Privacy Enhancing Technologies for the Internet III:
David R. Cheriton School of Computer Science
In 1997 with Wagner and Brewer, and again in 2002, we looked at the then-current state ofprivacy-enhancing technologies (PETs) for the Internet. [27, 26] Now, in 2007, we take a third look.
Technologies to help users maintain their privacy online are as important today as ever before—if not moreso. Identity theft is the fastest-growing crime in the US today  and it is all too easy for would-beidentity thieves to harvest personal information from the online trails Internet users leave every day. Lossesof large databases of personal information are an almost daily occurrence ; for example, retailers’servers are penetrated , databases are traded between government and private companies  andlaptops containing social security numbers are stolen .
In 1997, we discussed the dossier effect
: all available information about a person gets cross-referenced, andthe resulting dossier ends up being used for many purposes, lawful and not. This practice has expandedover the years; the companies that compile and sell these dossiers are known as data brokers
. Choicepointis a prime example—in 2005, this data broker sold dossiers on over 150,000 Americans to a group ofcriminals.  The PETs we discuss here give people a way to control how much of their personalinformation is revealed when they use the Internet. By controlling the spread of this information, they canlimit the size of the data brokers’ dossiers about them.
In this article, we examine different classes of privacy-enhancing technologies. For each class, we look atthe state of the technology in 2002 and see what has happened in the intervening five years. In section 2,we look at a range of systems to protect the identities of senders and recipients of electronic mail. Insection 3, we examine systems which attempt the solve the more complex problem of protecting your
∗This is a preprint, which may differ slightly from the final version published as Chapter 1 of Digital Privacy: Theory, Tech-
nologies, and Practices
, Alessandro Acquisti, Stefanos Gritzalis, Costos Lambrinoudakis, Sabrina di Vimercati, editors. Auerbach,December 2007.
identity when accessing interactive Internet services. Section 4 surveys a number of technologies whichprotect the contents of Internet conversations, as opposed to the identities of the participants. In section 5,we look to the future, and examine three particular technologies in which we hope to see progress in thenext five years. Section 6 outlines the principles researchers should keep in mind when designing futuresecurity and privacy technologies in order to maximize their usefulness, and section 7 concludes.
Email anonymity and pseudonymity systems
The first class of PETs we will examine are systems to provide anonymity
for electronicmail. Email anonymity systems allow a user to send
email without revealing his or her own personalinformation, such as identity, email address or IP address. Email pseudonymity systems also allow the userto set up a persistent pseudonym, or nym
, which can be used to receive
email as well. With thesepseudonymous systems, users can participate in ongoing email conversations while maintaining theirprivacy.
The oldest and simplest email anonymity systems were the type-0 remailers
. The term remailer
stems fromthe basic operation of these systems: a user sends email to the remailer, which strips off the user’sidentifying information and re-mails the message to its intended recipient. The remailer also assigns arandom pseudonym to the sender. By keeping a master list matching the pseudonyms to senders’ real emailaddresses, replies to remailed messages can be delivered to the original sender.
While these type-0 remailers provided some amount of protection against casual observers, the master listprovided a tempting target for attackers; anyone who could get his hands on the list could reveal the realemail addresses of all the users of the remailer. The most well-known of these remailers,anon.penet.fi, was shut down after its operator lost a legal fight that required him to turn over parts ofthe list. 
In order to better protect the privacy of email users, the type-I
, or cypherpunk remailers
They work on the same principle—a message arrives at a type-I remailer, which removes the sender’sidentifying information and then sends the message out. But these remailers add a number of keyimprovements. The first is chaining
: a user sends his message to a remailer with instructions to send it, notto the intended recipient, but rather to a second remailer (run by an operator independent from the first).That
remailer is instructed to send it to a third remailer, and so on. Only the last remailer in the chainreceives the email address of the intended final recipient. Therefore, compromising any remailer or itsoperator does not allow linking the sender to the recipient. The first remailer knows only that the sender is
a user of the remailer network, but not with whom he is communicating. The last remailer in the chainknows that somebody sent an anonymous message to a particular recipient, but cannot identify who.
Remailers in the middle of the chain know only that they are forwarding anonymous email, but do notknow the sender or recipient. The goal is that all
of the remailers in the chain need to be compromised forthe privacy of the sender to be breached.
The second improvement made by the type-I remailers is encryption
. Without encryption, the first remailerin the chain could simply read the instructions to the later remailers, including the address of the finalrecipient. Instead, the first remailer receives an encrypted message. When it decrypts it, it finds only theaddress of the second remailer and another encrypted message. This inner message, however, is encryptedto the second
remailer, so the first remailer cannot read it. The first remailer sends that message to thesecond remailer, which decrypts it to find the address of the third remailer and another encrypted message(that only the third remailer can read), and so on. Finally, when the last remailer decrypts its message, itfinds the address of the final recipient, as well as the (unencrypted) message to send.
The third improvement made by the type-I remailers is mixing
: incoming messages to any remailer arebatched together and randomly reordered before being sent out. This was to attempt to prevent a passiveobserver of a given remailer from determining which outgoing message corresponds to which incomingmessage. An attacker could perform a timing correlation attack
by comparing the order in which messageswere received by the remailer to the order in which they were subsequently sent out. By introducing delaysand reordering, this attack is hindered.
Unlike the type-0 remailers, the type-I remailers require technical sophistication to use. Users have toeither manually construct all of the encrypted parts of a message before sending it, or install a tool likepremail  that handles the message construction automatically.
Although the type-I remailers were, privacy-wise, a great improvement over the type-0 system, they werestill vulnerable to size correlation attacks
or replay attacks
. In a size correlation attack, an adversary triesto match the messages sent by a given remailer to the messages it received by matching the sizes of themessages. In a replay attack, the adversary makes a copy of one of the messages received by the remailer,and sends many copies of it to that same remailer. The adversary then observes which outgoing messagefrom that remailer gets repeated many times.
or Mixmaster remailers
were deployed to address these problems.  Type-II remailers divide allmessages into a number of fixed-sized packets that are sent separately through the network of remailers inorder to defeat size correlations. These remailers also employ more complex techniques to defeat replayattacks.
Messages for type-II remailers can not be constructed manually in any reasonable way; users needspecially customized software in order to send anonymous mail.
Type-II remailers were the current state of the art in 2002. What has happened in the last five years? Adesign for type-III
or Mixminion remailers
has been proposed , which improves privacy protection in anumber of ways. First, type-III remailers provide a better system for handling replies to anonymousmessages. Type-II remailers only support anonymity—not pseudonymity. In order to receive replies to atype-II message, senders have to set up a pseudonym with the older type-I remailer network.
Type-III remailers also provide improved protection against replay attacks and against key compromiseattacks
, where an attacker learns the private decryption key of one or more of the remailers. The type-IIIsystem has several other new features to prevent other forms of attack, and to aid in the management of thenetwork.
Unfortunately, support for type-III remailers is not yet widespread. The implementation of the publisheddesign has never been released past the testing stage, and has had almost no work done on it in the last year.
Although there are about thirty type-III remailers scattered around the world (about the same as the numberof type-II remailers), the authors of Mixminion specifically warn users that “you shouldn’t trust Mixminionwith your anonymity yet” .
Interactive anonymity and pseudonymity systems
Today’s online communication is increasingly interactive and real-time, using technologies like instantmessaging. Protecting these types of communication, as well as other interactive Internet applications, suchas the world-wide web, remote logins, voice-over-IP and games, poses a much more significant challengethan the corresponding problem for email. Whereas remailers obtain much of their security from delayingand reordering messages, such delays are unacceptable in the context of low-latency interactive services,and tradeoffs often have to be made.
In 1995, Wei Dai presented a design of an anonymity system for low-latency traffic, which he called“PipeNet” . The design of PipeNet emphasized security over all else: if the system detected anyanomaly that could
be an attacker trying to compromise privacy, the entire network would shut itself down.
Of course, no realistic system could work this way; people simply wouldn’t use it. There have been anumber of systems that have been implemented and fielded over the years to provide practical security andprivacy to users of interactive Internet applications. We examine several of these next.
Anonymizer.com, a company we mentioned in the 2002 survey, continues to run the Anonymizer proxyservice, a system we first mentioned in the 1997 survery.  They continue to be one of the fewcommercially successful anonymity technology providers. The Anonymizer works much like the type-0
remailers: a web browser makes a request to the Anonymizer, which relays the request to the intended webserver. This service protects the user’s privacy from that web server, but not from Anonymizer.com itself,or from anyone watching the Internet near it. As we saw in 2002, by providing protection only against thissimpler threat model, Anonymizer.com is able to keep costs and complexity down.
The US Naval Research Lab’s Onion Routing project [45, 28] was the first PipeNet-like system to bewidely deployed. Although its primary use was for anonymizing web traffic, it also allowed users toanonymously connect to any TCP/IP server on the Internet. A user configures his Internet applications touse the SOCKS proxy protocol  to connect to an Onion Proxy
. Analogously to remailer systems, theOnion Proxy creates a path through several Onion Routers
situated around the Internet.
Unlike remailer systems, however, this path is long-lived. Once it is created, any data sent through this pathis anonymously delivered to the intended TCP/IP server. Any replies from that server are returned alongthe path to the Onion Proxy, and from there to the user’s application. When the application is finishedcommunicating with the server, the path is torn down, freeing the resources allocated for it at the OnionRouters.
The original deployed Onion Routing network was primarily a proof-of-concept; it later evolved into theTor network (see below).
The Freedom Network
The Freedom Network was a commercial venture by Zero-Knowledge Systems, Inc.  Also aPipeNet-inspired system, it incorporated some of the ideas from the Onion Routing project, but its designdiffered in important ways. For example, while Onion Routing was a TCP/IP-based system that couldanonymously transport any TCP/IP protocol, the Freedom Network was an IP-based system that couldtransport UDP/IP as well. Unlike Onion Routing’s pure anonymity, the Freedom Network provided apersistent pseudonymity service, enabling users to maintain separate online personas. It also usedprotocol-specific techniques to protect both the users of the network and the network itself. Importantly,Freedom removed the need for users to configure their Internet applications, which removed the potentialfor privacy-degrading mistakes.
The Freedom Network recruited operators all over the world to run its AIP nodes
(Anonymous InternetProxies, again analogous to remailers), and paid them to do so. Unfortunately, as we mentioned in the 2002survey, these costs proved to be prohibitive; there were not enough paid users to support the high-qualitynetwork that a commercial venture requires, and the network had already been shut down by that time.
Java Anon Proxy
Java Anon Proxy (JAP) is a project of Technical University Dresden.  It is one of the fewprivacy-enhancing technologies that was around in 2002, and is still in use today. Unlike PipeNet-basedsystems, JAP is a web-only anonymization tool that uses the techniques of type-II remailers to do its job.
Web requests and replies are divided into fixed-sized chunks, and sent through a series of mix nodes. Eachsuch node collects a batch of these chunks, encrypts or decrypts them as appropriate, reorders them, andsends them on to the next mix node.
As with Onion Routing, users protect their privacy with JAP by running the JAP client program, andconfiguring their web browsers to use the JAP client as an HTTP proxy. In this way, each of the user’s webrequests is sent to the JAP client, which divides it up into chunks, and sends those chunks through the mixnetwork.
Tor [19, 18] is a new system that has appeared since the 2002 paper. It is the next generation of the OnionRouting project, and it is the most successful (in terms of number of users) interactive anonymity tool todate. Hundreds of thousands of users send about 8 terabytes of traffic per day through hundreds of Tornodes. As it is an extension of the Onion Routing project, it shares many of that project’s characteristics: itonly anonymizes TCP/IP protocols, it requires configuration of users’ Internet applications, and so on.
Unlike the erstwhile Freedom Network, the Tor nodes are run by volunteers and all of the software is freeand open-source. Although somewhat cumbersome for novice users to install and use on its own, graphicaluser interfaces such as Vidalia  and other helpful tools like Torbutton  greatly enhance Tor’s ease ofuse.
Currently, one of Tor’s biggest drawbacks is its noticeable degradation to web browsing speeds. Ideally,Tor could be used in an “always on” mode, with users not even noticing its presence. Although Tor’ssluggish performance prevents this today, work is being done to improve the situation. One possible way toaccomplish this is to use peer-to-peer techniques to improve its scalability, as we suggested in 2002. Adifferent project, MorphMix , proposed such a design, but not only was it never widely deployed forgeneral use, it was later shown to contain flaws in its privacy protection .
In addition to protecting the users of TCP/IP-based Internet services, Tor also contains a facility to protectproviders
of such services. The most common such hidden services
are web servers; a user runs a webserver somewhere in the world which is only accessible through Tor, and Tor protects the identities of boththe user and the provider of the service. In this way, Tor provides a censorship-resistant publishing
service,which has been used by whistleblowers, for example, to distribute information of public importance .
Other censorship-resistant publishing services include the Free Haven , FreeNet , and Publius projects mentioned in 2002. Of those latter three projects, however, only FreeNet is still being developedand used today. The Wikileaks project [51, 50] uses both Tor and FreeNet in order to provide acensorship-resistant repository of leaked documents, which anyone can easily add to.
Communication privacy systems
When communicating over the Internet, the above technologies can help keep identity information private,possibly from third parties, and possibly also from other parties to the communication. In addition,correspondents may wish to keep the contents
of the communication private from third parties. Thetechnologies in this section allow you to do this. Note that it is usually the case that these technologies canbe combined with those of the previous sections to protect both a user’s identity and the contents of hiscommunication.
It is important to note that with these technologies, all parties to the communication need to have the same(or compatible) systems installed. This is not the case with the technologies in the previous sections; thosesystems protect their users’ privacy without requiring the other parties’ cooperation.
PGP and compatible systems
Pretty Good Privacy (PGP) [25, 39] has been available in one form or another for over 25 years. Althoughnewer versions have many more features, PGP’s fundamental purpose is to encrypt and/or digitally signemail (and to decrypt it and verify the signatures at the other end, of course). PGP has evolved from acommand-line-only program to one with a full-featured graphical user interface, and there are a number ofcompatible implementations, such as GNU Privacy Guard (gpg)  and Hushmail .
Users install some PGP-compatible software, and use it to encrypt their email messages before sendingthem. This can be done manually, but some email programs, including Outlook, Eudora, mutt and pine,have incorporated PGP support, greatly improving its ease of use.
SSL and TLS
As the world-wide web turned into a platform for e-commerce in the late 1990s, it became important toprotect the contents of web transactions. Netscape invented the Secure Sockets Layer (SSL) protocol,which in later versions was renamed Transport Layer Security (TLS) [24, 16]. Though not withoutproblems, SSL and TLS are the single most widely used privacy-enhancing technology to date. Theirsuccess stems from the fact that every major web browser comes with support for these technologies builtright in and that their use is largely invisible to the user. That is, no special installation or configurationneeds to be done by end users before they can benefit from these technologies. A web browser willautomatically encrypt web requests when communicating with an SSL/TLS web server, and the server willautomatically encrypt its responses; no user intervention is needed at all. Later, we will come back to thistheme when we examine properties of useful security and privacy technologies.
In the last five years, online communication has increasingly moved from email to instant messaging,especially among younger users.  First released in 2004, Off-the-Record Messaging (OTR) [4, 3] is atechnology to protect the contents of these instant messaging communications. As the name implies, OTRprovides instant messaging users with an “off-the-record” conversation. Much like conversing face-to-face,OTR users can communicate privately and can also repudiate any claims as to the content of theirconversation.
Fundamentally, OTR allows instant messaging users to communicate in an encrypted and authenticatedmanner. When a user Alice sends a message to her buddy Bob using OTR, she is assured that only Bob willbe able to read it. In turn, Bob is assured that the message came from Alice and has not been modified enroute.
Moreover, OTR offers deniability
. If Bob tells his friend Charlie what Alice sent him, Bob is able to offerno proof
of that assertion—Charlie just has to trust him. OTR avoids using traditional non-repudiabledigital signatures for authentication of messages; if messages from Alice had been digitally signed, Charliecould easily check the signatures for himself. Instead, OTR uses inherently repudiable messageauthentication codes to assure Bob that the message really came from Alice, but render him unable to provethat fact to anyone else.
In addition, by taking advantage of the fact that instant messaging conversations are interactive, OTR isable to provide perfect forward secrecy
to its messages. If Bob’s computer is lost, is hacked into, gets avirus, or any such thing, and all of his secrets are stolen, any messages Alice had previously sent Bobwould remain secret.
Users clearly could not manually encrypt every instant message they send, so the OTR encryption must behandled in an automatic way. There are three ways that users can integrate OTR into their instantmessaging. The first is by using a proxy: the user runs an OTR proxy on her computer, and configures herinstant messaging client to talk to that proxy instead of talking directly to the instant messaging server.
This technique can be used by users of proprietary instant messaging clients like iChat and Trillian in orderto obtain OTR functionality. The second method is by using a plugin: many instant messaging clients havethe ability to have their functionality extended by third-party plugin modules. There are OTR pluginsavailable for the gaim, Trillian, and Miranda instant messaging clients. The third method is to have OTRfunctionality built directly in to the user’s client. This is of course the best option, since, like SSL/TLS, theuser does not have to install or configure anything special in order to gain some benefit from OTR. Thepopular Adium X instant messaging client for the OS X operating system has OTR built in.
Other privacy-enhancing technologies
There are many more privacy-enhancing technologies that have been proposed, but are not yet inwidespread use. In this section, we look at three particular technologies; we hope to see progress on these
In 2002, we discussed the disappointing lack of adoption of electronic cash. Today, there are still noserious electronic cash services. It is important to fill this gap in the set of available privacy-enhancingtechnologies. Not only is it undesirable for there to be centralized records of everything one purchasesonline, but databases of payment records—including credit card numbers—are routinely stolen frommerchants and from credit card processing firms.  These losses can lead to both credit card fraud andidentity theft.
While alternatives to online credit card transactions, such as PayPal , are gaining popularity, a trueprivacy-protecting electronic cash solution remains elusive. Although the last of the patents protectingDigiCash’s original electronic cash protocol has recently expired, the patents were not the only barrier toentry for a potential electronic cash provider. As we mentioned in 2002, making a system widely acceptedand interoperable with the “real” money system is a difficult task. In fact, PayPal itself may be in the bestposition to offer true privacy-friendly payments online; it already has the payment infrastructure, it couldeasily provide an interface between electronic cash and the rest of the financial system and it has a largeinstalled user base. Skype is also considering adding a payment system to its voice-and-chat offering ,though no information is yet available about privacy properties that system may or may not have.
As we saw in 2002, private credentials  are a way to separate authorization
. Theyallow users to prove that they are authorized to access a certain service or gain a certain benefit, whilerevealing no unnecessary personal information, such as their identities. Rather than Alice proving “I amAlice” to some server, and the server checking that Alice is on the approved-access list, Alice insteadproves “I am approved to access this server” without revealing who she is. This obviates any personalinformation about Alice being stored on the server, removing the possibility of that information beingdisclosed or stolen. Credentica  is expected to release a line of privacy-friendly Digital Credentialproducts based on this technology in the near future.
A phishing attack
occurs when a user is directed to a malicious website, often via a link in email or chat.
The site appears to be a common site, like a bank, eBay, or PayPal, but is really run by an attacker—thephisher
. The message encourages the user to log in to the site to address an urgent problem with theiraccount; when the user complies, the phisher captures the login name and password. From there thephisher can hijack the account, steal money or mount an identity theft.
There are a number of tools available to help a user determine if he is looking at an authentic website or ata phishing site. These tools often appear as a toolbar in the user’s web browser that turns one of threecolours: one colour if the tool determines the site is probably genuine, one if it determines the site isprobably a phishing site, and one if it cannot make a determination.
The way these tools make these determinations vary. Some, like eBay’s Account Guard , compare theURL being visited to centrally maintained lists of good and bad sites. Users can suggest sites to be addedto either list, and the list maintainers generally manually verify them before adding them. Other tools, likethe Cloudmark Anti-Fraud Toolbar , use the collective ratings of its users to automatically mark sites as“genuine” or “phishing”. Some, like Google’s Safe Browsing toolbar , use the fact that genuine sitesgenerally have higher Google PageRank than phishing sites. Many tools use combinations of thesetechniques.
Zhang et al.  present an evaluation of ten of these anti-phishing toolbars and find that they “left a lot tobe desired”. They give some suggestions for further improvements to toolbars like these; we can only hopethe state of the art will advance in the next five years.
Useful security and privacy technologies
Since 2002, we have indeed seen a small amount of progress; there are a handful of new technologies thatpeople are actually using in order to protect their privacy when they use the Internet. In comparison,research
in privacy-enhancing technologies in the last five years has been booming. New technologies havebeen proposed in a number of different academic settings, but many do not make it out of the lab. Worse,some do not even make it from design into working code at all. These technologies do not improvepeople’s security and privacy.
What we would like to see more of are security and privacy technologies that make a real difference to real
people. We call such systems useful security and privacy technologies
, and we have identified a number
of properties such technologies must have.
It has long been known that many security and privacy technologies are hard to use, or hard to
use correctly. Difficult-to-use technologies frustrate users, and can even put them in the unfortunatesituation of believing they are being protected when they in fact are not. [49, 42] In order for atechnology to be useful, users need to be able
to use it, and be able to use it properly. In addition,users have to want
to use it; if a system protects their privacy at the expense of greatly slowing downtheir Internet experience, for example, users will simply turn it off.
In order for a technology to be useful, it must be possible for everyday users doing
everyday things to obtain it and benefit from it. This means it needs to be compatible with theirpreferred operating system, their preferred web browser, their preferred instant messaging client, andso on. Ideally, the technology would be built right in so that the user doesn’t even need to find andinstall separate software packages.
Many designed, and even widely deployed, security and privacy technologies contain flaws
that can render their ostensible protection moot. For a technology to be useful, it of course has towork and to give the user the benefit it promises. Open design and open implementation can helpexperts spot problems before too many users are left vulnerable.
Some technologies will work as advertised, but only so long as thing go “according to plan”.
But most technology designers’ plans overlook the realities of users on the Internet today: theircomputers contract worms and viruses, they forget their passwords, they get tricked by phishingattacks, they misunderstand (or just “click through”) security-critical dialog boxes, and so on. Auseful system needs to maintain as much protection as possible in these situations, since they willoccur unfortunately often in practice.
In order to close the gap between the number of systems proposed by researchers and the number ofsystems giving benefit to users, developers of privacy-enhancing technologies should design with theseprinciples in mind.
The last five years have seen a small increase in the availability of privacy-enhancing technologies for theInternet, including at least one, Tor, which is seeing significant use. This improvement over the previoushalf-decade is encouraging, but much work remains. We need more technologies that move all the wayfrom design to widespread use and we suggest that the four principles of useful security and privacytechnologies—usability, deployability, effectiveness and robustness—may guide us in the right direction.
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Δ p u “ div `| ∇u | p ´2 ∇u ˘ , arises in non-Newtonian ﬂuid ﬂows, turbulent ﬁltration in porous media,plasticity theory, rheology, glaciology, and in many other application areas;see, e.g., Esteban and V´azquez [ 48 ] and Padial, Tak´ c, and Tello [ 90 ]. Prob- lems involving the p -Laplacian have been studied extensively in the literature during the last ﬁfty years. How