[tor-dev] Proposal 227: Include package fingerprints in consensus documents

[Nick Mathewson]

Here's a new proposal for a thing that Mike wants for TBB.  Please review!


Filename: 227-vote-on-package-fingerprints.txt
Title: Include package fingerprints in consensus documents
Author: Nick Mathewson, Mike Perry
Created: 2014-02-14
Status: Open

0. Abstract

   We propose extending the Tor consensus document to include
   digests of the latest versions of one or more package files, to
   allow software using Tor to determine its up-to-dateness, and
   help users verify that they are getting the correct software.

1. Introduction

   To improve the integrity and security of updates, it would be
   useful to provide a way to authenticate the latest versions of
   core Tor software through the consensus. By listing a location
   with this information for each version of each package, we can
   augment the update process of Tor software to authenticate the
   packages it downloads through the Tor consensus.

2. Proposal

   We introduce a new line for inclusion in votes and consensuses.
   Its format is:

     "package" SP PACKAGENAME SP VERSION SP URL SP DIGESTS NL

      PACKAGENAME = NONSPACE
      VERSION = NONSPACE
      URL = NONSPACE
      DIGESTS = DIGEST | DIGESTS SP DIGEST
      DIGEST = DIGESTTYPE "=" BASE64

      NONSPACE = one or more non-space printing characters

      BASE64 = one or more base-64 characters, with trailing =s
               removed.

      SP = " "
      NL = a newline

   Votes and consensuses may include any number of "package" lines,
   but no vote or consensus may include more than one "package" line
   with the same PACKAGENAME and VERSION values.  All "package"
   lines must be sorted by PACKAGENAME, and then by VERSION, in
   lexical (strcmp) order.

   If the consensus-method is at least (TBD), then when computing
   the consensus, package lines for a given PACKAGENAME/VERSION pair
   should be included if at least three authorities list such a
   package in their votes.  (Call these lines the "input" lines for
   PACKAGENAME.)  That consensus should contain every "package" line
   that is listed verbatim by more than half of the authorities
   listing a line for the PACKAGENAME/VERSION pair, and no
   others.

   These lines appear immediately following the client-versions and
   server-versions lines.

3. Recommended usage

   Programs that want to use this facility should pick their
   PACKAGENAME values, and arrange to have their versions listed in
   the consensus by at least three friendly authority operators.

   Programs may want to have multiple PACKAGENAME values in order to
   keep separate lists. These lists could correspond to how the
   software is used (as tor has client-versions and
   server-versions); or to a release series (as in tbb-alpha,
   tbb-beta, and tbb-stable); or to how bad it is to use versions
   not listed (as in foo-noknownexploits, foo-recommended).

   Programs MUST NOT use "package" lines from consensuses that have
   not been verified and accepted as valid according to the rules in
   dir-spec.txt, and SHOULD NOT fetch their own consensuses if there
   is a tor process also running that can fetch the consensus
   itself.

   For safety, programs MAY want to disable functionality until
   confirming that their versions are acceptable.

   To avoid synchronization problems, programs that use the DIGEST
   field to store a digest of the contents of the URL SHOULD NOT use
   any URLs whose contents are expected to change while any valid
   consensus lists them.

3.1. Intended usage by the Tor Browser Bundle

   Tor Browser Bundle packages will be listed with package names
   'tbb-stable, 'tbb-beta', and 'tbb-alpha'. We will list a line for
   the latest version of each release series.

   When the updater downloads a new update, it always downloads the
   latest version of the Tor Browser Bundle. Because of this, and
   because we will only use these lines to authenticate updates, we
   should not need to list more than one version per series in the
   consensus.

   After completing a package download and verifying the download
   signatures (which are handled independently from the Tor
   Consensus), it will consult the appropriate current consensus
   document through the control port.

   If the current consensus timestamp is not yet more recent than
   the proposed update timestamp, the updater will delay installing
   the package until a consensus timestamp that is more recent than
   the update timestamp has been obtained by the Tor client.

   If the consensus document has a package line for the current
   release series with a matching version, it will then download the
   file at the specified URL, and then compute its hash to make sure
   it matches the value in the consensus.

   If the hash matches, the Tor Browser will download the file and
   parse its contents, which will be a JSON file which lists
   information needed to verify the hashes of the downloaded update
   file.

   If the hash does not match, the Tor Browser Bundle should display
   an error to the user and not install the package.

   If there are no package lines in the consensus for the expected
   version, the updater will delay installing the update (but the
   bundle should still inform the user they are out of date and may
   update manually).

   If there are no package lines in the consensus for the current
   release series at all, the updater should install the package
   using only normal signature verification.

4. Limitations and open questions

   This proposal won't tell users how to upgrade, or even exactly
   what version to upgrade to.

   If software is so broken that it won't start at all, or shouldn't
   be started at all, this proposal can't help with that.

   This proposal is not a substitute for a proper software update
   tool.