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321 lines
14 KiB
Plaintext
321 lines
14 KiB
Plaintext
$Id$
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Tor Rendezvous Spec
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0. Overview and preliminaries
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Rendezvous points provide location-hidden services (server
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anonymity) for the onion routing network. With rendezvous points,
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Bob can offer a TCP service (say, a webserver) via the onion
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routing network, without revealing the IP of that service.
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Bob does this by anonymously advertising a public key for his
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service, along with a list of onion routers to act as "Introduction
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Points" for his service. He creates forward OR circuits to those
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introduction points, and tells them about his public key. To
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connect to Bob, Alice first builds an OR circuit to an OR to act as
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her "Rendezvous Point", then connects to one of Bob's chosen
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introduction points, and asks it to tell him about her Rendezvous
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Point (RP). If Bob chooses to answer, he builds an OR circuit to her
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RP, and tells it to connect him to Alice. The RP joints their
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circuits together, and begins relaying cells. Alice's 'BEGIN'
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cells are received directly by Bob's OP, which responds by
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communication with the local server implementing Bob's service.
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Below, we describe a network-level specification of this service,
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along with interfaces to make this process transparent to Alice
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(so long as she is using an OP).
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0.1. Notation, conventions and prerequisites
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In the specifications below, we use the same notation as in
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"tor-spec.txt". The service specified here also requires the existence of
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an onion routing network as specified in "tor-spec.txt".
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H(x) is a SHA1 digest of x.
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PKSign(SK,x) is a PKCS.1-padded RSA signature of x with SK.
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PKEncrypt(SK,x) is a PKCS.1-padded RSA encryption of x with SK.
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Public keys are all RSA, and encoded in ASN.1.
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All integers are stored in network (big-endian) order.
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All symmetric encryption uses AES in counter mode, except where
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otherwise noted.
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In all discussions, "Alice" will refer to a user connecting to a
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location-hidden service, and "Bob" will refer to a user running a
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location-hidden service.
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0.2. Protocol outline
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1. Bob->Bob's OP: "Offer IP:Port as public-key-name:Port". [configuration]
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(We do not specify this step; it is left to the implementor of
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Bob's OP.)
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2. Bob's OP generates keypair and rendezvous service descriptor:
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"Meet public-key X at introduction point A, B, or C." (signed)
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3. Bob's OP->Introduction point via Tor: [introduction setup]
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"This pk is me."
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4. Bob's OP->directory service via Tor: publishes Bob's service descriptor
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[advertisement]
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5. Out of band, Alice receives a y.onion:port address. She opens a
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SOCKS connection to her OP, and requests y.onion:port.
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6. Alice's OP retrieves Bob's descriptor via Tor: [descriptor lookup.]
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7. Alice's OP chooses a rendezvous point, opens a circuit to that
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rendezvous point, and establishes a rendezvous circuit. [rendezvous
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setup.]
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8. Alice connects to the Introduction point via Tor, and tells it about
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her rendezvous point. (Encrypted to Bob.) [Introduction 1]
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9. The Introduction point passes this on to Bob's OP via Tor, along the
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introduction circuit. [Introduction 2]
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10. Bob's OP decides whether to connect to Alice, and if so, creates a
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circuit to Alice's RP via Tor. Establishes a shared circuit.
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[Rendezvous.]
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11. Alice's OP sends begin cells to Bob's OP. [Connection]
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0.3. Constants and new cell types
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Relay cell types
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32 -- RELAY_ESTABLISH_INTRO
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33 -- RELAY_ESTABLISH_RENDEZVOUS
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34 -- RELAY_INTRODUCE1
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35 -- RELAY_INTRODUCE2
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36 -- RELAY_RENDEZVOUS1
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37 -- RELAY_RENDEZVOUS2
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1. The Protocol
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1.1. Bob configures his local OP.
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We do not specify a format for the OP configuration file. However,
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OPs SHOULD allow Bob to provide more than one advertised service
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per OP, and MUST allow Bob to specify one or more virtual ports per
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service. Bob provides a mapping from each of these virtual ports
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to a local IP:Port pair.
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1.2. Bob's OP generates service descriptors.
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The first time the OP provides an advertised service, it generates
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a public/private keypair (stored locally). Periodically, the OP
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generates service descriptor, containing:
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KL Key length [2 octets]
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PK Bob's public key [KL octets]
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TS A timestamp [4 octets]
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NI Number of introduction points [2 octets]
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Ipt A list of NUL-terminated OR nicknames [variable]
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SIG Signature of above fields [KL octets]
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KL is the length of PK, in octets. (Currently, KL must be 128.)
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TS is the number of seconds elapsed since Jan 1, 1970.
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[Shouldn't the nicknames be hostname:port's instead? That way, Alice's
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directory servers don't need to know Bob's chosen introduction points.
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Not important now, but essential if we ever have a non-total-knowledge
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design. -NM]
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1.3. Bob's OP establishes his introduction points.
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The OP establishes a new introduction circuit to each introduction
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point. These circuits MUST NOT be used for anything but rendezvous
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introduction. To establish the introduction, Bob sends a
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RELAY_ESTABLISH_INTRO cell, containing:
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KL Key length [2 octets
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PK Bob's public key [KL octets]
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HS Hash of session info [20 octets]
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SIG Signature of above information [KL octets]
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To prevent replay attacks, the HS field contains a SHA-1 hash based on the
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shared secret g^xy between Bob's OP and the introduction point, as
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follows:
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HS = H(g^xy | "INTRODUCE")
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That is:
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HS = H(g^xy | [49 4E 54 52 4F 44 55 43 45])
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Upon receiving such a cell, the OR first checks that the signature is
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correct with the included public key. If so, it checks whether HS is
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correct given the shared state between Bob's OP and the OR. If either
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check fails, the OP discards the cell; otherwise, it associates the
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circuit with Bob's public key, and dissociates any other circuits
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currently associated with PK.
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1.4. Bob's OP advertises his server descriptor
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Bob's OP opens a stream to each directory server's directory port via Tor.
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(He may re-use old circuits for this.)
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Over this stream, Bob's OP makes an HTTP 'POST' request, to the URL
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'/rendezvous/publish' (relative to the directory server's root),
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containing as its body Bob's service descriptor. Upon receiving a
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descriptor, the directory server checks the signature, and discards the
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descriptor if the signature does not match the enclosed public key. Next,
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the directory server checks the timestamp. If the timestamp is more than
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24 hours in the past or more than 1 hour in the future, or the directory
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server already has a newer descriptor with the same public key, the server
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discards the descriptor. Otherwise, the server discards any older
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descriptors with the same public key, and associates the new descriptor
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with the public key. The directory server remembers this descriptor for
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at least 24 hours after its timestamp. At least every 24 hours, Bob's OP
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uploads a fresh descriptor.
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1.5. Alice receives a y.onion address
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When Alice receives a pointer to a location-hidden service, it is as a
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hostname of the form "y.onion", where y is a base-32 encoding of a
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10-octet hash of Bob's service's public key, computed as follows:
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1. Let H = H(PK).
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2. Let H' = the first 80 bits of H, considering each octet from
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most significant bit to least significant bit.
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2. Generate a 16-character encoding of H', taking H' 5 bits at
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a time, and mapping each 5-bit value to a character as follows:
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0..25 map to the characters 'a'...'z', respectively.
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26..31 map to the characters '0'...'5', respectively.
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(We only use 80 bits instead of the 160 bits from SHA1 because we don't
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need to worry about man-in-the-middle attacks, and because it will make
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handling the url's more convenient.)
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[Yes, numbers are allowed at the beginning. See RFC1123. -NM]
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1.6. Alice's OP retrieves a service descriptor
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Alice opens a stream to a directory server via Tor, and makes an HTTP GET
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request for the document '/rendezvous/<y>', where '<y> is replaced with the
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encoding of Bob's public key as described above. (She may re-use old
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circuits for this.) The directory replies with a 404 HTTP response if
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it does not recognize <y>, and otherwise returns Bob's most recently
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uploaded service descriptor.
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If Alice's OP receives a 404 response, it tries the other directory
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servers, and only fails the lookup if none recognizes the public key hash.
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Upon receiving a service descriptor, Alice verifies with the same process
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as the directory server uses, described above in section 1.4.
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The directory server gives a 400 response if it cannot understand Alice's
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request.
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Alice should cache the descriptor locally, but should not use
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descriptors that are more than 24 hours older than their timestamp.
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[Caching may make her partitionable, but she fetched it anonymously,
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and we can't very well *not* cache it. -RD]
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1.7. Alice's OP establishes a rendezvous point.
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When Alice requests a connection to a given location-hidden service,
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and Alice's OP does not have an established circuit to that service,
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the OP builds a rendezvous circuit. It does this by establishing
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a circuit to a randomly chosen OR, and sending a
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RELAY_ESTABLISH_RENDEZVOUS cell to that OR. The body of that cell
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contains:
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RC Rendezvous cookie [20 octets]
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The rendezvous cookie is an arbitrary 20-byte value, chosen randomly by
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Alice's OP.
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Upon receiving a RELAY_ESTABLISH_RENDEZVOUS cell, the OR associates the
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RC with the circuit that sent it.
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Alice's OP MUST NOT use the circuit which sent the cell for any purpose
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other than rendezvous with the given location-hidden service.
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1.8. Introduction: from Alice's OP to Introduction Point
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Alice builds a separate circuit to one of Bob's chosen introduction
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points, and sends it a RELAY_INTRODUCE1 cell containing:
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Cleartext
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PK_ID Identifier for Bob's PK [20 octets]
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Encrypted to Bob's PK:
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RP Rendezvous point's nickname [variable]
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RC Rendezvous cookie [20 octets]
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g^x Diffie-Hellman data, part 1 [128 octetes]
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PK_ID is the hash of Bob's public key. RP is NUL-terminated.
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The data is encrypted to Bob's PK as follows: Suppose Bob's PK is L octets
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long. If the data to be encrypted is shorter than L, then it is encrypted
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directly (with no padding). If the data is at least as long as L, then a
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randomly generated 16-byte symmetric key is prepended to the data, after
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which the first L-16 bytes of the data are encrypted with Bob's PK; and the
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rest of the data is encrypted with the symmetric key.
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[There's very little here is that is recognizable by Bob to let him
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know if he decrypted it correctly. Perhaps we should add OAEP padding
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to the RSA -- there is space -RD]
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1.9. Introduction: From the Introduction Point to Bob's OP
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If the Introduction Point recognizes PK_ID as a public key which has
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established a circuit for introductions as in 1.3 above, it sends the
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encrypted portion of the cell in a new RELAY_INTRODUCE2 cell down the
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corresponding circuit. (If the PK_ID is unrecognized, the
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RELAY_INTRODUCE1 cell is discarded.)
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When Bob's OP receives the RELAY_INTRODUCE2 cell, it decrypts it with
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the private key for the corresponding hidden service, and extracts the
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rendezvous point's nickname, the rendezvous cookie, and the value of g^x
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chosen by Alice.
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1.10. Rendezvous
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Bob's OP build a new Tor circuit ending at Alice's chosen rendezvous
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point, and sends a RELAY_RENDEZVOUS1 cell along this circuit, containing:
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RC Rendezvous cookie [20 octets]
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g^y Diffie-Hellman [128 octets]
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H(KH) Handshake digest [20 octets]
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(Bob's OP MUST NOT use this circuit for any other purpose.)
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If the RP recognizes RC, it relays the rest of the cell down the
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corresponding circuit in a RELAY_RENDEZVOUS2 cell, containing:
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g^y Diffie-Hellman [128 octets]
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H(KH) Handshake digest [20 octets]
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(If the RP does not recognize the RC, it discards the cell and
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tears down the circuit.)
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When Alice's OP receives a RELAY_RENDEZVOUS2 cell on a circuit which
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has sent a RELAY_ESTABLISH_RENDEZVOUS cell but which has not yet received
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a reply, it uses g^y and H(g^xy) to complete the handshake as in the Tor
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circuit extend process: they establish a 60-octet string as
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K = SHA1(g^xy | [00]) | SHA1(g^xy | [01]) | SHA1(g^xy | [02])
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and generate
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KH = K[0..15]
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Kf = K[16..31]
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Kb = K[32..47]
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Subsequently, the rendezvous point passes relay cells, unchanged, from
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each of the two circuits to the other. When Alice's OP sends
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RELAY cells along the circuit, it first encrypts them with the
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Kf, then with all of the keys for the ORs in Alice's side of the circuit;
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and when Alice's OP receives RELAY cells from the circuit, it decrypts
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them with the keys for the ORs in Alice's side of the circuit, then
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decrypts them with Kb. Bob's OP does the same, with Kf and Kb
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interchanged.
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1.11. Creating streams
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To open TCP connections to Bob's location-hidden service, Alice's OP sends
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a RELAY_BEGIN cell along the established circuit, using the special
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address "", and a chosen port. Bob's OP chooses a destination IP and
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port, based on the configuration of the service connected to the circuit,
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and opens a TCP stream. From then on, Bob's OP treats the stream as an
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ordinary exit connection.
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Alice MAY send multiple RELAY_BEGIN cells along the circuit, to open
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multiple streams to Bob. Alice SHOULD NOT send RELAY_BEGIN cells for any
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other address along her circuit to Bob; if she does, Bob MUST reject them.
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