tor/doc/tor-spec.txt

$Id$ 

TOR Spec

Note: This is an attempt to specify TOR as it exists as implemented in
early June, 2003.  It is not recommended that others implement this
design as it stands; future versions of TOR will implement improved
protocols.

TODO: (very soon)
      - Specify truncate/truncated
      - Sendme w/stream0 is circuit sendme
      - Integrate -NM and -RD comments

0. Notation:

   PK -- a public key.
   SK -- a private key 
   K  -- a key for a symmetric cypher

   a|b -- concatenation of 'a' with 'b'.
   a[i:j] -- Bytes 'i' through 'j'-1 (inclusive) of the string a.

   All numeric values are encoded in network (big-endian) order.

   Unless otherwise specified, all symmetric ciphers are AES in counter
   mode, with an IV of all 0 bytes.  Asymmetric ciphers are either RSA
   with 1024-bit keys and exponents of 65537, or DH with the safe prime
   from rfc2409, section 6.2, whose hex representation is:

     "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
     "8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
     "302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
     "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
     "49286651ECE65381FFFFFFFFFFFFFFFF"


1. System overview

Tor is a connection-oriented anonymizing communication service. Users
build a path known as a "virtual circuit" through the network, in which
each node knows its predecessor and successor, but no others. Traffic
flowing down the circuit is unwrapped by a symmetric key at each node,
which reveals the downstream node.


2. Connections

2.1. Establishing connections to onion routers (ORs)

   There are two ways to connect to an OR. The first is as an onion
   proxy (OP), which allows any node to connect without providing any
   authentication or name. The second is as another OR, which allows
   strong authentication. In both cases the initiating party (called
   the 'client') sets up shared keys with the listening OR (called the
   'server').

   Before the handshake begins, assume all parties know the {(1024-bit)
   public key, IPV4 address, and port} triplet of each OR.

     1. Client connects to server:

        The client generates a pair of 16-byte symmetric keys (one
        [K_f] for the 'forward' stream from client to server, and one
        [K_b] for the 'backward' stream from server to client) to be
        used for link encryption.

        The client then generates a 'Client authentication' message [M]
        containing: 

        (If client is an OP)
           The number 1 to signify OP handshake [2 bytes]
           Maximum bandwidth (bytes/s)          [4 bytes]
           Forward link key [K_f]               [16 bytes]
           Backward link key [K_b]              [16 bytes]
                                             [Total: 38 bytes]

        (If client is an OR)
           The number 2 to signify OR handshake [2 bytes]
           The client's published IPV4 address  [4 bytes]
           The client's published port          [2 bytes]
           The server's published IPV4 address  [4 bytes]
           The server's published port          [2 bytes]
           The forward key [K_f]                [16 bytes]
           The backward key [K_b]               [16 bytes]
           The maximum bandwidth (bytes/s)      [4 bytes]
                                             [Total: 50 bytes] 

        The client then RSA-encrypts [M] with the server's public key
        and PKCS1 padding to give an encrypted message.

        The client then opens a TCP connection to the server, sends
        the 128-byte RSA-encrypted data to the server, and waits for a
        reply.

     2. The server receives the first handshake:

        The OR waits for 128 bytes of data, and decrypts the resulting
        data with its private key, checking the PKCS1 padding. If
        the padding is invalid, it closes the connection. If the tag
        indicates the client is an OP, and the message is 38 bytes long,
        it performs step 2a. If the tag indicates the client is an OR,
        and the message is 50 bytes long, it performs step 2b. Else,
        it closes the connection.

     2a. If client is an OP:

        The connection is established, and the OR is ready to receive
        cells. The server sets its keys for this connection, setting K_f
        to the client's K_b, and K_b to the client's K_f. The handshake
        is complete.

     2b. If the client is an OR:

        The server checks the list of known ORs for one with the address
        and port given in the client's authentication. If no such OR
        is known, or if the server is already connected to that OR, the
        server closes the current TCP connection and stops handshaking.

        The server sets its keys for this connection, setting K_f to
        the client's K_b, and K_b to the client's K_f.

        The server then creates a server authentication message [M2] as
        follows: 
               Modified client authentication         [48 bytes]
               A random nonce [N]                     [8 bytes]
                                                  [Total: 56 bytes]
        The client authentication is generated from M by replacing
        the client's preferred bandwidth [B_c] with the server's
        preferred bandwidth [B_s], if B_s < B_c. 

        The server encrypts M2 with the client's public key (found
        from the list of known routers), using PKCS1 padding.

        The server sends the 128-byte encrypted message to the client,
        and waits for a reply.

     3. Client authenticates to server.

        Once the client has received 128 bytes, it decrypts them with
        its public key, and checks the PKCS1 padding.  If the padding
        is invalid, or the decrypted message's length is other than 56
        bytes, the client closes the TCP connection.

        The client checks that the addresses and keys in the reply
        message are the same as the ones it originally sent.  If not,
        it closes the TCP connection.

        The client updates the connection's bandwidth to that set by
        the server, and generates the following authentication message [M3]:
           The client's published IPV4 address    [4 bytes]
           The client's published port            [2 bytes]
           The server's published IPV4 address    [4 bytes]
           The server's published port            [2 bytes]
           The server-generated nonce [N]         [8 bytes]
                                             [Total: 20 bytes]

        Once again, the client encrypts this message using the
        server's public key and PKCS1 padding, and sends the resulting
        128-byte message to the server.

     4. Server checks client authentication

        The server once again waits to receive 128 bytes from the
        client, decrypts the message with its private key, and checks
        the PKCS1 padding.  If the padding is incorrect, or if the
        message's length is other than 20 bytes, the server closes the
        TCP connection and stops handshaking.

        If the addresses in the decrypted message M3 match those in M
        and M2, and if the nonce in M3 is the same as in M2, the
        handshake is complete, and the client and server begin sending
        cells to one another.  Otherwise, the server closes the TCP
        connection.

2.2. Sending cells and link encryption

   Once the handshake is complete, the two sides send cells
   (specified below) to one another.  Cells are sent serially,
   encrypted with the AES-CNT keystream specified by the handshake
   protocol.  Over a connection, communicants encrypt outgoing cells
   with the connection's K_f, and decrypt incoming cells with the
   connection's K_b.

   [Commentary: This means that OR/OP->OR connections are malleable; I
    can flip bits in cells as they go across the wire, and see flipped
    bits coming out the cells as they are decrypted at the next
    server.  I need to look more at the data format to see whether
    this is exploitable, but if there's no integrity checking there
    either, I suspect we may have an attack here. -NM]
   [Yes, this protocol is open to tagging attacks. The payloads are
    encrypted inside the network, so it's only at the edge node and beyond
    that it's a worry. But adversaries can already count packets and
    observe/modify timing. It's not worth putting in hashes; indeed, it
    would be quite hard, because one of the sides of the circuit doesn't
    know the keys that are used for de/encrypting at each hop, so couldn't
    craft hashes anyway. See the Bandwidth Throttling (threat model)
    thread on http://archives.seul.org/or/dev/Jul-2002/threads.html. -RD]
   [Even if I don't control both sides of the connection, I can still
    do evil stuff.  For instance, if I can guess that a cell is a
    TOPIC_COMMAND_BEGIN cell to www.slashdot.org:80 , I can change the
    address and port to point to a machine I control. -NM]
   [We're going to address this tagging issue with e2e-only hashes.
    See TODO file. -RD]


3. Cell Packet format

   The basic unit of communication for onion routers and onion
   proxies is a fixed-width "cell".  Each cell contains the following
   fields:

        ACI (anonymous circuit identifier)    [2 bytes]
        Command                               [1 byte]
        Length                                [1 byte]
        Sequence number (unused, set to 0)    [4 bytes]
        Payload (padded with 0 bytes)         [248 bytes]
                                         [Total size: 256 bytes]

   The 'Command' field holds one of the following values:
         0 -- PADDING     (Padding)                 (See Sec 6.2)
         1 -- CREATE      (Create a circuit)        (See Sec 4)
         2 -- CREATED     (Acknowledge create)      (See Sec 4)
         3 -- RELAY       (End-to-end data)         (See Sec 5)
         4 -- DESTROY     (Stop using a circuit)    (See Sec 4)

   The interpretation of 'Length' and 'Payload' depend on the type of
   the cell.
      PADDING: Neither field is used.
      CREATE: Length is 144; the payload contains the first phase of the
        DH handshake.
      CREATED: Length is 128; the payload contains the second phase of
        the DH handshake.
      RELAY: Length is a value between 8 and 248; the first 'length'
        bytes of payload contain useful data.
      DESTROY: Neither field is used.

   Unused fields are filled with 0 bytes.  The payload is padded with
   0 bytes.

   PADDING cells are currently used to implement connection
   keepalive.  ORs and OPs send one another a PADDING cell every few
   minutes.

   CREATE and DESTROY cells are used to manage circuits; see section
   4 below.

   RELAY cells are used to send commands and data along a circuit; see
   section 5 below.


4. Circuit management

4.1. CREATE and CREATED cells

   Users set up circuits incrementally, one hop at a time. To create
   a new circuit, users send a CREATE cell to the first node, with the
   first half of the DH handshake; that node responds with a CREATED cell
   with the second half of the DH handshake. To extend a circuit past
   the first hop, the user sends an EXTEND relay cell (see section 5)
   which instructs the last node in the circuit to send a CREATE cell
   to extend the circuit.

   The payload for a CREATE cell is an 'onion skin', consisting of:
         RSA-encrypted data            [128 bytes]
         Symmetrically-encrypted data  [16 bytes]
   The RSA-encrypted portion contains:
         Symmetric key                 [16 bytes]
         First part of DH data (g^x)   [112 bytes]
   The symmetrically encrypted portion contains:
         Second part of DH data (g^x)  [16 bytes] 

   The two parts of the DH data, once decrypted and concatenated, form
   g^x as calculated by the client.

   The relay payload for an EXTEND relay cell consists of:
         Address                       [4 bytes]
         Port                          [2 bytes]
         Onion skin                    [144 bytes]

   The port and address field denote the IPV4 address and port of the
   next onion router in the circuit.  

4.2. Setting circuit keys

   Once the handshake between the OP and an OR is completed, both
   servers can now calculate g^xy with ordinary DH.  They divide the
   last 32 bytes of this shared secret into two 16-byte keys, the
   first of which (called Kf) is used to encrypt the stream of data
   going from the OP to the OR, and second of which (called Kb) is
   used to encrypt the stream of data going from the OR to the OP.

4.3. Creating circuits

   When creating a circuit through the network, the circuit creator
   performs the following steps:

      1. Choose a chain of N onion routers (R_1...R_N) to constitute
         the path, such that no router appears in the path twice.

      2. If not already connected to the first router in the chain,
         open a new connection to that router.

      3. Choose an ACI not already in use on the connection with the
         first router in the chain.  If our address/port pair is
         numerically higher than the address/port pair of the other
         side, then let the high bit of the ACI be 1, else 0.

      4. Send a CREATE cell along the connection, to be received by
         the first onion router.

      5. Wait until a CREATED cell is received; finish the handshake
         and extract the forward key Kf_1 and the back key Kb_1.

      6. For each subsequent onion router R (R_2 through R_N), extend
         the circuit to R.

   To extend the circuit by a single onion router R_M, the circuit
   creator performs these steps:

      1. Create an onion skin, encrypting the RSA-encrypted part with
         R's public key.

      2. Encrypt and send the onion skin in a RELAY_CREATE cell along
         the circuit (see section 5).

      3. When a RELAY_CREATED cell is received, calculate the shared
         keys.  The circuit is now extended.

   When an onion router receives an EXTEND relay cell, it sends a
   CREATE cell to the next onion router, with the enclosed onion skin
   as its payload.  The initiating onion router chooses some random
   ACI not yet used on the connection between the two onion routers.

   When an onion router receives a CREATE cell, if it already has a
   circuit on the given connection with the given ACI, it drops the
   cell.  Otherwise, sometime after receiving the CREATE cell, it completes
   the DH handshake, and replies with a CREATED cell, containing g^y
   as its [128 byte] payload.  Upon receiving a CREATED cell, an onion
   router packs it payload into an EXTENDED relay cell (see section 5),
   and sends that cell up the circuit.  Upon receiving the EXTENDED
   relay cell, the OP can retrieve g^y.

   (As an optimization, OR implementations may delay processing onions
   until a break in traffic allows time to do so without harming
   network latency too greatly.)

4.4. Tearing down circuits

   Circuits are torn down when an unrecoverable error occurs along
   the circuit, or when all streams on a circuit are closed and the
   circuit's intended lifetime is over.  Circuits may be torn down
   either completely or hop-by-hop.

   To tear down a circuit completely, an OR or OP sends a DESTROY
   cell to the adjacent nodes on that circuit, using the appropriate
   direction's ACI.

   Upon receiving an outgoing DESTROY cell, an OR frees resources
   associated with the corresponding circuit. If it's not the end of
   the circuit, it sends a DESTROY cell for that circuit to the next OR
   in the circuit. If the node is the end of the circuit, then it tears
   down any associated edge connections (see section 5.1).

   After a DESTROY cell has been processed, an OR ignores all data or
   destroy cells for the corresponding circuit.

   To tear down part of a circuit, the OP sends a RELAY_TRUNCATE cell
   signaling a given OR (Stream ID zero).  That OR sends a DESTROY
   cell to the next node in the circuit, and replies to the OP with a
   RELAY_TRUNCATED cell.

   When an unrecoverable error occurs along one connection in a
   circuit, the nodes on either side of the connection should, if they
   are able, act as follows:  the node closer to the OP should send a
   RELAY_TRUNCATED cell towards the OP; the node farther from the OP
   should send a DESTROY cell down the circuit.

   [We'll have to reevaluate this section once we figure out cleaner
    circuit/connection killing conventions. -RD]

4.5. Routing data cells

   When an OR receives a RELAY cell, it checks the cell's ACI and
   determines whether it has a corresponding circuit along that
   connection.  If not, the OR drops the RELAY cell.

   Otherwise, if the OR is not at the OP edge of the circuit (that is,
   either an 'exit node' or a non-edge node), it de/encrypts the length
   field and the payload with AES/CNT, as follows:
        'Forward' relay cell (same direction as CREATE):
            Use Kf as key; encrypt.
        'Back' relay cell (opposite direction from CREATE):
            Use Kb as key; decrypt.
   If the OR recognizes the stream ID on the cell (it is either the ID
   of an open stream or the signaling (zero) ID), the OR processes the
   contents of the relay cell.  Otherwise, it passes the decrypted
   relay cell along the circuit if the circuit continues, or drops the
   cell if it's the end of the circuit. [Getting an unrecognized
   relay cell at the end of the circuit must be allowed for now;
   we can reexamine this once we've designed full tcp-style close
   handshakes. -RD]

   Otherwise, if the data cell is coming from the OP edge of the
   circuit, the OP decrypts the length and payload fields with AES/CNT as
   follows:
         OP sends data cell to node R_M:
            For I=1...M, decrypt with Kf_I.

   Otherwise, if the data cell is arriving at the OP edge if the
   circuit, the OP encrypts the length and payload fields with AES/CNT as
   follows:
         OP receives data cell:
            For I=N...1, 
                Encrypt with Kb_I.  If the stream ID is a recognized
                stream for R_I, or if the stream ID is the signaling
                ID (zero), then stop and process the payload.

   For more information, see section 5 below.

5. Application connections and stream management

5.1. Streams

   Within a circuit, the OP and the exit node use the contents of
   RELAY packets to tunnel end-to-end commands and TCP connections
   ("Streams") across circuits.  End-to-end commands can be initiated
   by either edge; streams are initiated by the OP.

   The first 8 bytes of each relay cell are reserved as follows:
         Relay command           [1 byte]
         Stream ID               [7 bytes]

   The recognized relay commands are:
         1 -- RELAY_BEGIN
         2 -- RELAY_DATA
         3 -- RELAY_END
         4 -- RELAY_CONNECTED
         5 -- RELAY_SENDME
         6 -- RELAY_EXTEND
         7 -- RELAY_EXTENDED
         8 -- RELAY_TRUNCATE
         9 -- RELAY_TRUNCATED

   All RELAY cells pertaining to the same tunneled stream have the
   same stream ID.  Stream ID's are chosen randomly by the OP.  A
   stream ID is considered "recognized" on a circuit C by an OP or an
   OR if it already has an existing stream established on that
   circuit, or if the stream ID is equal to the signaling stream ID,
   which is all zero: [00 00 00 00 00 00 00]

   To create a new anonymized TCP connection, the OP sends a
   RELAY_BEGIN data cell with a payload encoding the address and port
   of the destination host.  The stream ID is zero.  The payload format is:
         ADDRESS | ':' | PORT | '\000'
   where ADDRESS may be a DNS hostname, or an IPv4 address in
   dotted-quad format; and where PORT is encoded in decimal.

   Upon receiving this packet, the exit node resolves the address as
   necessary, and opens a new TCP connection to the target port.  If
   the address cannot be resolved, or a connection can't be
   established, the exit node replies with a RELAY_END cell.
   Otherwise, the exit node replies with a RELAY_CONNECTED cell.

   The OP waits for a RELAY_CONNECTED cell before sending any data.
   Once a connection has been established, the OP and exit node
   package stream data in RELAY_DATA cells, and upon receiving such
   cells, echo their contents to the corresponding TCP stream.  

5.2. Closing streams

   [Note -- TCP streams can only be half-closed for reading.  Our
   Bickford's conversation was incorrect. -NM]

   Because TCP connections can be half-open, we follow an equivalent
   to TCP's FIN/FIN-ACK/ACK protocol to close streams.

   A exit conneection can have a TCP stream in one of three states:
   'OPEN', 'DONE_PACKAGING', and 'DONE_DELIVERING'.  For the purposes
   of modeling transitions, we treat 'CLOSED' as a fourth state,
   although connections in this state are not, in fact, tracked by the
   onion router.

   A stream begins in the 'OPEN' state.  Upon receiving a 'FIN' from
   the corresponding TCP connection, the edge node sends a 'RELAY_END'
   cell along the circuit and changes its state to 'DONE_PACKAGING'.
   Upon receiving a 'RELAY_END' cell, an edge node sends a 'FIN' to
   the corresponding TCP connection (e.g., by calling
   shutdown(SHUT_WR)) and changing its state to 'DONE_DELIVERING'.

   When a stream in already in 'DONE_DELIVERING' receives a 'FIN', it
   also sends a 'RELAY_END' along the circuit, and changes its state
   to 'CLOSED'.  When a stream already in 'DONE_PACKAGING' receives a
   'RELAY_END' cell, it sends a 'FIN' and changes its state to
   'CLOSED'.

   [Note: Please rename 'RELAY_END2'. :) -NM ]

   If an edge node encounters an error on any stram, it sends a
   'RELAY_END2' cell along the circuit (if possible) and closes the
   TCP connection immediately.  If an edge node receives a
   'RELAY_END2' cell for any stream, it closes the TCP connection
   completely, and sends nothing along the circuit.

6. Flow control

6.1. Link throttling

   As discussed above in section 2.1, ORs and OPs negotiate a maximum
   bandwidth upon startup.  The communicants only read up to that
   number of bytes per second on average, though they may use mechanisms
   to handle spikes (eg token buckets).

   [This isn't true anymore. Each node has a total bandwidth it's willing
    to accept from all nodes per second; it ignores negotiated
    per-connection bandwidths. -RD]

   Communicants rely on TCP's default flow control to push back when they
   stop reading, so nodes that don't obey this bandwidth limit can't do
   too much damage.

6.2. Link padding

   Currently nodes are not required to do any sort of link padding or
   dummy traffic. Because strong attacks exist even with link padding,
   and because link padding greatly increases the bandwidth requirements
   for running a node, we plan to leave out link padding until this
   tradeoff is better understood.

6.3. Circuit-level flow control

   To control a circuit's bandwidth usage, each OR keeps track of
   two 'windows', consisting of how many RELAY_DATA cells it is
   allowed to package for transmission, and how many RELAY_DATA cells
   it is willing to deliver to streams outside the network.  
   Each 'window' value is initially set to 1000 data cells
   in each direction (cells that are not data cells do not affect
   the window).  When an OR is willing to deliver more cells, it sends a
   RELAY_SENDME cell towards the OP, with Stream ID zero.  When an OR 
   receives a RELAY_SENDME cell with stream ID zero, it increments its
   packaging window.

   Either of these cells increment the corresponding window by 100.

   The OP behaves identically, except that it must track a packaging
   window and a delivery window for every OR in the circuit.
 
   An OR or OP sends cells to increment its delivery window when the
   corresponding window value falls under some threshold (900).

   If a packaging window reaches 0, the OR or OP stops reading from
   TCP connections for all streams on the corresponding circuit, and
   sends no more RELAY_DATA cells until receiving a RELAY_SENDME cell.

6.4. Stream-level flow control

   Edge nodes use RELAY_SENDME cells to implement end-to-end flow
   control for individual connections across circuits. Similarly to
   circuit-level flow control, edge nodes begin with a window of cells
   (500) per stream, and increment the window by a fixed value (50)
   upon receiving a RELAY_SENDME cell. Edge nodes initiate RELAY_SENDME
   cells when both a) the window is <= 450, and b) there are less than
   ten cell payloads remaining to be flushed at that edge.


7. Directories and routers

7.1. Router descriptor format.

(Unless otherwise noted, tokens on the same line are space-separated.)

Router ::= Router-Line  Public-Key  Signing-Key?  Exit-Policy NL
Router-Line ::= "router" address ORPort OPPort APPort DirPort bandwidth
                NL
Public-key ::= a public key in PEM format   NL
Signing-Key ::= "signing-key"  NL  signing key in PEM format   NL
Exit-Policy ::= Exit-Line*
Exit-Line ::= ("accept"|"reject")  string  NL

ORport ::= port where the router listens for other routers (speaking cells)
OPPort ::= where the router listens for onion proxies (speaking cells)
APPort ::=  where the router listens for applications (speaking socks)
DirPort ::= where the router listens for directory download requests
bandwidth ::= maximum bandwidth, in bytes/s


Example:
router moria.mit.edu 9001 9011 9021 9031 100000
-----BEGIN RSA PUBLIC KEY-----
MIGJAoGBAMBBuk1sYxEg5jLAJy86U3GGJ7EGMSV7yoA6mmcsEVU3pwTUrpbpCmwS
7BvovoY3z4zk63NZVBErgKQUDkn3pp8n83xZgEf4GI27gdWIIwaBjEimuJlEY+7K
nZ7kVMRoiXCbjL6VAtNa4Zy1Af/GOm0iCIDpholeujQ95xew7rQnAgMA//8=
-----END RSA PUBLIC KEY-----
signing-key
-----BEGIN RSA PUBLIC KEY-----
7BvovoY3z4zk63NZVBErgKQUDkn3pp8n83xZgEf4GI27gdWIIwaBjEimuJlEY+7K
MIGJAoGBAMBBuk1sYxEg5jLAJy86U3GGJ7EGMSV7yoA6mmcsEVU3pwTUrpbpCmwS
f/GOm0iCIDpholeujQ95xew7rnZ7kVMRoiXCbjL6VAtNa4Zy1AQnAgMA//8=
-----END RSA PUBLIC KEY-----
reject 18.0.0.0/24

Note: The extra newline at the end of the router block is intentional.

7.2. Directory format

Directory ::= Directory-Header  Directory-Router  Router*  Signature
Directory-Header ::= "signed-directory" NL Software-Line NL
Software-Line: "recommended-software"  comma-separated-version-list
Directory-Router ::= Router
Signature ::= "directory-signature" NL "-----BEGIN SIGNATURE-----" NL
              Base-64-encoded-signature NL "-----END SIGNATURE-----" NL

Note:  The router block for the directory server must appear first.
The signature is computed by computing the SHA-1 hash of the
directory, from the characters "signed-directory", through the newline
after "directory-signature".  This digest is then padded with PKCS.1,
and signed with the directory server's signing key.