tor/doc/tor-spec.txt

$Id$ 

TOR (The Onion Router) Spec

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

0. Notation:

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

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

   Unless otherwise specified, all symmetric ciphers are DES in OFB
   mode, with an IV of all 0 bytes.  All asymmetric ciphers are RSA
   with 1024-bit keys, and exponents of 65537.

   [Comments: DES?  This should be AES.  Why are -NM]
   [We will move to AES once we can assume everybody will have it. -RD]

1. System overview

[Something to start with here. Do feel free to change/expand. -RD]

Tor is an implementation of version 2 of Onion Routing.

Onion Routing is a connection-oriented anonymizing communication
service. Users build a layered block of asymmetric encryptions
(an "onion") which describes a source-routed path through a set of
nodes. Those nodes build 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 OR-to-OR connections

   When one onion router opens a connection to another, the initiating
   OR (called the 'client') and the listening OR (called the 'server')
   perform the following handshake.

   Before the handshake begins, the client and server know one
   another's (1024-bit) public keys, IPV4 addresses, and ports.

     1. Client connects to server:

        The client generates a pair of 8-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.

        The client then generates a 'Client authentication' message [M]
        containing: 
           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)                  [8 bytes]
           The backward key (K_f)                 [8 bytes]
           The maximum bandwidth (bytes/s)        [4 bytes]
                                               [Total: 36 bytes] 

        The client then RSA-encrypts the message with the server's
        public key, and PKCS1 padding to given an encrypted message

        [Commentary: 1024 bytes is probably too short, and this protocol can't
        support IPv6. -NM]
        [1024 is too short for a high-latency remailer; but perhaps it's
         fine for us, given our need for speed and also given our greater
         vulnerability to other attacks? Onions are infrequent enough now
         that maybe we could handle it; but I worry it will impact
         scalability, and handling more users is important.-RD]
 
        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. Server authenticates to client:

        Upon receiving a TCP connection, the server waits to receive
        128 bytes from the client.  It 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 32 bytes,
        the server closes the TCP connection and stops handshaking.

        The server then 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. 

        For later use, 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         [32 bytes]
               A random nonce [N]                     [8 bytes]
                                                  [Total: 40 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 40
        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. Establishing OP-to-OR connections

   When an Onion Proxy (OP) needs to establish a connection to an OR,
   the handshake is simpler because the OR does not need to verify the
   OP's identity.  The OP and OR establish the following steps:

     1. OP connects to OR:

        First, the OP generates a pair of 8-byte symmetric keys (one
        [K_f] for the 'forward' stream from OP to OR, and one
        [K_b] for the 'backward' stream from OR to OP.

        The OP generates a message [M] in the following format:
           Maximum bandwidth (bytes/s)      [4 bytes]
           Forward key [K_f]                [8 bytes]
           Backward key [K_b]               [8 bytes]
                                        [Total: 20 bytes]

        The OP encrypts M with the OR's public key and PKCS1 padding,
        opens a TCP connection to the OR's TCP port, and sends the
        resulting 128-byte encrypted message to the OR.

     2. OR receives keys:

        When the OR receives a connection from an OP [This is on a
        different port, right? How does it know the difference? -NM],
        [Correct. The 'or_port' config variable specifies the OR port,
         and the op_port variable specified the OP port. -RD]
        it waits for 128 bytes of data, and decrypts the resulting
        data with its private key, checking the PKCS1 padding.  If the
        padding is invalid, or the message is not 20 bytes long, the
        OR closes the connection.

        Otherwise, the connection is established, and the O 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.

2.3. Sending cells and link encryption

   Once the handshake is complete, the ORs or OR and OP send cells
   (specified below) to one another.  Cells are sent serially,
   encrypted with the DES-OFB 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]


3. Cell Packet format

   The basic unit of communication between 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)              [4 bytes]
        Payload (padded with 0 bytes)         [120 bytes]
                                         [Total size: 128 bytes]

   The 'Command' field holds one of the following values:
         0 -- PADDING     (Padding)
         1 -- CREATE      (Create a circuit)
         2 -- DATA        (End-to-end data)
         3 -- DESTROY     (Stop using a circuit)
         4 -- SENDME      (For flow control)

   The interpretation of 'Length' and 'Payload' depend on....

4. Onions and circuit management


5. Topic management


6. Flow control