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revise spec. It still needs more explanation to be intelligible by the uninitiated, but it is at least far less incorrect.

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Nick Mathewson 2004-03-01 05:56:34 +00:00
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doc/tor-spec.txt
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@ -1,26 +1,23 @@
$Id$
sw$Id$
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
early March, 2004. It is not recommended that others implement this
design as it stands; future versions of Tor will implement improved
protocols.
This is not a design document; most design criteria are not examined. For
more information on why Tor acts as it does, see tor-design.pdf.
TODO: (very soon)
- Specify truncate/truncated payloads?
- Specify RELAY_END payloads. [It's 1 byte of reason, then X bytes of
data, right? -NM]
[Right, where X=4 and it's an IP, currently. -RD]
- Sendme w/stream0 is circuit sendme
- Integrate -NM and -RD comments
- EXTEND cells should have hostnames or nicknames, so that OPs never
resolve OR hostnames. Else DNS servers can give different answers to
different OPs, and compromise their anonymity.
- Alternatively, directories should include IPs.
- REASON_CONNECTFAILED should include an IP.
- Copy prose from tor-design to make everything more readable.
EVEN LATER:
- Do TCP-style sequencing and ACKing of DATA cells so that we can afford
to lose some data cells. [Actually, we'll probably never do this. -RD]
0. Notation:
@ -28,7 +25,10 @@ EVEN LATER:
SK -- a private key
K -- a key for a symmetric cypher
a|b -- concatenation of 'a' with 'b'.
a|b -- concatenation of 'a' and 'b'.
[A0 B1 C2] -- a three-byte sequence, containing the bytes with
hexadecimal values A0, B1, and C2, in that order.
All numeric values are encoded in network (big-endian) order.
@ -43,7 +43,6 @@ EVEN LATER:
"A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
"49286651ECE65381FFFFFFFFFFFFFFFF"
1. System overview
Onion Routing is a distributed overlay network designed to anonymize
@ -62,14 +61,16 @@ EVEN LATER:
without authenticating itself. The second is as another OR, which
allows mutual authentication.
Tor uses TLS for link encryption, using the cipher suite
"TLS_DHE_RSA_WITH_AES_128_CBC_SHA".
[That's cool, except it's not what we use currently. We use
3DES because most people don't have openssl 0.9.7 and thus
don't have AES. -RD]
An OR always sends a
self-signed X.509 certificate whose commonName is the server's
nickname, and whose public key is in the server directory.
Tor uses TLS for link encryption. All implementations MUST support
the TLS ciphersuite "TLS_EDH_RSA_WITH_DES_192_CBC3_SHA", and SHOULD
support "TLS_DHE_RSA_WITH_AES_128_CBC_SHA" if it is available.
Implementations MAY support other ciphersuites, but MUST NOT
support any suite without ephemeral keys, symmetric keys of at
least 128 bits, and digests of at least 160 bits.
An OR always sends a self-signed X.509 certificate whose commonName
is the server's nickname, and whose public key is in the server
directory.
All parties receiving certificates must confirm that the public
key is as it appears in the server directory, and close the
@ -79,13 +80,17 @@ EVEN LATER:
(specified below) to one another. Cells are sent serially. All
cells are 512 bytes long. Cells may be sent embedded in TLS
records of any size or divided across TLS records, but the framing
of TLS records must not leak information about the type or
contents of the cells.
of TLS records MUST NOT leak information about the type or contents
of the cells.
OR-to-OR connections are never deliberately closed. An OP should
close a connection to an OR if there are no circuits running over
the connection, and an amount of time (KeepalivePeriod, defaults to
5 minutes) has passed.
OR-to-OR connections are never deliberately closed. When an OR
starts or receives a new directory, it tries to open new
connections to any OR it is not already connected to.
OR-to-OP connections are not permanent. An OP should close a
connection to an OR if there are no circuits running over the
connection, and an amount of time (KeepalivePeriod, defaults to 5
minutes) has passed.
3. Cell Packet format
@ -93,11 +98,14 @@ EVEN LATER:
proxies is a fixed-width "cell". Each cell contains the following
fields:
CircID [2 bytes]
CircID [2 bytes]
Command [1 byte]
Payload (padded with 0 bytes) [509 bytes]
[Total size: 512 bytes]
The CircID field determines which circuit, if any, the cell is
associated with.
The 'Command' field holds one of the following values:
0 -- PADDING (Padding) (See Sec 6.2)
1 -- CREATE (Create a circuit) (See Sec 4)
@ -106,17 +114,18 @@ EVEN LATER:
4 -- DESTROY (Stop using a circuit) (See Sec 4)
The interpretation of 'Payload' depends on the type of the cell.
PADDING: Unused.
PADDING: Payload is unused.
CREATE: Payload contains the handshake challenge.
CREATED: Payload contains the handshake response.
RELAY: Payload contains the relay header and relay body.
DESTROY: Unused.
DESTROY: Payload is unused.
Upon receiving any other value for the command field, an OR must
drop the cell.
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.
PADDING cells are currently used to implement connection keepalive.
ORs and OPs send one another a PADDING cell every few minutes.
CREATE, CREATED, and DESTROY cells are used to manage circuits;
see section 4 below.
@ -124,17 +133,16 @@ EVEN LATER:
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
new circuit, OPs 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 plus the first 20 bytes
of derivative key data (see section 4.2). To extend a circuit past
the first hop, the user sends an EXTEND relay cell (see section 5)
the first hop, the OP 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.
@ -159,6 +167,19 @@ EVEN LATER:
The port and address field denote the IPV4 address and port of the
next onion router in the circuit.
The payload for a CREATED cell, or the relay payload for an
EXTENDED cell, contains:
DH data (g^y) [128 bytes]
Derivative key data (KH) [20 bytes] <see 4.2 below>
The CircID for a CREATE cell is an arbitrarily chosen 2-byte
integer, selected by the node (OP or OR) that sends the CREATE
cell. To prevent CircID collisions, when one OR sends a CREATE
cell to another, it chooses from only one half of the possible
values based on the ORs' nicknames: if the sending OR has a
lexicographically earlier nickname, it chooses a CircID with a high
bit of 0; otherwise, it chooses a CircID with a high bit of 1.
4.2. Setting circuit keys
Once the handshake between the OP and an OR is completed, both
@ -167,7 +188,7 @@ EVEN LATER:
First, the server represents g^xy as a big-endian unsigned integer.
Next, the server computes 60 bytes of key data as K = SHA1(g^xy |
[00]) | SHA1(g^xy | [01]) | SHA1(g^xy | [02]) where "00" is a single
octet whose value is zero, "01" is a single octet whose value is
octet whose value is zero, [01] is a single octet whose value is
one, etc. The first 20 bytes of K form KH, the next 16 bytes of K
form Kf, and the next 16 bytes of K form Kb.
@ -179,23 +200,22 @@ EVEN LATER:
4.3. Creating circuits
When creating a circuit through the network, the circuit creator
performs the following steps:
(OP) 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.
[this is wrong, now we choose the last hop and then choose
new hops lazily -RD]
1. Choose an onion router as an exit node (R_N), such that the onion
router's exit policy does not exclude all pending streams
that need a circuit.
2. If not already connected to the first router in the chain,
2. Choose a chain of (N-1) chain of N onion routers
(R_1...R_N-1) to constitute the path, such that no router
appears in the path twice.
3. If not already connected to the first router in the chain,
open a new connection to that router.
3. Choose a circID not already in use on the connection with the
first router in the chain. If we are an onion router and our
nickname is lexicographically greater than the nickname of the
other side, then let the high bit of the circID be 1, else 0.
4. Send a CREATE cell along the connection, to be received by
the first onion router.
4. Choose a circID not already in use on the connection with the
first router in the chain; 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 backward key Kb_1.
@ -203,8 +223,8 @@ EVEN LATER:
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:
To extend the circuit by a single onion router R_M, the OP performs
these steps:
1. Create an onion skin, encrypting the RSA-encrypted part with
R's public key.
@ -212,29 +232,27 @@ EVEN LATER:
2. Encrypt and send the onion skin in a relay EXTEND cell along
the circuit (see section 5).
3. When a relay EXTENDED cell is received, calculate the shared
keys. The circuit is now extended.
3. When a relay EXTENDED cell is received, verify KH, and
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 circID not
yet used on the connection between the two onion routers. (But see
section 4.3. above, concerning choosing circIDs. [What? This
is 4.3. Maybe we mean to remind about lexicographic order of
nicknames? -RD])
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 circID not yet
used on the connection between the two onion routers. (But see
section 4.1. above, concerning choosing circIDs based on
lexicographic order of nicknames.)
As an extension (called router twins), if the desired next onion
router R in the circuit is down, and some other onion router R'
has the same key as R, then it's ok to extend to R' rather than R.
has the same public keys as R, then it's ok to extend to R' rather than R.
When an onion router receives a CREATE cell, if it already has a
circuit on the given connection with the given circID, it drops the
cell. Otherwise, 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.
cell. Otherwise, after receiving the CREATE cell, it completes the
DH handshake, and replies with a CREATED cell. 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
@ -260,8 +278,10 @@ EVEN LATER:
After a DESTROY cell has been processed, an OR ignores all data or
destroy cells for the corresponding circuit.
[This next paragraph is never used, and should perhaps go away. -RD]
To tear down part of a circuit, the OP sends a RELAY_TRUNCATE cell
(The rest of this section is not currently used; on errors, circuits
are destroyed, not truncated.)
To tear down part of a circuit, the OP may send 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.
@ -272,11 +292,6 @@ EVEN LATER:
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. Possibly the right answer
is to not use most of the extensions. -RD]
[Correct. We should specify that OPs must not send truncate cells. -RD]
4.5. Routing relay cells
When an OR receives a RELAY cell, it checks the cell's circID and
@ -284,57 +299,46 @@ EVEN LATER:
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/CTR, as follows:
either an 'exit node' or a non-edge node), it de/encrypts the payload
with AES/CTR, 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.
[This part is now wrong. There's a 'recognized' field. If it crypts
to 0, then check the digest. Speaking of which, there's a digest
field. We should mention this. -RD]
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 [No longer true, an unrecognized relay cell at
the end is met with a destroy cell. -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/CTR as
follows:
OP sends data cell to node R_M:
For I=1...M, decrypt with Kf_I.
The OR then decides whether it recognizes the relay cell, by
inspecting the payload as described in section 5.1 below. If the OR
recognizes the cell, it processes the contents of the relay cell.
Otherwise, it passes the decrypted relay cell along the circuit if
the circuit continues. If the OR at the end of the circuit
encounters an unrecognized relay cell, an error has occurred: the OR
sends a DESTROY cell to tear down the circuit.
Otherwise, if the data cell is arriving at the OP edge if the
circuit, the OP encrypts the length and payload fields with AES/CTR as
follows:
When a relay cell arrives at an OP, it the OP encrypts the length and
payload fields with AES/CTR 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.
Encrypt with Kb_I. If the payload is recognized (see
section 5.1), then stop and process the payload.
For more information, see section 5 below.
5. Application connections and stream management
5.1. Streams
5.1. Relay cells
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:
The payload of each unencrypted RELAY cell consists of:
Relay command [1 byte]
Stream ID [7 bytes]
[command 1 byte, recognized 2 bytes, streamid 2 bytes, digest 4 bytes,
length 2 bytes == 11 bytes of header -RD]
'Recognized' [2 bytes]
StreamID [2 bytes]
Digest [4 bytes]
Length [2 bytes]
Data [498 bytes]
The relay commands are:
1 -- RELAY_BEGIN
@ -348,41 +352,81 @@ EVEN LATER:
9 -- RELAY_TRUNCATED
10 -- RELAY_DROP
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]
The 'Recognized' field in any unencrypted relay payload is always set
to zero; the 'digest' field is computed as the first four bytes of a
SHA-1 digest of the rest of the RELAY cell's payload, taken with the
digest field set to zero.
[This next paragraph is wrong: to begin a new stream, it simply
uses the new streamid. No need to send it separately. -RD]
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:
NEWSTREAMID | ADDRESS | ':' | PORT | '\000'
where NEWSTREAMID is the newly generated Stream ID to use for
this stream, ADDRESS may be a DNS hostname, or an IPv4 address in
When the 'recognized' field of a RELAY cell is zero, and the digest
is correct, the cell is considered "recognized" for the purposes of
decryption (see section 4.5 above).
All RELAY cells pertaining to the same tunneled stream have the
same stream ID. StreamIDs are chosen randomly by the OP. RELAY
cells that affect the entire circuit rather than a particular
stream use a StreamID of zero.
The 'Length' field of a relay cell contains the number of bytes in
the relay payload which contain real payload data. The remainder of
the payload is padded with random bytes.
5.2. Opening streams and transferring data
To open a new anonymized TCP connection, the OP chooses an open
circuit to an exit that may be able to connect to the destination
address, selects an arbitrary StreamID not yet used on that circuit,
and constructs a RELAY_BEGIN cell with a payload encoding the address
and port of the destination host. The payload format is:
ADDRESS | ':' | PORT | [00]
where ADDRESS is 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.
[What is the [00] for? -NM]
Upon receiving this cell, 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. (See 5.4 below.)
Otherwise, the exit node replies with a RELAY_CONNECTED cell, whose
payload is the 4-byte IP address to which the connection was made.
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.
RELAY_DATA cells sent to unrecognized streams are dropped.
Relay RELAY_DROP cells are long-range dummies; upon receiving such
a cell, the OR or OP must drop it.
5.2. Closing streams
5.3. Closing streams
[Note -- TCP streams can only be half-closed for reading. Our
Bickford's conversation was incorrect. -NM]
When an anonymized TCP connection is closed, or an edge node
encounters error on any stream, it sends a 'RELAY_END' cell along the
circuit (if possible) and closes the TCP connection immediately. If
an edge node receives a 'RELAY_END' cell for any stream, it closes
the TCP connection completely, and sends nothing more along the
circuit for that stream.
The payload of a RELAY_END cell begins with a single 'reason' byte to
describe why the stream is closing, plus optional data (depending on
the reason.) The values are:
1 -- REASON_MISC (catch-all for unlisted reasons)
2 -- REASON_RESOLVEFAILED (couldn't look up hostname)
3 -- REASON_CONNECTFAILED (couldn't connect to host/port)
4 -- REASON_EXITPOLICY (OR refuses to connect to host or port)
5 -- REASON_DESTROY (circuit is being destroyed [???-NM])
6 -- REASON_DONE (anonymized TCP connection was closed)
7 -- REASON_TIMEOUT (OR timed out while connecting [???-NM])
(With REASON_EXITPOLICY, the 4-byte IP address forms the optional
data; no other reason currently has extra data.)
*** [The rest of this section describes unimplemented functionality.]
Because TCP connections can be half-open, we follow an equivalent
to TCP's FIN/FIN-ACK/ACK protocol to close streams.
@ -394,25 +438,21 @@ EVEN LATER:
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'
the corresponding TCP connection, the edge node sends a 'RELAY_FIN'
cell along the circuit and changes its state to 'DONE_PACKAGING'.
Upon receiving a 'RELAY_END' cell, an edge node sends a 'FIN' to
Upon receiving a 'RELAY_FIN' 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
also sends a 'RELAY_FIN' 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
'RELAY_FIN' 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 stream, it sends a
'RELAY_END' cell (if possible) and closes the stream immediately.
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