Tor

What Tor provides

Tor sits between VPNs and mixnets. Slower than a VPN, faster than a mixnet, and usable enough that millions of people actually run it daily. The design choices:

• Three-hop onion routing. Each relay only knows its predecessor and successor. The entry node knows the user, the exit node knows the destination, no single node sees both.
• TLS to relays. Public-key directory authorities sign the consensus that lists relay identities and bandwidth weights.
• v3 hidden services. A server with no IP address, addressable through a 56-character base32 of an Ed25519 public key.
• Pluggable transports. Obfs4, meek, snowflake, all aimed at deep-packet-inspection in censoring countries.
• Bridges. Unpublished relay IPs for users in countries that block the public Tor network.

What Tor is not:

• A defense against a global passive adversary. Timing correlation between entry and exit is possible in principle.
• A high-bandwidth network. Typical 1-10 Mbps, latency in the seconds.
• A complete metadata-protection answer. Size and timing patterns leak under analysis.

For traffic-analysis defense beyond Tor's threat model, see Nym and Katzenpost.

Circuit construction

Client ── TLS ── Guard ── ─── Middle ── ─── Exit ── ─── Destination
                  ↑           ↑          ↑
             knows client   knows nothing  knows destination
                            of either

Each hop is a layered TLS-over-AES tunnel. The client picks circuits from the consensus, builds them lazily as needed, and rotates them roughly every 10 minutes (or per-stream).

Guards

The first hop in a circuit is a guard. To resist long-term Sybil-based deanonymization, clients pin a small set of guards (usually one) for months. If the guard is honest, attackers above the guard never see your traffic. If the guard is malicious, correlation is possible, but the attack is bounded to one node.

Exit policies

Exits voluntarily declare which destination ports they will forward. Most allow web traffic, some only specific ports. Operating an exit relay carries legal exposure in many jurisdictions, so the exit set is smaller than the relay set.

v3 hidden services

A v3 onion service is a server reachable only via Tor:

• The server generates an Ed25519 keypair.
• The address is base32(Ed25519-pubkey ‖ checksum ‖ version) + ".onion", 56 characters before .onion.
• The server publishes introduction points in the Tor directory (short-lived).
• The client picks a rendezvous point and tells the introduction point to ask the service to meet there.
• Both sides connect to the rendezvous through three-hop circuits, six hops end-to-end inside Tor.

Properties:

• Self-authenticating. The address proves the key.
• No CA, no DNS. Trust binds to the Ed25519 public key.
• Hidden by default. The server's IP is not revealed to clients or to the network.
• Client authorization. v3 supports an explicit allowlist of authorized client keys.

Common uses: SecureDrop, the Tor Project's own services, Briar's transport, ricochet-refresh, Nym's clearnet ingress, Cwtch peers.

Pluggable transports and bridges

When a network blocks Tor (China, Iran, others), pluggable transports disguise Tor traffic:

Bridges are non-public Tor relays whose IP isn't in the consensus. They are distributed via BridgeDB (email, web, Telegram). Censors who learn a bridge's IP can block it; new bridges are issued.

Protocol-level details

• Cell size: fixed 514-byte cells defeat trivial size-based traffic analysis.
• Padding: Tor v3 padding negotiation reduces visible inactive-period inference.
• TLS 1.3 between relays.
• Directory authorities: about 10 trusted operators that vote on consensus hourly.
• Bandwidth weighting: clients prefer higher-bandwidth relays, with weighting derived from authority measurements.

Trust model

For threat models that include a global passive adversary, see Nym, Katzenpost, and the MOC - Metadata Privacy discussion of cover-traffic defenses.

2026 maintenance status

Use the Tor Project download page for the current Tor Browser release and its signature. This entry does not pin a browser version.

Arti 2.3.0 (2026-05-07) continues the relay/directory/RPC development trajectory. New in 2.3.0: syslog logging support, a logging.protocol_warnings option to surface protocol violations, RPC API additions for tunnel-path inspection, and ongoing work toward Arti-as-relay and Arti-as-directory-authority. The minimum macOS version bumped to 10.14. A breaking change is flagged for the next release: TorClient will explicitly require Arc wrapping instead of implicit Arc-like semantics — downstream arti-client crate users should monitor #2469.

Tor remains actively maintained rather than "finished infrastructure." The pattern is clear: Rust rewrite (Arti) absorbs new protocol features (CGO, relay mode, directory authority) while Tor Browser keeps pace with Firefox security updates and emergency patches. The Tor Project's 2026 release cadence — particularly the Tails 7.7.x emergency series in April-May and the Tor Browser 15.0.11/12/13 rapid-response updates — reflects a live, responsive security posture under active attack.

Arti, the Rust implementation of Tor, is the strategic integration path for wallets, messengers, and embedded apps. It matters because many freedom-tech tools want Tor as a library instead of requiring users to manage a separate daemon. Maturity assessment: C Tor remains the production network implementation; Arti is increasingly important for embedding and future codebase safety.

Operational patterns

Tor Browser

The reference client. Standardized fingerprint to reduce browser-fingerprinting differentiation across users. Uses HTTPS-Everywhere by default and NoScript on the Safer/Safest security levels.

System-wide Tor

SocksPort 9050 exposed locally; configure individual apps to use SOCKS5 to that. Pair with proxychains for CLI tools.

Whonix and Tails

OS-level Tor enforcement. Whonix has two VMs (a gateway that routes everything via Tor, a workstation with no direct internet). Tails is an amnesic Live USB.

onion-only services

Services bound to a .onion only, with no clearnet listener. Pair with Overview - Decentralized DNS alternatives for double-resistance.

Tor + attested LLM endpoint

Confidential-AI inference reachable only at a .onion, with attested TLS pinned in the enclave. The pattern is documented in MOC - Composing Primitives.

Trade-offs

Strengths

• Mature. Over 20 years; the largest anonymity network ever deployed.
• Wide app support. SOCKS5 makes integration trivial.
• Hidden services. Server-side anonymity, not just client-side.
• Robust against censorship with bridges plus pluggable transports.
• Open, auditable, transparent.

Limitations

• Low latency is a feature for usability, a weakness for traffic analysis.
• Exit-policy bottleneck. Fewer exits than entry relays creates congestion and exposure.
• A compromised exit can MITM unencrypted traffic. Use HTTPS or authenticated protocols.
• State-sponsored attacks. Tor is in scope for major SIGINT agencies, so it is not the right tool for targets of sustained nation-state attention without additional layering.
• Bandwidth. A typical Tor circuit isn't suitable for video, large transfers, or low-latency real-time work.

Attack surface

Recent developments (2024-2026)

• Arti, the Rust rewrite of the Tor client, is on a production roadmap. Lighter and more embeddable.
• Onion-balancing improvements for high-traffic hidden services.
• WebTunnel, a newer pluggable transport for HTTPS-like camouflage.
• Snowflake, broadly adopted by NGOs. WebRTC-based circumvention in browsers.
• PoW for hidden-service DoS, which works around resource-exhaustion attacks during high-demand periods.
• Vanguards-Lite, a second-layer guard discipline for hidden services.

Staying current

Get Tor Browser and release information from the Tor Project’s download page. Check the release offered for your platform when installing or updating.

Primary sources

• Tor Browser downloads

• Dingledine, Mathewson, Syverson, Tor: The Second-Generation Onion Router, USENIX Security 2004. svn-archive.torproject.org/svn/projects/design-paper/tor-design.pdf

• Tor Specification — spec.torproject.org
• Onion Service v3 Specification — same site.
• The Tor Project — torproject.org
• Arti — gitlab.torproject.org/tpo/core/arti

For bridge distribution, transport choices and connection steps, see Pluggable transports and bridges. To contribute a bridge or Snowflake proxy, follow Run a private circumvention server for people you know.

Also in Censorship Circumvention