From c82fbcd057696c5b2c2143e7c5eddeab73d84a1c Mon Sep 17 00:00:00 2001 From: Nick Mathewson Date: Fri, 26 Jan 2007 05:50:40 +0000 Subject: Make a new directory for specification proposals, and move some proposals there. Also, move dir-spec-v1.txt to spec. svn:r9415 --- proposals/101-dir-voting.txt | 388 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 388 insertions(+) create mode 100644 proposals/101-dir-voting.txt (limited to 'proposals/101-dir-voting.txt') diff --git a/proposals/101-dir-voting.txt b/proposals/101-dir-voting.txt new file mode 100644 index 0000000..4909701 --- /dev/null +++ b/proposals/101-dir-voting.txt @@ -0,0 +1,388 @@ +$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). + + It also creates a scaling problem: most of the downloaded networkstatus + are probably quite similar, and the redundancy grows as we add more + authorities. + + 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 a few authorities die or are 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 "trusted": signed by more than half the + authorities they recognize. If clients can't find any such consensus, + they use the most recent trusted consensus they have. If they don't + have any trusted consensus, they warn the user and refuse to operate + (and if DirServers is not the default, beg the user to adapt the list + of authorities). + +2. Details. + +2.1. Vote specifications + + Votes in v2.1 are similar to 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 based on version-spec.txt. + + "dir-options" and "known-flags" are not included. +[XXX really? why not list the ones that are used in the consensus? +For example, right now BadExit is in use, but no servers would be +labelled BadExit, and it's still worth knowing that it was considered +by the authorities. -RD] + + 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 + of 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 an + incentive for attackers to DOS authorities whose votes they don't like. + Can we remember what flags people set the last time we saw them? -NM] + [Which 'we' are we talking here? The end-users never learn which + authority sets which flags. So you're thinking the authorities + should record the last vote they saw from each authority and if it's + within a week or so, count all the flags that it advertised as 'no' + votes? Plausible. -RD] + + The signature hash covers from the "network-status-version" line through + the characters "directory-signature" in the first "directory-signature" + line. + + Consensus directories SHOULD be rejected if they are not signed by more + than half of the known authorities. + +2.2.1. Detached signatures + + Assuming full connectivity, every authority should compute and sign the + same consensus directory in each period. Therefore, it isn't necessary to + download the consensus computed by each authority; instead, the authorities + only push/fetch each others' signatures. A "detached signature" document + contains a single "consensus-digest" entry and one or more + directory-signature entries. [XXXX specify more.] + +2.3. URLs and timelines + +2.3.1. URLs and timeline used for agreement + + An authority SHOULD publish its vote immediately at the start of each voting + period. It does this by making it available at + http:///tor/status-vote/current/authority.z + and sending it in an HTTP POST request to each other authority at the URL + http:///tor/post/vote + + If, N minutes after the voting period has begun, an authority does not have + a current statement from another authority, the first authority retrieves + the other's statement. + + Once an authority has a vote from another authority, it makes it available + at + http:///tor/status-vote/current/.z + where is the fingerprint of the other authority's identity key. + + The consensus network status, along with as many signatures as the server + currently knows, should be available at + http:///tor/status-vote/current/consensus.z + All of the detached signatures it knows for consensus status should be + available at: + http:///tor/status-vote/current/consensus-signatures.z + + Once an authority has computed and signed a consensus network status, it + should send its detached signature to each other authority in an HTTP POST + request to the URL: + http:///tor/post/consensus-signature + + + [XXXX Store votes to disk.] + +2.3.2. Serving a consensus directory + + Once the authority is done getting signatures on the consensus directory, + it should serve it from: + http:///tor/status/consensus.z + + Caches SHOULD download consensus directories from an authority and serve + them from the same URL. + +2.3.3. Timeline and synchronization + + [XXXX] + +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. + + [We should do a thorough read-through of dir-spec again to make sure + that the authorities converge on which descriptor to "prefer" for + each router. Right now the decision happens at the client, which is + no longer the right place for it. -RD] + +3. Questions and concerns + +3.1. Push or pull? + + The URLs above define a push mechanism for publishing votes and consensus + signatures via HTTP POST requests, and a pull mechanism for downloading + these documents via HTTP GET requests. As specified, every authority will + post to every other. The "download if no copy has been received" mechanism + exists only as a fallback. + +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.) We can ensure that the short descriptors are used by + only recommending those in the network statuses. + + 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 + +3.5. Compression + + Gzip would be easier to work with than zlib; bzip2 would result in smaller + data lengths. [Concretely, we're looking at about 10-15% space savings at + the expense of 3-5x longer compression time for using bzip2.] Doing + on-the-fly gzip requires zlib 1.2 or later; doing bzip2 requires bzlib. + Pre-compressing status documents in multiple formats would force us to use + more memory to hold them. + +4. Migration + + For directory voting: + * It would be cool if caches could get ready to download consensus + status docs, verify enough signatures, and serve them now. That way + once stuff works all we need to do is upgrade the authorities. Caches + don't need to verify the correctness of the format so long as it's + signed (or maybe multisigned?). We need to make sure that caches back + off very quickly from downloading consensus docs until they're + actually implemented. + + For dropping the "opt" requirement: + * stopped requiring it as of 0.1.2.5-alpha. Stop generating it once + earlier formats are obsolete. + + For multilevel keys: + * no idea + + For long/short descriptors: + * In 0.1.2.x: + * Authorities should accept both, now, and silently drop short + descriptors. + * Routers should upload both once authorities accept them. + * There should be a "long descriptor" url and the current "normal" URL. + Authorities should serve long descriptors from both URLs. + * Once tools that want long descriptors support fetching them from the + "long descriptor" URL: + * Have authorities remember short descriptors, and serve them from the + 'normal' URL. + -- cgit v1.2.3-54-g00ecf