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torspec/padding-spec.txt
Mike Perry 2c1d626b9f Document relay commands for circpad cells.
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Tor Padding Specification
Mike Perry, George Kadianakis
Note: This is an attempt to specify Tor as currently implemented. Future
versions of Tor will implement improved algorithms.
This document tries to cover how Tor chooses to use cover traffic to obscure
various traffic patterns from external and internal observers. Other
implementations MAY take other approaches, but implementors should be aware of
the anonymity and load-balancing implications of their choices.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
1. Overview
Tor supports two classes of cover traffic: connection-level padding, and
circuit-level padding.
Connection-level padding uses the CELL_PADDING cell command for cover
traffic, where as circuit-level padding uses the RELAY_COMMAND_DROP relay
command. CELL_PADDING is single-hop only and can be differentiated from
normal traffic by Tor relays ("internal" observers), but not by entities
monitoring Tor OR connections ("external" observers).
RELAY_COMMAND_DROP is multi-hop, and is not visible to intermediate Tor
relays, because the relay command field is covered by circuit layer
encryption. Moreover, Tor's 'recognized' field allows RELAY_COMMAND_DROP
padding to be sent to any intermediate node in a circuit (as per Section
6.1 of tor-spec.txt).
Tor uses both connection level and circuit level padding. Connection
level padding is described in section 2. Circuit level padding is
described in section 3.
The circuit-level padding system is completely orthogonal to the
connection-level padding. The connection-level padding system regards
circuit-level padding as normal data traffic, and hence the connection-level
padding system will not add any additional overhead while the circuit-level
padding system is actively padding.
2. Connection-level padding
2.1. Background
Tor clients and relays make use of CELL_PADDING to reduce the resolution of
connection-level metadata retention by ISPs and surveillance infrastructure.
Such metadata retention is implemented by Internet routers in the form of
Netflow, jFlow, Netstream, or IPFIX records. These records are emitted by
gateway routers in a raw form and then exported (often over plaintext) to a
"collector" that either records them verbatim, or reduces their granularity
further[1].
Netflow records and the associated data collection and retention tools are
very configurable, and have many modes of operation, especially when
configured to handle high throughput. However, at ISP scale, per-flow records
are very likely to be employed, since they are the default, and also provide
very high resolution in terms of endpoint activity, second only to full packet
and/or header capture.
Per-flow records record the endpoint connection 5-tuple, as well as the
total number of bytes sent and received by that 5-tuple during a particular
time period. They can store additional fields as well, but it is primarily
timing and bytecount information that concern us.
When configured to provide per-flow data, routers emit these raw flow
records periodically for all active connections passing through them
based on two parameters: the "active flow timeout" and the "inactive
flow timeout".
The "active flow timeout" causes the router to emit a new record
periodically for every active TCP session that continuously sends data. The
default active flow timeout for most routers is 30 minutes, meaning that a
new record is created for every TCP session at least every 30 minutes, no
matter what. This value can be configured from 1 minute to 60 minutes on
major routers.
The "inactive flow timeout" is used by routers to create a new record if a
TCP session is inactive for some number of seconds. It allows routers to
avoid the need to track a large number of idle connections in memory, and
instead emit a separate record only when there is activity. This value
ranges from 10 seconds to 600 seconds on common routers. It appears as
though no routers support a value lower than 10 seconds.
For reference, here are default values and ranges (in parenthesis when
known) for common routers, along with citations to their manuals.
Some routers speak other collection protocols than Netflow, and in the
case of Juniper, use different timeouts for these protocols. Where this
is known to happen, it has been noted.
Inactive Timeout Active Timeout
Cisco IOS[3] 15s (10-600s) 30min (1-60min)
Cisco Catalyst[4] 5min 32min
Juniper (jFlow)[5] 15s (10-600s) 30min (1-60min)
Juniper (Netflow)[6,7] 60s (10-600s) 30min (1-30min)
H3C (Netstream)[8] 60s (60-600s) 30min (1-60min)
Fortinet[9] 15s 30min
MicroTik[10] 15s 30min
nProbe[14] 30s 120s
Alcatel-Lucent[2] 15s (10-600s) 30min (1-600min)
The combination of the active and inactive netflow record timeouts allow us
to devise a low-cost padding defense that causes what would otherwise be
split records to "collapse" at the router even before they are exported to
the collector for storage. So long as a connection transmits data before the
"inactive flow timeout" expires, then the router will continue to count the
total bytes on that flow before finally emitting a record at the "active
flow timeout".
This means that for a minimal amount of padding that prevents the "inactive
flow timeout" from expiring, it is possible to reduce the resolution of raw
per-flow netflow data to the total amount of bytes send and received in a 30
minute window. This is a vast reduction in resolution for HTTP, IRC, XMPP,
SSH, and other intermittent interactive traffic, especially when all
user traffic in that time period is multiplexed over a single connection
(as it is with Tor).
2.2. Implementation
Tor clients currently maintain one TLS connection to their Guard node to
carry actual application traffic, and make up to 3 additional connections to
other nodes to retrieve directory information.
We pad only the client's connection to the Guard node, and not any other
connection. We treat Bridge node connections to the Tor network as client
connections, and pad them, but otherwise not pad between normal relays.
Both clients and Guards will maintain a timer for all application (ie:
non-directory) TLS connections. Every time a non-padding packet is sent or
received by either end, that endpoint will sample a timeout value from
between 1.5 seconds and 9.5 seconds using the max(X,X) distribution
described in Section 2.3. The time range is subject to consensus
parameters as specified in Section 2.6.
If the connection becomes active for any reason before this timer
expires, the timer is reset to a new random value between 1.5 and 9.5
seconds. If the connection remains inactive until the timer expires, a
single CELL_PADDING cell will be sent on that connection.
In this way, the connection will only be padded in the event that it is
idle, and will always transmit a packet before the minimum 10 second inactive
timeout.
2.3. Padding Cell Timeout Distribution Statistics
It turns out that because the padding is bidirectional, and because both
endpoints are maintaining timers, this creates the situation where the time
before sending a padding packet in either direction is actually
min(client_timeout, server_timeout).
If client_timeout and server_timeout are uniformly sampled, then the
distribution of min(client_timeout,server_timeout) is no longer uniform, and
the resulting average timeout (Exp[min(X,X)]) is much lower than the
midpoint of the timeout range.
To compensate for this, instead of sampling each endpoint timeout uniformly,
we instead sample it from max(X,X), where X is uniformly distributed.
If X is a random variable uniform from 0..R-1 (where R=high-low), then the
random variable Y = max(X,X) has Prob(Y == i) = (2.0*i + 1)/(R*R).
Then, when both sides apply timeouts sampled from Y, the resulting
bidirectional padding packet rate is now a third random variable:
Z = min(Y,Y).
The distribution of Z is slightly bell-shaped, but mostly flat around the
mean. It also turns out that Exp[Z] ~= Exp[X]. Here's a table of average
values for each random variable:
R Exp[X] Exp[Z] Exp[min(X,X)] Exp[Y=max(X,X)]
2000 999.5 1066 666.2 1332.8
3000 1499.5 1599.5 999.5 1999.5
5000 2499.5 2666 1666.2 3332.8
6000 2999.5 3199.5 1999.5 3999.5
7000 3499.5 3732.8 2332.8 4666.2
8000 3999.5 4266.2 2666.2 5332.8
10000 4999.5 5328 3332.8 6666.2
15000 7499.5 7995 4999.5 9999.5
20000 9900.5 10661 6666.2 13332.8
In this way, we maintain the property that the midpoint of the timeout range
is the expected mean time before a padding packet is sent in either
direction.
2.4. Maximum overhead bounds
With the default parameters and the above distribution, we expect a
padded connection to send one padding cell every 5.5 seconds. This
averages to 103 bytes per second full duplex (~52 bytes/sec in each
direction), assuming a 512 byte cell and 55 bytes of TLS+TCP+IP headers.
For a client connection that remains otherwise idle for its expected
~50 minute lifespan (governed by the circuit available timeout plus a
small additional connection timeout), this is about 154.5KB of overhead
in each direction (309KB total).
With 2.5M completely idle clients connected simultaneously, 52 bytes per
second amounts to 130MB/second in each direction network-wide, which is
roughly the current amount of Tor directory traffic[11]. Of course, our
2.5M daily users will neither be connected simultaneously, nor entirely
idle, so we expect the actual overhead to be much lower than this.
2.5. Reducing or Disabling Padding via Negotiation
To allow mobile clients to either disable or reduce their padding overhead,
the CELL_PADDING_NEGOTIATE cell (tor-spec.txt section 7.2) may be sent from
clients to relays. This cell is used to instruct relays to cease sending
padding.
If the client has opted to use reduced padding, it continues to send
padding cells sampled from the range [9000,14000] milliseconds (subject to
consensus parameter alteration as per Section 2.6), still using the
Y=max(X,X) distribution. Since the padding is now unidirectional, the
expected frequency of padding cells is now governed by the Y distribution
above as opposed to Z. For a range of 5000ms, we can see that we expect to
send a padding packet every 9000+3332.8 = 12332.8ms. We also half the
circuit available timeout from ~50min down to ~25min, which causes the
client's OR connections to be closed shortly there after when it is idle,
thus reducing overhead.
These two changes cause the padding overhead to go from 309KB per one-time-use
Tor connection down to 69KB per one-time-use Tor connection. For continual
usage, the maximum overhead goes from 103 bytes/sec down to 46 bytes/sec.
If a client opts to completely disable padding, it sends a
CELL_PADDING_NEGOTIATE to instruct the relay not to pad, and then does not
send any further padding itself.
2.6. Consensus Parameters Governing Behavior
Connection-level padding is controlled by the following consensus parameters:
* nf_ito_low
- The low end of the range to send padding when inactive, in ms.
- Default: 1500
* nf_ito_high
- The high end of the range to send padding, in ms.
- Default: 9500
- If nf_ito_low == nf_ito_high == 0, padding will be disabled.
* nf_ito_low_reduced
- For reduced padding clients: the low end of the range to send padding
when inactive, in ms.
- Default: 9000
* nf_ito_high_reduced
- For reduced padding clients: the high end of the range to send padding,
in ms.
- Default: 14000
* nf_conntimeout_clients
- The number of seconds to keep circuits opened and available for
clients to use. Note that the actual client timeout is randomized
uniformly from this value to twice this value. This governs client
OR conn lifespan. Reduced padding clients use half the consensus
value.
- Default: 1800
* nf_pad_before_usage
- If set to 1, OR connections are padded before the client uses them
for any application traffic. If 0, OR connections are not padded
until application data begins.
- Default: 1
* nf_pad_relays
- If set to 1, we also pad inactive relay-to-relay connections
- Default: 0
* nf_conntimeout_relays
- The number of seconds that idle relay-to-relay connections are kept
open.
- Default: 3600
3. Circuit-level padding
The circuit padding system in Tor is an extension of the WTF-PAD
event-driven state machine design[15]. At a high level, this design places
one or more padding state machines at the client, and one or more padding
state machines at a relay, on each circuit.
State transition and histogram generation has been generalized to be fully
programmable, and probability distribution support was added to support more
compact representations like APE[16]. Additionally, packet count limits,
rate limiting, and circuit application conditions have been added.
At present, Tor uses this system to deploy two pairs of circuit padding
machines, to obscure differences between the setup phase of client-side
onion service circuits, up to the first 10 cells.
This specification covers only the resulting behavior of these padding
machines, and thus does not cover the state machine implementation details or
operation. For full details on using the circuit padding system to develop
future padding defenses, see the research developer documentation[17].
3.1. Circuit Padding Negotiation
Circuit padding machines are advertised as "Padding" subprotocol versions
(see tor-spec.txt Section 9). The onion service circuit padding machines are
advertised as "Padding=2".
Because circuit padding machines only become active at certain points in
circuit lifetime, and because more than one padding machine may be active at
any given point in circuit lifetime, there is also a padding negotiation
cell and a negotiated response. These are relay commands 41 and 42, with
relay headers as per section 6.1 of tor-spec.txt.
The fields of the relay cell Data payload of a negotiate request are
as follows:
const CIRCPAD_COMMAND_STOP = 1;
const CIRCPAD_COMMAND_START = 2;
const CIRCPAD_RESPONSE_OK = 1;
const CIRCPAD_RESPONSE_ERR = 2;
const CIRCPAD_MACHINE_CIRC_SETUP = 1;
struct circpad_negotiate {
u8 version IN [0];
u8 command IN [CIRCPAD_COMMAND_START, CIRCPAD_COMMAND_STOP];
u8 machine_type IN [CIRCPAD_MACHINE_CIRC_SETUP];
u8 unused; // Formerly echo_request
u32 machine_ctr;
};
When a client wants to start a circuit padding machine, it first checks that
the desired destination hop advertises the appropriate subprotocol version for
that machine. It then sends a circpad_negotiate cell to that hop with
command=CIRCPAD_COMMAND_START, and machine_type=CIRCPAD_MACHINE_CIRC_SETUP (for
the circ setup machine, the destination hop is the second hop in the
circuit). The machine_ctr is the count of which machine instance this is on
the circuit. It is used to disambiguate shutdown requests.
When a relay receives a circpad_negotiate cell, it checks that it supports
the requested machine, and sends a circpad_negotiated cell, which is formatted
in the data payload of a relay cell with command number 42 (see tor-spec.txt
section 6.1), as follows:
struct circpad_negotiated {
u8 version IN [0];
u8 command IN [CIRCPAD_COMMAND_START, CIRCPAD_COMMAND_STOP];
u8 response IN [CIRCPAD_RESPONSE_OK, CIRCPAD_RESPONSE_ERR];
u8 machine_type IN [CIRCPAD_MACHINE_CIRC_SETUP];
u32 machine_ctr;
};
If the machine is supported, the response field will contain
CIRCPAD_RESPONSE_OK. If it is not, it will contain CIRCPAD_RESPONSE_ERR.
Either side may send a CIRCPAD_COMMAND_STOP to shut down the padding machines
(clients MUST only send circpad_negotiate, and relays MUST only send
circpad_negotiated for this purpose).
If the machine_ctr does not match the current machine instance count
on the circuit, the command is ignored.
3.2. Circuit Padding Machine Message Management
Clients MAY send padding cells towards the relay before receiving the
circpad_negotiated response, to allow for outbound cover traffic before
negotiation completes.
Clients MAY send another circpad_negotiate cell before receiving the
circpad_negotiated response, to allow for rapid machine changes.
Relays MUST NOT send padding cells or circpad_negotiated cells, unless a
padding machine is active. Any padding-related cells that arrive at the client
from unexpected relay sources are protocol violations, and clients MAY
immediately tear down such circuits to avoid side channel risk.
3.3. Obfuscating client-side onion service circuit setup
The circuit padding currently deployed in Tor attempts to hide client-side
onion service circuit setup. Service-side setup is not covered, because doing
so would involve significantly more overhead, and/or require interaction with
the application layer.
The approach taken aims to make client-side introduction and rendezvous
circuits match the cell direction sequence and cell count of 3 hop general
circuits used for normal web traffic, for the first 10 cells only. The
lifespan of introduction circuits is also made to match the lifespan
of general circuits.
Note that inter-arrival timing is not obfuscated by this defense.
3.3.1. Common general circuit construction sequences
Most general Tor circuits used to surf the web or download directory
information start with the following 6-cell relay cell sequence (cells
surrounded in [brackets] are outgoing, the others are incoming):
[EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [BEGIN] -> CONNECTED
When this is done, the client has established a 3-hop circuit and also opened
a stream to the other end. Usually after this comes a series of DATA cell that
either fetches pages, establishes an SSL connection or fetches directory
information:
[DATA] -> [DATA] -> DATA -> DATA...(inbound cells continue)
The above stream of 10 relay cells defines the grand majority of general
circuits that come out of Tor browser during our testing, and it's what we use
to make introduction and rendezvous circuits blend in.
Please note that in this section we only investigate relay cells and not
connection-level cells like CREATE/CREATED or AUTHENTICATE/etc. that are used
during the link-layer handshake. The rationale is that connection-level cells
depend on the type of guard used and are not an effective fingerprint for a
network/guard-level adversary.
3.3.2. Client-side onion service introduction circuit obfuscation
Two circuit padding machines work to hide client-side introduction circuits:
one machine at the origin, and one machine at the second hop of the circuit.
Each machine sends padding towards the other. The padding from the origin-side
machine terminates at the second hop and does not get forwarded to the actual
introduction point.
From Section 3.3.1 above, most general circuits have the following initial
relay cell sequence (outgoing cells marked in [brackets]):
[EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [BEGIN] -> CONNECTED
-> [DATA] -> [DATA] -> DATA -> DATA...(inbound data cells continue)
Whereas normal introduction circuits usually look like:
[EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2
-> [INTRO1] -> INTRODUCE_ACK
This means that up to the sixth cell (first line of each sequence above),
both general and intro circuits have identical cell sequences. After that
we want to mimic the second line sequence of
-> [DATA] -> [DATA] -> DATA -> DATA...(inbound data cells continue)
We achieve this by starting padding INTRODUCE1 has been sent. With padding
negotiation cells, in the common case of the second line looks like:
-> [INTRO1] -> [PADDING_NEGOTIATE] -> PADDING_NEGOTIATED -> INTRO_ACK
Then, the middle node will send between INTRO_MACHINE_MINIMUM_PADDING (7) and
INTRO_MACHINE_MAXIMUM_PADDING (10) cells, to match the "...(inbound data cells
continue)" portion of the trace (aka the rest of an HTTPS response body).
We also set a special flag which keeps the circuit open even after the
introduction is performed. With this feature the circuit will stay alive for
the same duration as normal web circuits before they expire (usually 10
minutes).
3.3.3. Client-side rendezvous circuit hiding
Following a similar argument as for intro circuits, we are aiming for padded
rendezvous circuits to blend in with the initial cell sequence of general
circuits which usually look like this:
[EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [BEGIN] -> CONNECTED
-> [DATA] -> [DATA] -> DATA -> DATA...(incoming cells continue)
Whereas normal rendezvous circuits usually look like:
[EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [EST_REND] -> REND_EST
-> REND2 -> [BEGIN]
This means that up to the sixth cell (the first line), both general and
rend circuits have identical cell sequences.
After that we want to mimic a [DATA] -> [DATA] -> DATA -> DATA sequence.
With padding negotiation right after the REND_ESTABLISHED, the sequence
becomes:
[EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [EST_REND] -> REND_EST
-> [PADDING_NEGOTIATE] -> [DROP] -> PADDING_NEGOTIATED -> DROP...
After which normal application DATA cells continue on the circuit.
Hence this way we make rendezvous circuits look like general circuits up
till the end of the circuit setup.
After that our machine gets deactivated, and we let the actual rendezvous
circuit shape the traffic flow. Since rendezvous circuits usually imitate
general circuits (their purpose is to surf the web), we can expect that they
will look alike.
3.3.4. Circuit setup machine overhead
For the intro circuit case, we see that the origin-side machine just sends a
single [PADDING_NEGOTIATE] cell, whereas the origin-side machine sends a
PADDING_NEGOTIATED cell and between 7 to 10 DROP cells. This means that the
average overhead of this machine is 11 padding cells per introduction circuit.
For the rend circuit case, this machine is quite light. Both sides send 2
padding cells, for a total of 4 padding cells.
3.4. Circuit padding consensus parameters
The circuit padding system has a handful of consensus parameters that can
either disable circuit padding entirely, or rate limit the total overhead
at relays and clients.
* circpad_padding_disabled
- If set to 1, no circuit padding machines will negotiate, and all
current padding machines will cease padding immediately.
- Default: 0
* circpad_padding_reduced
- If set to 1, only circuit padding machines marked as "reduced"/"low
overhead" will be used. (Currently no such machines are marked
as "reduced overhead").
- Default: 0
* circpad_global_allowed_cells
- This is the number of padding cells that must be sent before
the 'circpad_global_max_padding_percent' parameter is applied.
- Default: 0
* circpad_global_max_padding_percent
- This is the maximum ratio of padding cells to total cells, specified
as a percent. If the global ratio of padding cells to total cells
across all circuits exceeds this percent value, no more padding is sent
until the ratio becomes lower. 0 means no limit.
- Default: 0
* circpad_max_circ_queued_cells
- This is the maximum number of cells that can be in the circuitmux queue
before padding stops being sent on that circuit.
- Default: CIRCWINDOW_START_MAX (1000)
A. Acknowledgments
This research was supported in part by NSF grants CNS-1111539,
CNS-1314637, CNS-1526306, CNS-1619454, and CNS-1640548.
1. https://en.wikipedia.org/wiki/NetFlow
2. http://infodoc.alcatel-lucent.com/html/0_add-h-f/93-0073-10-01/7750_SR_OS_Router_Configuration_Guide/Cflowd-CLI.html
3. http://www.cisco.com/en/US/docs/ios/12_3t/netflow/command/reference/nfl_a1gt_ps5207_TSD_Products_Command_Reference_Chapter.html#wp1185203
4. http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/70974-netflow-catalyst6500.html#opconf
5. https://www.juniper.net/techpubs/software/erx/junose60/swconfig-routing-vol1/html/ip-jflow-stats-config4.html#560916
6. http://www.jnpr.net/techpubs/en_US/junos15.1/topics/reference/configuration-statement/flow-active-timeout-edit-forwarding-options-po.html
7. http://www.jnpr.net/techpubs/en_US/junos15.1/topics/reference/configuration-statement/flow-active-timeout-edit-forwarding-options-po.html
8. http://www.h3c.com/portal/Technical_Support___Documents/Technical_Documents/Switches/H3C_S9500_Series_Switches/Command/Command/H3C_S9500_CM-Release1648%5Bv1.24%5D-System_Volume/200901/624854_1285_0.htm#_Toc217704193
9. http://docs-legacy.fortinet.com/fgt/handbook/cli52_html/FortiOS%205.2%20CLI/config_system.23.046.html
10. http://wiki.mikrotik.com/wiki/Manual:IP/Traffic_Flow
11. https://metrics.torproject.org/dirbytes.html
12. http://freehaven.net/anonbib/cache/murdoch-pet2007.pdf
13. https://gitweb.torproject.org/torspec.git/tree/proposals/188-bridge-guards.txt
14. http://www.ntop.org/wp-content/uploads/2013/03/nProbe_UserGuide.pdf
15. http://arxiv.org/pdf/1512.00524
16. https://www.cs.kau.se/pulls/hot/thebasketcase-ape/
17. https://github.com/torproject/tor/tree/master/doc/HACKING/CircuitPaddingDevelopment.md
18. https://www.usenix.org/node/190967
https://blog.torproject.org/technical-summary-usenix-fingerprinting-paper
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