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Michael G􏰇 Reed􏰆
munication over a public network􏰇 It provides anony􏰘 mous connections that are strongly resistant to both eavesdropping and tra􏰔c analysis􏰇 Onion routing􏰕s anonymous connections are bidirectional and near real􏰘 time􏰆 and can be used anywhere a socket connection can be used􏰇 Any identifying information must be in the data stream carried over an anonymous connec􏰘 tion􏰇 An onion is a data structure that is treated as the destination address by onion routers􏰜 thus􏰆 it is used to establish an anonymous connection􏰇 Onions them􏰘 selves appear di􏰓erently to each onion router as wel l as to network observers􏰇 The same goes for data carried over the connections they establish􏰇 Proxy aware ap􏰘 plications􏰆 such as web browsing and e􏰘mail􏰆 require no
Intro duction
is an infrastructure for private com􏰘

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reveal who they are communicating with to the rest of the world􏰇 In certain cases anonymity may b e desir􏰘 able also􏰛 anonymous e􏰘cash is not very anonymous if delivered with a return address􏰇 Web based shopping or browsing of public databases should not require re􏰘 vealing one􏰕s identity􏰇
modi􏰉cation to use
a series of proxies􏰇 A prototype onion routing network is running between our lab and other sites􏰇 This paper describes anonymous connections and their implemen􏰘
con􏰉gurations of
Internet communication
eavesdropping 􏰤􏰊􏰑􏰦􏰆 i􏰇e􏰇􏰆
the existence con􏰉dential􏰇
Address􏰛 ratory􏰆 Center
of inter􏰘company collab oration Similarly􏰆 e􏰘mail users may not
may b e wish to
Although onion routing may b e used for anony􏰘 mous communication􏰆 it di􏰓ers from anonymous re􏰘
ington􏰆 D􏰇C􏰇 􏰋􏰚􏰌􏰐􏰎􏰘􏰎􏰌􏰌􏰐􏰆 USA􏰆
􏰨􏰊 􏰋􏰚􏰋􏰇􏰍􏰚􏰍􏰇􏰐􏰒􏰍􏰋􏰆
Goldschlag􏰗 Divx􏰆 􏰎􏰐􏰚 SA􏰆 phone􏰛 􏰨􏰊 􏰐􏰚􏰌􏰘􏰐􏰚􏰑􏰘􏰍􏰚􏰋􏰑􏰇 david􏰇goldschlag􏰝divx􏰇com
􏰖For Herndon􏰆 VA 􏰋􏰚􏰊􏰐􏰚􏰆 􏰐􏰚􏰌􏰘􏰐􏰚􏰑􏰘􏰍􏰚􏰑􏰑􏰆 e􏰘mail􏰛
Preliminary 􏰤􏰋􏰑􏰆 􏰊􏰍 􏰆 􏰋􏰍 􏰦􏰇
onion routing􏰆 and do so through
tation using onion routing􏰇 This paper also several application proxies for onion routing􏰆 as
concerns fo cus on preventing
outsiders listening in on electronic conversations􏰇 But encrypted messages can still b e tracked􏰆 revealing who is talking to whom􏰇 This tracking is called tra􏰔c analy􏰘 sis and may reveal sensitive information􏰇 For example􏰆
e􏰘mail􏰛 flast
nameg􏰝itd􏰇nrl􏰇navy􏰇mil􏰇
Onion of p ortions of
Assurance Computer
Connections and Onion Routing
􏰇 Syverson􏰆 and 􏰇 Goldschlag
Naval Research
Lab oratory
describes wel l as
Most security
This pap er
onion routing 􏰆 can b e used to protect a variety of In􏰘
describ es how a freely available system􏰆
ternet services
analysis attacks􏰆 from b oth the network and outside ob􏰘 servers􏰇 This pap er includes a sp eci􏰉cation su􏰔cient to guide b oth re􏰘implementations and new applications of onion routing􏰇 We also discuss con􏰉gurations of onion routing networks and applications of onion routing􏰆 in􏰘 cluding Virtual Private Networks 􏰖VPN􏰗􏰆 Web brows􏰘 ing􏰆 e􏰘mail􏰆 remote login􏰆 and electronic cash􏰇􏰊
A purp ose of tra􏰔c analysis is to reveal who is talk􏰘 ing to whom􏰇 The anonymous connections describ ed here are designed to be resistant to tra􏰔c analysis􏰆 i􏰇e􏰇􏰆 to make it di􏰔cult for observers to learn identifying in􏰘 formation from the connection 􏰖e􏰇g􏰇􏰆 by reading packet headers􏰆 tracking encrypted payloads􏰆 etc􏰇􏰗􏰇 Any iden􏰘 tifying information must b e passed as data through the anonymous connections􏰇 Our implementation of anonymous connections􏰆 onion routing􏰆 provides pro􏰘 tection against eavesdropping as a side e􏰓ect􏰇 Onion routing provides bidirectional and near real􏰘time com􏰘 munication similar to TCP􏰙IP so cket connections or ATM AAL􏰎 􏰤􏰏􏰦􏰇 The anonymous connections can sub􏰘 stitute for so ckets in a wide variety of unmo di􏰉ed Inter􏰘 net applications by means of proxies􏰇 Data may also b e passed through a privacy 􏰉lter b efore b eing sent over
against b oth eavesdropping and tra􏰔c
an anonymous connection􏰇 This removes information from the data stream􏰆 to make cation anonymous to o􏰇
identifying communi􏰘
􏰖For Reed and Syverson􏰗 Naval
Research Systems􏰆
Lab o􏰘 Wash􏰘 phone􏰛 􏰨􏰊 􏰋􏰚􏰋􏰇􏰐􏰏􏰐􏰇􏰋􏰌􏰑􏰒􏰆 fax􏰛
Parkway􏰆 fax􏰛 􏰨􏰊
mailers 􏰤􏰐􏰆 􏰊􏰏􏰦 in two ways􏰛 and bidirectional􏰆 and the application indep endent􏰇
Communication is real􏰘time anonymous connections are
routing􏰕s anonymous this pap er have app eared

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Anonymous Connections and Onion Routing
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connections can supp ort anonymous mail as well as other applications􏰇 For example􏰆 onion routing may b e used for anonymous Web browsing􏰇 A user may wish to browse public Web sites without revealing his identity to those Web sites􏰇 That requires removing informa􏰘 tion that identi􏰉es him from his requests to Web servers and removing information from the connection itself that may identify him􏰇 Hence􏰆 anonymous Web brows􏰘 ing uses anonymized communication over anonymous connections􏰇 The Anonymizer 􏰤􏰊􏰦 only anonymizes the data stream􏰆 not the connection itself􏰇 So it do es not
2.1 Operational Overview
prevent tra􏰔c analysis attacks like tracking moves through the network􏰇
data as it
This pap er is organized in the following
tion 􏰋 presents an overview of onion routing􏰇 Section 􏰌 presents empirical data ab out our prototyp e􏰇 Sec􏰘 tion 􏰍 de􏰉nes our threat mo del􏰇 Section 􏰎 describ es onion routing and the application sp eci􏰉c proxies in
more detail􏰇
choices that were made for security reasons􏰇 Section 􏰐 describ es how onion routing may b e used in a wide va􏰘 riety of Internet applications􏰇 Section 􏰑 contrasts onion routing with related work􏰆 and section 􏰒 presents con􏰘 cluding remarks􏰇
Section 􏰏 describ es the implementation
In onion routing􏰆 instead of
tions directly to a resp onding
plications make connections through a sequence of ma􏰘 chines called onion routers 􏰇 The onion routing net􏰘 work allows the connection b etween the initiator and responder to remain anonymous􏰇 Anonymous connec􏰘 tions hide who is connected to whom􏰆 and for what purp ose􏰆 from b oth outside eavesdropp ers and com􏰘 promised onion routers􏰇 If the initiator also wants to remain anonymous to the resp onder􏰆 then all identify􏰘 ing information must b e removed from the data stream b efore b eing sent over the anonymous connection􏰇
Onion routers in the network are connected by long􏰘 standing 􏰖p ermanent􏰗 so cket connections􏰇 Anonymous connections through the network are multiplexed over the longstanding connections􏰇 For any anonymous con􏰘 nection􏰆 the sequence of onion routers in a route is strictly de􏰉ned at connection setup􏰇 However􏰆 each onion router can only identify the previous and next hops along a route􏰇 Data passed along the anonymous connection app ears di􏰓erent at each onion router􏰆 so data cannot b e tracked en route􏰆 and compromised onion routers cannot co op erate by correlating the data stream each sees􏰇 We will also see that they cannot make use of replayed onions or replayed data􏰇
connection is established􏰆 it sending data over an anony􏰘 mous connection􏰆 the onion proxy adds a layer of en􏰘
machine􏰆 initiating ap􏰘
series so cket connection to an application proxy 􏰇 This proxy mas􏰘 sages connection message format 􏰖and later data􏰗 to a generic form that can be passed through the onion rout􏰘 ing network􏰇 It then connects to an onion proxy 􏰆 which
and the resp onder􏰇
In addition to carrying next hop information􏰆 each
onion layer contains key seed material from which keys
are generated for crypting data sent forward or back􏰘
ward along the anonymous connection􏰇 􏰖We de􏰉ne for􏰘 ward to b e the direction in which the onion travels and
proxies 􏰇 An initiating application makes a
The onion routing network is accessed via a
de􏰉nes a route
constructing a
The onion is passed to the
pies one of the longstanding connections to an
backward as the opp osite Once the anonymous
direction􏰇􏰗
can carry data􏰇
through the
layered data structure called an onion 􏰇
o ccu􏰘 onion rout􏰘 ing network at that onion router􏰇 That onion router will b e the one for whom the outermost layer of the onion is intended􏰇 Each layer of the onion de􏰉nes the next hop in a route􏰇 An onion router that receives an onion p eels o􏰓 its layer􏰆 identi􏰉es the next hop􏰆 and sends the emb edded onion to that onion router􏰇 The last onion router forwards data to an exit funnel 􏰆 whose job is to pass data b etween the onion routing network
router and multiplexes connections to the
cryption for each
moves through the anonymous connection􏰆 each onion router removes one layer of encryption􏰆 so it arrives at the resp onder as plaintext􏰇 This layering o ccurs in the reverse order for data moving back to the initiator􏰇 So data that has passed backward through the anonymous
onion routing network by
entry funnel 􏰆
onion router in the route􏰇 As data
connection must b e rep eatedly p ost􏰘crypted the plaintext􏰇
By layering cryptographic op erations in this way􏰆 we gain an advantage over link encryption􏰇 As data moves through the network it app ears di􏰓erent to each onion router􏰇 Therefore􏰆 an anonymous connection is as strong as its strongest link􏰆 and even one honest no de
is enough to maintain the privacy encrypted systems􏰆 compromised to uncover route information􏰇
Onion routers keep track of received
they expire􏰇 Replayed or expired onions
warded􏰆 so they cannot b e used to uncover route in􏰘
formation􏰆 either by outsiders or compromised
no des can co op erate
route􏰇 In link
onions until are not for􏰘
􏰋 We de􏰉ne the verb crypt to mean the application of a cryp􏰘 tographic op eration􏰆 b e it encryption or decryption􏰇

routers􏰇 Note that clo ck skew b etween onion routers can only cause an onion router to reject a fresh onion or to keep track of pro cessed onions longer than nec􏰘
tracking of tra􏰔c originating or terminating within the sensitive site􏰆 this onion router should also route data b etween other onion routers􏰇 This con􏰉guration might represent the system interface from a typical corp orate or government site􏰇 Here the application proxies 􏰖to􏰘 gether with any privacy 􏰉lters􏰗􏰆 and the onion proxies would typically live at the 􏰉rewall as well􏰇 􏰖Typically􏰆 there might only b e one onion proxy􏰇􏰗
There are three imp ortant features of this basic con􏰘 􏰉guration􏰛
essary􏰇 Also􏰆 since data is
encrypted using stream ci􏰘
phers􏰆 passes
routing that
layer of the
More sp eci􏰉cally􏰆 we rely up on IP routing to route data
passed through the longstanding so cket
An anonymous connection is comprised of p ortions of several linked longstanding multiplexed so cket connec􏰘 tions􏰇 Therefore􏰆 although the series of onion routers in an anonymous connection is 􏰉xed for the lifetime of that anonymous connection􏰆 the route that data ac􏰘 tually travels b etween individual onion routers is de􏰘 termined by the underlying IP network􏰇 Thus􏰆 onion routing may b e compared to lo ose source routing􏰇
are not the dressed in a
key parts of onion routing later prototyp e􏰇
replayed data will through a prop erly
lo ok di􏰓erent each time op erating onion router􏰇
2.2 Configurations
we call this system onion routing􏰆 the o ccurs here do es so at the application proto col stack and not at the IP layer􏰇
vices that deliver data uncorrupted and in􏰘order􏰇 This simpli􏰉es the sp eci􏰉cation of the system􏰇 TCP so cket connections􏰆 which are layered on top of a connection􏰘 less service like IP􏰆 provide these guarantees􏰇 Similarly􏰆 onion routing could easily b e layered on top of other connection based services􏰆 like ATM AAL􏰎􏰇
Our current prototyp e of onion routing considers the network top ology to b e static and do es not have mecha􏰘
Connections b etween machines b ehind onion routers are protected against b oth eavesdropping and tra􏰔c analysis􏰇 Since the data stream never app ears in the clear on the public network􏰆 this data may carry identifying information􏰆 but com􏰘 munication is still private􏰇 􏰖This feature is used in section 􏰐􏰇􏰊􏰇􏰗
The onion router at the originating protected site knows b oth the source and destination of a con􏰘 nection􏰇 This protects the anonymity of con􏰘 nections from observers outside the 􏰉rewall but also simpli􏰉es enforcement of and monitoring for compliance with corp orate or governmental usage p olicy􏰇
routing dep ends up on connection based ser􏰘
nisms to automatically distribute or
or network top ology􏰇 These issues􏰆 though
public keys imp ortant􏰆 will b e ad􏰘
e􏰓ectively hides
siders􏰇 However􏰆 if the resp onder is not in a sen􏰘 sitive site 􏰖e􏰇g􏰇􏰆 the resp onder is some arbitrary
As mentioned ab ove
neighb ors in virtue of having longstanding
nections b etween them􏰆 and the network as a whole is accessed from the outside through a series of proxies􏰇 By adjusting where those proxies reside it is p ossible to vary which elements of the system are trusted by users and in what way􏰇 􏰖For some con􏰉gurations it may b e ef􏰘 􏰉cient to combine proxies that reside in the same place􏰆 thus they may b e only conceptually distinct􏰇􏰗
􏰋􏰇􏰋􏰇􏰊 Firewall Con􏰉guration
neighb oring onion
so cket con􏰘
In the 􏰉rewal l con􏰉guration 􏰆 an onion router
the 􏰉rewall of a sensitive site􏰇 This onion router serves as an interface b etween machines b ehind the 􏰉rewall and the external network􏰇 Connections from machines b ehind the 􏰉rewall to the onion router are protected by other means 􏰖e􏰇g􏰇􏰆 physical security􏰗􏰇 To complicate
connections􏰇
The use of anonymous
connections
sites that b oth control onion routers
their communication from out􏰘
Web server􏰗 the data stream from
initiator must also b e anonymized􏰇
nection b etween the exit funnel and the resp ond􏰘 ing server is unencrypted􏰆 the data stream might otherwise identify the initiator􏰇 For example􏰆 an attacker could simply listen in on the connections to a Web server and identify initiators of any con􏰘 nection to it􏰇
Remote Proxy Con􏰉guration
router􏰣 If the initiator can make encrypted connections to some remote onion router􏰆 then he can function as if he is in the 􏰉rewall con􏰉guration just describ ed􏰆 ex􏰘 cept that b oth observers and the network can tell when he makes connections to the onion router􏰇 However􏰆 if the initiator trusts the onion router to build onions􏰆 his asso ciation with the anonymous connection from that onion router to the responder is hidden from observers and the network􏰇 In a similar way􏰆 an encrypted con􏰘 nection from an exit funnel to a responder hides the
happ ens if an initiator do es not control an onion
b etween two
the sensitive If the con􏰘

asso ciation of the resp onder with the anonymous con􏰘 nection 􏰇
Therefore􏰆 if an initiator makes an anonymous con􏰘 nection to some resp onder􏰆 and layers end􏰘to􏰘end en􏰘 cryption over that anonymous connection􏰆 the initia􏰘 tor and resp onder can identify themselves to one an􏰘 other􏰆 yet hide their communication from the rest of the world􏰇 So we can build virtual private networks without protected sites􏰇
Notice􏰆 however􏰆 that the initiator trusts the remote onion router to conceal that the initiator wants to com􏰘 municate with the resp onder􏰆 and to build an anony􏰘
􏰌 Empirical Data
We invite readers to exp eriment with our pro􏰘 totyp e of onion routing by using it to anony􏰘 mously surf the Web􏰆 send anonymous e􏰘mail􏰆 and do remote logins􏰇 For instructions please see http􏰛􏰙􏰙www􏰇itd􏰇nrl􏰇navy􏰇mil􏰙ITD􏰙􏰎􏰎􏰍􏰚􏰙projects􏰙 onion􏰘routing􏰙􏰇
One should b e aware that accessing a remote onion router do es not completely preserve anonymity􏰆 b e􏰘
from the 􏰉rst
Supp ose􏰆 for
􏰖ISP􏰗 runs a
onion proxies
this con􏰉guration􏰆 users generate onions sp ecifying a path through the ISP to the destination􏰇 Although the ISP would know who initiates the connection􏰆 the ISP would not know with whom the customer is communi􏰘 cating􏰆 nor would it b e able to see data content􏰇 So the customer need not trust the ISP to maintain her pri􏰘 vacy􏰇 Furthermore􏰆 the ISP b ecomes a common carrier􏰆 who carries data for its customers􏰇 This may relieve the ISP of resp onsibility b oth for whom users are commu􏰘 nicating with and the content of those conversations􏰇 The ISP may or may not b e running an onion router as
remote proxy
connection through
section describ es how to shift some of this trust
the funnel can b e directly router􏰇
incorp orated
initiator􏰇
Customer􏰟ISP
Con􏰉guration
other onion routers􏰇 The
example􏰆 an
funnel that
running on subscrib ers􏰕 machines􏰇 In
at press time we have not yet gathered p er􏰘 data for this network􏰇 The data we present
well􏰇 If he is
di􏰔cult to identify connections that terminate with his customers􏰜 however􏰆 he is serving as a routing p oint for other tra􏰔c􏰇 On the other hand􏰆 if he simply runs a funnel to an onion router elsewhere􏰆 it will b e p ossible to identify connections terminating with him􏰆 but his overall tra􏰔c load will b e less􏰇 Which of these would b e the case for a given ISP would probably dep end on a va􏰘 riety of service􏰆 cost􏰆 and pricing considerations􏰇 Note that in this con􏰉guration the entry funnel must have an established longstanding connection to an onion router just like any neighb oring onion router􏰇 􏰖Cf􏰇 section 􏰎􏰇􏰏 for a description of how these are established􏰇􏰗 But􏰆 in most other cases􏰆 where the funnel resides on the same machine as the onion router􏰆 establishing an encrypted longstanding connection should not be necessary since
Internet accepts
Services Provider connections from
running an onion router􏰆 then it is more
cause the connection b etween a remote
the 􏰉rst onion router is not protected􏰇 If that connec􏰘
tion were protected􏰆 one con􏰉guration􏰆 but there son to trust the remote secured connection to an
onion router could b e
ate routers We have network of
complicate tra􏰔c analysis􏰇
would would onion onion
to further
recently set up a thirteen no de distributed government􏰆 academic􏰆 and private sites􏰇
router one trusted􏰆 our used as one of several intermedi􏰘
are for a network running on a single machine􏰇 In our exp erimental onion routing network􏰆 􏰉ve onion routers run on a single Sun Ultra 􏰋 􏰋􏰊􏰐􏰚􏰇 This machine has two 􏰊􏰏􏰐 MHz pro cessors􏰆 an

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