Tor specifications

This repository holds the specifications that describe how Tor works. They try to present Tor's protocols in sufficient detail to allow the reader to implement a compatible implementation of Tor without ever having to read the Tor source code.

The proposals directory holds our design proposals. These include historical documents that have now been merged into . For more information on the proposal process, including an explanation of how to make new proposals, see, see 001-process.txt.

What you can find here

Tor's specification is pretty big, and we've broken it into a bunch of files.

• General interest
  • tor-spec.txt contains the specification for the core Tor protocol itself; this is a good place to start reading.
  • cert-spec.txt describes a certificate format used in the other parts of the protocol.
  • dir-spec.txt specifies the operations and formats used to maintain a view of the network directory.
  • padding-spec.txt describes a set of padding mechanisms used to impede traffic analysis.
  • version-spec.txt explains how to parse Tor version numbers.
  • glossary.txt is a glossary of terms used in the other specifications.
• Client operations
  • address-spec.txt lists a set of special addresses that Tor handles differently from the regular DNS system.
  • guard-spec.txt explains the "guard node" algorithm that Tor clients use to avoid sampling attacks.
  • path-spec.txt explains how clients choose their paths through the Tor network.
  • socks-extensions specifies Tor-specific extensions to the SOCKS protocol.
• Onion services
  • rend-spec-v2.txt is the old, deprecated version of the onion service protocol.
  • rend-spec-v3.txt is the current version of the onion service protocol.
• Censorship resistance
  • bridgedb-spec.txt explains how the bridgedb server gives out bridges to censored clients.
  • gettor-spec.txt describes the gettor tool, which is used to download Tor in censored areas.
  • pt-spec.txt describes the protocol that Tor clients and relays use to communicate with pluggable transports used for traffic obfuscation.
• Directory authorities
  • bandwidth-file-spec.txt specifies the file format used by bandwidth-measuring tools to report their observations to directory authorities.
  • srv-spec.txt specifies the protocol that directory authorities use to securely compute shared random values for the network.
• Controller protocol
  • control-spec.txt explains the protocol used by controllers to communicate with a running Tor process.
• Miscellaneous
  • dir-list-spec.txt explains the format used by tools like the fallback directory scripts to output a list of Tor directories for inclusion in the Tor source code.
  • The attic directory has obsolete or historical documents.