Tor Directory Authorities: How Many Exist?

Directory authorities are the backbone of the Tor network, responsible for maintaining a trusted list of active relays to ensure secure routing for privacy-focused users. As of today, there are currently 9 directory authorities operating on the public Tor network, along with 1 dedicated bridge authority. This article explains the exact number of authorities, their crucial role in compiling network consensus, and why their count remains strictly limited.

The Role of Directory Authorities

The Tor network relies on thousands of volunteer-run relays to route encrypted traffic. To ensure that clients connect to legitimate, functioning nodes rather than malicious impostors, directory authorities regularly scan, verify, and document the status of all available relays.

Every hour, these specialized servers vote to create a unified document known as the consensus. This document lists all valid relays, their operational flags (such as “Exit”, “Guard”, or “Fast”), cryptographic keys, and routing capabilities. Tor clients download this consensus file to safely build random three-hop circuits through the network.

Current Breakdown of Tor Authorities

The system is configured with a fixed set of trusted authorities:

• 9 Main Directory Authorities: These servers participate directly in the hourly consensus voting process for standard relays. They are operated by independent, trusted individuals and organizations within the privacy community across various geographic locations to reduce the risk of coordinated compromise.
• 1 Bridge Authority: Known as Bifroest, this specialized authority does not publish its relay list publicly. Instead, it tracks hidden entry points (bridges) to assist users bypassing censorship in regions where standard Tor access is blocked.

Why Are There Only 9 Authorities?

The number of directory authorities is kept deliberately small and odd for specific structural reasons:

• Majority Voting: Having an odd number (9) prevents ties during consensus creation. A consensus requires a majority vote (at least 5 out of 9 authorities) to be valid, ensuring high Byzantine fault tolerance.
• Trust and Security: Because directory authorities hold significant influence over the network’s state, each operator must be vetted and highly trusted. Expanding the list too widely increases the attack surface and potential for malicious subversion.
• Resilience: If one or several authorities fail, go offline for maintenance, or suffer network partitioning, the remaining majority can continue to generate hourly consensus files without disrupting user connections.