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[tor-dev] Proposal: Authenticating sendme cells to mitigate bandwidth attacks
Filename: xxx-authenticated-sendmes.txt
Title: Authenticating sendme cells to mitigate bandwidth attacks
Author: Rob Jansen, Roger Dingledine
Created: 2016-12-01
Status: open
1. Overview and Motivation
In Rob's "Sniper attack", a malicious Tor client builds a circuit,
fetches a large file from some website, and then refuses to read any
of the cells from the entry guard, yet sends "sendme" (flow control
acknowledgement) cells down the circuit to encourage the exit relay
to keep sending more cells. Eventually enough cells queue at the
entry guard that it runs out of memory and exits [0, 1].
We resolved the "runs out of memory and exits" part of the attack with
our Out-Of-Memory (OOM) manager introduced in Tor 0.2.4.18-rc. But
the earlier part remains unresolved: a malicious client can launch
an asymmetric bandwidth attack by creating circuits and streams and
sending a small number of sendme cells on each to cause the target
relay to receive a large number of data cells.
This attack could be used for general mischief in the network (e.g.,
consume Tor network bandwidth resources or prevent access to relays),
and it could probably also be leveraged to harm anonymity a la the
"congestion attack" designs [2, 3].
This proposal describes a way to verify that the client has seen all
of the cells that its sendme cell is acknowledging, based on the
authenticated sendmes design from [1].
2. Sniper Attack Variations
There are some variations on the attack involving the number and
length of the circuits and the number of Tor clients used. We explain
them here to help understand which of them this proposal attempts to
defend against.
We compare the efficiency of these attacks in terms of the number of
cells transferred by the adversary and by the network, where receiving
and sending a cell counts as two transfers of that cell.
2.1 Single Circuit, without Sendmes
The simplest attack is where the adversary starts a single Tor client,
creates one circuit and two streams to some website, and stops
reading from the TCP connection to the entry guard. The adversary
gets 1000 "attack" cells "for free" (until the stream and circuit
windows close). The attack data cells are both received and sent by the
exit and the middle, while being received and queued by the guard.
Adversary:
6 transfers to create the circuit
2 to begin the two exit connections
2 to send the two GET requests
---
10 total
Network:
18 transfers to create the circuit
22 to begin the two exit connections (assumes two for the exit TCP connect)
12 to send the two GET requests to the website
5000 for requested data (until the stream and circuit windows close)
---
5052 total
2.2 Single Circuit, with Sendmes
A slightly more complex version of the attack in 2.1 is where the
adversary continues to send sendme cells to the guard (toward the exit),
and then gets another 100 attack data cells sent across the network for every
three additional exitward sendme cells that it sends (two stream-level
sendmes and one circuit-level sendme). The adversary also gets another
three clientward sendme cells sent by the exit for every 100 exitward
sendme cells it sends.
If the adversary sends N sendmes, then we have:
Adversary:
10 for circuit and stream setup
N for circuit and stream sendmes
---
10+N
Network:
5052 for circuit and stream setup and initial depletion of circuit windows
N*100/3*5 for transferring additional data cells from the website
N*3/100*4 for transferring sendmes from exit to client
---
5052 + N*166.79
It is important to note that once the adversary stops reading from the
guard, it will no longer get feedback on the speed at which the data
cells are able to be transferred through the circuit from the exit
to the guard. It needs to approximate when it should send sendmes
to the exit; if too many sendmes are sent such that the circuit
window would open farther than 1000 cells (500 for streams), then the
circuit may be closed by the exit. In practice, the adversary could
take measurements during the circuit setup process and use them to
estimate a conservative sendme sending rate.
2.3 Multiple Circuits
The adversary could parallelize the above attacks using multiple
circuits. Because the adversary needs to stop reading from the TCP
connection to the guard, they would need to do a pre-attack setup
phase during which they construct the attack circuits. Then, they
would stop reading from the guard and send all of the GET requests
across all of the circuits they created.
The number of cells from 2.1 and 2.2 would then be multiplied by the
number of circuits C that the adversary is able to build and sustain
during the attack.
2.4 Multiple Guards
The adversary could use the "UseEntryGuards 0" torrc option, or build
custom circuits with stem to parallelize the attack across multiple
guard nodes. This would slightly increase the bandwidth usage of the
adversary, since it would be creating additional TCP connections to
guard nodes.
2.5 Multiple Clients
The adversary could run multiple attack clients, each of which would
choose its own guard. This would slightly increase the bandwidth
usage of the adversary, since it would be creating additional TCP
connections to guard nodes and would also be downloading directory
info, creating testing circuits, etc.
2.6 Short Two-hop Circuits
If the adversary uses two-hop circuits, there is less overhead
involved with the circuit setup process.
Adversary:
4 transfers to create the circuit
2 to begin the two exit connections
2 to send the two GET requests
---
8
Network:
8 transfers to create the circuit
14 to begin the two exit connections (assumes two for the exit TCP connect)
8 to send the two GET requests to the website
5000 for requested data (until the stream and circuit windows close)
---
5030
2.7 Long >3-hop Circuits
The adversary could use a circuit longer than three hops to cause more
bandwidth usage across the network. Let's use an 8 hop circuit as an
example.
Adversary:
16 transfers to create the circuit
2 to begin the two exit connections
2 to send the two GET requests
---
20
Network:
128 transfers to create the circuit
62 to begin the two exit connections (assumes two for the exit TCP connect)
32 to send the two GET requests to the website
15000 for requested data (until the stream and circuit windows close)
---
15222
The adversary could also target a specific relay, and use it multiple
times as part of the long circuit, e.g., as hop 1, 4, and 7.
Target:
54 transfers to create the circuit
22 to begin the two exit connections (assumes two for the exit TCP connect)
12 to send the two GET requests to the website
5000 for requested data (until the stream and circuit windows close)
---
5088
3. Design
This proposal aims to defend against the versions of the attack that
utilize sendme cells without reading. It does not attempt to handle
the case of multiple circuits per guard, or try to restrict the number
of guards used by a client, or prevent a sybil attack across multiple
client instances.
The proposal involves three components: first, the client needs to add
a token to the sendme payload, to prove that it knows the contents
of the cells that it has received. Second, the exit relay needs to
verify this token. Third, to resolve the case where the client already
knows the contents of the file so it only pretends to read the cells,
the exit relay needs to be able to add unexpected randomness to the
circuit.
(Note: this proposal talks about clients and exit relays, but since
sendmes go in both directions, both sides of the circuit should do
these changes.)
3.1. Changing the sendme payload to prove receipt of cells
In short: clients put the latest received relay cell digest in the
payload of their circuit-level sendme cells.
Each relay cell header includes a 4-byte digest which represents
the rolling hash of all bytes received on that circuit. So knowledge
of that digest is an indication that you've seen the bytes that go
into it.
We pick circuit-level sendme cells, as opposed to stream-level sendme
cells, because we think modifying just circuit-level sendmes is
sufficient to accomplish the properties we need, and modifying just
stream-level sendmes is not sufficient: a client could send a bunch
of begin cells and fake their circuit-level sendmes, but never send
any stream-level sendmes, attracting 500*n queued cells to the entry
guard for the n streams that it opens.
Which digest should the client put in the sendme payload? Right now
circuit-level sendmes are sent whenever one window worth of relay cells
(100) has arrived. So the client should use the digest from the cell
that triggers the sendme.
How shall we version the sendme payload so we can change the format of
it later? Right now sendme payloads are empty. Here's a simple design:
we use five bytes in the payload, where the first byte indicates the
sendme payload version (0 in the original design, and 1 once we've
implemented this proposal), and the rest of the payload is formatted
based on the payload version number: in this case, it's simply the
four bytes of digest.
Is there a better way to version the payload, e.g. a way that is
already standard in other parts of the design, so we aren't adding
a new paint color to keep track of on the bike shed?
3.2. Verifying the sendme payload
In the current Tor, the exit relay keeps no memory of the cells it
has sent down the circuit, so it won't be in a position to verify
the digest that it gets back.
But fortunately, the exit relay can count also, so it knows which cell
is going to trigger the sendme response. Each circuit can have at most
10 sendmes worth of data outstanding. So the exit relay will keep
a per-circuit fifo queue of the digests from the appropriate cells,
and when a new sendme arrives, it pulls off the next digest in line,
and verifies that it matches.
If a sendme payload has a payload version of 1 yet its digest
doesn't match the expected digest, or if the sendme payload has
an unexpected payload version (see below about deployment phases),
the exit relay must tear down the circuit. (If we later find that
we need to introduce a newer payload version in an incompatible way,
we would do that by bumping the circuit protocol version.)
3.3. Making sure there are enough unpredictable bytes in the circuit
So far, the design as described fails to a very simple attacker:
the client fetches a file whose contents it already knows, and it
uses that knowledge to calculate the correct digests and fake its
sendmes just like in the original attack.
The fix is that the exit relay needs to be able to add some randomness
into its cells. It can add this randomness, in a way that's completely
orthogonal to the rest of this design, simply by choosing one relay
cell every so often and not using the entire relay cell payload for
actual data (i.e. using a Length field of less than 498), and putting
some random bytes in the remainder of the payload.
How many random bytes should the exit relay use, and how often should
it use them? There is a tradeoff between security when under attack,
and efficiency when not under attack. We think 1 byte of randomness
every 1000 cells is a good starting plan, and we can always improve
it later without needing to change any of the rest of this design.
(Note that the spec currently says "The remainder of the payload
is padded with NUL bytes." We think "is" doesn't mean MUST, so we
should just be sure to update that part of the spec to reflect our
new plans here.)
4. Deployment Plan
In phase one, both sides begin remembering their expected digests,
and they learn how to parse sendme payloads. When they receive a
sendme with payload version 1, they verify its digest and tear down
the circuit if it's wrong. But they continue to send and accept
payload version 0 sendmes.
In phase two, we flip a switch in the consensus, and everybody starts
sending payload version 1 sendmes. Payload version 0 sendmes are
still accepted.
In phase three, we flip a different switch in the consensus, and
everybody starts refusing payload version 0 sendmes.
(It has to be two separate switches, not one unified one, because
otherwise we'd have a race where relays learn about the update before
clients know to start the new behavior.)
We'll want to do a bunch of testing in chutney before flipping the
switches in the real network: I've long suspected we still have bugs
in our sendme timing, and this proposal might expose some of them.
Alas, this deployment plan leaves a pretty large window until relays
are protected from attack. It's not all bad news though, since we
could flip the switches earlier than intended if we encounter a
network-wide attack.
5. Security Discussion
Does our design enable any new adversarial capabilities?
An adversarial middle relay could attempt to trick the exit into
killing an otherwise valid circuit.
An adversarial relay can already kill a circuit, but here it could make
it appear that the circuit was killed for a legitimate reason (invalid
or missing sendme), and make someone else (the exit) do the killing.
There are two ways it might do this: by trying to make a valid sendme
appear invalid; and by blocking the delivery of a valid sendme. Both of
these depend on the ability for the adversary to guess which exitward
cell is a sendme cell, which it could do by counting clientward cells.
* Making a valid sendme appear invalid
A malicious middle could stomp bits in the exitward sendme so
that the exit sendme validation fails. However, bit stomping would
be detected at the protocol layer orthogonal to this design, and
unrecognized exitward cells would currently cause the circuit to be
torn down. Therefore, this attack has the same end result as blocking
the delivery of a valid sendme.
(Note that, currently, clientward unrecognized cells are dropped but
the circuit is not torn down.)
* Blocking delivery of a valid sendme
A malicious middle could simply drop a exitward sendme, so that
the exit is unable to verify the digest in the sendme payload. The
following exitward sendme cell would then be misaligned with the
sendme that the exit is expecting to verify. The exit would kill the
circuit because the client failed to prove it has read all of the
clientward cells.
The benefits of such an attack over just directly killing the circuit
seem low, and we feel that the added benefits of the defense outweigh
the risks.
6. Open problems
With the proposed defenses in place, an adversary will be unable to
successfully use the "continue sending sendmes" part of these attacks.
But this proposal won't resolve the "build up many circuits over time,
and then use them to attack all at once" issue, nor will it stop
sybil attacks like if an attacker makes many parallel connections to
a single target relay, or reaches out to many guards in parallel.
We spent a while trying to figure out if we can enforce some
upper bound on how many circuits a given connection is allowed
to have open at once, to limit every connection's potential for
launching a bandwidth attack. But there are plausible situations
where well-behaving clients accumulate many circuits over time:
Ricochet clients with many friends, popular onion services, or even
Tor Browser users with a bunch of tabs open.
Even though a per-conn circuit limit would produce many false
positives, it might still be useful to have it deployed and available
as a consensus parameter, as another tool for combatting a wide-scale
attack on the network: a parameter to limit the total number of
open circuits per conn (viewing each open circuit as a threat) would
complement the current work in #24902 to rate limit circuit creates
per client address.
But we think the threat of parallel attacks might be best handled by
teaching relays to react to actual attacks, like we've done in #24902:
we should teach Tor relays to recognize when somebody is *doing* this
attack on them, and to squeeze down or outright block the client IP
addresses that have tried it recently.
An alternative direction would be to await research ideas on how guards
might coordinate to defend against attacks while still preserving
user privacy.
In summary, we think authenticating the sendme cells is a useful
building block for these future solutions, and it can be (and should
be) done orthogonally to whatever sybil defenses we pick later.
7. References
[0] https://blog.torproject.org/blog/new-tor-denial-service-attacks-and-defenses
[1] https://www.freehaven.net/anonbib/#sniper14
[2] https://www.freehaven.net/anonbib/#torta05
[3] https://www.freehaven.net/anonbib/#congestion-longpaths
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