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authorRoger Dingledine <arma@torproject.org>2004-05-18 06:18:30 +0000
committerRoger Dingledine <arma@torproject.org>2004-05-18 06:18:30 +0000
commiteac75309f812d4ab8d2cdcd162b80e126ad49577 (patch)
treebe17b9d1a418cfc8d1092cffa5683e8481de3171
parent92c4b3f1392394dcc495e28ad630c9ac0c85812d (diff)
downloadtor-eac75309f812d4ab8d2cdcd162b80e126ad49577.tar
tor-eac75309f812d4ab8d2cdcd162b80e126ad49577.tar.gz
remove obsolete rendezvous pre-spec
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- How to make rendezvous points work
-
-0. Overview
-
- Rendezvous points are an implementation of location-hidden services
- (server anonymity) in the onion routing network. Location-hidden
- services means Bob can offer a tcp service (say, a webserver) via the
- onion routing network, without revealing the IP of that service.
-
- The basic idea is to provide censorship resistance for Bob by allowing
- him to advertise a variety of onion routers as his public location
- (nodes known as his Introduction Points, see Section 1). Alice,
- the client, chooses a node known as a Meeting Point (see Section
- 2). This extra level of indirection is needed so Bob doesn't serve
- files directly from his public locations (so these nodes don't open
- themselves up to abuse, eg from serving Nazi propaganda in France). The
- extra level of indirection also allows Bob to choose which requests
- to respond to, and which to ignore.
-
- We provide the necessary glue code so that Alice can view webpages
- on a location-hidden webserver, and Bob can run a location-hidden
- server, with minimal invasive changes (see Section 3). Both Alice
- and Bob must run local onion proxies (OPs) -- software that knows
- how to talk to the onion routing network.
-
- The big picture follows. We direct the reader to the rest of the
- document for more details and explanation.
-
- 1) Bob chooses some Introduction Points, and advertises them on a
- Distributed Hash Table (DHT).
- 2) Bob establishes onion routing connections to each of his
- Introduction Points, and waits.
- 3) Alice learns about Bob's service out of band (perhaps Bob gave her
- a pointer, or she found it on a website). She looks up the details
- of Bob's service from the DHT.
- 4) Alice chooses and establishes a Meeting Point for this transaction.
- 5) Alice goes to one of Bob's Introduction Points, and gives it a blob
- (encrypted for Bob) which tells him about herself and the Meeting
- Point she chose. The Introduction Point sends the blob to Bob.
- 6) Bob chooses whether to ignore the blob, or to onion route to MP.
- Let's assume the latter.
- 7) MP plugs together Alice and Bob. Note that MP doesn't know (or care)
- who Alice is, or who Bob is; and it can't read anything they
- transmit either, because they share a session key.
- 8) Alice sends a 'begin' cell along the circuit. It makes its way
- to Bob's onion proxy. Bob's onion proxy connects to Bob's webserver.
- 9) Data goes back and forth as usual.
-
-1. Introduction service
-
- Bob wants to learn about client requests for communication, but
- wants to avoid responding unnecessarily to unauthorized clients.
- Bob's proxy opens a circuit, and tells some onion router on that
- circuit to expect incoming connections, and notify Bob of them.
-
- When establishing such an introduction point, Bob provides the onion
- router with a public "introduction" key. The hash of this public
- key identifies a unique Bob, and (since Bob is required to sign his
- messages) prevents anybody else from usurping Bob's introduction
- point in the future. Additionally, Bob can use the same public key
- to establish an introduction point on another onion router (OR),
- and Alice can still be confident that Bob is the same server.
-
- (The set-up-an-introduction-point command should come via a
- RELAY_BIND_INTRODUCTION cell. This cell creates a new stream on the
- circuit from Bob to the introduction point.)
-
- ORs that support introduction run an introduction service on a
- separate port. When Alice wants to notify Bob of a meeting point,
- she connects (directly or via Tor) to the introduction port, and
- sends the following:
-
- MEETING REQUEST
- RSA-OAEP encrypted with server's public key:
-[20 bytes] Hash of Bob's public key (identifies which Bob to notify)
-[ 0 bytes] Initial authentication [optional]
- RSA encrypted with Bob's public key:
-[16 bytes] Symmetric key for encrypting blob past RSA
-[ 6 bytes] Meeting point (IP/port)
-[ 8 bytes] Meeting cookie
-[ 0 bytes] End-to-end authentication [optional]
-[98 bytes] g^x part 1 (inside the RSA)
-[30 bytes] g^x part 2 (symmetrically encrypted)
-
- The meeting point and meeting cookie allow Bob to contact Alice and
- prove his identity; the end-to-end authentication enables Bob to
- decide whether to talk to Alice; the initial authentication enables
- the introduction point to pre-screen introduction requests before
- sending them to Bob. (See Section 2 for a discussion of meeting
- points; see Section 1.1 for an example authentication mechanism.)
-
- The authentication steps are the appropriate places for the
- introduction server or Bob to do replay prevention, if desired.
-
- When the introduction point receives a valid meeting request, it
- sends the portion intended for Bob along the stream
- created by Bob's RELAY_BIND_INTRODUCTION. Bob then, at his
- discretion, connects to Alice's meeting point.
-
-1.1. An example authentication scheme for introduction services
-
- Bob makes two short-term secrets SB and SN, and tells the
- introduction point about SN. Bob gives Alice a cookie consisting
- of A,B,C such that H(A|SB)=B and H(A|SN)=C. Alice's initial
- authentication is <A,C>; Alice's end-to-end authentication is <A,B>.
-
- [Maybe] Bob keeps a replay cache of A values, and doesn't allow any
- value to be used twice. Over time, Bob rotates SB and SN.
-
- [Maybe] Each 'A' has an expiration time built in to it.
-
- In reality, we'll want to pick a scheme that (a) wasn't invented from
- scratch in an evening, and (b) doesn't require Alice to remember this
- many bits (see section 3.2).
-
-2. Meeting points
-
- For Bob to actually reply to Alice, Alice first establishes a
- circuit to an onion router R, and sends a RELAY_BIND_MEETING cell
- to that onion router. The RELAY_BIND_MEETING cell contains a
- 'Meeting cookie' (MC) that Bob can use to authenticate to R. R
- remembers the cookie and associates it with Alice.
-
- Later, Bob also routes to R and sends R a RELAY_JOIN_MEETING cell with
- the meeting cookie MC. After this point, R routes all traffic from
- Bob's circuit or Alice's circuit as if the two circuits were joined:
- any RELAY cells that are not for a recognized topic are passed down
- Alice or Bob's circuit. Bob's first cell to Alice contains g^y.
-
- To prevent R from reading their traffic, Alice and Bob derive two
- end-to-end keys from g^{xy}, and they each treat R as just another
- hop on the circuit. (These keys are used in addition to the series
- of encryption keys already in use on Alice and Bob's circuits.)
-
- Bob's OP accepts RELAY_BEGIN, RELAY_DATA, RELAY_END, and
- RELAY_SENDME cells from Alice. Alice's OP accepts RELAY_DATA,
- RELAY_END, and RELAY_SENDME cells from Bob. All RELAY_BEGIN cells
- to Bob must have target IP and port of zero; Bob's OP will redirect
- them to the actual target IP and port of Bob's server.
-
- Alice and Bob's OPs disallow CREATE or RELAY_EXTEND cells as usual.
-
-3. Application interface
-
-3.1. Application interface: server side
-
- Bob has a service that he wants to offer to the world but keep its
- location hidden. He configures his local OP to know about this
- service, including the following data:
-
- Local IP and port of the service
- Strategy for choosing introduction points
- (for now, just randomly pick among the ORs offering it)
- Strategy for user authentication
- (for now, just accept all users)
- Public (RSA) key (one for each service Bob offers)
-
- Bob chooses a set of N Introduction servers on which to advertise
- his service.
-
- We assume the existence of a robust decentralized efficient lookup
- system (call it "DHT" for distributed hash table -- note that the
- onion routers can run nodes). Bob publishes
- * Bob's Public Key for that service
- * Expiration date ("don't use after")
- * Introduction server 0 ... Introduction server N
- (All signed by Bob's Public Key)
- into DHT, indexed by the hash of Bob's Public Key. Bob should
- periodically republish his introduction information with a new
- expiration date (and possibly with new/different introduction servers
- if he wants), so Alice can trust that DHT is giving her an up-to-date
- version. The Chord CFS paper describes a sample DHT that allows
- authenticated updating.
-
-3.2. Application interface: client side
-
- We require that the client interface remain a SOCKS proxy, and we
- require that Alice shouldn't have to modify her applications. Thus
- we encode all of the necessary information into the hostname (more
- correctly, fqdn) that Alice uses, eg when clicking on a url in her
- browser.
-
- To establish a connection to Bob, Alice needs to know an Introduction
- point, Bob's PK, and some authentication cookie. Because encoding this
- information into the hostname will be too long for a typical hostname,
- we instead use a layer of indirection. We encode a hash of Bob's PK
- (10 bytes is sufficient since we're not worrying about collisions),
- and also the authentication token (empty for now). Location-hidden
- services use the special top level domain called '.onion': thus
- hostnames take the form x.y.onion where x is the hash of PK, and y
- is the authentication cookie. If no cookie is required, the hostname
- can simply be of the form x.onion. Assuming only case insensitive
- alphanumeric and hyphen, we get a bit more than 6 bits encoded
- per character, meaning the x part of the hostname will be about
- 13 characters.
-
- Alice's onion proxy examines hostnames and recognizes when they're
- destined for a hidden server. If so, it decodes the PK and performs
- the steps in Section 0 above.
-