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Initial rendezvous spec version. Needs another review and revision; but first I must sleep

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$Id$
Tor Rendezvous Spec
0. Overview and preliminaries
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
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 service descriptor, containing:
KL Key length [2 octets]
PK Bob's public key [KL octets]
TS A timestamp [8 octets]
NI Number of introduction points [2 octets]
Ipt A list of NUL-terminated OR nicknames [variable]
SIG Signature of above fields [KL octets]
KL is the length of PK, in octets. (Currently, KL must be 128.)
TS is the number of milliseconds elapsed since Jan 1, 1970.
[Shouldn't the nicknames be hostname:port's instead? That way, Alice's
directory servers don't need to know Bob's chosen introduction points.
Not important now, but essential if we ever have a non-total-knowledge
design. -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 [KL octets]
To prevent replay attacks, the HS field contains a SHA-1 hash based on the
shared secret g^xy between Bob's OP and the introduction point, as
follows:
HS = H(g^xy | "INTRODUCE")
That is:
HS = H(g^xy | [49 4E 54 52 4F 44 55 43 45])
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.
1.4. Bob's OP advertises his server descriptor
Bob's OP opens a stream to each directory server's directory port via Tor.
Over this stream, Bob's OP makes an HTTP 'POST' request, to the URL
'/rendezvous/publish' (relative to the directory server's root),
containing as its body Bob's service descriptor. Upon receiving a
descriptor, the director 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
~16-octet hash of Bob's service's public key, computed as follows:
1. Let H = H(PK).
2. Let H' = the first 130 bits of H, considering each octet from
most significant bit to least significant big.
2. Generate a 26-character encoding of H', taking H' 5 bits at
a time, and mapping each 5-bit value to a character as follows:
0..25 map to the characters 'a'...'z', respectively.
26..31 map to the characters '0'...'5', respectively.
(We only use 130 bits instead of the 160 bits from SHA1 because we don't
need to worry about man-in-the-middle attacks. We use 130 instead of 128
in order to have an even multiple of 5.)
[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 '/rendevous/<y>', where '<y> is replaced with the
encoding of Bob's public key as described above. 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'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.
[XXXX can Alice cache the descriptor? It may make her
partitionable. -NM]
1.7. Alice's OP establishes a rendezvous point.
When Alice requests a connection to a given location-hidden service,
and Alice's OP has not already established a circuit to that circuit,
the OP establishes a rendezvous service. 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 OP associates the
RC with the circuit that sent it.
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
Via another circuit, Alice sends a RELAY_INTRODUCE1 cell to once of Bob's
chosen introduction points. This cell contains:
Cleartext
PK_ID Identifier for Bob's PK [16 octets]
Encrypted to Bob's PK:
RP Rendezvous point's nickname [variable]
RC Rendezvous cookie [20 octets]
g^x Diffie-Hellman data, part 1 [128 octetes]
PK_ID is the first 16 octets of the hash of Bob's public key. RP is
NUL-terminated.
The data is encrypted to Bob's PK as follows: Suppose Bob's PK is L octets
long. If the data to be encrypted is shorter than L, then it is encrypted
directly (with no padding). If the data is at least as long as L, then a
randomly generated 16-byte symmetric key is prepended to the data, after
which the first L-1 bytes of the data are encrypted with Bob's PK; and the
rest of the data is encrypted with the symmetric key.
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
encrypted portion 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.)
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]
H(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]
H(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, unchained, from
each of the two circuits to the other. When Alice's OP sends
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.
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.