Michael G Reed
munication over a public network It provides anony mous connections that are strongly resistant to both eavesdropping and trac analysis Onion routings 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 dierently 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 email 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 ecash is not very anonymous if delivered with a return address Web based shopping or browsing of public databases should not require re vealing ones identity
modication 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
congurations of
Internet communication
eavesdropping ie
the existence condential
Address ratory Center
of intercompany collab oration Similarly email users may not
may b e wish to
Although onion routing may b e used for anony mous communication it diers from anonymous re
ington DC USA
Goldschlag Divx SA phone davidgoldschlagdivxcom
For Herndon VA email
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 trac analy sis and may reveal sensitive information For example
email flast
namegitdnrlnavymil
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 ecication sucient to guide b oth reimplementations and new applications of onion routing We also discuss congurations of onion routing networks and applications of onion routing in cluding Virtual Private Networks VPN Web brows ing email remote login and electronic cash
A purp ose of trac analysis is to reveal who is talk ing to whom The anonymous connections describ ed here are designed to be resistant to trac analysis ie to make it dicult for observers to learn identifying in formation from the connection eg 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 eect Onion routing provides bidirectional and near realtime com munication similar to TCPIP so cket connections or ATM AAL The anonymous connections can sub stitute for so ckets in a wide variety of unmo died 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 trac
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 realtime anonymous connections are
routings anonymous this pap er have app eared
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Anonymous Connections and Onion Routing
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Naval Research Laboratory,Center for High Assurance Computer Systems,4555 Overlook Avenue, SW,Washington,DC,20375
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Prescribed by ANSI Std Z39-18
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 identies 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 trac 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 denes our threat mo del Section describ es onion routing and the application sp ecic 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 dened 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 dierent 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 dene 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
denes 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 denes the next hop in a route An onion router that receives an onion p eels o its layer identies 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 ostcrypted 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 dierent 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 dene 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 trac originating or terminating within the sensitive site this onion router should also route data b etween other onion routers This conguration 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 ecically 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 dierent 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 inorder This simplies the sp ecication 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 trac 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 simplies 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
eectively hides
siders However if the resp onder is not in a sen sitive site eg 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 congurations it may b e ef cient to combine proxies that reside in the same place thus they may b e only conceptually distinct
Firewall Conguration
neighb oring onion
so cket con
In the rewal l conguration 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 eg 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 Conguration
router If the initiator can make encrypted connections to some remote onion router then he can function as if he is in the rewall conguration 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 endtoend 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 email and do remote logins For instructions please see httpwwwitdnrlnavymilITDprojects onionrouting
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 conguration 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
CustomerISP
Conguration
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
dicult to identify connections that terminate with his customers however he is serving as a routing p oint for other trac 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 trac 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 conguration 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 conguration but there son to trust the remote secured connection to an
onion router could b e
ate routers We have network of
complicate trac 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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