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sw$Id$
Tor Spec
Note: This is an attempt to specify Tor as it exists as implemented in
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)
- 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.
0. Notation:
PK -- a public key.
SK -- a private key
K -- a key for a symmetric cypher
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.
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
Onion Routing is a distributed overlay network designed to anonymize
low-latency TCP-based applications such as web browsing, secure shell,
and instant messaging. Clients choose a path through the network and
build a ``circuit'', in which each node (or ``onion router'' or ``OR'')
in the path knows its predecessor and successor, but no other nodes in
the circuit. Traffic flowing down the circuit is sent in fixed-size
``cells'', which are unwrapped by a symmetric key at each node (like
the layers of an onion) and relayed downstream.
2. Connections
There are two ways to connect to an onion router (OR). The first is
as an onion proxy (OP), which allows the OP to authenticate the OR
without authenticating itself. The second is as another OR, which
allows mutual authentication.
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
connection if it is not.
Once a TLS connection is established, the two sides send cells
(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.
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
The basic unit of communication for onion routers and onion
proxies is a fixed-width "cell". Each cell contains the following
fields:
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)
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 'Payload' depends on the type of the cell.
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: 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.
CREATE, CREATED, 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, 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 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.
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 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.
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
servers can now calculate g^xy with ordinary DH. From the base key
material g^xy, they compute derivative key material as follows.
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
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.
KH is used in the handshake response to demonstrate knowledge of the
computed shared key. Kf is used to encrypt the stream of data going
from the OP to the OR, and 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
(OP) performs the following steps:
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. 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.
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.
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 OP 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 EXTEND cell along
the circuit (see section 5).
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 4.1. 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 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. 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 circID.
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.
(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.
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.
4.5. Routing relay cells
When an OR receives a RELAY cell, it checks the cell's circID 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 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.
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.
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 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. 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 payload of each unencrypted RELAY cell consists of:
Relay command [1 byte]
'Recognized' [2 bytes]
StreamID [2 bytes]
Digest [4 bytes]
Length [2 bytes]
Data [498 bytes]
The 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
10 -- RELAY_DROP
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.
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.
[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.
5.3. Closing streams
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.
An exit connection 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_FIN'
cell along the circuit and changes its state to 'DONE_PACKAGING'.
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_FIN' along the circuit, and changes its state
to 'CLOSED'. When a stream already in 'DONE_PACKAGING' receives a
'RELAY_FIN' cell, it sends a 'FIN' and changes its state to
'CLOSED'.
If an edge node encounters an error on any stream, it sends a
'RELAY_END' cell (if possible) and closes the stream immediately.
6. Flow control
6.1. Link throttling
Each node should do appropriate bandwidth throttling to keep its
user happy.
Communicants rely on TCP's default flow control to push back when they
stop reading.
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.
Each of these cells increments 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.
[this stuff is badly worded; copy in the tor-design section -RD]
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. Extensible information format
Router descriptors and directories both obey the following lightweight
extensible information format.
The highest level object is a Document, which consists of one or more Items.
Every Item begins with a KeywordLine, followed by one or more Objects. A
KeywordLine begins with a Keyword, optionally followed by a space and more
non-newline characters, and ends with a newline. A Keyword is a sequence of
one or more characters in the set [A-Za-z0-9-]. An Object is a block of
encoded data in pseudo-Open-PGP-style armor. (cf. RFC 2440)
More formally:
Document ::= (Item | NL)+
Item ::= KeywordLine Object*
KeywordLine ::= Keyword NL | Keyword SP ArgumentsChar+ NL
Keyword = KeywordChar+
KeywordChar ::= 'A' ... 'Z' | 'a' ... 'z' | '0' ... '9' | '-'
ArgumentChar ::= any printing ASCII character except NL.
Object ::= BeginLine Base-64-encoded-data EndLine
BeginLine ::= "-----BEGIN " Keyword "-----" NL
EndLine ::= "-----END " Keyword "-----" NL
The BeginLine and EndLine of an Object must use the same keyword.
When interpreting a Document, software MUST reject any document containing a
KeywordLine that starts with a keyword it doesn't recognize.
7.1. Router descriptor format.
Every router descriptor MUST start with a "router" Item; MUST end with a
"router-signature" Item and an extra NL; and MUST contain exactly one
instance of each of the following Items: "published" "onion-key" "link-key"
"signing-key". Additionally, a router descriptor MAY contain any number of
"accept", "reject", and "opt" Items.
The items' formats are as follows:
"router" nickname address (ORPort SocksPort DirPort bandwidth)?
"ports" ORPort SocksPort DirPort
"bandwidth" bandwidth
"platform" string
"published" YYYY-MM-DD HH:MM:SS
"onion-key" NL a public key in PEM format
"link-key" NL a public key in PEM format
"signing-key" NL a public key in PEM format
"accept" string
"reject" string
"router-signature" NL "-----BEGIN SIGNATURE-----" NL Signature NL
"-----END SIGNATURE-----"
"opt" SP keyword string? NL,Object?
ORport ::= port where the router listens for routers/proxies (speaking cells)
SocksPort ::= where the router listens for applications (speaking socks)
DirPort ::= where the router listens for directory download requests
bandwidth ::= maximum bandwidth, in bytes/s
nickname ::= between 1 and 32 alphanumeric characters. case-insensitive.
Bandwidth and ports are required; if they are not included in the router
line, they must appear in "bandwidth" and "ports" lines.
"opt" is reserved for non-critical future extensions.
7.2. Directory format
A Directory begins with a "signed-directory" item, followed by one each of
the following, in any order: "recommended-software". It may include any
number of "opt" items. After these items, a directory includes any number
of router descriptors, and a singer "directory-signature" item.
"signed-directory"
"recommended-software" comma-separated-version-list
"directory-signature" NL Signature
Note: The router descriptor 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.
If software encounters an unrecognized keyword in a single router descriptor,
it should reject only that router descriptor, and continue using the
others. If it encounters an unrecognized keyword in the directory header,
it should reject the entire directory.
7.3. Behavior of a directory server
lists nodes that are connected currently
speaks http on a socket, spits out directory on request
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