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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