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r12489@catbus: nickm | 2007-04-21 13:48:39 -0400

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The ten thousandth Tor commit: add two new proposals (one from Mike Perry about randomized path length, and one from me about simplifyin authority operation) and expand and/or refine serveral older ones.  Most notable  there are changes to 103 that will allow us to make authorities more resistant to key compromise.


svn:r10000
This commit is contained in:
Nick Mathewson 2007-04-21 17:48:50 +00:00
parent f9cf90b597
commit c277b742f4
6 changed files with 399 additions and 7 deletions
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2
doc/spec/proposals/000-index.txt
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@ -30,3 +30,5 @@ Proposals by number:
109 No more than one server per IP address [ACCEPTED]
110 Avoiding infinite length circuits [OPEN]
111 Prioritizing local traffic over relayed traffic [OPEN]
112 Bring Back Patlen Coin Weight [OPEN]
113 Simplifying directory authority administration [OPEN]

9
doc/spec/proposals/101-dir-voting.txt
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@ -90,9 +90,14 @@ Proposal:
2. Details.
2.0. Versioning
All documents generated here have version "3" given in their
network-status-version entries.
2.1. Vote specifications
Votes in v2.1 are similar to v2 network status documents. We add these
Votes in v3 are similar to v2 network status documents. We add these
fields to the preamble:
"vote-status" -- the word "vote".
@ -122,7 +127,7 @@ Proposal:
2.2. Consensus directory specifications
Consensuses are like v2.1 votes, except for the following fields:
Consensuses are like v3 votes, except for the following fields:
"vote-status" -- the word "consensus".

99
doc/spec/proposals/103-multilevel-keys.txt
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@ -57,10 +57,99 @@ Proposal:
(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.
A possible solution:
Oh, and of course, we'll want to make sure that the keys are
cross-certified. :)
One thing to consider is that router identity keys are not very sensitive:
if an OR disappears and reappears with a new key, the network treats it as
though an old router had disappeared and a new one had joined the network.
The Tor network continues unharmed; this isn't a disaster.
Ideas? -NM
Thus, the ideas above are mostly relevant for authorities.
The most straightforward solution for the authorities is probably to take
advantage of the protocol transition that will come with proposal 101, and
introduce a new set of signing _and_ identity keys used only to sign votes
and consensus network-status documents. Signing and identity keys could be
delivered to users in a separate, rarely changing "keys" document, so that
the consensus network-status documents wouldn't need to include N signing
keys, N identity keys, and N certifications.
Note also that there is no reason that the identity/signing keys used by
directory authorities would necessarily have to be the same as the identity
keys those authorities use in their capacity as routers. Decoupling these
keys would give directory authorities the following set of keys:
Directory authority identity:
Highly confidential; stored encrypted and/or offline. Used to
identity directory authorities. Shipped with clients. Used to
sign Directory authority signing keys.
Directory authority signing key:
Stored online, accessible to regular Tor process. Used to sign
votes and consensus directories. Downloaded as part of a "keys"
document.
[Administrators SHOULD rotate their signing keys every month or
two, just to keep in practice and keep from forgetting the
password to the authority identity.]
V1-V2 directory authority identity:
Stored online, never changed. Used to sign legacy network-status
and directory documents.
Router identity:
Stored online, seldom changed. Used to sign server descriptors
for this authority in its role as a router. Implicitly certified
by being listed in network-status documents.
Onion key, link key:
As in tor-spec.txt
Extensions to Proposal 101.
Add the following elements to vote documents:
"dir-identity-key": The long-term identity key for this authority.
"dir-key-published": The time when this directory's signing key was last
changed.
"dir-key-certification": A signature of the fields "fingerprint",
"dir-key-published", "dir-signing-key", and "dir-identity-key",
concatenated, in that order. The signed material extends from the
beginning of "fingerprint" through the newline after
"dir-key-certification". The identity key is used to generate this
signature.
The elements "fingerprint", "dir-key-published", "dir-signing-key",
"dir-identity-key", and "dir-key-certification" together constitute a
"key certificate". These are generated offline when starting a v2.1
authority.
The elements "dir-signing-key", "dir-key-published", and
"dir-identity-key", "dir-key-certification" and MUST NOT appear in
consensus documents.
The "fingerprint" field is generated based on the identity key, not
the signing key.
Consensus network statues change as follows:
Remove dir-signing-key.
Change "directory-signature" to take a fingerprint of the authority's
identity key rather than the authority's nickname.
Add a new document type:
A "keys" document contains all currently known key certification
certificates. All authorities serve it at
http://<hostname>/tor/status/keys.z
Caches and clients download the keys document whenever they receive a
consensus vote that uses a key they do not recognize. Caches download
from authorities; clients download from caches.
Verification:
[XXXX write me]

7
doc/spec/proposals/104-short-descriptors.txt
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@ -122,6 +122,13 @@ Specification:
the digest of the extra-info document.
* The published fields in the two documents match.
Authorities SHOULD drop extra-info documents that do not meet these
criteria.
Extra-info documents MAY be uploaded as part of the same HTTP post as
the router descriptor, or separately. Authorities MUST accept both
methods.
Authorities SHOULD try to fetch extra-info documents from one another if
they do not have one matching the digest declared in a router
descriptor.

209
doc/spec/proposals/112-bring-back-pathlencoinweight.txt Normal file
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@ -0,0 +1,209 @@
Filename: 112-bring-back-pathlencoinweight.txt
Title: Bring Back Pathlen Coin Weight
Version:
Last-Modified:
Author: Mike Perry
Created:
Status: Open
Overview:
The idea is that users should be able to choose a weight which
probabilistically chooses their path lengths to be 2 or 3 hops. This
weight will essentially be a biased coin that indicates an
additional hop (beyond 2) with probability P. The user should be
allowed to choose 0 for this weight to always get 2 hops and 1 to
always get 3.
This value should be modifiable from the controller, and should be
available from Vidalia.
Motivation:
The Tor network is slow and overloaded. Increasingly often I hear
stories about friends and friends of friends who are behind firewalls,
annoying censorware, or under surveillance that interferes with their
productivity and Internet usage, or chills their speech. These people
know about Tor, but they choose to put up with the censorship because
Tor is too slow to be usable for them. In fact, to download a fresh,
complete copy of levine-timing.pdf for the Anonymity Implications
section of this proposal over Tor took me 3 tries.
There are many ways to improve the speed problem, and of course we
should and will implement as many as we can. Johannes's GSoC project
and my reputation system are longer term, higher-effort things that
will still provide benefit independent of this proposal.
However, reducing the path length to 2 for those who do not need the
(questionable) extra anonymity 3 hops provide not only improves
their Tor experience but also reduces their load on the Tor network by
33%, and can be done in less than 10 lines of code. That's not just
Win-Win, it's Win-Win-Win.
Furthermore, when blocking resistance measures insert an extra relay
hop into the equation, 4 hops will certainly be completely unusable
for these users, especially since it will be considerably more
difficult to balance the load across a dark relay net than balancing
the load on Tor itself (which today is still not without its flaws).
Anonymity Implications:
It has long been established that timing attacks against mixed
networks are extremely effective, and that regardless of path
length, if the adversary has compromised your first and last
hop of your path, you can assume they have compromised your
identity for that connection.
In [1], it is demonstrated that for all but the slowest, lossiest
networks, error rates for false positives and false negatives were
very near zero. Only for constant streams of traffic over slow and
(more importantly) extremely lossy network links did the error rate
hit 20%. For loss rates typical to the Internet, even the error rate
for slow nodes with constant traffic streams was 13%.
When you take into account that most Tor streams are not constant,
but probably much more like their "HomeIP" dataset, which consists
mostly of web traffic that exists over finite intervals at specific
times, error rates drop to fractions of 1%, even for the "worst"
network nodes.
Therefore, the user has little benefit from the extra hop, assuming
the adversary does timing correlation on their nodes. The real
protection is the probability of getting both the first and last hop,
and this is constant whether the client chooses 2 hops, 3 hops, or 42.
Partitioning attacks form another concern. Since Tor uses telescoping
to build circuits, it is possible to tell a user is constructing only
two hop paths at the entry node. It is questionable if this data is
actually worth anything though, especially if the majority of users
have easy access to this option, and do actually choose their path
lengths semi-randomly.
Nick has postulated that exits may also be able to tell that you are
using only 2 hops by the amount of time between sending their
RELAY_CONNECTED cell and the first bit of RELAY_DATA traffic they
see from the OP. I doubt that they will be able to make much use
of this timing pattern, since it will likely vary widely depending
upon the type of node selected for that first hop, and the user's
connection rate to that first hop. It is also questionable if this
data is worth anything, especially if many users are using this
option (and I imagine many will).
Perhaps most seriously, two hop paths do allow malicious guards
to easily fail circuits if they do not extend to their colluding peers
for the exit hop. Since guards can detect the number of hops in a
path, they could always fail the 3 hop circuits and focus on
selectively failing the two hop ones until a peer was chosen.
I believe currently guards are rotated if circuits fail, which does
provide some protection, but this could be changed so that an entry
guard is completely abandoned after a certain number of extend or
general circuit failures, though perhaps this also could be gamed
to increase guard turnover. Such a game would be much more noticeable
than an individual guard failing circuits, though, since it would
affect all clients, not just those who chose a particular guard.
Why not fix Pathlen=2?:
The main reason I am not advocating that we always use 2 hops is that
in some situations, timing correlation evidence by itself may not be
considered as solid and convincing as an actual, uninterrupted, fully
traced path. Are these timing attacks as effective on a real network
as they are in simulation? Would an extralegal adversary or authoritarian
government even care? In the face of these situation-dependent unknowns,
it should be up to the user to decide if this is a concern for them or not.
Implementation:
new_route_len() can be modified directly with a check of the
PathlenCoinWeight option (converted to percent) and a call to
crypto_rand_int(0,100) for the weighted coin.
The Vidalia setting should probably be in the network status window
as a slider, complete with tooltip, help documentation, and perhaps
an "Are you Sure?" checkbox.
The entry_guard_t structure could have a num_circ_failed member
such that if it exceeds N circuit extend failure to a second hop,
it is removed from the entry list. N should be sufficiently high
to avoid churn from normal Tor circuit failure, and could possibly be
represented as a ratio of failed to successful circuits through that
guard.
Migration:
Phase one: Re-enable config and modify new_route_len() to add an
extra hop if coin comes up "heads".
Phase two: Experiment with the proper ratio of circuit failures
used to expire garbage or malicious guards.
Phase three: Make slider or entry box in Vidalia, along with help entry
that explains in layman's terms the risks involved.
[1] http://www.cs.umass.edu/~mwright/papers/levine-timing.pdf
============================================================
I love replying to myself. I can't resist doing it. Sorry. "Think twice
post once" is a concept totally lost on me, especially when I'm wrong
the first two times ;)
Thus spake Mike Perry (mikepery@fscked.org):
> Why not fix Pathlen=2?:
>
> The main reason I am not advocating that we always use 2 hops is that
> in some situations, timing correlation evidence by itself may not be
> considered as solid and convincing as an actual, uninterrupted, fully
> traced path. Are these timing attacks as effective on a real network
> as they are in simulation? Would an extralegal adversary or authoritarian
> government even care? In the face of these situation-dependent unknowns,
> it should be up to the user to decide if this is a concern for them or not.
Hrmm.. it should probably also be noted that even a false positive
rate of 1% for a 200k concurrent-user network could mean that for a
given node, a given stream could be confused with something like 10
users, assuming ~200 nodes carry most of the traffic (ie 1000 users
each). Though of course to really know for sure, someone needs to do
an attack on a real network, unfortunately.
For this reason this option should instead be represented not as a
slider, but as a straight boolean value, at least in Vidalia.
Perhaps something like a radiobutton:
* "I use Tor for Censorship Resistance, not Anonymity. Speed is more
important to me than Anonymity."
* "I use Tor for Anonymity. I need extra protection at the cost of speed."
and then some explanation in the help for exactly what this means, and
the risks involved with eliminating the adversary's need for timing attacks
wrt to false positives, etc.
This radio button can then also be used to toggle Johannes's work,
should it be discovered that using latency/bandwidth measurements
gives the adversary some information as to your location or likely
node choices. Or we can create a series of choices along these lines
as more load balancing/path choice optimizations are developed.
----
So what does this change mean wrt to the proposal process? Should I
submit a new proposal? I'm still on the fence if the underlying torrc
option and Tor implementation should be a coin weight or a fixed
value, so at this point really all this changes is the proposed
Vidalia behavior (Vidalia is an imporant part of this proposal,
because it would be nice to take 33% of the load off the network for
all users who do not need 3 hops).

80
doc/spec/proposals/113-fast-authority-interface.txt Normal file
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@ -0,0 +1,80 @@
Filename: 113-fast-authority-interface.txt
Title: Simplifying directory authority administration
Version: $Revision: 12412 $
Last-Modified: $Date: 2007-04-16T19:11:29.511998Z $
Author: Nick Mathewson
Created:
Status: Open
Overview
The problem:
Administering a directory authority is a pain: you need to go through
emails and manually add new nodes as "named". When bad things come up,
you need to mark nodes (or whole regions) as invalid, badexit, etc.
This means that mostly, authority admins don't: only 2/4 current authority
admins actually bind names or list bad exits, and those two have often
complained about how annoying it is to do so.
Worse, name binding is a common path, but it's a pain in the neck: nobody
has done it for a couple of months.
Digression: who knows what?
It's trivial for Tor to automatically keep track of all of the
following information about a server:
name, fingerprint, IP, last-seen time, first-seen time, declared
contact.
All we need to have the administrator set is:
- Is this name/fingerprint pair bound?
- Is this fingerprint/IP a bad exit?
- Is this fingerprint/IP an invalid node?
- Is this fingerprint/IP to be rejected?
The workflow for authority admins has two parts:
- Periodically, go through tor-ops and add new names. This doesn't
need to be done urgently.
- Less often, mark badly behaved serves as badly behaved. This is more
urgent.
Possible solution #1: Web-interface for name binding.
Deprecate use of the tor-ops mailing list; instead, have operators go to a
webform and enter their server info. This would put the information in a
standardized format, thus allowing quick, nearly-automated approval and
reply.
Possible solution #2: Self-binding names.
Peter Palfrader has proposed that names be assigned automatically to nodes
that have been up and running and valid for a while.
Possible solution #3: Self-maintaining approved-routers file
Mixminion alpha has a neat feature where whenever a new server is seen,
a stub line gets added to a configuration file. For Tor, it could look
something like this:
## First seen with this key on 2007-04-21 13:13:14
## Stayed up for at least 12 hours on IP 192.168.10.10
#RouterName AAAABBBBCCCCDDDDEFEF
(Note that the implementation needs to parse commented lines to make sure
that it doesn't add duplicates, but that's not so hard.)
To add a router as named, administrators would only need to uncomment the
entry. This automatically maintained file could be kept separately from a
manually maintained one.
This could be combined with solution #2, such that Tor would do the hard
work of uncommenting entries for routers that should get Named, but
operators could override its decisions.
Possible solution #4: A separate mailing list for authority operators.
Right now, the tor-ops list is very high volume. There should be another
list that's only for dealing with problems that need prompt action, like
marking a router as !badexit.