```
Filename: 152-single-hop-circuits.txt
Title: Optionally allow exit from single-hop circuits 
Author: Geoff Goodell
Created: 13-Jul-2008
Status: Closed
Implemented-In: 0.2.1.6-alpha

Overview

    Provide a special configuration option that adds a line to descriptors
    indicating that a router can be used as an exit for one-hop circuits,
    and allow clients to attach streams to one-hop circuits provided
    that the descriptor for the router in the circuit includes this
    configuration option.

Motivation

    At some point, code was added to restrict the attachment of streams
    to one-hop circuits.

    The idea seems to be that we can use the cost of forking and
    maintaining a patch as a lever to prevent people from writing
    controllers that jeopardize the operational security of routers
    and the anonymity properties of the Tor network by creating and
    using one-hop circuits rather than the standard three-hop circuits.
    It may be, for example, that some users do not actually seek true
    anonymity but simply reachability through network perspectives
    afforded by the Tor network, and since anonymity is stronger in
    numbers, forcing users to contribute to anonymity and decrease the
    risk to server operators by using full-length paths may be reasonable.

    As presently implemented, the sweeping restriction of one-hop circuits
    for all routers limits the usefulness of Tor as a general-purpose
    technology for building circuits.  In particular, we should allow
    for controllers, such as Blossom, that create and use single-hop
    circuits involving routers that are not part of the Tor network.

Design

    Introduce a configuration option for Tor servers that, when set,
    indicates that a router is willing to provide exit from one-hop
    circuits.  Routers with this policy will not require that a circuit
    has at least two hops when it is used as an exit.

    In addition, routers for which this configuration option
    has been set will have a line in their descriptors, "opt
    exit-from-single-hop-circuits".  Clients will keep track of which
    routers have this option and allow streams to be attached to
    single-hop circuits that include such routers.

Security Considerations

    This approach seems to eliminate the worry about operational router
    security, since server operators will not set the configuraiton
    option unless they are willing to take on such risk.

    To reduce the impact on anonymity of the network resulting
    from including such "risky" routers in regular Tor path
    selection, clients may systematically exclude routers with "opt
    exit-from-single-hop-circuits" when choosing random paths through
    the Tor network.

```