A Padding Machine from Scratch

A quickstart guide by Tobias Pulls.

This document describes the process of building a "padding machine" in tor's new circuit padding framework from scratch. Notes were taken as part of porting Adaptive Padding Early (APE) from basket2 to the circuit padding framework. The goal is just to document the process and provide useful pointers along the way, not create a useful machine.

The quick and dirty plan is to:

clone and compile tor
use newly built tor in TB and at small (non-exit) relay we run
add a bare-bones APE padding machine
run the machine, inspect logs for activity
port APE's state machine without thinking much about parameters

Clone and compile tor

$ git clone https://gitlab.torproject.org/tpo/core/tor.git
$ cd tor
$ git checkout tor-0.4.1.5

Above we use the tag for tor-0.4.1.5 where the circuit padding framework was released. Note that this version of the framework is missing many features and fixes that have since been merged to origin/master. If you need the newest framework features, you should use that master instead.

$ sh autogen.sh
$ ./configure
$ make

When you run ./configure you'll be told of missing dependencies and packages to install on debian-based distributions. Important: if you plan to run tor on a relay as part of the real Tor network and your server runs a distribution that uses systemd, then I'd recommend that you apt install dpkg dpkg-dev libevent-dev libssl-dev asciidoc quilt dh-apparmor libseccomp-dev dh-systemd libsystemd-dev pkg-config dh-autoreconf libfakeroot zlib1g zlib1g-dev automake liblzma-dev libzstd-dev and ensure that tor has systemd support enabled: ./configure --enable-systemd. Without this, on a recent Ubuntu, my tor service was forcefully restarted (SIGINT interrupt) by systemd every five minutes.

If you want to install on your localsystem, run make install. For our case we just want the tor binary at src/app/tor.

Use tor in TB and at a relay

Download and install a fresh Tor Browser (TB) from torproject.org. Make sure it works. From the command line, relative to the folder created when you extracted TB, run ./Browser/start-tor-browser --verbose to get some basic log output. Note the version of tor, in my case, Tor 0.4.0.5 (git-bf071e34aa26e096) as part of TB 8.5.4. Shut down TB, copy the tor binary that you compiled earlier and replace Browser/TorBrowser/Tor/tor. Start TB from the command line again, you should see a different version, in my case Tor 0.4.1.5 (git-439ca48989ece545).

The relay we run is also on linux, and tor is located at /usr/bin/tor. To view relevant logs since last boot sudo journalctl -b /usr/bin/tor, where we find Tor 0.4.0.5 running on Linux. Copy the locally compiled tor to the relay at a temporary location and then make sure it's ownership and access rights are identical to /usr/bin/tor. Next, shut down the running tor service with sudo service tor stop, wait for it to stop (typically 30s), copy our locally compiled tor to replace /usr/bin/tor then start the service again. Checking the logs we see or 0.4.1.5 (git-439ca48989ece545).

Repeatedly shutting down a relay is detrimental to the network and should be avoided. Sorry about that.

We have one more step left before we move on the machine: configure TB to always use our middle relay. Edit Browser/TorBrowser/Data/Tor/torrc and set MiddleNodes <fingerprint>, where <fingerprint> is the fingerprint of the relay. Start TB, visit a website, and manually confirm that the middle is used by looking at the circuit display.

Add a bare-bones APE padding machine

Now the fun part. We have several resources at our disposal (mind that links might be broken in the future, just search for the headings):

The official Circuit Padding Developer Documentation.
Notes we made on the implementation of the circuit padding framework.
The implementation of the current circuit padding machines in tor: circuitpadding.c and circuitpadding_machines.h.

Please consult the above links for details. Moving forward, the focus is to describe what was done, not necessarily explaining all the details why.

Since we plan to make changes to tor, create a new branch git checkout -b circuit-padding-ape-machine tor-0.4.1.5.

We start with declaring two functions, one for the machine at the client and one at the relay, in circuitpadding_machines.h:

void circpad_machine_relay_wf_ape(smartlist_t *machines_sl);
void circpad_machine_client_wf_ape(smartlist_t *machines_sl);

The definitions go into circuitpadding_machines.c:

/**************** Adaptive Padding Early (APE) machine ****************/

/**
 * Create a relay-side padding machine based on the APE design.
 */
void
circpad_machine_relay_wf_ape(smartlist_t *machines_sl)
{
  circpad_machine_spec_t *relay_machine
  = tor_malloc_zero(sizeof(circpad_machine_spec_t));

  relay_machine->name = "relay_wf_ape";
  relay_machine->is_origin_side = 0; // relay-side

  // Pad to/from the middle relay, only when the circuit has streams
  relay_machine->target_hopnum = 2;
  relay_machine->conditions.min_hops = 2;
  relay_machine->conditions.state_mask = CIRCPAD_CIRC_STREAMS;

  // limits to help guard against excessive padding
  relay_machine->allowed_padding_count = 1;
  relay_machine->max_padding_percent = 1;

  // one state to start with: START (-> END, never takes a slot in states)
  circpad_machine_states_init(relay_machine, 1);
  relay_machine->states[CIRCPAD_STATE_START].
    next_state[CIRCPAD_EVENT_NONPADDING_SENT] =
    CIRCPAD_STATE_END;

  // register the machine
  relay_machine->machine_num = smartlist_len(machines_sl);
  circpad_register_padding_machine(relay_machine, machines_sl);

  log_info(LD_CIRC,
           "Registered relay WF APE padding machine (%u)",
           relay_machine->machine_num);
}

/**
 * Create a client-side padding machine based on the APE design.
 */
void
circpad_machine_client_wf_ape(smartlist_t *machines_sl)
{
    circpad_machine_spec_t *client_machine
  = tor_malloc_zero(sizeof(circpad_machine_spec_t));

  client_machine->name = "client_wf_ape";
  client_machine->is_origin_side = 1; // client-side

  /** Pad to/from the middle relay, only when the circuit has streams, and only
  * for general purpose circuits (typical for web browsing)
  */
  client_machine->target_hopnum = 2;
  client_machine->conditions.min_hops = 2;
  client_machine->conditions.state_mask = CIRCPAD_CIRC_STREAMS;
  client_machine->conditions.purpose_mask =
    circpad_circ_purpose_to_mask(CIRCUIT_PURPOSE_C_GENERAL);

  // limits to help guard against excessive padding
  client_machine->allowed_padding_count = 1;
  client_machine->max_padding_percent = 1;

  // one state to start with: START (-> END, never takes a slot in states)
  circpad_machine_states_init(client_machine, 1);
  client_machine->states[CIRCPAD_STATE_START].
    next_state[CIRCPAD_EVENT_NONPADDING_SENT] =
    CIRCPAD_STATE_END;

  client_machine->machine_num = smartlist_len(machines_sl);
  circpad_register_padding_machine(client_machine, machines_sl);
  log_info(LD_CIRC,
           "Registered client WF APE padding machine (%u)",
           client_machine->machine_num);
}

We also have to modify circpad_machines_init() in circuitpadding.c to register our machines:

/* Register machines for the APE WF defense */
circpad_machine_client_wf_ape(origin_padding_machines);
circpad_machine_relay_wf_ape(relay_padding_machines);

We run make to get a new tor binary and copy it to our local TB.

Run the machine

To be able to view circuit info events in the console as we launch TB, we add Log [circ]info notice stdout to torrc of TB.

Running TB to visit example.com we first find in the log:

Aug 30 18:36:43.000 [info] circpad_machine_client_hide_intro_circuits(): Registered client intro point hiding padding machine (0)
Aug 30 18:36:43.000 [info] circpad_machine_relay_hide_intro_circuits(): Registered relay intro circuit hiding padding machine (0)
Aug 30 18:36:43.000 [info] circpad_machine_client_hide_rend_circuits(): Registered client rendezvous circuit hiding padding machine (1)
Aug 30 18:36:43.000 [info] circpad_machine_relay_hide_rend_circuits(): Registered relay rendezvous circuit hiding padding machine (1)
Aug 30 18:36:43.000 [info] circpad_machine_client_wf_ape(): Registered client WF APE padding machine (2)
Aug 30 18:36:43.000 [info] circpad_machine_relay_wf_ape(): Registered relay WF APE padding machine (2)

All good, our machine is running. Looking further we find:

Aug 30 18:36:55.000 [info] circpad_setup_machine_on_circ(): Registering machine client_wf_ape to origin circ 2 (5)
Aug 30 18:36:55.000 [info] circpad_node_supports_padding(): Checking padding: supported
Aug 30 18:36:55.000 [info] circpad_negotiate_padding(): Negotiating padding on circuit 2 (5), command 2
Aug 30 18:36:55.000 [info] circpad_machine_spec_transition(): Circuit 2 circpad machine 0 transitioning from 0 to 65535
Aug 30 18:36:55.000 [info] circpad_machine_spec_transitioned_to_end(): Padding machine in end state on circuit 2 (5)
Aug 30 18:36:55.000 [info] circpad_circuit_machineinfo_free_idx(): Freeing padding info idx 0 on circuit 2 (5)
Aug 30 18:36:55.000 [info] circpad_handle_padding_negotiated(): Middle node did not accept our padding request on circuit 2 (5)

We see that our middle support padding (since we upgraded to tor-0.4.1.5), that we attempt to negotiate, our machine starts on the client, transitions to the end state, and is freed. The last line shows that the middle doesn't have a padding machine that can run.

Next, we follow the same steps as earlier and replace the modified tor at our middle relay. We don't update the logging there to avoid logging on the info level on the live network. Looking at the client log again we see that negotiation works as before except for the last line: it's missing, so the machine is running at the middle as well.

Implementing the APE state machine

Porting is fairly straightforward: define the states for all machines, add two more machines (for the receive portion of WTFP-PAD, beyond AP), and pick reasonable parameters for the distributions (I completely winged it now, as when implementing APE). The circuit-padding-ape-machine branch contains the commits for the full machines with plenty of comments.

Some comments on the process:

tor-0.4.1.5 did not support two machines on the same circuit, the following fix had to be made: https://bugs.torproject.org/tpo/core/tor/31111 . The good news is that everything else seems to work after the small change in the fix.
APE randomizes its distributions. Currently, this can only be done during start of tor. This makes sense in the censorship circumvention setting (obfs4), less so for WF defenses: further randomizing each circuit is likely a PITA for attackers with few downsides.
it was annoying to figure out that the lack of systemd support in my compiled tor caused systemd to interrupt (SIGINT) my tor process at the middle relay every five minutes. Updated build steps above to hopefully save others the pain.
there's for sure some bug on relays when sending padding cells too early (?). It can happen with some probability with the APE implementation due to circpad_machine_relay_wf_ape_send(). Will investigate next.
Moving the registration of machines from the definition of the machines to circpad_machines_init() makes sense, as suggested in the circuit padding doc draft.

Remember that APE is just a proof-of-concept and we make zero claims about its ability to withstand WF attacks, in particular those based on deep learning.

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