Why does Whonix use Tor?
Whonix uses Tor because it is the best anonymity network available today.
The Whonix Project wants to enforce good security by default for our users. That is why a fundamental Whonix design is to force all outgoing traffic through the Tor anonymity network.
Virtual Private Networks (VPNs) are usually faster than Tor, but they are not anonymity networks. VPN administrators can log both where a user is connecting from and the destination website, breaking anonymity in the process.  Tor provides anonymity by design rather than policy, making it impossible for a single point in the network to know both the origin and the destination of a connection. Anonymity by design provides a higher standard, since trust is removed from the equation.
When using a VPN, an adversary can break anonymity by monitoring the incoming and outgoing connections of the limited set of servers. On the other hand, the Tor network is formed by over 6000 relays and 2000 bridges run worldwide by volunteers.  This makes it far more difficult to conduct successful, end-to-end correlation (confirmation) attacks, although not impossible.
Poisoned Tor nodes are another concern. If an adversary runs a malicious Tor entry guard and exit node in a network of 7,000 relays (2,000 entry guards and 1,000 exit nodes), the odds of the Tor circuit crossing both are around one in 2 million. If an adversary can increase their malicious entry and exit relays to comprise 10 percent of the bandwidth, they could deanonymize 1 percent of all Tor circuits. 
Tor has the largest user base of all available anonymity networks. More than 2 million users connected to Tor daily in 2018. Tor's adoption by a substantial audience proves its maturity, stability, and usability. It has also lead to rapid development and significant community contributions.
Tor is equally used by journalists, law enforcement, governments, human rights activists, business leaders, militaries, abuse victims and average citizens concerned about online privacy.  This diversity actually provides stronger anonymity because it makes it more difficult to identify or target a specific profile of Tor user. Anonymity loves company. 
Technical Merits and Recognition
Tor has partnered with leading research institutions, and has been subjected to intensive academic research. It is the anonymity network which benefits from the most auditing and peer review.
An extract of a Top Secret appraisal by the NSA characterized Tor as "the King of high secure, low latency Internet anonymity" with "no contenders for the throne in waiting".
Relationship between the Tor Project and Whonix
- The Tor software is made by The Tor Project.
- The Tor network is run by a worldwide community of volunteers.
- Whonix is a completely separate project developed by a different developer team.
- Whonix is a complete operating system which uses Tor as its default networking application.
Many people use Tor outside of Whonix. Similarly, a significant number use Whonix for activities other than accessing the Internet through Tor, for example hosting onion services, tunnelling I2P through Tor, tunnelling VPNs or other anonymity networks through Tor, watching flash videos anonymously, and more.
- Comparison Of Tor with CGI Proxies, Proxy Chains, and VPN Services.
- Review of different Anonymity Networks.
- Promises made by VPN operators are meaningless, since they cannot be verified.
- Admittedly the network has an unknown proportion of malicious relay operators.
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