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Tor Rendezvous Specification
0. Overview and 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.
Read
https://svn.torproject.org/svn/projects/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 circuits to those
introduction points, and tells them about his service. To
connect to Bob, Alice first builds a circuit to an OR to act as
her "Rendezvous Point." She 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 a circuit to her
RP, and tells it to connect him to Alice. The RP joins their
circuits together, and begins relaying cells. Alice's 'BEGIN'
cells are received directly by Bob's OP, which passes data to
and from 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 and terminology
as in "tor-spec.txt". The service specified here also requires the
existence of an onion routing network as specified in that file.
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.
An OP is (as defined elsewhere) an "Onion Proxy" or Tor client.
An OR is (as defined elsewhere) an "Onion Router" or Tor server.
An "Introduction point" is a Tor server chosen to be Bob's medium-term
'meeting place'. A "Rendezvous point" is a Tor server chosen by Alice to
be a short-term communication relay between her and Bob. All Tor servers
potentially act as introduction and rendezvous points.
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 a long-term keypair.
3. Bob's OP->Introduction point via Tor: [introduction setup]
"This public key is (currently) associated to me."
4. Bob's OP->directory service via Tor: publishes Bob's service descriptor
[advertisement]
"Meet public-key X at introduction point A, B, or C." (signed)
5. Out of band, Alice receives a z.onion:port address.
She opens a SOCKS connection to her OP, and requests z.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 1]
11. The Rendezvous point forwards Bob's confirmation to Alice's OP.
[Rendezvous 2]
12. Alice's OP sends begin cells to Bob's OP. [Connection]
0.3. Constants and new cell types
Relay cell types
32 -- RELAY_COMMAND_ESTABLISH_INTRO
33 -- RELAY_COMMAND_ESTABLISH_RENDEZVOUS
34 -- RELAY_COMMAND_INTRODUCE1
35 -- RELAY_COMMAND_INTRODUCE2
36 -- RELAY_COMMAND_RENDEZVOUS1
37 -- RELAY_COMMAND_RENDEZVOUS2
38 -- RELAY_COMMAND_INTRO_ESTABLISHED
39 -- RELAY_COMMAND_RENDEZVOUS_ESTABLISHED
40 -- RELAY_COMMAND_INTRODUCE_ACK
0.4. Version overview
There are several parts in the hidden service protocol that have
changed over time, each of them having its own version number, whereas
other parts remained the same. The following list of potentially
versioned protocol parts should help reduce some confusion:
- Hidden service descriptor: the binary-based v0 was the default for a
long time, and an ASCII-based v2 has been added by proposal 114. The
v0 descriptor format has been deprecated in 0.2.2.1-alpha. See 1.3.
- Hidden service descriptor propagation mechanism: currently related to
the hidden service descriptor version -- v0 publishes to the original
hs directory authorities, whereas v2 publishes to a rotating subset
of relays with the "HSDir" flag; see 1.4 and 1.6.
- Introduction protocol for how to generate an introduction cell:
v0 specified a nickname for the rendezvous point and assumed the
relay would know about it, whereas v2 now specifies IP address,
port, and onion key so the relay doesn't need to already recognize
it. See 1.8.
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 establishes his introduction points.
The first time the OP provides an advertised service, it generates
a public/private keypair (stored locally).
The OP choses a small number of Tor servers as introduction points.
The OP establishes a new introduction circuit to each introduction
point. These circuits MUST NOT be used for anything but hidden service
introduction. To establish the introduction, Bob sends a
RELAY_COMMAND_ESTABLISH_INTRO cell, containing:
KL Key length [2 octets]
PK Bob's public key or service key [KL octets]
HS Hash of session info [20 octets]
SIG Signature of above information [variable]
KL is the length of PK, in octets.
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_COMMAND_INTRO_ESTABLISHED cell with an empty payload.
Bob's OP uses either Bob's public key or a freshly generated, single-use
service key in the RELAY_COMMAND_ESTABLISH_INTRO cell, depending on the
configured hidden service descriptor version. The public key is used for
v0 descriptors, the service key for v2 descriptors. In the latter case, the
service keys of all introduction points are included in the v2 hidden
service descriptor together with the other introduction point information.
The reason is that the introduction point does not need to and therefore
should not know for which hidden service it works, so as to prevent it from
tracking the hidden service's activity. If the hidden service is configured
to publish both v0 and v2 descriptors, two separate sets of introduction
points are established.
1.3. Bob's OP generates service descriptors.
For versions before 0.2.2.1-alpha, Bob's OP periodically generates and
publishes a descriptor of type "V0".
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]
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]
Beginning with 0.2.0.10-alpha, Bob's OP encodes "V2" descriptors in
addition to (or instead of) "V0" descriptors. The format of a "V2"
descriptor is as follows:
"rendezvous-service-descriptor" descriptor-id NL
[At start, exactly once]
Indicates the beginning of the descriptor. "descriptor-id" is a
periodically changing identifier of 160 bits formatted as 32 base32
chars that is calculated by the hidden service and its clients. The
"descriptor-id" is calculated by performing the following operation:
descriptor-id =
H(permanent-id | H(time-period | descriptor-cookie | replica))
"permanent-id" is the permanent identifier of the hidden service,
consisting of 80 bits. It can be calculated by computing the hash value
of the public hidden service key and truncating after the first 80 bits:
permanent-id = H(public-key)[:10]
"H(time-period | descriptor-cookie | replica)" is the (possibly
secret) id part that is necessary to verify that the hidden service is
the true originator of this descriptor and that is therefore contained
in the descriptor, too. The descriptor ID can only be created by the
hidden service and its clients, but the "signature" below can only be
created by the service.
"time-period" changes periodically as a function of time and
"permanent-id". The current value for "time-period" can be calculated
using the following formula:
time-period = (current-time + permanent-id-byte * 86400 / 256)
/ 86400
"current-time" contains the current system time in seconds since
1970-01-01 00:00, e.g. 1188241957. "permanent-id-byte" is the first
(unsigned) byte of the permanent identifier (which is in network
order), e.g. 143. Adding the product of "permanent-id-byte" and
86400 (seconds per day), divided by 256, prevents "time-period" from
changing for all descriptors at the same time of the day. The result
of the overall operation is a (network-ordered) 32-bit integer, e.g.
13753 or 0x000035B9 with the example values given above.
"descriptor-cookie" is an optional secret password of 128 bits that
is shared between the hidden service provider and its clients. If the
descriptor-cookie is left out, the input to the hash function is 128
bits shorter.
"replica" denotes the number of the replica. A service publishes
multiple descriptors with different descriptor IDs in order to
distribute them to different places on the ring.
"version" version-number NL
[Exactly once]
The version number of this descriptor's format. In this case: 2.
"permanent-key" NL a public key in PEM format
[Exactly once]
The public key of the hidden service which is required to verify the
"descriptor-id" and the "signature".
"secret-id-part" secret-id-part NL
[Exactly once]
The result of the following operation as explained above, formatted as
32 base32 chars. Using this secret id part, everyone can verify that
the signed descriptor belongs to "descriptor-id".
secret-id-part = H(time-period | descriptor-cookie | replica)
"publication-time" YYYY-MM-DD HH:MM:SS NL
[Exactly once]
A timestamp when this descriptor has been created.
"protocol-versions" version-string NL
[Exactly once]
A comma-separated list of recognized and permitted version numbers
for use in INTRODUCE cells; these versions are described in section
1.8 below.
"introduction-points" NL encrypted-string
[At most once]
A list of introduction points. If the optional "descriptor-cookie" is
used, this list is encrypted with AES in CTR mode with a random
initialization vector of 128 bits that is written to
the beginning of the encrypted string, and the "descriptor-cookie" as
secret key of 128 bits length.
The string containing the introduction point data (either encrypted
or not) is encoded in base64, and surrounded with
"-----BEGIN MESSAGE-----" and "-----END MESSAGE-----".
The unencrypted string may begin with:
"service-authentication" auth-type auth-data NL
[Any number]
The service-specific authentication data can be used to perform
client authentication. This data is independent of the selected
introduction point as opposed to "intro-authentication" below. The
format of auth-data (base64-encoded or PEM format) depends on
auth-type. See section 2 of this document for details on auth
mechanisms.
Subsequently, an arbitrary number of introduction point entries may
follow, each containing the following data:
"introduction-point" identifier NL
[At start, exactly once]
The identifier of this introduction point: the base-32 encoded
hash of this introduction point's identity key.
"ip-address" ip-address NL
[Exactly once]
The IP address of this introduction point.
"onion-port" port NL
[Exactly once]
The TCP port on which the introduction point is listening for
incoming onion requests.
"onion-key" NL a public key in PEM format
[Exactly once]
The public key that can be used to encrypt messages to this
introduction point.
"service-key" NL a public key in PEM format
[Exactly once]
The public key that can be used to encrypt messages to the hidden
service.
"intro-authentication" auth-type auth-data NL
[Any number]
The introduction-point-specific authentication data can be used
to perform client authentication. This data depends on the
selected introduction point as opposed to "service-authentication"
above. The format of auth-data (base64-encoded or PEM format)
depends on auth-type. See section 2 of this document for details
on auth mechanisms.
(This ends the fields in the encrypted portion of the descriptor.)
[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]
"signature" NL signature-string
[At end, exactly once]
A signature of all fields above with the private key of the hidden
service.
1.3.1. Other descriptor formats we don't use.
Support for the V0 descriptor format was dropped in 0.2.2.0-alpha-dev:
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.
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 '$'.
The V1 descriptor format was understood and accepted from
0.1.1.5-alpha-cvs to 0.2.0.6-alpha-dev, but no Tors generated it and
it was removed:
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 Introduction point's address [4 octets]
PORT Introduction point's OR port [2 octets]
ID Introduction point identity ID [20 octets]
KLEN Length of onion key [2 octets]
KEY Introduction point onion key [KLEN octets]
SIG Signature of above fields [variable]
A hypothetical "V1" descriptor, that has never been used but might
be useful for historical reasons, 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 Rendezvous protocol versions: bitmask [2 octets]
NA Number of auth mechanisms accepted [1 octet]
For each auth mechanism:
AUTHT The auth type that is supported [2 octets]
AUTHL Length of auth data [1 octet]
AUTHD Auth data [variable]
NI Number of introduction points [2 octets]
For each introduction point: (as in INTRODUCE2 cells)
ATYPE An address type (typically 4) [1 octet]
ADDR Introduction point's IP address [4 or 16 octets]
PORT Introduction point's OR port [2 octets]
AUTHT The auth type that is supported [2 octets]
AUTHL Length of auth data [1 octet]
AUTHD Auth data [variable]
ID Introduction point identity ID [20 octets]
KLEN Length of onion key [2 octets]
KEY Introduction point onion key [KLEN octets]
SIG Signature of above fields [variable]
AUTHT specifies which authentication/authorization mechanism is
required by the hidden service or the introduction point. AUTHD
is arbitrary data that can be associated with an auth approach.
Currently only AUTHT of [00 00] is supported, with an AUTHL of 0.
See section 2 of this document for details on auth mechanisms.
1.4. Bob's OP advertises his service descriptor(s).
Bob's OP advertises his service descriptor to a fixed set of v0 hidden
service directory servers and/or a changing subset of all v2 hidden service
directories.
For versions before 0.2.2.1-alpha, Bob's OP opens a stream to each v0
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
version format, 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 18 hours, Bob's OP uploads a
fresh descriptor.
If Bob's OP is configured to publish v2 descriptors, it does so to a
changing subset of all v2 hidden service directories instead of the
authoritative directory servers. Therefore, Bob's OP opens a stream via
Tor to each responsible hidden service directory. (He may re-use old
circuits for this.) Over this stream, Bob's OP makes an HTTP 'POST'
request to a URL "/tor/rendezvous2/publish" relative to the hidden service
directory's root, containing as its body Bob's service descriptor.
At any time, there are 6 hidden service directories responsible for
keeping replicas of a descriptor; they consist of 2 sets of 3 hidden
service directories with consecutive onion IDs. Bob's OP learns about
the complete list of hidden service directories by filtering the
consensus status document received from the directory authorities. A
hidden service directory is deemed responsible for all descriptor IDs in
the interval from its direct predecessor, exclusive, to its own ID,
inclusive; it further holds replicas for its 2 predecessors. A
participant only trusts its own routing list and never learns about
routing information from other parties.
Bob's OP publishes a new v2 descriptor once an hour or whenever its
content changes. V2 descriptors can be found by clients within a given
time period of 24 hours, after which they change their ID as described
under 1.3. If a published descriptor would be valid for less than 60
minutes (= 2 x 30 minutes to allow the server to be 30 minutes behind
and the client 30 minutes ahead), Bob's OP publishes the descriptor
under the ID of both, the current and the next publication period.
1.5. Alice receives a z.onion address.
When Alice receives a pointer to a location-hidden service, it is as a
hostname of the form "z.onion", where z 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.
3. 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 arbitrary collisions, and because it will
make handling the url's more convenient.)
[Yes, numbers are allowed at the beginning. See RFC 1123. -NM]
1.6. Alice's OP retrieves a service descriptor.
Alice's OP fetches the service descriptor from the fixed set of v0 hidden
service directory servers and/or a changing subset of all v2 hidden service
directories.
For versions before 0.2.2.1-alpha, Alice's OP opens a stream to a directory
server via Tor, and makes an HTTP GET request for the document
'/tor/rendezvous/<z>', where '<z>' 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 <z>,
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 recognize 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]
If Alice's OP is running 0.2.1.10-alpha or higher, it fetches v2 hidden
service descriptors. Versions before 0.2.2.1-alpha are fetching both v0 and
v2 descriptors in parallel. Similar to the description in section 1.4,
Alice's OP fetches a v2 descriptor from a randomly chosen hidden service
directory out of the changing subset of 6 nodes. If the request is
unsuccessful, Alice retries the other remaining responsible hidden service
directories in a random order. Alice relies on Bob to care about a potential
clock skew between the two by possibly storing two sets of descriptors (see
end of section 1.4).
Alice's OP opens a stream via Tor to the chosen v2 hidden service
directory. (She may re-use old circuits for this.) Over this stream,
Alice's OP makes an HTTP 'GET' request for the document
"/tor/rendezvous2/<z>", where z is replaced with the encoding of the
descriptor ID. The directory replies with a 404 HTTP response if it does
not recognize <z>, and otherwise returns Bob's most recently uploaded
service descriptor.
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_COMMAND_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. Alice SHOULD choose a new rendezvous cookie for each new
connection attempt.
Upon receiving a RELAY_COMMAND_ESTABLISH_RENDEZVOUS cell, the OR associates
the RC with the circuit that sent it. It replies to Alice with an empty
RELAY_COMMAND_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_COMMAND_INTRODUCE1 cell containing:
Cleartext
PK_ID Identifier for Bob's PK [20 octets]
Encrypted to Bob's PK: (in the v0 intro protocol)
RP Rendezvous point's nickname [20 octets]
RC Rendezvous cookie [20 octets]
g^x Diffie-Hellman data, part 1 [128 octets]
OR (in the v1 intro protocol)
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 (in the v2 intro protocol)
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]
OR (in the v3 intro protocol)
VER Version byte: set to 3. [1 octet]
AUTHT The auth type that is used [1 octet]
AUTHL Length of auth data [2 octets]
AUTHD Auth data [variable]
TS A timestamp [4 octets]
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 or the service key, depending on the
hidden service descriptor version. In case of a v0 descriptor, Alice's OP
uses Bob's public key. If Alice has downloaded a v2 descriptor, she uses
the contained public key ("service-key").
RP is NUL-padded and terminated. In version 0 of the intro protocol, RP
must contain a nickname. In version 1, it must contain EITHER a nickname or
an identity key digest that is encoded in hex and prefixed with a '$'.
The hybrid encryption to Bob's PK works just like the hybrid
encryption in CREATE cells (see tor-spec). Thus the payload of the
version 0 RELAY_COMMAND_INTRODUCE1 cell on the wire will contain
20+42+16+20+20+128=246 bytes, and the version 1 and version 2
introduction formats have other sizes.
Through Tor 0.2.0.6-alpha, clients only generated the v0 introduction
format, whereas hidden services have understood and accepted v0,
v1, and v2 since 0.1.1.x. As of Tor 0.2.0.7-alpha and 0.1.2.18,
clients switched to using the v2 intro format.
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 1.2 above, it sends the body
of the cell in a new RELAY_COMMAND_INTRODUCE2 cell down the corresponding
circuit. (If the PK_ID is unrecognized, the RELAY_COMMAND_INTRODUCE1 cell is
discarded.)
After sending the RELAY_COMMAND_INTRODUCE2 cell, the OR replies to Alice
with an empty RELAY_COMMAND_INTRODUCE_ACK cell. If no
RELAY_COMMAND_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_COMMAND_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 builds a new Tor circuit ending at Alice's chosen rendezvous
point, and sends a RELAY_COMMAND_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_COMMAND_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_COMMAND_RENDEZVOUS2 cell on a circuit which
has sent a RELAY_COMMAND_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_COMMAND_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_COMMAND_BEGIN cells along the circuit, to open
multiple streams to Bob. Alice SHOULD NOT send RELAY_COMMAND_BEGIN cells
for any other address along her circuit to Bob; if she does, Bob MUST reject
them.
2. Authentication and authorization.
The rendezvous protocol as described in Section 1 provides a few options
for implementing client-side authorization. There are two steps in the
rendezvous protocol that can be used for performing client authorization:
when downloading and decrypting parts of the hidden service descriptor and
at Bob's Tor client before contacting the rendezvous point. A service
provider can restrict access to his service at these two points to
authorized clients only.
There are currently two authorization protocols specified that are
described in more detail below:
1. The first protocol allows a service provider to restrict access
to clients with a previously received secret key only, but does not
attempt to hide service activity from others.
2. The second protocol, albeit being feasible for a limited set of about
16 clients, performs client authorization and hides service activity
from everyone but the authorized clients.
2.1. Service with large-scale client authorization
The first client authorization protocol aims at performing access control
while consuming as few additional resources as possible. A service
provider should be able to permit access to a large number of clients
while denying access for everyone else. However, the price for
scalability is that the service won't be able to hide its activity from
unauthorized or formerly authorized clients.
The main idea of this protocol is to encrypt the introduction-point part
in hidden service descriptors to authorized clients using symmetric keys.
This ensures that nobody else but authorized clients can learn which
introduction points a service currently uses, nor can someone send a
valid INTRODUCE1 message without knowing the introduction key. Therefore,
a subsequent authorization at the introduction point is not required.
A service provider generates symmetric "descriptor cookies" for his
clients and distributes them outside of Tor. The suggested key size is
128 bits, so that descriptor cookies can be encoded in 22 base64 chars
(which can hold up to 22 * 5 = 132 bits, leaving 4 bits to encode the
authorization type (here: "0") and allow a client to distinguish this
authorization protocol from others like the one proposed below).
Typically, the contact information for a hidden service using this
authorization protocol looks like this:
v2cbb2l4lsnpio4q.onion Ll3X7Xgz9eHGKCCnlFH0uz
When generating a hidden service descriptor, the service encrypts the
introduction-point part with a single randomly generated symmetric
128-bit session key using AES-CTR as described for v2 hidden service
descriptors in rend-spec. Afterwards, the service encrypts the session
key to all descriptor cookies using AES. Authorized client should be able
to efficiently find the session key that is encrypted for him/her, so
that 4 octet long client ID are generated consisting of descriptor cookie
and initialization vector. Descriptors always contain a number of
encrypted session keys that is a multiple of 16 by adding fake entries.
Encrypted session keys are ordered by client IDs in order to conceal
addition or removal of authorized clients by the service provider.
ATYPE Authorization type: set to 1. [1 octet]
ALEN Number of clients := 1 + ((clients - 1) div 16) [1 octet]
for each symmetric descriptor cookie:
ID Client ID: H(descriptor cookie | IV)[:4] [4 octets]
SKEY Session key encrypted with descriptor cookie [16 octets]
(end of client-specific part)
RND Random data [(15 - ((clients - 1) mod 16)) * 20 octets]
IV AES initialization vector [16 octets]
IPOS Intro points, encrypted with session key [remaining octets]
An authorized client needs to configure Tor to use the descriptor cookie
when accessing the hidden service. Therefore, a user adds the contact
information that she received from the service provider to her torrc
file. Upon downloading a hidden service descriptor, Tor finds the
encrypted introduction-point part and attempts to decrypt it using the
configured descriptor cookie. (In the rare event of two or more client
IDs being equal a client tries to decrypt all of them.)
Upon sending the introduction, the client includes her descriptor cookie
as auth type "1" in the INTRODUCE2 cell that she sends to the service.
The hidden service checks whether the included descriptor cookie is
authorized to access the service and either responds to the introduction
request, or not.
2.2. Authorization for limited number of clients
A second, more sophisticated client authorization protocol goes the extra
mile of hiding service activity from unauthorized clients. With all else
being equal to the preceding authorization protocol, the second protocol
publishes hidden service descriptors for each user separately and gets
along with encrypting the introduction-point part of descriptors to a
single client. This allows the service to stop publishing descriptors for
removed clients. As long as a removed client cannot link descriptors
issued for other clients to the service, it cannot derive service
activity any more. The downside of this approach is limited scalability.
Even though the distributed storage of descriptors (cf. proposal 114)
tackles the problem of limited scalability to a certain extent, this
protocol should not be used for services with more than 16 clients. (In
fact, Tor should refuse to advertise services for more than this number
of clients.)
A hidden service generates an asymmetric "client key" and a symmetric
"descriptor cookie" for each client. The client key is used as
replacement for the service's permanent key, so that the service uses a
different identity for each of his clients. The descriptor cookie is used
to store descriptors at changing directory nodes that are unpredictable
for anyone but service and client, to encrypt the introduction-point
part, and to be included in INTRODUCE2 cells. Once the service has
created client key and descriptor cookie, he tells them to the client
outside of Tor. The contact information string looks similar to the one
used by the preceding authorization protocol (with the only difference
that it has "1" encoded as auth-type in the remaining 4 of 132 bits
instead of "0" as before).
When creating a hidden service descriptor for an authorized client, the
hidden service uses the client key and descriptor cookie to compute
secret ID part and descriptor ID:
secret-id-part = H(time-period | descriptor-cookie | replica)
descriptor-id = H(client-key[:10] | secret-id-part)
The hidden service also replaces permanent-key in the descriptor with
client-key and encrypts introduction-points with the descriptor cookie.
ATYPE Authorization type: set to 2. [1 octet]
IV AES initialization vector [16 octets]
IPOS Intro points, encr. with descriptor cookie [remaining octets]
When uploading descriptors, the hidden service needs to make sure that
descriptors for different clients are not uploaded at the same time (cf.
Section 1.1) which is also a limiting factor for the number of clients.
When a client is requested to establish a connection to a hidden service
it looks up whether it has any authorization data configured for that
service. If the user has configured authorization data for authorization
protocol "2", the descriptor ID is determined as described in the last
paragraph. Upon receiving a descriptor, the client decrypts the
introduction-point part using its descriptor cookie. Further, the client
includes its descriptor cookie as auth-type "2" in INTRODUCE2 cells that
it sends to the service.
2.3. Hidden service configuration
A hidden service that is meant to perform client authorization adds a
new option HiddenServiceAuthorizeClient to its hidden service
configuration. This option contains the authorization type which is
either "1" for the protocol described in 2.1 or "2" for the protocol in
2.2 and a comma-separated list of human-readable client names, so that
Tor can create authorization data for these clients:
HiddenServiceAuthorizeClient auth-type client-name,client-name,...
If this option is configured, HiddenServiceVersion is automatically
reconfigured to contain only version numbers of 2 or higher.
Tor stores all generated authorization data for the authorization
protocols described in Sections 2.1 and 2.2 in a new file using the
following file format:
"client-name" human-readable client identifier NL
"descriptor-cookie" 128-bit key ^= 22 base64 chars NL
If the authorization protocol of Section 2.2 is used, Tor also generates
and stores the following data:
"client-key" NL a public key in PEM format
2.4. Client configuration
Clients need to make their authorization data known to Tor using another
configuration option that contains a service name (mainly for the sake of
convenience), the service address, and the descriptor cookie that is
required to access a hidden service (the authorization protocol number is
encoded in the descriptor cookie):
HidServAuth service-name service-address descriptor-cookie
3. Hidden service directory operation
This section has been introduced with the v2 hidden service descriptor
format. It describes all operations of the v2 hidden service descriptor
fetching and propagation mechanism that are required for the protocol
described in section 1 to succeed with v2 hidden service descriptors.
3.1. Configuring as hidden service directory
Every onion router that has its directory port open can decide whether it
wants to store and serve hidden service descriptors. An onion router which
is configured as such includes the "hidden-service-dir" flag in its router
descriptors that it sends to directory authorities.
The directory authorities include a new flag "HSDir" for routers that
decided to provide storage for hidden service descriptors and that
have been running for at least 24 hours.
3.2. Accepting publish requests
Hidden service directory nodes accept publish requests for v2 hidden service
descriptors and store them to their local memory. (It is not necessary to
make descriptors persistent, because after restarting, the onion router
would not be accepted as a storing node anyway, because it has not been
running for at least 24 hours.) All requests and replies are formatted as
HTTP messages. Requests are initiated via BEGIN_DIR cells directed to
the router's directory port, and formatted as HTTP POST requests to the URL
"/tor/rendezvous2/publish" relative to the hidden service directory's root,
containing as its body a v2 service descriptor.
A hidden service directory node parses every received descriptor and only
stores it when it thinks that it is responsible for storing that descriptor
based on its own routing table. See section 1.4 for more information on how
to determine responsibility for a certain descriptor ID.
3.3. Processing fetch requests
Hidden service directory nodes process fetch requests for hidden service
descriptors by looking them up in their local memory. (They do not need to
determine if they are responsible for the passed ID, because it does no harm
if they deliver a descriptor for which they are not (any more) responsible.)
All requests and replies are formatted as HTTP messages. Requests are
initiated via BEGIN_DIR cells directed to the router's directory port,
and formatted as HTTP GET requests for the document "/tor/rendezvous2/<z>",
where z is replaced with the encoding of the descriptor ID.
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