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-Filename: 134-robust-voting.txt
-Title: More robust consensus voting with diverse authority sets
-Author: Peter Palfrader
-Created: 2008-04-01
-Status: Rejected
-
-History:
-  2009 May 27: Added note on rejecting this proposal -- Nick
-
-Overview:
-
-  A means to arrive at a valid directory consensus even when voters
-  disagree on who is an authority.
-
-
-Motivation:
-
-  Right now there are about five authoritative directory servers in the
-  Tor network, tho this number is expected to rise to about 15 eventually.
-
-  Adding a new authority requires synchronized action from all operators of
-  directory authorities so that at any time during the update at least half of
-  all authorities are running and agree on who is an authority.  The latter
-  requirement is there so that the authorities can arrive at a common
-  consensus:  Each authority builds the consensus based on the votes from
-  all authorities it recognizes, and so a different set of recognized
-  authorities will lead to a different consensus document.
-
-
-Objective:
-
-  The modified voting procedure outlined in this proposal obsoletes the
-  requirement for most authorities to exactly agree on the list of
-  authorities.
-
-
-Proposal:
-
-  The vote document each authority generates contains a list of 
-  authorities recognized by the generating authority.  This will be 
-  a list of authority identity fingerprints.
-
-  Authorities will accept votes from and serve/mirror votes also for
-  authorities they do not recognize.  (Votes contain the signing,
-  authority key, and the certificate linking them so they can be 
-  verified even without knowing the authority beforehand.)
-
-  Before building the consensus we will check which votes to use for
-  building:
-
-   1) We build a directed graph of which authority/vote recognizes
-      whom.
-   2) (Parts of the graph that aren't reachable, directly or
-      indirectly, from any authorities we recognize can be discarded
-      immediately.)
-   3) We find the largest fully connected subgraph.
-      (Should there be more than one subgraph of the same size there
-      needs to be some arbitrary ordering so we always pick the same.
-      E.g. pick the one who has the smaller (XOR of all votes' digests)
-      or something.)
-   4) If we are part of that subgraph, great.  This is the list of 
-      votes we build our consensus with.
-   5) If we are not part of that subgraph, remove all the nodes that
-      are part of it and go to 3.
-
-  Using this procedure authorities that are updated to recognize a
-  new authority will continue voting with the old group until a
-  sufficient number has been updated to arrive at a consensus with
-  the recently added authority.
-
-  In fact, the old set of authorities will probably be voting among
-  themselves until all but one has been updated to recognize the
-  new authority.  Then which set of votes is used for consensus 
-  building depends on which of the two equally large sets gets 
-  ordered before the other in step (3) above.
-
-  It is necessary to continue with the process in (5) even if we
-  are not in the largest subgraph.  Otherwise one rogue authority
-  could create a number of extra votes (by new authorities) so that
-  everybody stops at 5 and no consensus is built, even tho it would
-  be trusted by all clients.
-
-
-Anonymity Implications:
-
-  The author does not believe this proposal to have anonymity
-  implications.
-
-
-Possible Attacks/Open Issues/Some thinking required:
-
- Q: Can a number (less or exactly half) of the authorities cause an honest
-    authority to vote for "their" consensus rather than the one that would
-    result were all authorities taken into account?
-
-
- Q: Can a set of votes from external authorities, i.e of whom we trust either
-    none or at least not all, cause us to change the set of consensus makers we
-    pick?
- A: Yes, if other authorities decide they rather build a consensus with them
-    then they'll be thrown out in step 3.  But that's ok since those other
-    authorities will never vote with us anyway.
-    If we trust none of them then we throw them out even sooner, so no harm done.
-
- Q: Can this ever force us to build a consensus with authorities we do not
-    recognize?
- A: No, we can never build a fully connected set with them in step 3.
-
-------------------------------
-
-I'm rejecting this proposal as insecure.
-
-Suppose that we have a clique of size N, and M hostile members in the
-clique.  If these hostile members stop declaring trust for up to M-1
-good members of the clique, the clique with the hostile members will
-in it will be larger than the one without them.
-
-The M hostile members will constitute a majority of this new clique
-when M > (N-(M-1)) / 2, or when M > (N + 1) / 3.  This breaks our
-requirement that an adversary must compromise a majority of authorities
-in order to control the consensus.
-
--- Nick