Filename: 153-automatic-software-update-protocol.txt
Title: Automatic software update protocol
Author: Jacob Appelbaum 
Created: 14-July-2008
Status: Superseded

[Superseded by thandy-spec.txt]

                      Automatic Software Update Protocol Proposal

0.0 Introduction

The Tor project and its users require a robust method to update shipped
software bundles. The software bundles often includes Vidalia, Privoxy, Polipo,
Torbutton and of course Tor itself. It is not inconcievable that an update
could include all of the Tor Browser Bundle. It seems reasonable to make this 
a standalone program that can be called in shell scripts, cronjobs or by
various Tor controllers.

0.1 Minimal Tasks To Implement Automatic Updating

At the most minimal, an update must be able to do the following: 

    0 - Detect the curent Tor version, note the working status of Tor.
    1 - Detect the latest Tor version. 
    2 - Fetch the latest version in the form of a platform specific package(s).
    3 - Verify the itegrity of the downloaded package(s).
    4 - Install the verified package(s).
    5 - Test that the new package(s) works properly.

0.2 Specific Enumeration Of Minimal Tasks

To implement requirement 0, we need to detect the current Tor version of both 
the updater and the current running Tor. The update program itself should be 
versioned internally. This requirement should also test connecting through Tor 
itself and note if such connections are possible.

To implement requirement 1, we need to learn the concensus from the directory 
authorities or fail back to a known good URL with cryptographically signed 

To implement requirement 2, we need to download Tor - hopefully over Tor.

To implement requirement 3, we need to verify the package signature.

To implement requirement 4, we need to use a platform specific method of 
installation. The Tor controller performing the update perform these platform 
specific methods.

To implement requirement 5, we need to be able to extend circuits and reach 
the internet through Tor.

0.x Implementation Goals

The update system will be cross platform and rely on as little external code 
as possible. If the update system uses it, it must be updated by the update 
system itself. It will consist only of free software and will not rely on any 
non-free components until the actual installation phase. If a package manager 
is in use, it will be platform specific and thus only invoked by the update 
system implementing the update protocol.

The update system itself will attempt to perform update related network 
activity over Tor. Possibly it will attempt to use a hidden service first.
It will attempt to use novel and not so novel caching 
when possible, it will always verify cryptographic signatures before any 
remotely fetched code is executed. In the event of an unusable Tor system, 
it will be able to attempt to fetch updates without Tor. This should be user 
configurable, some users will be unwilling to update without the protection of 
using Tor - others will simply be unable because of blocking of the main Tor 

The update system will track current version numbers of Tor and supporting 
software. The update system will also track known working versions to assist 
with automatic The update system itself will be a standalone library. It will be 
strongly versioned internally to match the Tor bundle it was shiped with. The 
update system will keep track of the given platform, cpu architecture, lsb_release, 
package management functionality and any other platform specific metadata.

We have referenced two popular automatic update systems, though neither fit 
our needs, both are useful as an idea of what others are doing in the same 

The first is sparkle[0] but it is sadly only available for Cocoa 
environments and is written in Objective C. This doesn't meet our requirements 
because it is directly tied into the private Apple framework.

The second is the Mozilla Automatic Update System[1]. It is possibly useful 
as an idea of how other free software projects automatically update. It is 
however not useful in its currently documented form.


0.x Previous methods of Tor and related software update

Previously, Tor users updated their Tor related software by hand. There has
been no fully automatic method for any user to update. In addition, there
hasn't been any specific way to find out the most current stable version of Tor
or related software as voted on by the directory authority concensus.

0.x Changes to the directory specification

We will want to supplement client-versions and server-versions in the 
concensus voting with another version identifier known as 
'auto-update-versions'. This will keep track of the current concensus of 
specific versions that are best per platform and per architecture. It should 
be noted that while the Mac OS X universal binary may be the best for x86 
processers with Tiger, it may not be the best for PPC users on Panther. This 
goes for all of the package updates. We want to prevent updates that cause Tor 
to break even if the updating program can recover gracefully.

x.x Assumptions About Operating System Package Management

It is assumed that users will use their package manager unless they are on 
Microsoft Windows (any version) or Mac OS X (any version). Microsoft Windows 
users will have integration with the normal "add/remove program" functionality 
that said users would expect.

x.x Package Update System Failure Modes

The package update will try to ensure that a user always has a working Tor at 
the very least. It will keep state to remember versions of Tor that were able 
to bootstrap properly and reach the rest of the Tor network. It will also keep 
note of which versions broke. It will select the best Tor that works for the 
user. It will also allow for anonymized bug reporting on the packages 
available and tested by the auto-update system.

x.x Package Signature Verification

The update system will be aware of replay attacks against the update signature 
system itself. It will not allow package update signatures that are radically 
out of date. It will be a multi-key system to prevent any single party from 
forging an update. The key will be updated regularly. This is like authority 
key (see proposal 103) usage.

x.x Package Caching

The update system will iterate over different update methods. Whichever method 
is picked will have caching functionality. Each Tor server itself should be 
able to serve cached update files. This will be an option that friendly server 
administrators can turn on should they wish to support caching. In addition, 
it is possible to cache the full contents of a package in an 
authoratative DNS zone. Users can then query the DNS zone for their package. 
If we wish to further distribute the update load, we can also offer packages 
with encrypted bittorrent. Clients who wish to share the updates but do not 
wish to be a server can help distribute Tor updates. This can be tied together 
with the DNS caching[2][3] if needed.


x.x Helping Our Users Spread Tor

There should be a way for a user to participate in the packaging caching as 
described in section x.x. This option should be presented by the Tor 

x.x Simple HTTP Proxy To The Tor Project Website

It has been suggested that we should provide a simple proxy that allows a user 
to visit the main Tor website to download packages. This was part of a 
previous proposal and has not been closely examined.

x.x Package Installation

Platform specific methods for proper package installation will be left to the 
controller that is calling for an update. Each platform is different, the 
installation options and user interface will be specific to the controller in 

x.x Other Things

Other things should be added to this proposal. What are they?