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102 lines
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Plaintext
102 lines
4.0 KiB
Plaintext
Filename: 178-param-voting.txt
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Title: Require majority of authorities to vote for consensus parameters
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Author: Sebastian Hahn
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Created: 16-Feb-2011
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Status: Closed
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Implemented-In: 0.2.3.9-alpha
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Overview:
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The consensus that the directory authorities create may contain one or
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more parameters (32-bit signed integers) that influence the behavior
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of Tor nodes (see proposal 167, "Vote on network parameters in
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consensus" for more details).
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Currently (as of consensus method 11), a consensus will end up
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containing a parameter if at least one directory authority votes for
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that paramater. The value of the parameter will be the low-median of
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all the votes for this parameter.
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This proposal aims at changing this voting process to be more secure
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against tampering by a small fraction of directory authorities.
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Motivation:
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To prevent a small fraction of the directory authorities from
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influencing the value of a parameter unduly, a big enough fraction
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of all directory authorities authorities has to vote for that
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parameter. This is not currently happening, and it is in fact not
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uncommon for a single authority to govern the value of a consensus
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parameter.
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Design:
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When the consensus is generated, the directory authorities ensure that
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a param is only included in the list of params if at least three of the
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authorities (or a simple majority, whichever is the smaller number)
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votes for that param. The value chosen is the low-median of all the
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votes. We don't mandate that the authorities have to vote on exactly
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the same value for it to be included because some consensus parameters
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could be the result of active measurements that individual authorities
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make.
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Security implications:
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This change is aimed at improving the security of Tor nodes against
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attacks carried out by a small fraction of directory authorities. It
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is possible that a consensus parameter that would be helpful to the
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network is not included because not enough directory authorities
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voted for it, but since clients are required to have sane defaults
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in case the parameter is absent this does not carry a security risk.
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This proposal makes a security vs coordination effort tradeoff. When
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considering only the security of the design, it would be better to
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require a simple majority of directory authorities to agree on
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voting on a parameter, but it would involve requiring more
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directory authority operators to coordinate their actions to set the
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parameter successfully.
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Specification:
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dir-spec section 3.4 currently says:
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Entries are given on the "params" line for every keyword on which any
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authority voted. The values given are the low-median of all votes on
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that keyword.
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It is proposed that the above is changed to:
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Entries are given on the "params" line for every keyword on which a
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majority of authorities (total authorities, not just those
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participating in this vote) voted on, or if at least three
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authorities voted for that parameter. The values given are the
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low-median of all votes on that keyword.
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Consensus methods 11 and before, entries are given on the "params"
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line for every keyword on which any authority voted, the value given
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being the low-median of all votes on that keyword.
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The following should be added to the bottom of section 3.4.:
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* If consensus method 12 or later is used, only consensus
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parameters that more than half of the total number of
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authorities voted for are included in the consensus.
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The following line should be added to the bottom of section 3.4.1.:
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"12" -- Params are only included if enough auths voted for them
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Compatibility:
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A sufficient number of directory authorities must upgrade to the new
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consensus method used to calculate the params in the way this proposal
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calls for, otherwise the old mechanism is used. Nodes that do not act
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as directory authorities do not need to be upgraded and should
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experience no change in behaviour.
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Implementation:
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An example implementation of this feature can be found in
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https://gitweb.torproject.org/sebastian/tor.git, branch safer_params.
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