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torspec/rend-spec.txt

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$Id$
Tor Rendezvous Specification
0. Overview and preliminaries
Read http://tor.eff.org/doc/design-paper/tor-design.html#sec:rendezvous
before you read this specification. It will make more sense.
Rendezvous points provide location-hidden services (server
anonymity) for the onion routing network. With rendezvous points,
Bob can offer a TCP service (say, a webserver) via the onion
routing network, without revealing the IP of that service.
Bob does this by anonymously advertising a public key for his
service, along with a list of onion routers to act as "Introduction
Points" for his service. He creates forward OR circuits to those
introduction points, and tells them about his public key. To
connect to Bob, Alice first builds an OR circuit to an OR to act as
her "Rendezvous Point", then connects to one of Bob's chosen
introduction points, and asks it to tell him about her Rendezvous
Point (RP). If Bob chooses to answer, he builds an OR circuit to her
RP, and tells it to connect him to Alice. The RP joints their
circuits together, and begins relaying cells. Alice's 'BEGIN'
cells are received directly by Bob's OP, which responds by
communication with the local server implementing Bob's service.
Below, we describe a network-level specification of this service,
along with interfaces to make this process transparent to Alice
(so long as she is using an OP).
0.1. Notation, conventions and prerequisites
In the specifications below, we use the same notation as in
"tor-spec.txt". The service specified here also requires the existence of
an onion routing network as specified in "tor-spec.txt".
H(x) is a SHA1 digest of x.
PKSign(SK,x) is a PKCS.1-padded RSA signature of x with SK.
PKEncrypt(SK,x) is a PKCS.1-padded RSA encryption of x with SK.
Public keys are all RSA, and encoded in ASN.1.
All integers are stored in network (big-endian) order.
All symmetric encryption uses AES in counter mode, except where
otherwise noted.
In all discussions, "Alice" will refer to a user connecting to a
location-hidden service, and "Bob" will refer to a user running a
location-hidden service.
0.2. Protocol outline
1. Bob->Bob's OP: "Offer IP:Port as public-key-name:Port". [configuration]
(We do not specify this step; it is left to the implementor of
Bob's OP.)
2. Bob's OP generates keypair and rendezvous service descriptor:
"Meet public-key X at introduction point A, B, or C." (signed)
3. Bob's OP->Introduction point via Tor: [introduction setup]
"This pk is me."
4. Bob's OP->directory service via Tor: publishes Bob's service descriptor
[advertisement]
5. Out of band, Alice receives a y.onion:port address. She opens a
SOCKS connection to her OP, and requests y.onion:port.
6. Alice's OP retrieves Bob's descriptor via Tor: [descriptor lookup.]
7. Alice's OP chooses a rendezvous point, opens a circuit to that
rendezvous point, and establishes a rendezvous circuit. [rendezvous
setup.]
8. Alice connects to the Introduction point via Tor, and tells it about
her rendezvous point. (Encrypted to Bob.) [Introduction 1]
9. The Introduction point passes this on to Bob's OP via Tor, along the
introduction circuit. [Introduction 2]
10. Bob's OP decides whether to connect to Alice, and if so, creates a
circuit to Alice's RP via Tor. Establishes a shared circuit.
[Rendezvous.]
11. Alice's OP sends begin cells to Bob's OP. [Connection]
0.3. Constants and new cell types
Relay cell types
32 -- RELAY_ESTABLISH_INTRO
33 -- RELAY_ESTABLISH_RENDEZVOUS
34 -- RELAY_INTRODUCE1
35 -- RELAY_INTRODUCE2
36 -- RELAY_RENDEZVOUS1
37 -- RELAY_RENDEZVOUS2
38 -- RELAY_INTRO_ESTABLISHED
39 -- RELAY_RENDEZVOUS_ESTABLISHED
40 -- RELAY_COMMAND_INTRODUCE_ACK
1. The Protocol
1.1. Bob configures his local OP.
We do not specify a format for the OP configuration file. However,
OPs SHOULD allow Bob to provide more than one advertised service
per OP, and MUST allow Bob to specify one or more virtual ports per
service. Bob provides a mapping from each of these virtual ports
to a local IP:Port pair.
1.2. Bob's OP generates service descriptors.
The first time the OP provides an advertised service, it generates
a public/private keypair (stored locally). Periodically, the OP
generates a pair of service descriptors, one "V1" and one "V0".
The "V1" descriptor contains:
V Format byte: set to 255 [1 octet]
V Version byte: set to 1 [1 octet]
KL Key length [2 octets]
PK Bob's public key [KL octets]
TS A timestamp [4 octets]
PROTO Protocol versions: bitmask [2 octets]
NI Number of introduction points [2 octets]
For each introduction point: (as in INTRODUCE2 cells)
IP Rendezvous point's address [4 octets]
PORT Rendezvous point's OR port [2 octets]
ID Rendezvous point identity ID [20 octets]
KLEN Length of onion key [2 octets]
KEY Rendezvous point onion key [KLEN octets]
SIG Signature of above fields [variable]
The "V0" descriptor contains:
KL Key length [2 octets]
PK Bob's public key [KL octets]
TS A timestamp [4 octets]
NI Number of introduction points [2 octets]
Ipt A list of NUL-terminated ORs [variable]
SIG Signature of above fields [variable]
KL is the length of PK, in octets. (Currently, KL must be 128.)
TS is the number of seconds elapsed since Jan 1, 1970.
The members of Ipt may be either (a) nicknames, or (b) identity key
digests, encoded in hex, and prefixed with a '$'. Clients must
accept both forms. Services must only generate the second form.
Once 0.0.9.x is obsoleted, we can drop the first form.
[It's ok for Bob to advertise 0 introduction points. He might want
to do that if he previously advertised some introduction points,
and now he doesn't have any. -RD]
[Once Tor 0.1.0.x is obsolete, we can stop generating or using V0
descriptors. -NM]
1.3. Bob's OP establishes his introduction points.
The OP establishes a new introduction circuit to each introduction
point. These circuits MUST NOT be used for anything but rendezvous
introduction. To establish the introduction, Bob sends a
RELAY_ESTABLISH_INTRO cell, containing:
KL Key length [2 octets]
PK Bob's public key [KL octets]
HS Hash of session info [20 octets]
SIG Signature of above information [variable]
To prevent replay attacks, the HS field contains a SHA-1 hash based on the
shared secret KH between Bob's OP and the introduction point, as
follows:
HS = H(KH | "INTRODUCE")
That is:
HS = H(KH | [49 4E 54 52 4F 44 55 43 45])
(KH, as specified in tor-spec.txt, is H(g^xy | [00]) .)
Upon receiving such a cell, the OR first checks that the signature is
correct with the included public key. If so, it checks whether HS is
correct given the shared state between Bob's OP and the OR. If either
check fails, the OP discards the cell; otherwise, it associates the
circuit with Bob's public key, and dissociates any other circuits
currently associated with PK. On success, the OR sends Bob a
RELAY_INTRO_ESTABLISHED cell with an empty payload.
1.4. Bob's OP advertises his service descriptor(s)
Bob's OP opens a stream to each directory server's directory port via Tor.
(He may re-use old circuits for this.) Over this stream, Bob's OP makes
an HTTP 'POST' request, to a URL "/tor/rendezvous/publish" relative to the
directory server's root, containing as its body Bob's service descriptor.
Upon receiving a descriptor, the directory server checks the signature,
and discards the descriptor if the signature does not match the enclosed
public key. Next, the directory server checks the timestamp. If the
timestamp is more than 24 hours in the past or more than 1 hour in the
future, or the directory server already has a newer descriptor with the
same public key, the server discards the descriptor. Otherwise, the
server discards any older descriptors with the same public key, and
associates the new descriptor with the public key. The directory server
remembers this descriptor for at least 24 hours after its timestamp. At
least every 24 hours, Bob's OP uploads a fresh descriptor.
1.5. Alice receives a y.onion address
When Alice receives a pointer to a location-hidden service, it is as a
hostname of the form "y.onion", where y is a base-32 encoding of a
10-octet hash of Bob's service's public key, computed as follows:
1. Let H = H(PK).
2. Let H' = the first 80 bits of H, considering each octet from
most significant bit to least significant bit.
2. Generate a 16-character encoding of H', using base32 as defined
in RFC 3548.
(We only use 80 bits instead of the 160 bits from SHA1 because we don't
need to worry about man-in-the-middle attacks, and because it will make
handling the url's more convenient.)
[Yes, numbers are allowed at the beginning. See RFC1123. -NM]
1.6. Alice's OP retrieves a service descriptor
Alice opens a stream to a directory server via Tor, and makes an HTTP GET
request for the document '/tor/rendezvous/<y>' or '/tor/rendezvous1/<y>',
where '<y> is replaced with the encoding of Bob's public key as described
above. (She may re-use old circuits for this.) The directory replies with
a 404 HTTP response if it does not recognize <y>, and otherwise returns
Bob's most recently uploaded service descriptor. (If Alice requests
'rendezvous1', the directory server provides a V1 descriptor or a V0
descriptor if no V1 descriptor is available. If Alice requests
'rendezvous', the directory server returns a V0 descriptor.)
If Alice's OP receives a 404 response, it tries the other directory
servers, and only fails the lookup if none recognizes the public key hash.
Upon receiving a service descriptor, Alice verifies with the same process
as the directory server uses, described above in section 1.4.
The directory server gives a 400 response if it cannot understand Alice's
request.
Alice should cache the descriptor locally, but should not use
descriptors that are more than 24 hours older than their timestamp.
[Caching may make her partitionable, but she fetched it anonymously,
and we can't very well *not* cache it. -RD]
1.7. Alice's OP establishes a rendezvous point.
When Alice requests a connection to a given location-hidden service,
and Alice's OP does not have an established circuit to that service,
the OP builds a rendezvous circuit. It does this by establishing
a circuit to a randomly chosen OR, and sending a
RELAY_ESTABLISH_RENDEZVOUS cell to that OR. The body of that cell
contains:
RC Rendezvous cookie [20 octets]
The rendezvous cookie is an arbitrary 20-byte value, chosen randomly by
Alice's OP.
Upon receiving a RELAY_ESTABLISH_RENDEZVOUS cell, the OR associates the
RC with the circuit that sent it. It replies to Alice with an empty
RELAY_RENDEZVOUS_ESTABLISHED cell to indicate success.
Alice's OP MUST NOT use the circuit which sent the cell for any purpose
other than rendezvous with the given location-hidden service.
1.8. Introduction: from Alice's OP to Introduction Point
Alice builds a separate circuit to one of Bob's chosen introduction
points, and sends it a RELAY_INTRODUCE1 cell containing:
Cleartext
PK_ID Identifier for Bob's PK [20 octets]
Encrypted to Bob's PK:
RP Rendezvous point's nickname [20 octets]
RC Rendezvous cookie [20 octets]
g^x Diffie-Hellman data, part 1 [128 octets]
OR
VER Version byte: set to 1. [1 octet]
RP Rendezvous point nick or ID [42 octets]
RC Rendezvous cookie [20 octets]
g^x Diffie-Hellman data, part 1 [128 octets]
OR
VER Version byte: set to 2. [1 octet]
IP Rendezvous point's address [4 octets]
PORT Rendezvous point's OR port [2 octets]
ID Rendezvous point identity ID [20 octets]
KLEN Length of onion key [2 octets]
KEY Rendezvous point onion key [KLEN octets]
RC Rendezvous cookie [20 octets]
g^x Diffie-Hellman data, part 1 [128 octets]
PK_ID is the hash of Bob's public key. RP is NUL-padded and terminated,
and must contain EITHER a nickname, or an identity key digest, encoded in
hex, and prefixed with a '$'.
Implementations SHOULD accept all variants, and list the variants they
accept in their V1 descriptor. Implementations should only generate the
variants listed in the service's V1 descriptor; if no V1 descriptor is
available, only the first variant should be generated. No version should
generate the second variant (version byte=1).
The hybrid encryption to Bob's PK works just like the hybrid
encryption in CREATE cells (see main spec). Thus the payload of the
RELAY_INTRODUCE1 cell on the wire will contain 20+42+16+20+20+128=246
bytes. [XXXX not really]
1.9. Introduction: From the Introduction Point to Bob's OP
If the Introduction Point recognizes PK_ID as a public key which has
established a circuit for introductions as in 1.3 above, it sends the body
of the cell in a new RELAY_INTRODUCE2 cell down the corresponding circuit.
(If the PK_ID is unrecognized, the RELAY_INTRODUCE1 cell is discarded.)
After sending the RELAY_INTRODUCE2 cell, the OR replies to Alice with an
empty RELAY_COMMAND_INTRODUCE_ACK cell. If no RELAY_INTRODUCE2 cell can
be sent, the OR replies to Alice with a non-empty cell to indicate an
error. (The semantics of the cell body may be determined later; the
current implementation sends a single '1' byte on failure.)
When Bob's OP receives the RELAY_INTRODUCE2 cell, it decrypts it with
the private key for the corresponding hidden service, and extracts the
rendezvous point's nickname, the rendezvous cookie, and the value of g^x
chosen by Alice.
1.10. Rendezvous
Bob's OP build a new Tor circuit ending at Alice's chosen rendezvous
point, and sends a RELAY_RENDEZVOUS1 cell along this circuit, containing:
RC Rendezvous cookie [20 octets]
g^y Diffie-Hellman [128 octets]
KH Handshake digest [20 octets]
(Bob's OP MUST NOT use this circuit for any other purpose.)
If the RP recognizes RC, it relays the rest of the cell down the
corresponding circuit in a RELAY_RENDEZVOUS2 cell, containing:
g^y Diffie-Hellman [128 octets]
KH Handshake digest [20 octets]
(If the RP does not recognize the RC, it discards the cell and
tears down the circuit.)
When Alice's OP receives a RELAY_RENDEZVOUS2 cell on a circuit which
has sent a RELAY_ESTABLISH_RENDEZVOUS cell but which has not yet received
a reply, it uses g^y and H(g^xy) to complete the handshake as in the Tor
circuit extend process: they establish a 60-octet string as
K = SHA1(g^xy | [00]) | SHA1(g^xy | [01]) | SHA1(g^xy | [02])
and generate
KH = K[0..15]
Kf = K[16..31]
Kb = K[32..47]
Subsequently, the rendezvous point passes relay cells, unchanged, from
each of the two circuits to the other. When Alice's OP sends
RELAY cells along the circuit, it first encrypts them with the
Kf, then with all of the keys for the ORs in Alice's side of the circuit;
and when Alice's OP receives RELAY cells from the circuit, it decrypts
them with the keys for the ORs in Alice's side of the circuit, then
decrypts them with Kb. Bob's OP does the same, with Kf and Kb
interchanged.
1.11. Creating streams
To open TCP connections to Bob's location-hidden service, Alice's OP sends
a RELAY_BEGIN cell along the established circuit, using the special
address "", and a chosen port. Bob's OP chooses a destination IP and
port, based on the configuration of the service connected to the circuit,
and opens a TCP stream. From then on, Bob's OP treats the stream as an
ordinary exit connection.
[ Except he doesn't include addr in the connected cell or the end
cell. -RD]
Alice MAY send multiple RELAY_BEGIN cells along the circuit, to open
multiple streams to Bob. Alice SHOULD NOT send RELAY_BEGIN cells for any
other address along her circuit to Bob; if she does, Bob MUST reject them.
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