[or-cvs] r8960: Commit additional thoughts towards a revised directory proto (in tor/trunk: . doc)
nickm at seul.org
nickm at seul.org
Fri Nov 17 03:35:25 UTC 2006
Author: nickm
Date: 2006-11-16 22:35:19 -0500 (Thu, 16 Nov 2006)
New Revision: 8960
Added:
tor/trunk/doc/dir-voting.txt
Modified:
tor/trunk/
Log:
r9562 at Kushana: nickm | 2006-11-16 22:33:23 -0500
Commit additional thoughts towards a revised directory protocol, including voting.
Property changes on: tor/trunk
___________________________________________________________________
svk:merge ticket from /tor/trunk [r9562] on c95137ef-5f19-0410-b913-86e773d04f59
Added: tor/trunk/doc/dir-voting.txt
===================================================================
--- tor/trunk/doc/dir-voting.txt 2006-11-17 03:34:58 UTC (rev 8959)
+++ tor/trunk/doc/dir-voting.txt 2006-11-17 03:35:19 UTC (rev 8960)
@@ -0,0 +1,278 @@
+$Id: /tor/branches/eventdns/doc/dir-spec.txt 9469 2006-11-01T23:56:30.179423Z nickm $
+
+ Voting on the Tor Directory System
+
+0. Scope and preliminaries
+
+ This document describes a consensus voting scheme for Tor directories.
+ Once it's accepted, it should be merged with dir-spec.txt. Some
+ preliminaries for authority and caching support should be done during
+ the 0.1.2.x series; the main deployment should come during the 0.1.3.x
+ series.
+
+0.1. Goals and motivation: voting.
+
+ The current directory system relies on clients downloading separate
+ network status statements from the caches signed by each directory.
+ Clients download a new statement every 30 minutes or so, choosing to
+ replace the oldest statement they currently have.
+
+ This creates a partitioning problem: different clients have different
+ "most recent" networkstatus sources, and different versions of each
+ (since authorities change their statements often). Also, it is very
+ redundant: most of the downloaded networkstatus are probably quite
+ similar.
+
+ So if we have clients only download a single multiply signed consensus
+ network status statement, we can:
+ - Save bandwidth.
+ - Reduce client partitioning
+ - Reduce client-side and cache-side storage
+ - Simplify client-side voting code (by moving voting away from the
+ client)
+
+ We should try to do this without:
+ - Assuming that client-side or cache-side clocks are more correct
+ than we assume now.
+ - Assuming that authority clocks are perfectly correct.
+ - Degrading badly if an authority dies or is offline for a bit.
+
+ We do not have to perform well if:
+ - No clique of more than half the authorities can agree about who
+ the authorities are.
+
+1. The idea.
+
+ Instead of publishing a network status whenever something changes,
+ each authority instead publishes a fresh network status only once per
+ "period" (say, 60 minutes). Authorities either upload this network
+ status (or "vote") to every other authority, or download every other
+ authority's "vote" (see 3.1 below for discussion on push vs pull).
+
+ After an authority has (or has become convinced that it won't be able to
+ get) every other authority's vote, it deterministically computes a
+ consensus networkstatus, and signs it. Authorities download (or are
+ uploaded; see 3.1) one another's signatures, and form a multiply signed
+ consensus. This multiply-signed consensus is what caches cache and what
+ clients download.
+
+ If an authority is down, authorities vote based on what they *can*
+ download/get uploaded.
+
+ If an authority is "a little" down and only some authorities can reach
+ it, authorities try to get its info from other authorities.
+
+ If an authority computes the vote wrong, its signature isn't included on
+ the consensus.
+
+ Clients use a consensus if it is signed by more than half the
+ authorities they recognize. If they can't find any such consensus,
+ clients either use an older version, or beg the user to adapt the list
+ of authorities.
+
+2. Details.
+
+2.1. Vote specifications
+
+ Votes in v2.1 are just like v2 network status documents. We add these
+ fields to the preamble:
+
+ "vote-status" -- the word "vote".
+
+ "valid-until" -- the time when this authority expects to publish its
+ next vote.
+
+ "known-flags" -- a space-separated list of flags that will sometimes
+ be included on "s" lines later in the vote.
+
+ "dir-source" -- as before, except the "hostname" part MUST be the
+ authority's nickname, which MUST be unique among authorities, and
+ MUST match the nickname in the "directory-signature" entry.
+
+ Authorities SHOULD cache their most recently generated votes so they
+ can persist them across restarts. Authorities SHOULD NOT generate
+ another document until valid-until has passed.
+
+ Router entries in the vote MUST be sorted in ascending order by router
+ identity digest. The flags in "s" lines MUST appear in alphabetical
+ order.
+
+ Votes SHOULD be synchronized to half-hour publication intervals (one
+ hour? XXX say more; be more precise.)
+
+ XXXX some way to request older networkstatus docs?
+
+
+2.2. Consensus directory specifications
+
+ Consensuses are like v2.1 votes, except for the following fields:
+
+ "vote-status" -- the word "consensus".
+
+ "published" is the latest of all the published times on the votes.
+
+ "valid-until" is the earliest of all the valid-until times on the
+ votes.
+
+ "dir-source" and "fingerprint" and "dir-signing-key" and "contact"
+ are included for each authority that contributed to the vote.
+
+ "vote-digest" for each authority that contributed to the vote,
+ calculated as for the digest in the signature on the vote. [XXX
+ re-English this sentence]
+
+ "client-versions" and "server-versions" are sorted in ascending
+ order.
+
+ "dir-options" and "known-flags" are not included.
+
+ The fields MUST occur in the following order:
+ "network-status-version"
+ "vote-status"
+ "published"
+ "valid-until"
+ For each authority, sorted in ascending order of nickname, case-
+ insensitively:
+ "dir-source", "fingerprint", "contact", "dir-signing-key",
+ "vote-digest".
+ "client-versions"
+ "server-versions"
+
+ The signatures at the end of the document appear as multiple instances
+ directory-signature, sorted in ascending order by nickname,
+ case-insensitively.
+
+ A router entry should be included in the result if it is included by
+ more than half of the authorities (total authorities, not just those
+ whose votes we have). A router entry has a flag set if it is included
+ by more than half of the authorities who care about that flag. [XXXX
+ this creates a DOS incentive. Can we remember what flags people set the
+ last time we saw them?]
+
+ [What does the signature hash cover ? XXX]
+
+2.3. Agreement and timeline
+
+ [XXXX publish signed vote summaries.]
+ [XXXX URL list: vote, other people's votes, directory.]
+ [XXXX in-progress URL vs done URL]
+ [XXXX Store votes to disk.]
+
+2.4. Distributing routerdescs between authorities
+
+ Consensus will be more meaningful if authorities take steps to make sure
+ that they all have the same set of descriptors _before_ the voting
+ starts. This is safe, since all descriptors are self-certified and
+ timestamped: it's always okay to replace a signed descriptor with a more
+ recent one signed by the same identity.
+
+ In the long run, we might want some kind of sophisticated process here.
+ For now, since authorities already download one another's networkstatus
+ documents and use them to determine what descriptors to download from one
+ another, we can rely on this existing mechanism to keep authorities up to
+ date.
+
+3. Questions and concerns
+
+3.1. Push or pull?
+
+ [XXXX]
+
+3.2. Dropping "opt".
+
+ The "opt" keyword in Tor's directory formats was originally intended to
+ mean, "it is okay to ignore this entry if you don't understand it"; the
+ default behavior has been "discard a routerdesc if it contains entries you
+ don't recognize."
+
+ But so far, every new flag we have added has been marked 'opt'. It would
+ probably make sense to change the default behavior to "ignore unrecognized
+ fields", and add the statement that clients SHOULD ignore fields they don't
+ recognize. As a meta-principle, we should say that clients and servers
+ MUST NOT have to understand new fields in order to use directory documents
+ correctly.
+
+ Of course, this will make it impossible to say, "The format has changed a
+ lot; discard this quietly if you don't understand it." We could do that by
+ adding a version field.
+
+3.3. Multilevel keys.
+
+ Replacing a directory authority's identity key in the event of a compromise
+ would be tremendously annoying. We'd need to tell every client to switch
+ their configuration, or update to a new version with an uploaded list. So
+ long as some weren't upgraded, they'd be at risk from whoever had
+ compromised the key.
+
+ With this in mind, it's a shame that our current protocol forces us to
+ store identity keys unencrypted in RAM. We need some kind of signing key
+ stored unencrypted, since we need to generate new descriptors/directories
+ and rotate link and onion keys regularly. (And since, of course, we can't
+ ask server operators to be on-hand to enter a passphrase every time we
+ want to rotate keys or sign a descriptor.)
+
+ The obvious solution seems to be to have a signing-only key that lives
+ indefinitely (months or longer) and signs descriptors and link keys, and a
+ separate identity key that's used to sign the signing key. Tor servers
+ could run in one of several modes:
+ 1. Identity key stored encrypted. You need to pick a passphrase when
+ you enable this mode, and re-enter this passphrase every time you
+ rotate the signing key.
+ 1'. Identity key stored separate. You save your identity key to a
+ floppy, and use the floppy when you need to rotate the signing key.
+ 2. All keys stored unencrypted. In this case, we might not want to even
+ *have* a separate signing key. (We'll need to support no-separate-
+ signing-key mode anyway to keep old servers working.)
+ 3. All keys stored encrypted. You need to enter a passphrase to start
+ Tor.
+ (Of course, we might not want to implement all of these.)
+
+ Case 1 is probably most usable and secure, if we assume that people don't
+ forget their passphrases or lose their floppies. We could mitigate this a
+ bit by encouraging people to PGP-encrypt their passphrases to themselves,
+ or keep a cleartext copy of their secret key secret-split into a few
+ pieces, or something like that.
+
+ Migration presents another difficulty, especially with the authorities. If
+ we use the current set of identity keys as the new identity keys, we're in
+ the position of having sensitive keys that have been stored on
+ media-of-dubious-encryption up to now. Also, we need to keep old clients
+ (who will expect descriptors to be signed by the identity keys they know
+ and love, and who will not understand signing keys) happy.
+
+ I'd enumerate designs here, but I'm hoping that somebody will come up with
+ a better one, so I'll try not to prejudice them with more ideas yet.
+
+ Oh, and of course, we'll want to make sure that the keys are
+ cross-certified. :)
+
+ Ideas? -NM
+
+3.4. Long and short descriptors
+
+ Some of the costliest fields in the current directory protocol are ones
+ that no client actually uses. In particular, the "read-history" and
+ "write-history" fields are used only by the authorities for monitoring the
+ status of the network. If we took them out, the size of a compressed list
+ of all the routers would fall by about 60%. (No other disposable field
+ would save more than 2%.)
+
+ One possible solution here is that routers should generate and upload a
+ short-form and long-form descriptor. Only the short-form descriptor should
+ ever be used by anybody for routing. The long-form descriptor should be
+ used only for analytics and other tools. (If we allowed people to route with
+ long descriptors, we'd have to ensure that they stayed in sync with the
+ short ones somehow.)
+
+ Another possible solution would be to drop these fields from descriptors,
+ and have them uploaded as a part of a separate "bandwidth report" to the
+ authorities. This could help prevent the mistake of using long descriptors
+ in the place of short ones.
+
+ Thoughts? -NM
+
+4. Migration
+
+ For directory voting, ...
+
+caches need to start caching consensuses and accepting multisigned documents.
More information about the tor-commits
mailing list