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Tor Protocol Specification
Roger Dingledine
Nick Mathewson
Table of Contents
0. Preliminaries
0.1. Notation and encoding
0.2. Security parameters
0.3. Ciphers
0.4. A bad hybrid encryption algorithm, for legacy purposes
1. System overview
1.1. Keys and names
2. Connections
2.1. Picking TLS ciphersuites
2.2. TLS security considerations
3. Cell Packet format
4. Negotiating and initializing connections
4.1. Negotiating versions with VERSIONS cells
4.2. CERTS cells
4.3. AUTH_CHALLENGE cells
4.4. AUTHENTICATE cells
4.4.1. Link authentication type 1: RSA-SHA256-TLSSecret
4.4.2. Link authentication type 3: Ed25519-SHA256-RFC5705
4.5. NETINFO cells
5. Circuit management
5.1. CREATE and CREATED cells
5.1.1. Choosing circuit IDs in create cells
5.1.2. EXTEND and EXTENDED cells
5.1.3. The "TAP" handshake
5.1.4. The "ntor" handshake
5.1.5. CREATE_FAST/CREATED_FAST cells
5.2. Setting circuit keys
5.2.1. KDF-TOR
5.2.2. KDF-RFC5869
5.3. Creating circuits
5.3.1. Canonical connections
5.4. Tearing down circuits
5.5. Routing relay cells
5.5.1. Circuit ID Checks
5.5.2. Forward Direction
5.5.2.1. Routing from the Origin
5.5.2.2. Relaying Forward at Onion Routers
5.5.3. Backward Direction
5.5.3.1. Relaying Backward at Onion Routers
5.5.4. Routing to the Origin
5.6. Handling relay_early cells
6. Application connections and stream management
6.1. Relay cells
6.2. Opening streams and transferring data
6.2.1. Opening a directory stream
6.3. Closing streams
6.4. Remote hostname lookup
7. Flow control
7.1. Link throttling
7.2. Link padding
7.3. Circuit-level flow control
7.3.1. SENDME Cell Format
7.4. Stream-level flow control
8. Handling resource exhaustion
8.1. Memory exhaustion
9. Subprotocol versioning
9.1. "Link"
9.2. "LinkAuth"
9.3. "Relay"
9.4. "HSIntro"
9.5. "HSRend"
9.6. "HSDir"
9.7. "DirCache"
9.8. "Desc"
9.9. "Microdesc"
9.10. "Cons"
9.11. "Padding"
9.12. "FlowCtrl"
Note: This document aims to specify Tor as currently implemented, though it
may take it a little time to become fully up to date. Future versions of Tor
may implement improved protocols, and compatibility is not guaranteed.
We may or may not remove compatibility notes for other obsolete versions of
Tor as they become obsolete.
This specification 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.
0. Preliminaries
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
0.1. Notation and encoding
KP -- a public key for an asymmetric cipher.
KS -- a private key for an asymmetric cipher.
K -- a key for a symmetric cipher.
N -- a "nonce", a random value, usually deterministically chosen
from other inputs using hashing.
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.
H(m) -- a cryptographic hash of m.
We use "byte" and "octet" interchangeably. Possibly we shouldn't.
Some specs mention "base32". This means RFC4648, without "=" padding.
0.1.1. Encoding integers
Unless we explicitly say otherwise below, all numeric values in the
Tor protocol are encoded in network (big-endian) order. So a "32-bit
integer" means a big-endian 32-bit integer; a "2-byte" integer means
a big-endian 16-bit integer, and so forth.
0.2. Security parameters
Tor uses a stream cipher, a public-key cipher, the Diffie-Hellman
protocol, and a hash function.
KEY_LEN -- the length of the stream cipher's key, in bytes.
KP_ENC_LEN -- the length of a public-key encrypted message, in bytes.
KP_PAD_LEN -- the number of bytes added in padding for public-key
encryption, in bytes. (The largest number of bytes that can be encrypted
in a single public-key operation is therefore KP_ENC_LEN-KP_PAD_LEN.)
DH_LEN -- the number of bytes used to represent a member of the
Diffie-Hellman group.
DH_SEC_LEN -- the number of bytes used in a Diffie-Hellman private key (x).
HASH_LEN -- the length of the hash function's output, in bytes.
PAYLOAD_LEN -- The longest allowable cell payload, in bytes. (509)
CELL_LEN(v) -- The length of a Tor cell, in bytes, for link protocol
version v.
CELL_LEN(v) = 512 if v is less than 4;
= 514 otherwise.
0.3. Ciphers
These are the ciphers we use _unless otherwise specified_. Several of
them are deprecated for new use.
For a stream cipher, unless otherwise specified, we use 128-bit AES in
counter mode, with an IV of all 0 bytes. (We also require AES256.)
For a public-key cipher, unless otherwise specified, we use RSA with
1024-bit keys and a fixed exponent of 65537. We use OAEP-MGF1
padding, with SHA-1 as its digest function. We leave the optional
"Label" parameter unset. (For OAEP padding, see
ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1.pdf)
We also use the Curve25519 group and the Ed25519 signature format in
several places.
For Diffie-Hellman, unless otherwise specified, we use a generator
(g) of 2. For the modulus (p), we use the 1024-bit safe prime from
rfc2409 section 6.2 whose hex representation is:
"FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
"8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
"302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
"A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
"49286651ECE65381FFFFFFFFFFFFFFFF"
As an optimization, implementations SHOULD choose DH private keys (x) of
320 bits. Implementations that do this MUST never use any DH key more
than once.
[May other implementations reuse their DH keys?? -RD]
[Probably not. Conceivably, you could get away with changing DH keys once
per second, but there are too many oddball attacks for me to be
comfortable that this is safe. -NM]
For a hash function, unless otherwise specified, we use SHA-1.
KEY_LEN=16.
DH_LEN=128; DH_SEC_LEN=40.
KP_ENC_LEN=128; KP_PAD_LEN=42.
HASH_LEN=20.
We also use SHA256 and SHA3-256 in some places.
When we refer to "the hash of a public key", unless otherwise
specified, we mean the SHA-1 hash of the DER encoding of an ASN.1 RSA
public key (as specified in PKCS.1).
All "random" values MUST be generated with a cryptographically
strong pseudorandom number generator seeded from a strong entropy
source, unless otherwise noted.
0.4. A bad hybrid encryption algorithm, for legacy purposes.
Some specifications will refer to the "legacy hybrid encryption" of a
byte sequence M with a public key KP. It is computed as follows:
1. If the length of M is no more than KP_ENC_LEN-KP_PAD_LEN,
pad and encrypt M with KP.
2. Otherwise, generate a KEY_LEN byte random key K.
Let M1 = the first KP_ENC_LEN-KP_PAD_LEN-KEY_LEN bytes of M,
and let M2 = the rest of M.
Pad and encrypt K|M1 with KP. Encrypt M2 with our stream cipher,
using the key K. Concatenate these encrypted values.
Note that this "hybrid encryption" approach does not prevent
an attacker from adding or removing bytes to the end of M. It also
allows attackers to modify the bytes not covered by the OAEP --
see Goldberg's PET2006 paper for details. Do not use it as the basis
for new protocols! Also note that as used in Tor's protocols, case 1
never occurs.
1. System overview
Tor 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.
1.1. Keys and names
Every Tor relay has multiple public/private keypairs:
These are 1024-bit RSA keys:
- A long-term signing-only "Identity key" used to sign documents and
certificates, and used to establish relay identity.
KP_relayid_rsa, KS_relayid_rsa.
- A medium-term TAP "Onion key" used to decrypt onion skins when accepting
circuit extend attempts. (See 5.1.) Old keys MUST be accepted for a
while after they are no longer advertised. Because of this,
relays MUST retain old keys for a while after they're rotated. (See
"onion key lifetime parameters" in dir-spec.txt.)
KP_onion_tap, KS_onion_tap.
- A short-term "Connection key" used to negotiate TLS connections.
Tor implementations MAY rotate this key as often as they like, and
SHOULD rotate this key at least once a day.
KP_conn_tls, KS_conn_tls.
This is Curve25519 key:
- A medium-term ntor "Onion key" used to handle onion key handshakes when
accepting incoming circuit extend requests. As with TAP onion keys,
old ntor keys MUST be accepted for at least one week after they are no
longer advertised. Because of this, relays MUST retain old keys for a
while after they're rotated. (See "onion key lifetime parameters" in
dir-spec.txt.)
KP_ntor, KS_ntor.
These are Ed25519 keys:
- A long-term "master identity" key. This key never
changes; it is used only to sign the "signing" key below. It may be
kept offline.
KP_relayid_ed, KS_relayid_ed.
- A medium-term "signing" key. This key is signed by the master identity
key, and must be kept online. A new one should be generated
periodically. It signs nearly everything else.
KP_relaysign_ed, KS_relaysign_ed.
- A short-term "link authentication" key, used to authenticate
the link handshake: see section 4 below. This key is signed
by the "signing" key, and should be regenerated frequently.
KP_link_ed, KS_link_ed.
KP_relayid_* together identify a router uniquely. Once a router
has used a KP_relayid_ed (an Ed25519 master identity key)
together with a given KP_relayid_rsa (RSA identity key), neither of
those keys may ever be used with a different key.
We write KP_relayid to refer to a key which is either
KP_relayid_rsa or KP_relayid_ed.
The same key or keypair should never be used for separate roles within
the Tor protocol suite, unless specifically stated. For example,
a relay's identity keys K_relayid should not also be used as the
identity keypair for a hidden service K_hs_id (see rend-spec-v3.txt).
2. Connections
Connections between two Tor relays, or between a client and a relay,
use TLS/SSLv3 for link authentication and encryption. All
implementations MUST support the SSLv3 ciphersuite
"TLS_DHE_RSA_WITH_AES_128_CBC_SHA" if it is available. They SHOULD
support better ciphersuites if available.
There are three ways to perform TLS handshakes with a Tor server. In
the first way, "certificates-up-front", both the initiator and
responder send a two-certificate chain as part of their initial
handshake. (This is supported in all Tor versions.) In the second
way, "renegotiation", the responder provides a single certificate,
and the initiator immediately performs a TLS renegotiation. (This is
supported in Tor 0.2.0.21 and later.) And in the third way,
"in-protocol", the initial TLS negotiation completes, and the
parties bootstrap themselves to mutual authentication via use of the
Tor protocol without further TLS handshaking. (This is supported in
0.2.3.6-alpha and later.)
Each of these options provides a way for the parties to learn it is
available: a client does not need to know the version of the Tor
server in order to connect to it properly.
In "certificates up-front" (a.k.a "the v1 handshake"),
the connection initiator always sends a
two-certificate chain, consisting of an X.509 certificate using a
short-term connection public key and a second, self-signed X.509
certificate containing its identity key. The other party sends a similar
certificate chain. The initiator's ClientHello MUST NOT include any
ciphersuites other than:
TLS_DHE_RSA_WITH_AES_256_CBC_SHA
TLS_DHE_RSA_WITH_AES_128_CBC_SHA
SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA
In "renegotiation" (a.k.a. "the v2 handshake"),
the connection initiator sends no certificates, and
the responder sends a single connection certificate. Once the TLS
handshake is complete, the initiator renegotiates the handshake, with each
party sending a two-certificate chain as in "certificates up-front".
The initiator's ClientHello MUST include at least one ciphersuite not in
the list above -- that's how the initiator indicates that it can
handle this handshake. For other considerations on the initiator's
ClientHello, see section 2.1 below.
In "in-protocol" (a.k.a. "the v3 handshake"), the initiator sends no
certificates, and the
responder sends a single connection certificate. The choice of
ciphersuites must be as in a "renegotiation" handshake. There are
additionally a set of constraints on the connection certificate,
which the initiator can use to learn that the in-protocol handshake
is in use. Specifically, at least one of these properties must be
true of the certificate:
* The certificate is self-signed
* Some component other than "commonName" is set in the subject or
issuer DN of the certificate.
* The commonName of the subject or issuer of the certificate ends
with a suffix other than ".net".
* The certificate's public key modulus is longer than 1024 bits.
The initiator then sends a VERSIONS cell to the responder, which then
replies with a VERSIONS cell; they have then negotiated a Tor
protocol version. Assuming that the version they negotiate is 3 or higher
(the only ones specified for use with this handshake right now), the
responder sends a CERTS cell, an AUTH_CHALLENGE cell, and a NETINFO
cell to the initiator, which may send either CERTS, AUTHENTICATE,
NETINFO if it wants to authenticate, or just NETINFO if it does not.
For backward compatibility between later handshakes and "certificates
up-front", the ClientHello of an initiator that supports a later
handshake MUST include at least one ciphersuite other than those listed
above. The connection responder examines the initiator's ciphersuite list
to see whether it includes any ciphers other than those included in the
list above. If extra ciphers are included, the responder proceeds as in
"renegotiation" and "in-protocol": it sends a single certificate and
does not request
client certificates. Otherwise (in the case that no extra ciphersuites
are included in the ClientHello) the responder proceeds as in
"certificates up-front": it requests client certificates, and sends a
two-certificate chain. In either case, once the responder has sent its
certificate or certificates, the initiator counts them. If two
certificates have been sent, it proceeds as in "certificates up-front";
otherwise, it proceeds as in "renegotiation" or "in-protocol".
To decide whether to do "renegotiation" or "in-protocol", the
initiator checks whether the responder's initial certificate matches
the criteria listed above.
All new relay implementations of the Tor protocol MUST support
backwards-compatible renegotiation; clients SHOULD do this too. If
this is not possible, new client implementations MUST support both
"renegotiation" and "in-protocol" and use the router's
published link protocols list (see dir-spec.txt on the "protocols" entry)
to decide which to use.
In all of the above handshake variants, certificates sent in the clear
SHOULD NOT include any strings to identify the host as a Tor relay. In
the "renegotiation" and "backwards-compatible renegotiation" steps, the
initiator SHOULD choose a list of ciphersuites and TLS extensions
to mimic one used by a popular web browser.
Even though the connection protocol is identical, we will think of the
initiator as either an onion router (OR) if it is willing to relay
traffic for other Tor users, or an onion proxy (OP) if it only handles
local requests. Onion proxies SHOULD NOT provide long-term-trackable
identifiers in their handshakes.
In all handshake variants, once all certificates are exchanged, all
parties receiving certificates must confirm that the identity key is as
expected. If the key is not as expected, the party must close the
connection.
(When initiating a connection, if a reasonably live consensus is
available, then the expected identity key is taken from that
consensus. But when initiating a connection otherwise, the expected
identity key is the one given in the hard-coded authority or
fallback list. Finally, when creating a connection because of an
EXTEND/EXTEND2 cell, the expected identity key is the one given in
the cell.)
When connecting to an OR, all parties SHOULD reject the connection if that
OR has a malformed or missing certificate. When accepting an incoming
connection, an OR SHOULD NOT reject incoming connections from parties with
malformed or missing certificates. (However, an OR should not believe
that an incoming connection is from another OR unless the certificates
are present and well-formed.)
[Before version 0.1.2.8-rc, ORs rejected incoming connections from ORs and
OPs alike if their certificates were missing or malformed.]
Once a TLS connection is established, the two sides send cells
(specified below) to one another. Cells are sent serially. Standard
cells are CELL_LEN(link_proto) bytes long, but variable-length cells
also exist; see Section 3. 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.
TLS connections are not permanent. Either side MAY close a connection
if there are no circuits running over it and an amount of time
(KeepalivePeriod, defaults to 5 minutes) has passed since the last time
any traffic was transmitted over the TLS connection. Clients SHOULD
also hold a TLS connection with no circuits open, if it is likely that a
circuit will be built soon using that connection.
Client-only Tor instances are encouraged to avoid using handshake
variants that include certificates, if those certificates provide
any persistent tags to the relays they contact. If clients do use
certificates, they SHOULD NOT keep using the same certificates when
their IP address changes. Clients MAY send certificates using any
of the above handshake variants.
2.1. Picking TLS ciphersuites
Clients SHOULD send a ciphersuite list chosen to emulate some popular
web browser or other program common on the internet. Clients may send
the "Fixed Cipheruite List" below. If they do not, they MUST NOT
advertise any ciphersuite that they cannot actually support, unless that
cipher is one not supported by OpenSSL 1.0.1.
The fixed ciphersuite list is:
TLS1_ECDHE_ECDSA_WITH_AES_256_CBC_SHA
TLS1_ECDHE_RSA_WITH_AES_256_CBC_SHA
TLS1_DHE_RSA_WITH_AES_256_SHA
TLS1_DHE_DSS_WITH_AES_256_SHA
TLS1_ECDH_RSA_WITH_AES_256_CBC_SHA
TLS1_ECDH_ECDSA_WITH_AES_256_CBC_SHA
TLS1_RSA_WITH_AES_256_SHA
TLS1_ECDHE_ECDSA_WITH_RC4_128_SHA
TLS1_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
TLS1_ECDHE_RSA_WITH_RC4_128_SHA
TLS1_ECDHE_RSA_WITH_AES_128_CBC_SHA
TLS1_DHE_RSA_WITH_AES_128_SHA
TLS1_DHE_DSS_WITH_AES_128_SHA
TLS1_ECDH_RSA_WITH_RC4_128_SHA
TLS1_ECDH_RSA_WITH_AES_128_CBC_SHA
TLS1_ECDH_ECDSA_WITH_RC4_128_SHA
TLS1_ECDH_ECDSA_WITH_AES_128_CBC_SHA
SSL3_RSA_RC4_128_MD5
SSL3_RSA_RC4_128_SHA
TLS1_RSA_WITH_AES_128_SHA
TLS1_ECDHE_ECDSA_WITH_DES_192_CBC3_SHA
TLS1_ECDHE_RSA_WITH_DES_192_CBC3_SHA
SSL3_EDH_RSA_DES_192_CBC3_SHA
SSL3_EDH_DSS_DES_192_CBC3_SHA
TLS1_ECDH_RSA_WITH_DES_192_CBC3_SHA
TLS1_ECDH_ECDSA_WITH_DES_192_CBC3_SHA
SSL3_RSA_FIPS_WITH_3DES_EDE_CBC_SHA
SSL3_RSA_DES_192_CBC3_SHA
[*] The "extended renegotiation is supported" ciphersuite, 0x00ff, is
not counted when checking the list of ciphersuites.
If the client sends the Fixed Ciphersuite List, the responder MUST NOT
select any ciphersuite besides TLS_DHE_RSA_WITH_AES_256_CBC_SHA,
TLS_DHE_RSA_WITH_AES_128_CBC_SHA, and SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA:
such ciphers might not actually be supported by the client.
If the client sends a v2+ ClientHello with a list of ciphers other then
the Fixed Ciphersuite List, the responder can trust that the client
supports every cipher advertised in that list, so long as that ciphersuite
is also supported by OpenSSL 1.0.1.
Responders MUST NOT select any TLS ciphersuite that lacks ephemeral keys,
or whose symmetric keys are less then KEY_LEN bits, or whose digests are
less than HASH_LEN bits. Responders SHOULD NOT select any SSLv3
ciphersuite other than the DHE+3DES suites listed above.
2.2. TLS security considerations
Implementations MUST NOT allow TLS session resumption -- it can
exacerbate some attacks (e.g. the "Triple Handshake" attack from
Feb 2013), and it plays havoc with forward secrecy guarantees.
Implementations SHOULD NOT allow TLS compression -- although we don't
know a way to apply a CRIME-style attack to current Tor directly,
it's a waste of resources.
3. Cell Packet format
The basic unit of communication for onion routers and onion
proxies is a fixed-width "cell".
On a version 1 connection, each cell contains the following
fields:
CircID [CIRCID_LEN bytes]
Command [1 byte]
Payload (padded with padding bytes) [PAYLOAD_LEN bytes]
On a version 2 or higher connection, all cells are as in version 1
connections, except for variable-length cells, whose format is:
CircID [CIRCID_LEN octets]
Command [1 octet]
Length [2 octets; big-endian integer]
Payload (some commands MAY pad) [Length bytes]
Most variable-length cells MAY be padded with padding bytes, except
for VERSIONS cells, which MUST NOT contain any additional bytes.
(The payload of VPADDING cells consists of padding bytes.)
On a version 2 connection, variable-length cells are indicated by a
command byte equal to 7 ("VERSIONS"). On a version 3 or
higher connection, variable-length cells are indicated by a command
byte equal to 7 ("VERSIONS"), or greater than or equal to 128.
CIRCID_LEN is 2 for link protocol versions 1, 2, and 3. CIRCID_LEN
is 4 for link protocol version 4 or higher. The first VERSIONS cell,
and any cells sent before the first VERSIONS cell, always have
CIRCID_LEN == 2 for backward compatibility.
The CircID field determines which circuit, if any, the cell is
associated with.
The 'Command' field of a fixed-length cell holds one of the following
values:
0 -- PADDING (Padding) (See Sec 7.2)
1 -- CREATE (Create a circuit) (See Sec 5.1)
2 -- CREATED (Acknowledge create) (See Sec 5.1)
3 -- RELAY (End-to-end data) (See Sec 5.5 and 6)
4 -- DESTROY (Stop using a circuit) (See Sec 5.4)
5 -- CREATE_FAST (Create a circuit, no KP) (See Sec 5.1)
6 -- CREATED_FAST (Circuit created, no KP) (See Sec 5.1)
8 -- NETINFO (Time and address info) (See Sec 4.5)
9 -- RELAY_EARLY (End-to-end data; limited)(See Sec 5.6)
10 -- CREATE2 (Extended CREATE cell) (See Sec 5.1)
11 -- CREATED2 (Extended CREATED cell) (See Sec 5.1)
12 -- PADDING_NEGOTIATE (Padding negotiation) (See Sec 7.2)
Variable-length command values are:
7 -- VERSIONS (Negotiate proto version) (See Sec 4)
128 -- VPADDING (Variable-length padding) (See Sec 7.2)
129 -- CERTS (Certificates) (See Sec 4.2)
130 -- AUTH_CHALLENGE (Challenge value) (See Sec 4.3)
131 -- AUTHENTICATE (Client authentication)(See Sec 4.5)
132 -- AUTHORIZE (Client authorization) (Not yet used)
The interpretation of 'Payload' depends on the type of the cell.
VPADDING/PADDING:
Payload contains padding bytes.
CREATE/CREATE2: Payload contains the handshake challenge.
CREATED/CREATED2: Payload contains the handshake response.
RELAY/RELAY_EARLY: Payload contains the relay header and relay body.
DESTROY: Payload contains a reason for closing the circuit.
(see 5.4)
Upon receiving any other value for the command field, an OR must
drop the cell. Since more cell types may be added in the future, ORs
should generally not warn when encountering unrecognized commands.
The cell is padded up to the cell length with padding bytes.
Senders set padding bytes depending on the cell's command:
VERSIONS: Payload MUST NOT contain padding bytes.
AUTHORIZE: Payload is unspecified and reserved for future use.
Other variable-length cells:
Payload MAY contain padding bytes at the end of the cell.
Padding bytes SHOULD be set to NUL.
RELAY/RELAY_EARLY: Payload MUST be padded to PAYLOAD_LEN with padding
bytes. Padding bytes SHOULD be set to random values.
Other fixed-length cells:
Payload MUST be padded to PAYLOAD_LEN with padding bytes.
Padding bytes SHOULD be set to NUL.
We recommend random padding in RELAY/RELAY_EARLY cells, so that the cell
content is unpredictable. See the format of relay cells in section 6.1
for detail.
For other cells, TLS authenticates cell content, so randomized padding
bytes are redundant.
Receivers MUST ignore padding bytes.
PADDING cells are currently used to implement connection keepalive.
If there is no other traffic, ORs and OPs send one another a PADDING
cell every few minutes.
CREATE, CREATE2, CREATED, CREATED2, and DESTROY cells are used to
manage circuits; see section 5 below.
RELAY cells are used to send commands and data along a circuit; see
section 6 below.
VERSIONS and NETINFO cells are used to set up connections in link
protocols v2 and higher; in link protocol v3 and higher, CERTS,
AUTH_CHALLENGE, and AUTHENTICATE may also be used. See section 4
below.
4. Negotiating and initializing connections
After Tor instances negotiate handshake with either the "renegotiation" or
"in-protocol" handshakes, they must exchange a set of cells to set up
the Tor connection and make it "open" and usable for circuits.
When the renegotiation handshake is used, both parties immediately
send a VERSIONS cell (4.1 below), and after negotiating a link
protocol version (which will be 2), each send a NETINFO cell (4.5
below) to confirm their addresses and timestamps. No other intervening
cell types are allowed.
When the in-protocol handshake is used, the initiator sends a
VERSIONS cell to indicate that it will not be renegotiating. The
responder sends a VERSIONS cell, a CERTS cell (4.2 below) to give the
initiator the certificates it needs to learn the responder's
identity, an AUTH_CHALLENGE cell (4.3) that the initiator must include
as part of its answer if it chooses to authenticate, and a NETINFO
cell (4.5). As soon as it gets the CERTS cell, the initiator knows
whether the responder is correctly authenticated. At this point the
initiator behaves differently depending on whether it wants to
authenticate or not. If it does not want to authenticate, it MUST
send a NETINFO cell. If it does want to authenticate, it MUST send a
CERTS cell, an AUTHENTICATE cell (4.4), and a NETINFO. When this
handshake is in use, the first cell must be VERSIONS, VPADDING, or
AUTHORIZE, and no other cell type is allowed to intervene besides
those specified, except for VPADDING cells.
The AUTHORIZE cell type is reserved for future use by scanning-resistance
designs.
[Tor versions before 0.2.3.11-alpha did not recognize the AUTHORIZE cell,
and did not permit any command other than VERSIONS as the first cell of
the in-protocol handshake.]
4.1. Negotiating versions with VERSIONS cells
There are multiple instances of the Tor link connection protocol. Any
connection negotiated using the "certificates up front" handshake (see
section 2 above) is "version 1". In any connection where both parties
have behaved as in the "renegotiation" handshake, the link protocol
version must be 2. In any connection where both parties have behaved
as in the "in-protocol" handshake, the link protocol must be 3 or higher.
To determine the version, in any connection where the "renegotiation"
or "in-protocol" handshake was used (that is, where the responder
sent only one certificate at first and where the initiator did not
send any certificates in the first negotiation), both parties MUST
send a VERSIONS cell. In "renegotiation", they send a VERSIONS cell
right after the renegotiation is finished, before any other cells are
sent. In "in-protocol", the initiator sends a VERSIONS cell
immediately after the initial TLS handshake, and the responder
replies immediately with a VERSIONS cell. (As an exception to this rule,
if both sides support the "in-protocol" handshake, either side may send
VPADDING cells at any time.)
The payload in a VERSIONS cell is a series of big-endian two-byte
integers. Both parties MUST select as the link protocol version the
highest number contained both in the VERSIONS cell they sent and in the
versions cell they received. If they have no such version in common,
they cannot communicate and MUST close the connection. Either party MUST
close the connection if the versions cell is not well-formed (for example,
if the payload contains an odd number of bytes).
Any VERSIONS cells sent after the first VERSIONS cell MUST be ignored.
(To be interpreted correctly, later VERSIONS cells MUST have a CIRCID_LEN
matching the version negotiated with the first VERSIONS cell.)
Since the version 1 link protocol does not use the "renegotiation"
handshake, implementations MUST NOT list version 1 in their VERSIONS
cell. When the "renegotiation" handshake is used, implementations
MUST list only the version 2. When the "in-protocol" handshake is
used, implementations MUST NOT list any version before 3, and SHOULD
list at least version 3.
Link protocols differences are:
1 -- The "certs up front" handshake.
2 -- Uses the renegotiation-based handshake. Introduces
variable-length cells.
3 -- Uses the in-protocol handshake.
4 -- Increases circuit ID width to 4 bytes.
5 -- Adds support for link padding and negotiation (padding-spec.txt).
4.2. CERTS cells
The CERTS cell describes the keys that a Tor instance is claiming
to have. It is a variable-length cell. Its payload format is:
N: Number of certs in cell [1 octet]
N times:
CertType [1 octet]
CLEN [2 octets]
Certificate [CLEN octets]
Any extra octets at the end of a CERTS cell MUST be ignored.
Relevant certType values are:
1: Link key certificate certified by RSA1024 identity
2: RSA1024 Identity certificate, self-signed.
3: RSA1024 AUTHENTICATE cell link certificate, signed with RSA1024 key.
4: Ed25519 signing key, signed with identity key.
5: TLS link certificate, signed with ed25519 signing key.
6: Ed25519 AUTHENTICATE cell key, signed with ed25519 signing key.
7: Ed25519 identity, signed with RSA identity.
The certificate format for certificate types 1-3 is DER encoded
X509. For others, the format is as documented in cert-spec.txt.
Note that type 7 uses a different format from types 4-6.
A CERTS cell may have no more than one certificate of each CertType.
To authenticate the responder as having a given Ed25519,RSA identity key
combination, the initiator MUST check the following.
* The CERTS cell contains exactly one CertType 2 "ID" certificate.
* The CERTS cell contains exactly one CertType 4 Ed25519
"Id->Signing" cert.
* The CERTS cell contains exactly one CertType 5 Ed25519
"Signing->link" certificate.
* The CERTS cell contains exactly one CertType 7 "RSA->Ed25519"
cross-certificate.
* All X.509 certificates above have validAfter and validUntil dates;
no X.509 or Ed25519 certificates are expired.
* All certificates are correctly signed.
* The certified key in the Signing->Link certificate matches the
SHA256 digest of the certificate that was used to
authenticate the TLS connection.
* The identity key listed in the ID->Signing cert was used to
sign the ID->Signing Cert.
* The Signing->Link cert was signed with the Signing key listed
in the ID->Signing cert.
* The RSA->Ed25519 cross-certificate certifies the Ed25519
identity, and is signed with the RSA identity listed in the
"ID" certificate.
* The certified key in the ID certificate is a 1024-bit RSA key.
* The RSA ID certificate is correctly self-signed.
To authenticate the responder as having a given RSA identity only,
the initiator MUST check the following:
* The CERTS cell contains exactly one CertType 1 "Link" certificate.
* The CERTS cell contains exactly one CertType 2 "ID" certificate.
* Both certificates have validAfter and validUntil dates that
are not expired.
* The certified key in the Link certificate matches the
link key that was used to negotiate the TLS connection.
* The certified key in the ID certificate is a 1024-bit RSA key.
* The certified key in the ID certificate was used to sign both
certificates.
* The link certificate is correctly signed with the key in the
ID certificate
* The ID certificate is correctly self-signed.
In both cases above, checking these conditions is sufficient to
authenticate that the initiator is talking to the Tor node with the
expected identity, as certified in the ID certificate(s).
To authenticate the initiator as having a given Ed25519,RSA
identity key combination, the responder MUST check the following:
* The CERTS cell contains exactly one CertType 2 "ID" certificate.
* The CERTS cell contains exactly one CertType 4 Ed25519
"Id->Signing" certificate.
* The CERTS cell contains exactly one CertType 6 Ed25519
"Signing->auth" certificate.
* The CERTS cell contains exactly one CertType 7 "RSA->Ed25519"
cross-certificate.
* All X.509 certificates above have validAfter and validUntil dates;
no X.509 or Ed25519 certificates are expired.
* All certificates are correctly signed.
* The identity key listed in the ID->Signing cert was used to
sign the ID->Signing Cert.
* The Signing->AUTH cert was signed with the Signing key listed
in the ID->Signing cert.
* The RSA->Ed25519 cross-certificate certifies the Ed25519
identity, and is signed with the RSA identity listed in the
"ID" certificate.
* The certified key in the ID certificate is a 1024-bit RSA key.
* The RSA ID certificate is correctly self-signed.
To authenticate the initiator as having an RSA identity key only,
the responder MUST check the following:
* The CERTS cell contains exactly one CertType 3 "AUTH" certificate.
* The CERTS cell contains exactly one CertType 2 "ID" certificate.
* Both certificates have validAfter and validUntil dates that
are not expired.
* The certified key in the AUTH certificate is a 1024-bit RSA key.
* The certified key in the ID certificate is a 1024-bit RSA key.
* The certified key in the ID certificate was used to sign both
certificates.
* The auth certificate is correctly signed with the key in the
ID certificate.
* The ID certificate is correctly self-signed.
Checking these conditions is NOT sufficient to authenticate that the
initiator has the ID it claims; to do so, the cells in 4.3 and 4.4
below must be exchanged.
4.3. AUTH_CHALLENGE cells
An AUTH_CHALLENGE cell is a variable-length cell with the following
fields:
Challenge [32 octets]
N_Methods [2 octets]
Methods [2 * N_Methods octets]
It is sent from the responder to the initiator. Initiators MUST
ignore unexpected bytes at the end of the cell. Responders MUST
generate every challenge independently using a strong RNG or PRNG.
The Challenge field is a randomly generated string that the
initiator must sign (a hash of) as part of authenticating. The
methods are the authentication methods that the responder will
accept. Only two authentication methods are defined right now:
see 4.4.1 and 4.4.2 below.
4.4. AUTHENTICATE cells
If an initiator wants to authenticate, it responds to the
AUTH_CHALLENGE cell with a CERTS cell and an AUTHENTICATE cell.
The CERTS cell is as a server would send, except that instead of
sending a CertType 1 (and possibly CertType 5) certs for arbitrary link
certificates, the initiator sends a CertType 3 (and possibly
CertType 6) cert for an RSA/Ed25519 AUTHENTICATE key.
This difference is because we allow any link key type on a TLS
link, but the protocol described here will only work for specific key
types as described in 4.4.1 and 4.4.2 below.
An AUTHENTICATE cell contains the following:
AuthType [2 octets]
AuthLen [2 octets]
Authentication [AuthLen octets]
Responders MUST ignore extra bytes at the end of an AUTHENTICATE
cell. Recognized AuthTypes are 1 and 3, described in the next
two sections.
Initiators MUST NOT send an AUTHENTICATE cell before they have
verified the certificates presented in the responder's CERTS
cell, and authenticated the responder.
4.4.1. Link authentication type 1: RSA-SHA256-TLSSecret
If AuthType is 1 (meaning "RSA-SHA256-TLSSecret"), then the
Authentication field of the AUTHENTICATE cell contains the following:
TYPE: The characters "AUTH0001" [8 octets]
CID: A SHA256 hash of the initiator's RSA1024 identity key [32 octets]
SID: A SHA256 hash of the responder's RSA1024 identity key [32 octets]
SLOG: A SHA256 hash of all bytes sent from the responder to the
initiator as part of the negotiation up to and including the
AUTH_CHALLENGE cell; that is, the VERSIONS cell, the CERTS cell,
the AUTH_CHALLENGE cell, and any padding cells. [32 octets]
CLOG: A SHA256 hash of all bytes sent from the initiator to the
responder as part of the negotiation so far; that is, the
VERSIONS cell and the CERTS cell and any padding cells. [32
octets]
SCERT: A SHA256 hash of the responder's TLS link certificate. [32
octets]
TLSSECRETS: A SHA256 HMAC, using the TLS master secret as the
secret key, of the following:
- client_random, as sent in the TLS Client Hello
- server_random, as sent in the TLS Server Hello
- the NUL terminated ASCII string:
"Tor V3 handshake TLS cross-certification"
[32 octets]
RAND: A 24 byte value, randomly chosen by the initiator. (In an
imitation of SSL3's gmt_unix_time field, older versions of Tor
sent an 8-byte timestamp as the first 8 bytes of this field;
new implementations should not do that.) [24 octets]
SIG: A signature of a SHA256 hash of all the previous fields
using the initiator's "Authenticate" key as presented. (As
always in Tor, we use OAEP-MGF1 padding; see tor-spec.txt
section 0.3.)
[variable length]
To check the AUTHENTICATE cell, a responder checks that all fields
from TYPE through TLSSECRETS contain their unique
correct values as described above, and then verifies the signature.
The server MUST ignore any extra bytes in the signed data after
the RAND field.
Responders MUST NOT accept this AuthType if the initiator has
claimed to have an Ed25519 identity.
(There is no AuthType 2: It was reserved but never implemented.)
4.4.2. Link authentication type 3: Ed25519-SHA256-RFC5705.
If AuthType is 3, meaning "Ed25519-SHA256-RFC5705", the
Authentication field of the AuthType cell is as below:
Modified values and new fields below are marked with asterisks.
TYPE: The characters "AUTH0003" [8 octets]
CID: A SHA256 hash of the initiator's RSA1024 identity key [32 octets]
SID: A SHA256 hash of the responder's RSA1024 identity key [32 octets]
CID_ED: The initiator's Ed25519 identity key [32 octets]
SID_ED: The responder's Ed25519 identity key, or all-zero. [32 octets]
SLOG: A SHA256 hash of all bytes sent from the responder to the
initiator as part of the negotiation up to and including the
AUTH_CHALLENGE cell; that is, the VERSIONS cell, the CERTS cell,
the AUTH_CHALLENGE cell, and any padding cells. [32 octets]
CLOG: A SHA256 hash of all bytes sent from the initiator to the
responder as part of the negotiation so far; that is, the
VERSIONS cell and the CERTS cell and any padding cells. [32
octets]
SCERT: A SHA256 hash of the responder's TLS link certificate. [32
octets]
TLSSECRETS: The output of an RFC5705 Exporter function on the
TLS session, using as its inputs:
- The label string "EXPORTER FOR TOR TLS CLIENT BINDING AUTH0003"
- The context value equal to the initiator's Ed25519 identity key.
- The length 32.
[32 octets]
RAND: A 24 byte value, randomly chosen by the initiator. [24 octets]
SIG: A signature of all previous fields using the initiator's
Ed25519 authentication key (as in the cert with CertType 6).
[variable length]
To check the AUTHENTICATE cell, a responder checks that all fields
from TYPE through TLSSECRETS contain their unique
correct values as described above, and then verifies the signature.
The server MUST ignore any extra bytes in the signed data after
the RAND field.
4.5. NETINFO cells
If version 2 or higher is negotiated, each party sends the other a
NETINFO cell. The cell's payload is:
TIME (Timestamp) [4 bytes]
OTHERADDR (Other OR's address) [variable]
ATYPE (Address type) [1 byte]
ALEN (Address length) [1 byte]
AVAL (Address value in NBO) [ALEN bytes]
NMYADDR (Number of this OR's addresses) [1 byte]
NMYADDR times:
ATYPE (Address type) [1 byte]
ALEN (Address length) [1 byte]
AVAL (Address value in NBO)) [ALEN bytes]
Recognized address types (ATYPE) are:
[04] IPv4.
[06] IPv6.
ALEN MUST be 4 when ATYPE is 0x04 (IPv4) and 16 when ATYPE is 0x06
(IPv6). If the ALEN value is wrong for the given ATYPE value, then
the provided address should be ignored.
The timestamp is a big-endian unsigned integer number of seconds
since the Unix epoch. Implementations MUST ignore unexpected bytes
at the end of the cell. Clients SHOULD send "0" as their timestamp, to
avoid fingerprinting.
Implementations MAY use the timestamp value to help decide if their
clocks are skewed. Initiators MAY use "other OR's address" to help
learn which address their connections may be originating from, if they do
not know it; and to learn whether the peer will treat the current
connection as canonical. Implementations SHOULD NOT trust these
values unconditionally, especially when they come from non-authorities,
since the other party can lie about the time or IP addresses it sees.
Initiators SHOULD use "this OR's address" to make sure
that they have connected to another OR at its canonical address.
(See 5.3.1 below.)
5. Circuit management
5.1. CREATE and CREATED cells
Users set up circuits incrementally, one hop at a time. To create a
new circuit, OPs send a CREATE/CREATE2 cell to the first node, with
the first half of an authenticated handshake; that node responds with
a CREATED/CREATED2 cell with the second half of the handshake. To
extend a circuit past the first hop, the OP sends an EXTEND/EXTEND2
relay cell (see section 5.1.2) which instructs the last node in the
circuit to send a CREATE/CREATE2 cell to extend the circuit.
There are two kinds of CREATE and CREATED cells: The older
"CREATE/CREATED" format, and the newer "CREATE2/CREATED2" format. The
newer format is extensible by design; the older one is not.
A CREATE2 cell contains:
HTYPE (Client Handshake Type) [2 bytes]
HLEN (Client Handshake Data Len) [2 bytes]
HDATA (Client Handshake Data) [HLEN bytes]
A CREATED2 cell contains:
HLEN (Server Handshake Data Len) [2 bytes]
HDATA (Server Handshake Data) [HLEN bytes]
Recognized HTYPEs (handshake types) are:
0x0000 TAP -- the original Tor handshake; see 5.1.3
0x0001 reserved
0x0002 ntor -- the ntor+curve25519+sha256 handshake; see 5.1.4
The format of a CREATE cell is one of the following:
HDATA (Client Handshake Data) [TAP_C_HANDSHAKE_LEN bytes]
or
HTAG (Client Handshake Type Tag) [16 bytes]
HDATA (Client Handshake Data) [TAP_C_HANDSHAKE_LEN-16 bytes]
The first format is equivalent to a CREATE2 cell with HTYPE of 'tap'
and length of TAP_C_HANDSHAKE_LEN. The second format is a way to
encapsulate new handshake types into the old CREATE cell format for
migration. See 5.1.2 below. Recognized HTAG values are:
ntor -- 'ntorNTORntorNTOR'
The format of a CREATED cell is:
HDATA (Server Handshake Data) [TAP_S_HANDSHAKE_LEN bytes]
(It's equivalent to a CREATED2 cell with length of TAP_S_HANDSHAKE_LEN.)
As usual with DH, x and y MUST be generated randomly.
In general, clients SHOULD use CREATE whenever they are using the TAP
handshake, and CREATE2 otherwise. Clients SHOULD NOT send the
second format of CREATE cells (the one with the handshake type tag)
to a server directly.
Servers always reply to a successful CREATE with a CREATED, and to a
successful CREATE2 with a CREATED2. On failure, a server sends a
DESTROY cell to tear down the circuit.
[CREATE2 is handled by Tor 0.2.4.7-alpha and later.]
5.1.1. Choosing circuit IDs in create cells
The CircID for a CREATE/CREATE2 cell is an arbitrarily chosen
nonzero integer, selected by the node (OP or OR) that sends the
CREATE/CREATE2 cell. In link protocol 3 or lower, CircIDs are 2
bytes long; in protocol 4 or higher, CircIDs are 4 bytes long.
To prevent CircID collisions, when one node sends a CREATE/CREATE2
cell to another, it chooses from only one half of the possible
values based on the ORs' public identity keys.
In link protocol version 3 or lower, if the sending node has a lower
key, it chooses a CircID with an MSB of 0; otherwise, it chooses a
CircID with an MSB of 1. (Public keys are compared numerically by
modulus.) With protocol version 3 or lower, a client with no public key
MAY choose any CircID it wishes, since clients never need to process a
CREATE/CREATE2 cell.
In link protocol version 4 or higher, whichever node initiated the
connection sets its MSB to 1, and whichever node didn't initiate the
connection sets its MSB to 0.
The CircID value 0 is specifically reserved for cells that do not
belong to any circuit: CircID 0 must not be used for circuits. No
other CircID value, including 0x8000 or 0x80000000, is reserved.
Existing Tor implementations choose their CircID values at random from
among the available unused values. To avoid distinguishability, new
implementations should do the same. Implementations MAY give up and stop
attempting to build new circuits on a channel, if a certain number of
randomly chosen CircID values are all in use (today's Tor stops after 64).
5.1.2. EXTEND and EXTENDED cells
To extend an existing circuit, the client sends an EXTEND or EXTEND2
relay cell to the last node in the circuit.
An EXTEND2 cell's relay payload contains:
NSPEC (Number of link specifiers) [1 byte]
NSPEC times:
LSTYPE (Link specifier type) [1 byte]
LSLEN (Link specifier length) [1 byte]
LSPEC (Link specifier) [LSLEN bytes]
HTYPE (Client Handshake Type) [2 bytes]
HLEN (Client Handshake Data Len) [2 bytes]
HDATA (Client Handshake Data) [HLEN bytes]
Link specifiers describe the next node in the circuit and how to
connect to it. Recognized specifiers are:
[00] TLS-over-TCP, IPv4 address
A four-byte IPv4 address plus two-byte ORPort
[01] TLS-over-TCP, IPv6 address
A sixteen-byte IPv6 address plus two-byte ORPort
[02] Legacy identity
A 20-byte SHA1 identity fingerprint. At most one may be listed.
[03] Ed25519 identity
A 32-byte Ed25519 identity fingerprint. At most one may
be listed.
Nodes MUST ignore unrecognized specifiers, and MUST accept multiple
instances of specifiers other than 'legacy identity' and
'Ed25519 identity'. (Nodes SHOULD reject link specifier lists
that include multiple instances of either one of those specifiers.)
For purposes of indistinguishability, implementations SHOULD send
these link specifiers, if using them, in this order: [00], [02], [03],
[01].
The relay payload for an EXTEND relay cell consists of:
Address [4 bytes]
Port [2 bytes]
Onion skin [TAP_C_HANDSHAKE_LEN bytes]
Identity fingerprint [HASH_LEN bytes]
The "legacy identity" and "identity fingerprint" fields are the
SHA1 hash of the PKCS#1 ASN1 encoding of the next onion router's
identity (signing) key. (See 0.3 above.) The "Ed25519 identity"
field is the Ed25519 identity key of the target node. Including
this key information allows the extending OR verify that it is
indeed connected to the correct target OR, and prevents certain
man-in-the-middle attacks.
Extending ORs MUST check _all_ provided identity keys (if they
recognize the format), and and MUST NOT extend the circuit if the
target OR did not prove its ownership of any such identity key.
If only one identity key is provided, but the extending OR knows
the other (from directory information), then the OR SHOULD also
enforce the key in the directory.
If an extending OR has a channel with a given Ed25519 ID and RSA
identity, and receives a request for that Ed25519 ID and a
different RSA identity, it SHOULD NOT attempt to make another
connection: it should just fail and DESTROY the circuit.
The client MAY include multiple IPv4 or IPv6 link specifiers in an
EXTEND cell; current OR implementations only consider the first
of each type.
After checking relay identities, extending ORs generate a
CREATE/CREATE2 cell from the contents of the EXTEND/EXTEND2 cell.
See section 5.3 for details.
The payload of an EXTENDED cell is the same as the payload of a
CREATED cell.
The payload of an EXTENDED2 cell is the same as the payload of a
CREATED2 cell.
[Support for EXTEND2/EXTENDED2 was added in Tor 0.2.4.8-alpha.]
Clients SHOULD use the EXTEND format whenever sending a TAP
handshake, and MUST use it whenever the EXTEND cell will be handled
by a node running a version of Tor too old to support EXTEND2. In
other cases, clients SHOULD use EXTEND2.
When generating an EXTEND2 cell, clients SHOULD include the target's
Ed25519 identity whenever the target has one, and whenever the
target supports LinkAuth subprotocol version "3". (See section 9.2.)
When encoding a non-TAP handshake in an EXTEND cell, clients SHOULD
use the format with 'client handshake type tag'.
5.1.3. The "TAP" handshake
This handshake uses Diffie-Hellman in Z_p and RSA to compute a set of
shared keys which the client knows are shared only with a particular
server, and the server knows are shared with whomever sent the
original handshake (or with nobody at all). It's not very fast and
not very good. (See Goldberg's "On the Security of the Tor
Authentication Protocol".)
Define TAP_C_HANDSHAKE_LEN as DH_LEN+KEY_LEN+KP_PAD_LEN.
Define TAP_S_HANDSHAKE_LEN as DH_LEN+HASH_LEN.
The payload for a CREATE cell is an 'onion skin', which consists of
the first step of the DH handshake data (also known as g^x). This
value is encrypted using the "legacy hybrid encryption" algorithm
(see 0.4 above) to the server's onion key, giving a client handshake:
KP-encrypted:
Padding [KP_PAD_LEN bytes]
Symmetric key [KEY_LEN bytes]
First part of g^x [KP_ENC_LEN-KP_PAD_LEN-KEY_LEN bytes]
Symmetrically encrypted:
Second part of g^x [DH_LEN-(KP_ENC_LEN-KP_PAD_LEN-KEY_LEN)
bytes]
The payload for a CREATED cell, or the relay payload for an
EXTENDED cell, contains:
DH data (g^y) [DH_LEN bytes]
Derivative key data (KH) [HASH_LEN bytes] <see 5.2 below>
Once the handshake between the OP and an OR is completed, both can
now calculate g^xy with ordinary DH. Before computing g^xy, both parties
MUST verify that the received g^x or g^y value is not degenerate;
that is, it must be strictly greater than 1 and strictly less than p-1
where p is the DH modulus. Implementations MUST NOT complete a handshake
with degenerate keys. Implementations MUST NOT discard other "weak"
g^x values.
(Discarding degenerate keys is critical for security; if bad keys
are not discarded, an attacker can substitute the OR's CREATED
cell's g^y with 0 or 1, thus creating a known g^xy and impersonating
the OR. Discarding other keys may allow attacks to learn bits of
the private key.)
Once both parties have g^xy, they derive their shared circuit keys
and 'derivative key data' value via the KDF-TOR function in 5.2.1.
5.1.4. The "ntor" handshake
This handshake uses a set of DH handshakes to compute a set of
shared keys which the client knows are shared only with a particular
server, and the server knows are shared with whomever sent the
original handshake (or with nobody at all). Here we use the
"curve25519" group and representation as specified in "Curve25519:
new Diffie-Hellman speed records" by D. J. Bernstein.
[The ntor handshake was added in Tor 0.2.4.8-alpha.]
In this section, define:
H(x,t) as HMAC_SHA256 with message x and key t.
H_LENGTH = 32.
ID_LENGTH = 20.
G_LENGTH = 32
PROTOID = "ntor-curve25519-sha256-1"
t_mac = PROTOID | ":mac"
t_key = PROTOID | ":key_extract"
t_verify = PROTOID | ":verify"
G = The preferred base point for curve25519 ([9])
KEYGEN() = The curve25519 key generation algorithm, returning
a private/public keypair.
m_expand = PROTOID | ":key_expand"
KEYID(A) = A
EXP(a, b) = The ECDH algorithm for establishing a shared secret.
To perform the handshake, the client needs to know an identity key
digest for the server, and an ntor onion key (a curve25519 public
key) for that server. Call the ntor onion key "B". The client
generates a temporary keypair:
x,X = KEYGEN()
and generates a client-side handshake with contents:
NODEID Server identity digest [ID_LENGTH bytes]
KEYID KEYID(B) [H_LENGTH bytes]
CLIENT_KP X [G_LENGTH bytes]
The server generates a keypair of y,Y = KEYGEN(), and uses its ntor
private key 'b' to compute:
secret_input = EXP(X,y) | EXP(X,b) | ID | B | X | Y | PROTOID
KEY_SEED = H(secret_input, t_key)
verify = H(secret_input, t_verify)
auth_input = verify | ID | B | Y | X | PROTOID | "Server"
The server's handshake reply is:
SERVER_KP Y [G_LENGTH bytes]
AUTH H(auth_input, t_mac) [H_LENGTH bytes]
The client then checks Y is in G^* [see NOTE below], and computes
secret_input = EXP(Y,x) | EXP(B,x) | ID | B | X | Y | PROTOID
KEY_SEED = H(secret_input, t_key)
verify = H(secret_input, t_verify)
auth_input = verify | ID | B | Y | X | PROTOID | "Server"
The client verifies that AUTH == H(auth_input, t_mac).
Both parties check that none of the EXP() operations produced the
point at infinity. [NOTE: This is an adequate replacement for
checking Y for group membership, if the group is curve25519.]
Both parties now have a shared value for KEY_SEED. They expand this
into the keys needed for the Tor relay protocol, using the KDF
described in 5.2.2 and the tag m_expand.
5.1.5. CREATE_FAST/CREATED_FAST cells
When initializing the first hop of a circuit, the OP has already
established the OR's identity and negotiated a secret key using TLS.
Because of this, it is not always necessary for the OP to perform the
public key operations to create a circuit. In this case, the
OP MAY send a CREATE_FAST cell instead of a CREATE cell for the first
hop only. The OR responds with a CREATED_FAST cell, and the circuit is
created.
A CREATE_FAST cell contains:
Key material (X) [HASH_LEN bytes]
A CREATED_FAST cell contains:
Key material (Y) [HASH_LEN bytes]
Derivative key data [HASH_LEN bytes] (See 5.2.1 below)
The values of X and Y must be generated randomly.
Once both parties have X and Y, they derive their shared circuit keys
and 'derivative key data' value via the KDF-TOR function in 5.2.1.
The CREATE_FAST handshake is currently deprecated whenever it is not
necessary; the migration is controlled by the "usecreatefast"
networkstatus parameter as described in dir-spec.txt.
[Tor 0.3.1.1-alpha and later disable CREATE_FAST by default.]
5.2. Setting circuit keys
5.2.1. KDF-TOR
This key derivation function is used by the TAP and CREATE_FAST
handshakes, and in the current hidden service protocol. It shouldn't
be used for new functionality.
If the TAP handshake is used to extend a circuit, both parties
base their key material on K0=g^xy, represented as a big-endian unsigned
integer.
If CREATE_FAST is used, both parties base their key material on
K0=X|Y.
From the base key material K0, they compute KEY_LEN*2+HASH_LEN*3 bytes of
derivative key data as
K = H(K0 | [00]) | H(K0 | [01]) | H(K0 | [02]) | ...
The first HASH_LEN bytes of K form KH; the next HASH_LEN form the forward
digest Df; the next HASH_LEN 41-60 form the backward digest Db; the next
KEY_LEN 61-76 form Kf, and the final KEY_LEN form Kb. Excess bytes from K
are discarded.
KH is used in the handshake response to demonstrate knowledge of the
computed shared key. Df is used to seed the integrity-checking hash
for the stream of data going from the OP to the OR, and Db seeds the
integrity-checking hash for the data stream from the OR to the OP. 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.
5.2.2. KDF-RFC5869
For newer KDF needs, Tor uses the key derivation function HKDF from
RFC5869, instantiated with SHA256. (This is due to a construction
from Krawczyk.) The generated key material is:
K = K_1 | K_2 | K_3 | ...
Where H(x,t) is HMAC_SHA256 with value x and key t
and K_1 = H(m_expand | INT8(1) , KEY_SEED )
and K_(i+1) = H(K_i | m_expand | INT8(i+1) , KEY_SEED )
and m_expand is an arbitrarily chosen value,
and INT8(i) is a octet with the value "i".
In RFC5869's vocabulary, this is HKDF-SHA256 with info == m_expand,
salt == t_key, and IKM == secret_input.
When used in the ntor handshake, the first HASH_LEN bytes form the
forward digest Df; the next HASH_LEN form the backward digest Db; the
next KEY_LEN form Kf, the next KEY_LEN form Kb, and the final
DIGEST_LEN bytes are taken as a nonce to use in the place of KH in the
hidden service protocol. Excess bytes from K are discarded.
5.3. Creating circuits
When creating a circuit through the network, the circuit creator
(OP) performs the following steps:
1. Choose an onion router as an end node (R_N):
* N MAY be 1 for non-anonymous directory mirror, introduction point,
or service rendezvous connections.
* N SHOULD be 3 or more for anonymous connections.
Some end nodes accept streams (see 6.1), others are introduction
or rendezvous points (see rend-spec-{v2,v3}.txt).
2. Choose a chain of (N-1) 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/CREATE2 cell along
the connection, to be received by the first onion router.
5. Wait until a CREATED/CREATED2 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, encrypted to R_M's public onion key.
2. Send the onion skin in a relay EXTEND/EXTEND2 cell along
the circuit (see sections 5.1.2 and 5.5).
3. When a relay EXTENDED/EXTENDED2 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.
When an onion router receives an EXTEND2 relay cell, it sends a CREATE2
cell to the next onion router, with the enclosed HLEN, HTYPE, and HDATA
as its payload. The initiating onion router chooses some circID not yet
used on the connection between the two onion routers. (But see section
5.1.1 above, concerning choosing circIDs.)
As special cases, if the EXTEND/EXTEND2 cell includes a legacy identity, or
identity fingerprint of all zeroes, or asks to extend back to the relay
that sent the extend cell, the circuit will fail and be torn down.
Ed25519 identity keys are not required in EXTEND2 cells, so all zero
keys SHOULD be accepted. If the extending relay knows the ed25519 key from
the consensus, it SHOULD also check that key. (See section 5.1.2.)
If an EXTEND2 cell contains the ed25519 key of the relay that sent the
extend cell, the circuit will fail and be torn down.
When an onion router receives a CREATE/CREATE2 cell, if it already has a
circuit on the given connection with the given circID, it drops the
cell. Otherwise, after receiving the CREATE/CREATE2 cell, it completes
the specified handshake, and replies with a CREATED/CREATED2 cell.
Upon receiving a CREATED/CREATED2 cell, an onion router packs it payload
into an EXTENDED/EXTENDED2 relay cell (see section 5.1.2), and sends
that cell up the circuit. Upon receiving the EXTENDED/EXTENDED2 relay
cell, the OP can retrieve the handshake material.
(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.)
5.3.1. Canonical connections
It is possible for an attacker to launch a man-in-the-middle attack
against a connection by telling OR Alice to extend to OR Bob at some
address X controlled by the attacker. The attacker cannot read the
encrypted traffic, but the attacker is now in a position to count all
bytes sent between Alice and Bob (assuming Alice was not already
connected to Bob.)
To prevent this, when an OR gets an extend request, it SHOULD use an
existing OR connection if the ID matches, and ANY of the following
conditions hold:
- The IP matches the requested IP.
- The OR knows that the IP of the connection it's using is canonical
because it was listed in the NETINFO cell.
ORs SHOULD NOT check the IPs that are listed in the server descriptor.
Trusting server IPs makes it easier to covertly impersonate a relay, after
stealing its keys.
5.4. 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.
ORs SHOULD also tear down circuits which attempt to create:
* streams with RELAY_BEGIN, or
* rendezvous points with ESTABLISH_RENDEZVOUS,
ending at the first hop. Letting Tor be used as a single hop proxy makes
exit and rendezvous nodes a more attractive target for compromise.
ORs MAY use multiple methods to check if they are the first hop:
* If an OR sees a circuit created with CREATE_FAST, the OR is sure to be
the first hop of a circuit.
* If an OR is the responder, and the initiator:
* did not authenticate the link, or
* authenticated with a key that is not in the consensus,
then the OR is probably the first hop of a circuit (or the second hop of
a circuit via a bridge relay).
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 6.1).
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 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.
[Note: If an OR receives a TRUNCATE cell and it has any RELAY cells
still queued on the circuit for the next node it will drop them
without sending them. This is not considered conformant behavior,
but it probably won't get fixed until a later version of Tor. Thus,
clients SHOULD NOT send a TRUNCATE cell to a node running any current
version of Tor if a) they have sent relay cells through that node,
and b) they aren't sure whether those cells have been sent on yet.]
When an unrecoverable error occurs along one a circuit, the nodes
must report it as follows:
* If possible, send a DESTROY cell to ORs _away_ from the client.
* If possible, send *either* a DESTROY cell towards the client, or
a RELAY_TRUNCATED cell towards the client.
Current versions of Tor do not reuse truncated RELAY_TRUNCATED
circuits: An OP, upon receiving a RELAY_TRUNCATED, will send
forward a DESTROY cell in order to entirely tear down the circuit.
Because of this, we recommend that relays should send DESTROY
towards the client, not RELAY_TRUNCATED.
NOTE:
In tor versions before 0.4.5.13, 0.4.6.11 and 0.4.7.9, relays would
handle an inbound DESTROY by sending the client a RELAY_TRUNCATED
message. Beginning with those versions, relays now propagate
DESTROY cells in either direction, in order to tell every
intermediary ORs to stop queuing data on the circuit. The earlier
behavior created queuing pressure on the intermediary ORs.
The payload of a DESTROY and RELAY_TRUNCATED cell contains a single
octet, describing the reason that the circuit was
closed. RELAY_TRUNCATED cells, and DESTROY cells sent _towards the
client, should contain the actual reason from the list of error codes
below. Reasons in DESTROY cell SHOULD NOT be propagated downward or
upward, due to potential side channel risk: An OR receiving a DESTROY
command should use the DESTROYED reason for its next cell. An OP
should always use the NONE reason for its own DESTROY cells.
The error codes are:
0 -- NONE (No reason given.)
1 -- PROTOCOL (Tor protocol violation.)
2 -- INTERNAL (Internal error.)
3 -- REQUESTED (A client sent a TRUNCATE command.)
4 -- HIBERNATING (Not currently operating; trying to save bandwidth.)
5 -- RESOURCELIMIT (Out of memory, sockets, or circuit IDs.)
6 -- CONNECTFAILED (Unable to reach relay.)
7 -- OR_IDENTITY (Connected to relay, but its OR identity was not
as expected.)
8 -- CHANNEL_CLOSED (The OR connection that was carrying this circuit
died.)
9 -- FINISHED (The circuit has expired for being dirty or old.)
10 -- TIMEOUT (Circuit construction took too long)
11 -- DESTROYED (The circuit was destroyed w/o client TRUNCATE)
12 -- NOSUCHSERVICE (Request for unknown hidden service)
5.5. Routing relay cells
5.5.1. Circuit ID Checks
When a node wants to send a RELAY or RELAY_EARLY cell, it checks the cell's
circID and determines whether the corresponding circuit along that
connection is still open. If not, the node drops the cell.
When a node receives a RELAY or RELAY_EARLY cell, it checks the cell's
circID and determines whether it has a corresponding circuit along
that connection. If not, the node drops the cell.
5.5.2. Forward Direction
The forward direction is the direction that CREATE/CREATE2 cells
are sent.
5.5.2.1. Routing from the Origin
When a relay cell is sent from an OP, the OP encrypts the payload
with the stream cipher as follows:
OP sends relay cell:
For I=N...1, where N is the destination node:
Encrypt with Kf_I.
Transmit the encrypted cell to node 1.
5.5.2.2. Relaying Forward at Onion Routers
When a forward relay cell is received by an OR, it decrypts the payload
with the stream cipher, as follows:
'Forward' relay cell:
Use Kf as key; decrypt.
The OR then decides whether it recognizes the relay cell, by
inspecting the payload as described in section 6.1 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.
For more information, see section 6 below.
5.5.3. Backward Direction
The backward direction is the opposite direction from
CREATE/CREATE2 cells.
5.5.3.1. Relaying Backward at Onion Routers
When a backward relay cell is received by an OR, it encrypts the payload
with the stream cipher, as follows:
'Backward' relay cell:
Use Kb as key; encrypt.
5.5.3. Routing to the Origin
When a relay cell arrives at an OP, the OP decrypts the payload
with the stream cipher as follows:
OP receives relay cell from node 1:
For I=1...N, where N is the final node on the circuit:
Decrypt with Kb_I.
If the payload is recognized (see section 6.1), then:
The sending node is I.
Stop and process the payload.
5.6. Handling relay_early cells
A RELAY_EARLY cell is designed to limit the length any circuit can reach.
When an OR receives a RELAY_EARLY cell, and the next node in the circuit
is speaking v2 of the link protocol or later, the OR relays the cell as a
RELAY_EARLY cell. Otherwise, older Tors will relay it as a RELAY cell.
If a node ever receives more than 8 RELAY_EARLY cells on a given
outbound circuit, it SHOULD close the circuit. If it receives any
inbound RELAY_EARLY cells, it MUST close the circuit immediately.
When speaking v2 of the link protocol or later, clients MUST only send
EXTEND/EXTEND2 cells inside RELAY_EARLY cells. Clients SHOULD send the first
~8 RELAY cells that are not targeted at the first hop of any circuit as
RELAY_EARLY cells too, in order to partially conceal the circuit length.
[Starting with Tor 0.2.3.11-alpha, relays should reject any
EXTEND/EXTEND2 cell not received in a RELAY_EARLY cell.]
6. Application connections and stream management
6.1. Relay cells
Within a circuit, the OP and the end 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.
End nodes that accept streams may be:
* exit relays (RELAY_BEGIN, anonymous),
* directory servers (RELAY_BEGIN_DIR, anonymous or non-anonymous),
* onion services (RELAY_BEGIN, anonymous via a rendezvous point).
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 [Length bytes]
Padding [PAYLOAD_LEN - 11 - Length bytes]
The relay commands are:
1 -- RELAY_BEGIN [forward]
2 -- RELAY_DATA [forward or backward]
3 -- RELAY_END [forward or backward]
4 -- RELAY_CONNECTED [backward]
5 -- RELAY_SENDME [forward or backward] [sometimes control]
6 -- RELAY_EXTEND [forward] [control]
7 -- RELAY_EXTENDED [backward] [control]
8 -- RELAY_TRUNCATE [forward] [control]
9 -- RELAY_TRUNCATED [backward] [control]
10 -- RELAY_DROP [forward or backward] [control]
11 -- RELAY_RESOLVE [forward]
12 -- RELAY_RESOLVED [backward]
13 -- RELAY_BEGIN_DIR [forward]
14 -- RELAY_EXTEND2 [forward] [control]
15 -- RELAY_EXTENDED2 [backward] [control]
16..18 -- Reserved for UDP; Not yet in use, see prop339.
19..22 -- Reserved for Conflux, see prop329.
32..40 -- Used for hidden services; see rend-spec-{v2,v3}.txt.
41..42 -- Used for circuit padding; see Section 3 of padding-spec.txt.
Used for flow control; see Section 4 of prop324.
43 -- XON [forward or backward]
44 -- XOFF [forward or backward]
Commands labelled as "forward" must only be sent by the originator
of the circuit. Commands labelled as "backward" must only be sent by
other nodes in the circuit back to the originator. Commands marked
as either can be sent either by the originator or other nodes.
The 'recognized' field is used as a simple indication that the cell
is still encrypted. It is an optimization to avoid calculating
expensive digests for every cell. When sending cells, the unencrypted
'recognized' MUST be set to zero.
When receiving and decrypting cells the 'recognized' will always be
zero if we're the endpoint that the cell is destined for. For cells
that we should relay, the 'recognized' field will usually be nonzero,
but will accidentally be zero with P=2^-16.
When handling a relay cell, if the 'recognized' in field in a
decrypted relay payload is zero, the 'digest' field is computed as
the first four bytes of the running digest of all the bytes that have
been destined for this hop of the circuit or originated from this hop
of the circuit, seeded from Df or Db respectively (obtained in
section 5.2 above), and including this RELAY cell's entire payload
(taken with the digest field set to zero). Note that these digests
_do_ include the padding bytes at the end of the cell, not only those up
to "Len". If the digest is correct, the cell is considered "recognized"
for the purposes of decryption (see section 5.5 above).
(The digest does not include any bytes from relay cells that do
not start or end at this hop of the circuit. That is, it does not
include forwarded data. Therefore if 'recognized' is zero but the
digest does not match, the running digest at that node should
not be updated, and the cell should be forwarded on.)
All RELAY cells pertaining to the same tunneled stream have the same
stream ID. StreamIDs are chosen arbitrarily by the OP. No stream
may have a StreamID of zero. Rather, RELAY cells that affect the
entire circuit rather than a particular stream use a StreamID of zero
-- they are marked in the table above as "[control]" style
cells. (Sendme cells are marked as "sometimes control" because they
can include a StreamID or not depending on their purpose -- see
Section 7.)
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 unencrypted payload is padded with padding bytes. Implementations
handle padding bytes of unencrypted relay cells as they do padding
bytes for other cell types; see Section 3.
The 'Padding' field is used to make relay cell contents unpredictable, to
avoid certain attacks (see proposal 289 for rationale). Implementations
SHOULD fill this field with four zero-valued bytes, followed by as many
random bytes as will fit. (If there are fewer than 4 bytes for padding,
then they should all be filled with zero.
Implementations MUST NOT rely on the contents of the 'Padding' field.
If the RELAY cell is recognized but the relay command is not
understood, the cell must be dropped and ignored. Its contents
still count with respect to the digests and flow control windows, though.
6.2. 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:
ADDRPORT [nul-terminated string]
FLAGS [4 bytes]
ADDRPORT is made of ADDRESS | ':' | PORT | [00]
where ADDRESS can be a DNS hostname, or an IPv4 address in
dotted-quad format, or an IPv6 address surrounded by square brackets;
and where PORT is a decimal integer between 1 and 65535, inclusive.
The ADDRPORT string SHOULD be sent in lower case, to avoid
fingerprinting. Implementations MUST accept strings in any case.
The FLAGS value has one or more of the following bits set, where
"bit 1" is the LSB of the 32-bit value, and "bit 32" is the MSB.
(Remember that all values in Tor are big-endian (see 0.1.1 above), so
the MSB of a 4-byte value is the MSB of the first byte, and the LSB
of a 4-byte value is the LSB of its last byte.)
bit meaning
1 -- IPv6 okay. We support learning about IPv6 addresses and
connecting to IPv6 addresses.
2 -- IPv4 not okay. We don't want to learn about IPv4 addresses
or connect to them.
3 -- IPv6 preferred. If there are both IPv4 and IPv6 addresses,
we want to connect to the IPv6 one. (By default, we connect
to the IPv4 address.)
4..32 -- Reserved. Current clients MUST NOT set these. Servers
MUST ignore them.
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 6.3 below.)
Otherwise, the exit node replies with a RELAY_CONNECTED cell, whose
payload is in one of the following formats:
The IPv4 address to which the connection was made [4 octets]
A number of seconds (TTL) for which the address may be cached [4 octets]
or
Four zero-valued octets [4 octets]
An address type (6) [1 octet]
The IPv6 address to which the connection was made [16 octets]
A number of seconds (TTL) for which the address may be cached [4 octets]
[Tor exit nodes before 0.1.2.0 set the TTL field to a fixed value. Later
versions set the TTL to the last value seen from a DNS server, and expire
their own cached entries after a fixed interval. This prevents certain
attacks.]
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.
If the exit node does not support optimistic data (i.e. its
version number is before 0.2.3.1-alpha), then the OP MUST wait
for a RELAY_CONNECTED cell before sending any data. If the exit
node supports optimistic data (i.e. its version number is
0.2.3.1-alpha or later), then the OP MAY send RELAY_DATA cells
immediately after sending the RELAY_BEGIN cell (and before
receiving either a RELAY_CONNECTED or RELAY_END cell).
RELAY_DATA cells sent to unrecognized streams are dropped. If
the exit node supports optimistic data, then RELAY_DATA cells it
receives on streams which have seen RELAY_BEGIN but have not yet
been replied to with a RELAY_CONNECTED or RELAY_END are queued.
If the stream creation succeeds with a RELAY_CONNECTED, the queue
is processed immediately afterwards; if the stream creation fails
with a RELAY_END, the contents of the queue are deleted.
Relay RELAY_DROP cells are long-range dummies; upon receiving such
a cell, the OR or OP must drop it.
6.2.1. Opening a directory stream
If a Tor relay is a directory server, it should respond to a
RELAY_BEGIN_DIR cell as if it had received a BEGIN cell requesting a
connection to its directory port. RELAY_BEGIN_DIR cells ignore exit
policy, since the stream is local to the Tor process.
Directory servers may be:
* authoritative directories (RELAY_BEGIN_DIR, usually non-anonymous),
* bridge authoritative directories (RELAY_BEGIN_DIR, anonymous),
* directory mirrors (RELAY_BEGIN_DIR, usually non-anonymous),
* onion service directories (RELAY_BEGIN_DIR, anonymous).
If the Tor relay is not running a directory service, it should respond
with a REASON_NOTDIRECTORY RELAY_END cell.
Clients MUST generate an all-zero payload for RELAY_BEGIN_DIR cells,
and relays MUST ignore the payload.
In response to a RELAY_BEGIN_DIR cell, relays respond either with a
RELAY_CONNECTED cell on success, or a RELAY_END cell on failure. They
MUST send a RELAY_CONNECTED cell all-zero payload, and clients MUST ignore
the payload.
[RELAY_BEGIN_DIR was not supported before Tor 0.1.2.2-alpha; clients
SHOULD NOT send it to routers running earlier versions of Tor.]
6.3. 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. For some reasons, it contains
additional 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_CONNECTREFUSED (remote host refused connection) [*]
4 -- REASON_EXITPOLICY (OR refuses to connect to host or port)
5 -- REASON_DESTROY (Circuit is being destroyed)
6 -- REASON_DONE (Anonymized TCP connection was closed)
7 -- REASON_TIMEOUT (Connection timed out, or OR timed out
while connecting)
8 -- REASON_NOROUTE (Routing error while attempting to
contact destination)
9 -- REASON_HIBERNATING (OR is temporarily hibernating)
10 -- REASON_INTERNAL (Internal error at the OR)
11 -- REASON_RESOURCELIMIT (OR has no resources to fulfill request)
12 -- REASON_CONNRESET (Connection was unexpectedly reset)
13 -- REASON_TORPROTOCOL (Sent when closing connection because of
Tor protocol violations.)
14 -- REASON_NOTDIRECTORY (Client sent RELAY_BEGIN_DIR to a
non-directory relay.)
[*] Older versions of Tor also send this reason when connections are
reset.
OPs and ORs MUST accept reasons not on the above list, since future
versions of Tor may provide more fine-grained reasons.
For most reasons, the format of RELAY_END is:
Reason [1 byte]
For REASON_EXITPOLICY, the format of RELAY_END is:
Reason [1 byte]
IPv4 or IPv6 address [4 bytes or 16 bytes]
TTL [4 bytes]
(If the TTL is absent, it should be treated as if it were 0xffffffff.
If the address is absent or is the wrong length, the RELAY_END message
should be processed anyway.)
Tors SHOULD NOT send any reason except REASON_MISC for a stream that they
have originated.
Implementations SHOULD accept empty RELAY_END messages, and treat them
as if they specified REASON_MISC.
Upon receiving a RELAY_END cell, the recipient may be sure that no further
cells will arrive on that stream, and can treat such cells as a protocol
violation.
After sending a RELAY_END cell, the sender needs to give the recipient
time to receive that cell. In the meantime, the sender SHOULD remember
how many cells of which types (CONNECTED, SENDME, DATA) that it would have
accepted on that stream, and SHOULD kill the circuit if it receives more
than permitted.
--- [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 (or onion service) 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.
6.4. Remote hostname lookup
To find the address associated with a hostname, the OP sends a
RELAY_RESOLVE cell containing the hostname to be resolved with a NUL
terminating byte. (For a reverse lookup, the OP sends a RELAY_RESOLVE
cell containing an in-addr.arpa address.) The OR replies with a
RELAY_RESOLVED cell containing any number of answers. Each answer is
of the form:
Type (1 octet)
Length (1 octet)
Value (variable-width)
TTL (4 octets)
"Length" is the length of the Value field.
"Type" is one of:
0x00 -- Hostname
0x04 -- IPv4 address
0x06 -- IPv6 address
0xF0 -- Error, transient
0xF1 -- Error, nontransient
If any answer has a type of 'Error', then no other answer may be
given.
The 'Value' field encodes the answer:
IP addresses are given in network order.
Hostnames are given in standard DNS order ("www.example.com")
and not NUL-terminated.
The content of Errors is currently ignored. Relays currently
set it to the string "Error resolving hostname" with no
terminating NUL. Implementations MUST ignore this value.
For backward compatibility, if there are any IPv4 answers, one of those
must be given as the first answer.
The RELAY_RESOLVE cell must use a nonzero, distinct streamID; the
corresponding RELAY_RESOLVED cell must use the same streamID. No stream
is actually created by the OR when resolving the name.
7. Flow control
7.1. Link throttling
Each client or relay 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.
The mainline Tor implementation uses token buckets (one for reads,
one for writes) for the rate limiting.
Since 0.2.0.x, Tor has let the user specify an additional pair of
token buckets for "relayed" traffic, so people can deploy a Tor relay
with strict rate limiting, but also use the same Tor as a client. To
avoid partitioning concerns we combine both classes of traffic over a
given OR connection, and keep track of the last time we read or wrote
a high-priority (non-relayed) cell. If it's been less than N seconds
(currently N=30), we give the whole connection high priority, else we
give the whole connection low priority. We also give low priority
to reads and writes for connections that are serving directory
information. See proposal 111 for details.
7.2. Link padding
Link padding can be created by sending PADDING or VPADDING cells
along the connection; relay cells of type "DROP" can be used for
long-range padding. The payloads of PADDING, VPADDING, or DROP
cells are filled with padding bytes. See Section 3.
If the link protocol is version 5 or higher, link level padding is
enabled as per padding-spec.txt. On these connections, clients may
negotiate the use of padding with a CELL_PADDING_NEGOTIATE command
whose format is as follows:
Version [1 byte]
Command [1 byte]
ito_low_ms [2 bytes]
ito_high_ms [2 bytes]
Currently, only version 0 of this cell is defined. In it, the command
field is either 1 (stop padding) or 2 (start padding). For the start
padding command, a pair of timeout values specifying a low and a high
range bounds for randomized padding timeouts may be specified as unsigned
integer values in milliseconds. The ito_low_ms field should not be lower
than the current consensus parameter value for nf_ito_low (default:
1500). The ito_high_ms field should not be lower than ito_low_ms.
(If any party receives an out-of-range value, they clamp it so
that it is in-range.)
For more details on padding behavior, see padding-spec.txt.
7.3. 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
originate or willing to consume.
These two windows are respectively named: the package window (packaged for
transmission) and the deliver window (delivered for local streams).
Because of our leaky-pipe topology, every relay on the circuit has a pair
of windows, and the OP has a pair of windows for every relay on the
circuit. These windows do not apply to relayed cells, however, and a relay
that is never used for streams will never decrement its window or cause the
client to decrement a window.
Each 'window' value is initially set based on the consensus parameter
'circwindow' in the directory (see dir-spec.txt), or to 1000 data cells if
no 'circwindow' value is given. In each direction, cells that are not
RELAY_DATA cells do not affect the window.
An OR or OP (depending on the stream direction) sends a RELAY_SENDME cell
to indicate that it is willing to receive more cells when its deliver
window goes down below a full increment (100). For example, if the window
started at 1000, it should send a RELAY_SENDME when it reaches 900.
When an OR or OP receives a RELAY_SENDME, it increments its package window
by a value of 100 (circuit window increment) and proceeds to sending the
remaining RELAY_DATA cells.
If a package 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.
If a deliver window goes below 0, the circuit should be torn down.
Starting with tor-0.4.1.1-alpha, authenticated SENDMEs are supported
(version 1, see below). This means that both the OR and OP need to remember
the rolling digest of the cell that precedes (triggers) a RELAY_SENDME.
This can be known if the package window gets to a multiple of the circuit
window increment (100).
When the RELAY_SENDME version 1 arrives, it will contain a digest that MUST
match the one remembered. This represents a proof that the end point of the
circuit saw the sent cells. On failure to match, the circuit should be torn
down.
To ensure unpredictability, random bytes should be added to at least one
RELAY_DATA cell within one increment window. In other word, every 100 cells
(increment), random bytes should be introduced in at least one cell.
7.3.1. SENDME Cell Format
A circuit-level RELAY_SENDME cell always has its StreamID=0.
An OR or OP must obey these two consensus parameters in order to know which
version to emit and accept.
'sendme_emit_min_version': Minimum version to emit.
'sendme_accept_min_version': Minimum version to accept.
If a RELAY_SENDME version is received that is below the minimum accepted
version, the circuit should be closed.
The RELAY_SENDME payload contains the following:
VERSION [1 byte]
DATA_LEN [2 bytes]
DATA [DATA_LEN bytes]
The VERSION tells us what is expected in the DATA section of length
DATA_LEN and how to handle it. The recognized values are:
0x00: The rest of the payload should be ignored.
0x01: Authenticated SENDME. The DATA section MUST contain:
DIGEST [20 bytes]
If the DATA_LEN value is less than 20 bytes, the cell should be
dropped and the circuit closed. If the value is more than 20 bytes,
then the first 20 bytes should be read to get the DIGEST value.
The DIGEST is the rolling digest value from the RELAY_DATA cell that
immediately preceded (triggered) this RELAY_SENDME. This value is
matched on the other side from the previous cell sent that the OR/OP
must remember.
If the VERSION is unrecognized or below the minimum accepted version (taken
from the consensus), the circuit should be torn down.
7.4. 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.
Stream-level RELAY_SENDME cells are distinguished by having nonzero
StreamID. They are still empty; the body still SHOULD be ignored.
8. Handling resource exhaustion
8.1. Memory exhaustion.
(See also dos-spec.md.)
If RAM becomes low, an OR should begin destroying circuits until
more memory is free again. We recommend the following algorithm:
- Set a threshold amount of RAM to recover at 10% of the total RAM.
- Sort the circuits by their 'staleness', defined as the age of the
oldest data queued on the circuit. This data can be:
* Bytes that are waiting to flush to or from a stream on that
circuit.
* Bytes that are waiting to flush from a connection created with
BEGIN_DIR.
* Cells that are waiting to flush or be processed.
- While we have not yet recovered enough RAM:
* Free all memory held by the most stale circuit, and send DESTROY
cells in both directions on that circuit. Count the amount of
memory we recovered towards the total.
9. Subprotocol versioning
This section specifies the Tor subprotocol versioning. They are broken down
into different types with their current version numbers. Any new version
number should be added to this section.
The dir-spec.txt details how those versions are encoded. See the
"proto"/"pr" line in a descriptor and the "recommended-relay-protocols",
"required-relay-protocols", "recommended-client-protocols" and
"required-client-protocols" lines in the vote/consensus format.
Here are the rules a relay and client should follow when encountering a
protocol list in the consensus:
- When a relay lacks a protocol listed in recommended-relay-protocols,
it should warn its operator that the relay is obsolete.
- When a relay lacks a protocol listed in required-relay-protocols, it
should warn its operator as above. If the consensus is newer than the
date when the software was released or scheduled for release, it must
not attempt to join the network.
- When a client lacks a protocol listed in recommended-client-protocols,
it should warn the user that the client is obsolete.
- When a client lacks a protocol listed in required-client-protocols,
it should warn the user as above. If the consensus is newer than the
date when the software was released, it must not connect to the
network. This implements a "safe forward shutdown" mechanism for
zombie clients.
- If a client or relay has a cached consensus telling it that a given
protocol is required, and it does not implement that protocol, it
SHOULD NOT try to fetch a newer consensus.
Software release dates SHOULD be automatically updated as part of the
release process, to prevent forgetting to move them forward. Software
release dates MAY be manually adjusted by maintainers if necessary.
Starting in version 0.2.9.4-alpha, the initial required protocols for
clients that we will Recommend and Require are:
Cons=1-2 Desc=1-2 DirCache=1 HSDir=1 HSIntro=3 HSRend=1 Link=4
LinkAuth=1 Microdesc=1-2 Relay=2
For relays we will Require:
Cons=1 Desc=1 DirCache=1 HSDir=1 HSIntro=3 HSRend=1 Link=3-4
LinkAuth=1 Microdesc=1 Relay=1-2
For relays, we will additionally Recommend all protocols which we
recommend for clients.
9.1. "Link"
The "link" protocols are those used by clients and relays to initiate and
receive OR connections and to handle cells on OR connections. The "link"
protocol versions correspond 1:1 to those versions.
Two Tor instances can make a connection to each other only if they have at
least one link protocol in common.
The current "link" versions are: "1" through "5". See section 4.1 for more
information. All current Tor versions support "1-3"; versions from
0.2.4.11-alpha and on support "1-4"; versions from 0.3.1.1-alpha and on
support "1-5". Eventually we will drop "1" and "2".
9.2. "LinkAuth"
LinkAuth protocols correspond to varieties of Authenticate cells used for
the v3+ link protocols.
Current versions are:
"1" is the RSA link authentication described in section 4.4.1 above.
"2" is unused, and reserved by proposal 244.
"3" is the ed25519 link authentication described in 4.4.2 above.
9.3. "Relay"
The "relay" protocols are those used to handle CREATE/CREATE2
cells, and those that handle the various RELAY cell types received
after a CREATE/CREATE2 cell. (Except, relay cells used to manage
introduction and rendezvous points are managed with the "HSIntro"
and "HSRend" protocols respectively.)
Current versions are:
"1" -- supports the TAP key exchange, with all features in Tor 0.2.3.
Support for CREATE and CREATED and CREATE_FAST and CREATED_FAST
and EXTEND and EXTENDED.
"2" -- supports the ntor key exchange, and all features in Tor
0.2.4.19. Includes support for CREATE2 and CREATED2 and
EXTEND2 and EXTENDED2.
Relay=2 has limited IPv6 support:
* Clients might not include IPv6 ORPorts in EXTEND2 cells.
* Relays (and bridges) might not initiate IPv6 connections in
response to EXTEND2 cells containing IPv6 ORPorts, even if they
are configured with an IPv6 ORPort.
However, relays support accepting inbound connections to their IPv6
ORPorts. And they might extend circuits via authenticated IPv6
connections to other relays.
"3" -- relays support extending over IPv6 connections in response to an
EXTEND2 cell containing an IPv6 ORPort.
Bridges might not extend over IPv6, because they try to imitate
client behaviour.
A successful IPv6 extend requires:
* Relay subprotocol version 3 (or later) on the extending relay,
* an IPv6 ORPort on the extending relay,
* an IPv6 ORPort for the accepting relay in the EXTEND2 cell, and
* an IPv6 ORPort on the accepting relay.
(Because different tor instances can have different views of the
network, these checks should be done when the path is selected.
Extending relays should only check local IPv6 information, before
attempting the extend.)
When relays receive an EXTEND2 cell containing both an IPv4 and an
IPv6 ORPort, and there is no existing authenticated connection with
the target relay, the extending relay may choose between IPv4 and
IPv6 at random. The extending relay might not try the other address,
if the first connection fails.
As is the case with other subprotocol versions, tor advertises,
recommends, or requires support for this protocol version, regardless
of its current configuration.
In particular:
* relays without an IPv6 ORPort, and
* tor instances that are not relays,
have the following behaviour, regardless of their configuration:
* advertise support for "Relay=3" in their descriptor
(if they are a relay, bridge, or directory authority), and
* react to consensuses recommending or requiring support for
"Relay=3".
This subprotocol version is described in proposal 311, and
implemented in Tor 0.4.5.1-alpha.
"4" -- support the ntorv3 (version 3) key exchange and all features in
0.4.7.3-alpha. This adds a new CREATE2 cell type. See proposal 332
for more details.
9.4. "HSIntro"
The "HSIntro" protocol handles introduction points.
"3" -- supports authentication as of proposal 121 in Tor
0.2.1.6-alpha.
"4" -- support ed25519 authentication keys which is defined by the HS v3
protocol as part of proposal 224 in Tor 0.3.0.4-alpha.
"5" -- support ESTABLISH_INTRO cell DoS parameters extension for onion
service version 3 only in Tor 0.4.2.1-alpha.
9.5. "HSRend"
The "HSRend" protocol handles rendezvous points.
"1" -- supports all features in Tor 0.0.6.
"2" -- supports RENDEZVOUS2 cells of arbitrary length as long as they
have 20 bytes of cookie in Tor 0.2.9.1-alpha.
9.6. "HSDir"
The "HSDir" protocols are the set of hidden service document types that can
be uploaded to, understood by, and downloaded from a tor relay, and the set
of URLs available to fetch them.
"1" -- supports all features in Tor 0.2.0.10-alpha.
"2" -- support ed25519 blinded keys request which is defined by the HS v3
protocol as part of proposal 224 in Tor 0.3.0.4-alpha.
9.7. "DirCache"
The "DirCache" protocols are the set of documents available for download
from a directory cache via BEGIN_DIR, and the set of URLs available to
fetch them. (This excludes URLs for hidden service objects.)
"1" -- supports all features in Tor 0.2.4.19.
"2" -- adds support for consensus diffs in Tor 0.3.1.1-alpha.
9.8. "Desc"
Describes features present or absent in descriptors.
Most features in descriptors don't require a "Desc" update -- only those
that need to someday be required. For example, someday clients will need
to understand ed25519 identities.
"1" -- supports all features in Tor 0.2.4.19.
"2" -- cross-signing with onion-keys, signing with ed25519
identities.
9.9. "Microdesc"
Describes features present or absent in microdescriptors.
Most features in descriptors don't require a "MicroDesc" update -- only
those that need to someday be required. These correspond more or less with
consensus methods.
"1" -- consensus methods 9 through 20.
"2" -- consensus method 21 (adds ed25519 keys to microdescs).
9.10. "Cons"
Describes features present or absent in consensus documents.
Most features in consensus documents don't require a "Cons" update -- only
those that need to someday be required.
These correspond more or less with consensus methods.
"1" -- consensus methods 9 through 20.
"2" -- consensus method 21 (adds ed25519 keys to microdescs).
9.11. "Padding"
Describes the padding capabilities of the relay.
"1" -- [DEFUNCT] Relay supports circuit-level padding. This version MUST NOT
be used as it was also enabled in relays that don't actually support
circuit-level padding. Advertised by Tor versions from
tor-0.4.0.1-alpha and only up to and including tor-0.4.1.4-rc.
"2" -- Relay supports the HS circuit setup padding machines (proposal 302).
Advertised by Tor versions from tor-0.4.1.5 and onwards.
9.12. "FlowCtrl"
Describes the flow control protocol at the circuit and stream level. If
there is no FlowCtrl advertised, tor supports the unauthenticated flow
control features (version 0).
"1" -- supports authenticated circuit level SENDMEs as of proposal 289 in
Tor 0.4.1.1-alpha.
"2" -- supports congestion control by the Exits which implies a new SENDME
format and algorithm. See proposal 324 for more details. Advertised
in tor 0.4.7.3-alpha.
9.13. "Datagram"
Describes the UDP protocol capabilities of a relay.
"1" -- [RESERVED] supports UDP by an Exit as in the relay command
CONNECT_UDP, CONNECTED_UDP and DATAGRAM. See proposal
339 for more details. (Not yet advertised, reserved)
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