The best known and most widespread of them is probably Tor, which takes its name from The Onion Routing protocol it is based upon. Those networks, often called darknets, implement suitable cryptographic protocols to the purpose of keeping anonymous the identity of both the services offering contents and the users enjoying them. “Dark web” is a generic term for the subset of the Web that, other than being non-indexed by popular search engines, is accessible only through specific privacy-preserving browsers and overlay networks. Nevertheless, if we only consider mutual connections, a more efficient subgraph emerges, that is, probably, the backbone of social interactions in Tor. From this viewpoint, the graph appears inefficient. 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. 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. The limited number of entry points that can be used to crawl the network, makes the study of this graph far from being simple. 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. Recent research works have evaluated Tor security, its evolution over time, and its thematic organization.

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. 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. Tor is an open source software that allows accessing various kinds of resources, known as hidden services, while guaranteeing sender and receiver anonymity.

Why are most tor links down