# Achieving Sender Anonymity in Tor against the Global Passive Adversary
## 2 Related Work  
The issues most relevant to this paper are the vulnerabilities which Tor suffers from.  
As stated by the creators themselves [1], the Tor overlay network, based on Onion routing [2], does not provide anonymous guarantees in the severe threat model of a global passive adversary [4], which is able to observe the entire traffic of the network.  
Anyway, even if we relax the powers of the adversary, many attacks are still effective [5; 12; 13]. The most famous class of attacks is represented by the traffic analysis attacks [14; 15; 16] in which the adversary analyzes the traffic to find correlations. Among the traffic analysis attacks there are the timing attacks [17; 18; 19], in which the adversary observes the timing of the messages arriving at and leaving from the nodes to find correlations.Other interesting subclasses of traffic analysis attacks are traffic confirmation attacks [20], in which the adversary controls and observes two possible end-relays of a Tor circuit to conclude that they really belong to the same circuit, and watermarking attacks [21], in which the adversary manipulates the traffic stream by introducing an identifiable pattern. Another category of attacks target the router selection used to build the Tor circuit. Indeed, the standard selection is based on network and CPU performance reported by the nodes themselves. This enables self-promotion attacks [22]. A countermeasure can be found in [23].  
The performance of Tor was investigated in [24; 25; 26]. Performance analysis in relation to de-anonymization attacks was performed in [27].  
In our approach, we extend Tor achieving sender anonymity (and then relationship anonymity) [7] in the sense of _communication k-anonymity_[8], against a global passive adversary. This goal can be reached only with the introduction of cover traffic [28] (as required by our approach).  
Among the approaches supporting cover traffic, the most significant are _mixnets_, originally proposed in [29], and _buses_[9; 10; 30].  
In the literature, several proposals include cover traffic in mixnets [31; 32; 33; 34]. The introduction of cover traffic makes traffic analysis more difficult. For example, a possible approach is to introduce cover traffic to maintain a constant transmission rate. A very recent mixnet-based approach designed for the network layer was presented in [35]. However, it does not provide sender anonymity against a global adversary. Another relevant approach in this category, even if dated (but still very solid), is Tarzan [32]. As discussed in [36], mixnets, in general, require a suitable amount of cover traffic.  
More related to our work are buses, as we also consider a pre-determined route that is used by the sender. However, buses are unrealistic in a large network (such as the Internet), since the fixed route is a Eulerian path passing through all the nodes, including thus all the possible pairs of sender-receiver.  
Similar considerations can be made for DC-Nets [37], based on a secure multi-party cryptographic protocol, in which it is required that all participants are involved in every run of the protocol and initially share a pairwise key.  
This paper considerably extends a work-in-progress paper [38]. Reference [38] just presented the rough idea underlying the protocol. Specifically, in that paper, the approach is only sketched out, and it refers to the original Onion approach, with no fault tolerance, no real-life contextualization in the Tor system. Moreover, no detailed security and complexity analyses were performed, and no experimental evaluation was included.