[tor-dev] Proposal 178: Require majority of authorities to vote for consensus parameters

[Sebastian Hahn]

On Nov 25, 2011, at 9:27 PM, Sebastian Hahn wrote:
> I have since become convinced that it would be better to get
> this implemented quickly, even if it doesn't have a generic
> "prevent this param from being set" mechanism. I would thus like
> to change the proposal to the following (also available in my
> prop178 branch in my torspec repository). The diff is available at
> https://gitweb.torproject.org/sebastian/torspec.git/commitdiff/db9a429d005ae05ad0841e6026941594d5740b0b

I meant to also send the proposal here, inline. Looks like I messed
that up. Here goes:



Filename: 178-param-voting.txt
Title: Require majority of authorities to vote for consensus parameters
Author: Sebastian Hahn
Created: 16-Feb-2011
Status: Open
Target: 0.2.3.x

Overview:

The consensus that the directory authorities create may contain one or
more parameters (32-bit signed integers) that influence the behavior
of Tor nodes (see proposal 167, "Vote on network parameters in
consensus" for more details).

Currently (as of consensus method 11), a consensus will end up
containing a parameter if at least one directory authority votes for
that paramater. The value of the parameter will be the low-median of
all the votes for this parameter.

This proposal aims at changing this voting process to be more secure
against tampering by a small fraction of directory authorities.

Motivation:

To prevent a small fraction of the directory authorities from
influencing the value of a parameter unduly, a big enough fraction
of all directory authorities authorities has to vote for that
parameter. This is not currently happening, and it is in fact not
uncommon for a single authority to govern the value of a consensus
parameter.

Design:

When the consensus is generated, the directory authorities ensure that
a param is only included in the list of params if at least three of the
authorities (or a simple majority, whichever is the smaller number)
votes for that param. The value chosen is the low-median of all the
votes. We don't mandate that the authorities have to vote on exactly
the same value for it to be included because some consensus parameters
could be the result of active measurements that individual authorities
make.

Security implications:

This change is aimed at improving the security of Tor nodes against
attacks carried out by a small fraction of directory authorities. It
is possible that a consensus parameter that would be helpful to the
network is not included because not enough directory authorities
voted for it, but since clients are required to have sane defaults
in case the parameter is absent this does not carry a security risk.

This proposal makes a security vs coordination effort tradeoff. When
considering only the security of the design, it would be better to
require a simple majority of directory authorities to agree on
voting on a parameter, but it would involve requiring more
directory authority operators to coordinate their actions to set the
parameter successfully.

Specification:

dir-spec section 3.4 currently says:

     Entries are given on the "params" line for every keyword on which any
     authority voted.  The values given are the low-median of all votes on
     that keyword.

It is proposed that the above is changed to:

     Entries are given on the "params" line for every keyword on which a
     majority of authorities (total authorities, not just those
     participating in this vote) voted on, or if at least three
     authorities voted for that parameter. The values given are the
     low-median of all votes on that keyword.

     Consensus methods 11 and before, entries are given on the "params"
     line for every keyword on which any authority voted, the value given
     being the low-median of all votes on that keyword.

The following should be added to the bottom of section 3.4.:

        * If consensus method 12 or later is used, only consensus
          parameters that more than half of the total number of
          authorities voted for are included in the consensus.

The following line should be added to the bottom of section 3.4.1.:

     "12" -- Params are only included if a enough auths voted for them

Compatibility:

A sufficient number of directory authorities must upgrade to the new
consensus method used to calculate the params in the way this proposal
calls for, otherwise the old mechanism is used. Nodes that do not act
as directory authorities do not need to be upgraded and should
experience no change in behaviour.

Implementation:

An example implementation of this feature can be found in
https://gitweb.torproject.org/sebastian/tor.git, branch safer_params.