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tor/doc/tor-spec.txt
Roger Dingledine 7137a57849 further spec cleanup 
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$Id$
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 3DES in OFB
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"
[We will move to AES once we can assume everybody will have it. -RD]
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 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.
[or when an op wants to connect to or]
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 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_f) [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. 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 50 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 [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. Establishing OP-to-OR connections
[wrap this with the above]
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:
The number 1 to signify OP handshake [2 bytes]
Maximum bandwidth (bytes/s) [4 bytes]
Forward key [K_f] [16 bytes]
Backward key [K_b] [16 bytes]
[Total: 38 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 38 bytes long, the
OR closes the connection.
Otherwise, 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.
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 3DES-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]
[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]
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 3DES/OFB, 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 3DES/OFB 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 3DES/OFB 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.
[XXX Mention zlib encoding. -NM]
When one side of the TCP stream is closed, the corresponding edge
node sends a RELAY_END cell along the circuit; upon receiving a
RELAY_END cell, the edge node closes its side of the corresponding
TCP stream (by sending a FIN packet), but continues to accept and
package incoming data until both sides of the TCP stream are
closed. At that point, the edge node sends a second RELAY_END
cell, and drops its record of the stream.
For creation and handling of RELAY_EXTEND and RELAY_EXTENDED cells,
see section 4. For creating and handling of RELAY_SENDME cells,
see section 6.
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).
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.
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