Update padding-spec.txt to cover hs circuit padding.

Also update padding proposals that are deprecated by padding-spec.txt,
to refer the reader to the new spec.

1 files changed, 262 insertions, 4 deletions

diff --git a/padding-spec.txt b/padding-spec.txt
index b85f0fd..455c5eb 100644
--- a/padding-spec.txt
+++ b/padding-spec.txt
@@ -1,6 +1,6 @@
                          Tor Padding Specification
 
-                                Mike Perry
+                        Mike Perry, George Kadianakis
 
 Note: This is an attempt to specify Tor as currently implemented.  Future
 versions of Tor will implement improved algorithms.
@@ -35,9 +35,15 @@ the anonymity and load-balancing implications of their choices.
   padding to be sent to any intermediate node in a circuit (as per Section
   6.1 of tor-spec.txt).
 
-  Currently, only single-hop CELL_PADDING is used by Tor. It is described in
-  Section 2. At a later date, further sections will be added to this document
-  to describe various uses of multi-hop circuit-level padding.
+  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
@@ -274,11 +280,257 @@ the anonymity and load-balancing implications of their choices.
         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,
+  with fields 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];
+     };
+
+  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).
+
+  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
+  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];
+     };
+
+  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).
+
+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.
 
+  XXX: There's more CNS numbers now..
+
 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
@@ -293,3 +545,9 @@ A. Acknowledgments
 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. XXX-dev-doc
+18. https://www.usenix.org/node/190967
+    https://blog.torproject.org/technical-summary-usenix-fingerprinting-paper
+