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