torspec/gettor-spec.txt

                              GetTor specification
                                 Jacob Appelbaum

Table of Contents

    0. Preface
    1. Overview
    2. Implementation
        2.1. Reference implementation
    3. SMTP transport
        3.1. SMTP transport security considerations
        3.2. SMTP transport privacy considerations
    4. Other transports
    5. Implementation suggestions

0. Preface

 This document describes GetTor and how to properly implementation GetTor.

1. Overview

 GetTor was created to resolve direct and indirect censorship of Tor's
 software.  In many countries and networks Tor's main website is blocked and
 would-be Tor users are unable to download even the source code to the Tor
 program. Other software hosted by the Tor Project is similarly censored. The
 filtering of the possible download sites is sometimes easy to bypass by using
 our TLS enabled website. In other cases the website and all of the mirrors are
 entirely blocked; this is a situation where a user seems to actually need Tor
 to fetch Tor. We discovered that it is feasible to use alternate transport
 methods such as SMTP between a non-trusted third party or with IRC and XDCC.

2. Implementation

 Any compliant GetTor implementation will implement at least a single transport
 to meet the needs of a certain class of users. It should be i18n and l10n
 compliant for all user facing interactions; users should be able to manually
 set their language and this should serve as their preference for localization
 of any software delivered. The implementation must be free software and it
 should be freely available by request from the implementation that they
 interface with to download any of the other software available from that
 GetTor instance.  Security and privacy considerations should be described on a
 per transport basis.

2.1. Reference implementation

 We have implemented[0] a compliant GetTor that supports SMTP as a transport.

3. SMTP transport

 The SMTP transport for GetTor should allow users to send any RFC822 compliant
 message in any known human language; GetTor should respond in whatever
 language is detected with supplementary translations in the same email.
 GetTor shall offer a list of all available software in the body of the email -
 it should offer the software as a list of packages and their subsequent
 descriptions.

3.1. SMTP transport security considerations

 Any GetTor instance that offers SMTP as a transport should optionally
 implement the checking of DKIM signatures to ensure that email is not forged.
 Optionally GetTor should take an OpenPGP key from the user and encrypt the
 response with a blinded message.

3.2. SMTP transport privacy considerations

 Any GetTor instance that offers SMTP as a transport must at least store the
 requester's address for the time that it takes to process a response. This
 should not be written to any permanent storage medium; GetTor should function
 without any long term storage excepting a cache of files that it will send to
 any user who requests it.

 GetTor may optionally collect anonymized usage statistics to better understand
 how GetTor[1] is in use. This must not include any personally identifying
 information about any of the requester beyond language selection.

4. Other transports

 At this time no other transports have been specified. IRC XDCC is a likely
 useful system as is XMPP/Jabber with the newest OTR file sharing transport.

5. Implementation suggestions

 It is suggested that any compliant GetTor instance should be written in a so
 called "safe" language such as Python.

[0] https://gitweb.torproject.org/gettor.git
[1] https://metrics.torproject.org/packages.html