evaluated by using PeerSim simulator.

In the push-pull-clone collaborative editing model widely used in distributed version control systems users replicate shared data, modify it and redistribute modified versions of this data without the need of a central authority. However, in this model no usage restriction mechanism is proposed to c

Nowadays, real-time editing systems are catching on. Tools such as Etherpad or Google Docs enable multiple authors at dispersed locations to collaboratively write shared documents. In such systems, a replication mechanism is required to ensure consistency when merging concurrent changes performed on the same document. Current editing systems make use of operational transformation (OT), a traditional replication mechanism for concurrent document editing.

In order to overcome the disadvantages of a central authority, a tendency is to move towards a peer-to-peer collaboration where control over data is given to users who can decide with whom they want to share their private data. In this peer-to-peer collaboration it is very difficult to ensure that after data is shared with other peers, these peers will not misbehave and violate data privacy. In this paper, we propose a mechanism that addresses the issue of data privacy violation by auditing the collaboration logs. In our approach, trust values between users are adjusted according to their previous activities on the shared data. Users share their private data by specifying some obligations the receivers are expected to follow. We log modifications done by users as well as the obligations and use a log-auditing mechanism to detect users who misbehaved. We adjust their associated trust values by using any existing decentralized trust model.

When involved in collaborative tasks, users often choose to use multi-synchronous applications in order to concurrently work in isolation. Hence, privacy of their changes is maintained until they decide to publish their contributions. Not being aware of changes made by their collaborators, they often create concurrent modifications which might generate conflicts or lead to redundant work. We propose an awareness mechanism that solves this problem by computing and providing awareness in multi-synchronous collaboration while at the same time respecting user privacy by allowing users to specify the detail of information made available to their collaborators. The computation of awareness is based on metrics that measure the effect of changes for the different types of changes, on the different syntactic document levels and document parts. For the visualisation of awareness, we employ the concept of edit profiles.

Wikis, a popular tool for sharing knowledge, are basically collaborative editing systems. However, existing Wiki systems offer limited support for co-operative authoring, and they do not scale well, because they are based on a centralised architecture. This paper compares the well-known centralised MediaWiki system with several peer-to-peer approaches to editing of wiki pages: an operational transformation approach (MOT2), a commutativity-oriented approach (WOOTO) and a conflict resolution approach (ACF). We evaluate and compare them, according to a number of qualitative and quantitative metrics.

Version control systems are widely used to support a group of people working together on a set of documents over a network by merging their changes into the same source repository. The existing versioning systems offer limited support concerning conflict resolution and tracking of user activity. In this paper we propose a multi-level editing approach that keeps the editing operations that refer to an element of a hierarchical document associated with that element. In this way, customisable merging is achieved, where the conflicts can be specified and resolved at different granularity levels

Awareness of the activities of other users and also document evolution is an important part of collaborative authoring. We introduce the concept of an editing profile that can be used to maintain and visualise measures of the changes made across a document by different users in both synchronous and asynchronous collaborative editing. It provides a simple means of making users aware of "hot areas" and also who is or has been active in various parts of the document, as well as a quick access point into parts of the document that have been changed. The profile can be adapted to display the most relevant information based on both user preference and situation

Collaborative object-based graphical editors offer good support for design teams to work concurrently on their design. However, not much research has been done on maintaining consistency when complex operations such as the grouping of objects or working on layers are involved. In this paper, we propose a novel operation serialisation algorithm for consistency maintenance based on the reordering of nodes in a graph. The nodes of a graph represent operations and the edges represent ordering constraints between operations. Users can specify types of conflicts between operations and the policy for the resolution of conflicts.

Often collaborative graphical systems lag behind well accepted single-user applications in terms of features supported. The frequently used operations of group/ungroup offered by almost every single-user graphical editor have not been considered by the collaborative graphical editing systems that try to preserve the intentions of the users involved in the concurrent editing. In this paper we present a novel algorithm based on operation serialisation for consistency maintenance in collaborative graphical editing dealing not only with simple operations such as create, delete, move, change colour or position, but also with group/ungroup operations. Based on the classification of conflicts into real and resolvable, an undo/redo mechanism is used in order to re-execute the operations in an imposed serialisation order.