Proposal 144: enforce distinct providers

[Nick Mathewson]

On Sat, Jun 28, 2008 at 04:28:57PM +0200, M Fr wrote:
> Hello,
> 
> like we've speak on bug
> http://bugs.noreply.org/flyspray/index.php?do=details&id=690
> 
> Here is a proposal to implement this functionality.
> 

Hi!  By request, I'm forwarding this to or-dev and adding it as proposal 144.

I've reformatted it, and included the reformatted version below.

Filename: 144-enforce-distinct-providers.txt
Title: Increase the diversity of circuits by detecting nodes belonging the
same provider
Author: Mfr
Created: 2008-06-15
Status: Draft

Overview:

  Increase network security by reducing the capacity of the relay or
  ISPs monitoring personally or requisition, a large part of traffic
  Tor trying to break circuits privacy.  A way to increase the
  diversity of circuits without killing the network performance.

Motivation:

  Since 2004, Roger an Nick publication about diversity [1], very fast
  relays Tor running are focused among an half dozen of providers,
  controlling traffic of some dozens of routers [2].

  In the same way the generalization of VMs clonables paid by hour,
  allowing starting in few minutes and for a small cost, a set of very
  high-speed relay whose in a few hours can attract a big traffic that
  can be analyzed, increasing the vulnerability of the network.

  Whether ISPs or domU providers, these usually have several groups of
  IP Class B.  Also the restriction in place EnforceDistinctSubnets
  automatically excluding IP subnet class B is only partially
  effective. By contrast a restriction at the class A will be too
  restrictive.

 Therefore it seems necessary to consider another approach.

Proposal:

  Add a provider control based on AS number added by the router on is
  descriptor, controlled by Directories Authorities, and used like the
  declarative family field for circuit creating.

Design:

Step 1 :

 Add to the router descriptor a provider information get request [4]
  by the router itself.

         "provider" name NL

            'names' is the AS number of the router formated like this:
            'ASxxxxxx' where AS is fixed and xxxxxx is the AS number,
            left aligned ( ex: AS98304 , AS4096,AS1 ) or if AS number
            is missing the network A class number is used like that:
            'ANxxx' where AN is fixed and xxx is the first 3 digits of
            the IP (ex: for the IP 1.1.1.2 AN1) or an 'L' value is set
            if it's a local network IP.

            If two ORs list one another in their "provider" entries,
            then OPs should treat them as a single OR for the purpose
            of path selection.

            For example, if node A's descriptor contains "provider B",
            and node B's descriptor contains "provider A", then node A
            and node B should never be used on the same circuit.

    Add the regarding config option in torrc

            EnforceDistinctProviders set to 1 by default.
            Permit building circuits with relays in the same provider
            if set to 0.
            Regarding to proposal 135 if TestingTorNetwork is set
            need to be EnforceDistinctProviders is unset.

    Control by Authorities Directories of the AS numbers

         The Directories Authority control the AS numbers of the new node
         descriptor uploaded.

            If an old version is operated by the node this test is
            bypassed.

            If AS number get by request is different from the
            description, router is flagged as non-Valid by the testing
            Authority for the voting process.

Step 2     When a ' significant number of nodes' of valid routers are
generating descriptor with provider information.

        Add missing provider information get by DNS request
functionality for the circuit user:

                During circuit building, computing, OP apply first
                family check and EnforceDistinctSubnets directives for
                performance, then if provider info is needed and
                missing in router descriptor try to get AS provider
                info by DNS request [4].  This information could be
                DNS cached.  AN ( class A number) is never generated
                during this process to prevent DNS block problems.  If
                DNS request fails ignore and continue building
                circuit.

Step 3 When the 'whole majority' of valid Tor clients are providing
DNS request.

        Older versions are deprecated and mark as no-Valid.

  EnforceDistinctProviders replace EnforceDistinctSubnets functionnality.

        EnforceDistinctSubnets is removed.

        Functionalities deployed in step 2 are removed.

Security implications:

      This providermeasure will increase the number of providers
      addresses that an attacker must use in order to carry out
      traffic analysis.

Compatibility:

        The presented protocol does not raise compatibility issues
        with current Tor versions. The compatibility is preserved by
        implementing this functionality in 3 steps, giving time to
        network users to upgrade clients and routers.

Performance and scalability notes:

        Provider change for all routers could reduce a little
        performance if the circuit to long.

        During step 2 Get missing provider information could increase
        building path time and should have a time out.

Possible Attacks/Open Issues/Some thinking required:

        These proposal seems be compatible with proposal 135 Simplify
        Configuration of Private Tor Networks.

        This proposal does not resolve multiples AS owners and top
        providers traffic monitoring attacks [5].

        Unresolved AS number are treated as a Class A network. Perhaps
        should be marked as invalid.  But there's only fives items on
        last check see [2].

        Need to define what's a 'significant number of nodes' and
        'whole majority' ;-)

References:
[1] Location Diversity in Anonymity Networks by Nick Feamster and Roger
Dingledine.
In the Proceedings of the Workshop on Privacy in the Electronic Society
(WPES 2004), Washington, DC, USA, October 2004
http://freehaven.net/anonbib/#feamster:wpes2004
[2] http://as4jtw5gc6efb267.onion/IPListbyAS.txt
[3] see Goodell Tor Exit Page
http://cassandra.eecs.harvard.edu/cgi-bin/exit.py
[4] see the great IP to ASN DNS Tool
http://www.team-cymru.org/Services/ip-to-asn.html
[5] Sampled Traffic Analysis by Internet-Exchange-Level Adversaries by
Steven J. Murdoch and Piotr Zielinski.
In the Proceedings of the Seventh Workshop on Privacy Enhancing Technologies

(PET 2007), Ottawa, Canada, June 2007.
http://freehaven.net/anonbib/#murdoch-pet2007
[5] http://bugs.noreply.org/flyspray/index.php?do=details&id=690