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

Steven Murdoch Steven.Murdoch at cl.cam.ac.uk
Thu Feb 20 12:48:27 UTC 2014

Hi Nick,

This sounds related to an old idea I presented at PETS 2006: http://www.lightbluetouchpaper.org/2006/07/13/protecting-software-distribution-with-a-cryptographic-build-process/

However there's an important difference. The proposal below is how the directory authorities advertise "good" versions, whereas the blog post discusses ORs publishing their version (independent of whether those versions are good or not).

The point of the blog post's idea is that it is harder to compromise a majority of directory authority keys, than it is to compromise one of the keys needed to publish a back-doored Tor. Therefore knowing that several OR operators have had the opportunity to review the source of a particular version adds to the security of someone else who downloads the the same version.

The blog post discusses hashes of source code rather than binary (and this was one of the weaknesses). Now that Tor has a deterministic build process it is possible to publish the hash of the binary instead.

I can see merits in both approaches. Proposal 227 is a top-down method, where the directory authority operators decide which hashes to include. The blog post is a bottom up method where OR operators decide what hashes to include implicitly as a result of running a particular version.

The former will be better if directory authorities only list good versions. The latter will be better if the directory authorities are subject to a targeted attack which causes them to list bad hashes but the attacker doesn't want to risk widely distributing the bad version to OR operators and the attacker does not have access to directory authority keys.

Best wishes,
On 14 Feb 2014, at 15:39, Nick Mathewson <nickm at torproject.org> wrote:

> 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:
>      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.
> _______________________________________________
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> tor-dev at lists.torproject.org
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