The exploration and analysis of Web graphs has flourished in the recent past, producing a large number of relevant and interesting research results. However, the unique characteristics of the Tor network limit the applicability of standard techniques and demand for specific algorithms to explore and analyze it. The attention of the research community has focused on assessing the security of the Tor infrastructure (i.e., its ability to actually provide the intended level of anonymity) and on discussing what Tor is currently being used for. Since there are no foolproof techniques for automatically discovering Tor hidden services, little or no information is available about the topology of the Tor Web graph. Even less is known on the relationship between content similarity and topological structure. The present article aims at addressing such lack of information. Among its contributions: a study on automatic Tor Web exploration/data collection approaches; the adoption of novel representative metrics for evaluating Tor data; a novel in-depth analysis of the hidden services graph; a rich correlation analysis of hidden services’ semantics and topology. Finally, a broad interesting set of novel insights/considerations over the Tor Web organization and content are provided.

Tor is an open source software that allows accessing various kinds of resources, known as hidden services, while guaranteeing sender and receiver anonymity. Tor relies on a free, worldwide, overlay network, managed by volunteers, that works according to the principles of onion routing in which messages are encapsulated in layers of encryption, analogous to layers of an onion. The Tor Web is the set of web resources that exist on the Tor network, and Tor websites are part of the so-called dark web. Recent research works have evaluated Tor security, its evolution over time, and its thematic organization. Nevertheless, limited information is available about the structure of the graph defined by the network of Tor websites, not to be mistaken with the network of nodes that supports the onion routing. The limited number of entry points that can be used to crawl the network, makes the study of this graph far from being simple. In the present paper we analyze two graph representations of the Tor Web and the relationship between contents and structural features, considering three crawling datasets collected over a five-month time frame. Among other findings, we show that Tor consists of a tiny strongly connected component, in which link directories play a central role, and of a multitude of services that can (only) be reached from there. From this viewpoint, the graph appears inefficient. Nevertheless, if we only consider mutual connections, a more efficient subgraph emerges, that is, probably, the backbone of social interactions in Tor.

Virtualization is being pervasively adopted in a variety of scenarios ranging from regular desktop PCs to server farms and clusters. Indeed, the security of guest virtual machines and of the applications and services they host can be improved by leveraging the additional architectural layer introduced by such a technology. This paper discusses security management for virtualized environments and provides several contributions. First, a novel architecture (Kvm-SMA) with the following features is detailed: it can protect guest integrity from both remote and local attacks such as root-kits, viruses, and worms; it is not circumventable and it is completely transparent to guest machines; it can asynchronously analyze guest data and monitor guest system behavior. Second, the proposed architecture has been implemented entirely on open source software and can be replicated to both Linux and Windows guests. Third the effectiveness and efficiency of the proposed architecture is shown. The former is proved showing the results of root-kit detection test, while the latter is supported by standard performance tests showing that the introduced overhead is small. Finally, a distinguishing feature of our monitoring system proposal is that it is immune to timing attacks: that is, an adversary cannot notice the monitoring system is active by analyzing the time required to perform system calls. We believe that security management of both single virtualized hosts and distributed virtualized systems can benefit from our proposal.