Privacy & Anonymity in the WWW

CSCI 5234 Web Security 1

Privacy & Anonymityin the WWW

Ch. 12, Oppliger


Privacy & Anonymity

12.1 Intro

12.2 Early work

12.4 Anonymous browsing

12.5 Anonymous Publishing

12.6 Voluntary privacy standards

12.3 Cookies


Intro 1/7

– When a client access a web site, all kinds of information regarding the client may be collected without the client’s knowledge

– Examples: client software (browser type, OS), IP address, computer name, screen width, length, Trace route, domain config., …


Intro 2/7 Legislations

– Many countries have data privacy or data protection laws that make it a legal obligation for entities storing, processing, and transmitting personal data to adequately protect the privacy of the data.

– The EU relies on comprehensive legislation that, for example, requires creation of government data protection agencies, registration of databases with those agencies, and in some instances prior approval before personal data processing may begin.

E.g., The European Commission’s Directive on Data Protection became effective in 10/98.

– The US takes a sectoral approach to privacy by relying on a mix of legislation (e.g., HIPPA), regulation, and self-regulation.

– Challenges for international businesses– Solution: a safe harbor framework to bridge the discrepancy

(still ongoing effort)


Intro 3/7

– http://privacy.net/ – Provides free privacy & other network-

related analysis– Sample privacy analysis– How does it work?


Intro 4/7

– Local network administrators, web managers, and ISPs have access to even more information about the users.

– Networking devices are usually configured to log relevant information.

– An ongoing legal discussion about how far they may go…


Intro 5/7

Traffic Analysis [RFC 2828 Internet Security Glossary]

$ traffic analysis

(I) Inference of information from observable characteristics of data flow(s), even when the data is encrypted or otherwise not directly available. Such characteristics include the identities and locations of the source(s) and destination(s), and the presence, amount, frequency, and duration of occurrence. (See: wiretapping.)

(O) "The inference of information from observation of traffic flows (presence, absence, amount, direction, and frequency)." [I7498 Part 2]

$ traffic flow confidentiality

(I) A data confidentiality service to protect against traffic analysis.

(O) "A confidentiality service to protect against traffic analysis." [I7498 Part 2]

$ traffic padding

(I) "The generation of spurious instances of communication, spurious data units, and/or spurious data within data units." [I7498 Part 2]


Intro 6/7

– Traffic analysis may reveal sensitive data.– Some protocols, such as electronic cash, must

guard against ‘traffic analysis’ in order to work properly

– A threat that is very difficult to protect against– Q: Would encrypting IP packets between a

browser and a web server protect against traffic analysis?


Intro 7/7

– Specialized security mechanisms are required to protect communicating peers against traffic analysis

– 3 types of anonymity services:1. Sender anonymity

2. Receiver anonymity

3. Connection anonymity - unlinkability of sender & receiver


Early attempts of anonymous emails 1/5

– Anonymous remailere.g., anon.penet.fiAn anonymous e-mail forwarding serviceA simple SMTP proxy server that stripped off

all header info of incoming e-mail messages before forwarding them toward their destinations

Q: What type of anonymity service is provided by anonymous remailer?

– Chaum mixing network


Early attempts 2/5

– Chaum mixing network• a more sophisticated approach for anony

mous emails• A Chaum mix is an anonymous remailer.• A Chaum mixing network consists of a set

of Chaum mixes.• The sender of the message chooses a rou

te through a series of mixes M1, …, Mn to the intended recipient.


Early attempts 3/5

– Chaum mixing network • The message is encrypted layer by layer usi

ng each mix’s pubic key• Example (where n = 2, B is the recipient):

M1, {M2, {B, {mesg}KB}KM2}KM1

• The message is first sent to M1, which decry

pts it using its private key, and then sends

{B, {mesg}KB}KM2 to M2

• M2 then decrypts it and forward {mesg}KB t

o B


Early attempts 4/5

– Issues: How would the recipient respond to the sender?

– Various approaches were proposed:• The recipient may post the response (with

a specific subject line) to a newsgroup• An inverse untraceable backward route

The return path information (RPI) contains block of information, which must accompany the original message.


Early attempts 5/5

– Can the ‘anonymous remailer’ approach be used in providing anonymity services on WWW?

– Ans: not quite…– c.f.,

Operation mode pull vs push

WWW Interactive Pull

email Store-and-forward Push


Anonymous browsing 1/7

– Technologies that can be used 1. To protect the privacy of Web users, and2. To provide support for anonymous browsi

ng accordingly

– Examples:• Anonymizing HTTP proxy servers• JAP• Crowds• Onion routing• Freedom Network



Anonymizing HTTP proxy servers • An HTTP proxy server that removes all parts of an

HTTP request message that may directly or indirec

tly reveals information about the browser

• Requirements: The removed info are not required

by the Web server to serve the request and to resp

ond appropriately.

• Such a server can hide the browser’s IP address.

• Responses from the Web server are forwarded by

the proxy server.

• Most anonymizing HTTP proxy servers rely on nes

ted URLs.



Anonymizing HTTP proxy servers • A nested URL is one where the document part refe

rs to another URL

• http://proxy.ABC.org/http://www.uhcl.edu

• The browser first connects to the proxy server (http:

//proxy.ABC.org), which in turn connects to the We

b server at /http://www.uhcl.edu.

• ‘Chained’ HTTP proxy servers: useful when the us

er does not trust any single proxy server

• http://proxy.ABC.org/http://proxy.XYZ.net/http://www.uh

cl.edu

• Overhead?


Anonymous browsing 4/7 JAP

– Developed by a group at Univ. of Technology Dresden– http://anon.inf.tu-dresden.de/index_en.html – Java-based– In essence, a Chaum mixing network for HTTP

– JAP uses a single static address which is shared by many JAP users. That way neither the visited website, nor an eavesdropper can determine which user visited which website.

– Instead of connecting directly to a Web server, users take a detour, connecting with encryption through several intermediaries mixes.

– A relationship between a connection and its user could only be determined if all intermediaries worked together to sabotage the anonymization. But, the intermediaries (mix providers) are generally provided by independent institutions which officially declare, that they do not keep connection log files or exchange such data with other mix providers.



Crowds– Developed in late 90s by a group at AT&T Research– A ‘crowd’ is a large group of geographically diverse us

ers.– Basic ideas:

• To probabilistically chain multiple anonymizing HTTP proxy servers a unique feature

• To encrypt all data that is sent forth and back between the proxy servers

– Procedure:• Each user is represented by a local process called jondo.

• Jondo contacts the blender server to request admittance to the crowd.

• Jondo works as a local proxy server; any request originating from the browser is sent directly to its jondo.



Crowds– Procedure (Cont.):

• The local Jondo picks a jondo from the crowd, possibly itself at random, and forwards the request to it.

• Each jondo then determines randomly whether to forward the request to another jondo or to the Web server.

• So, a random path of jondos between the browser and the Web server is established randomly.

• The return path is the same, only in reverse.

• All communications between two jondos (J1, J2) are enc

rypted by a shared key, KJ1, J2. membership managem

ent overhead

• To reduce the overhead, Crowds uses a simple and cent

ralized solution.



Crowds– membership management

• The blender serves as the centralized membership and key ma

nager of a Crowd.

• Each user’s jondo must be authenticated by the blender (id, pas

sword).

• The blender generates a list of shared keys for a new jondo; ea

ch of the keys is to be shared between the new jondo and anoth

er jondo.

– Strengths? Separation of key management from the actual Web tr

ansactions

– Issues? Corrupted blender, attacked blender, firewall bypassing, …

– Future improvements: Diffie-Hellman key exchange directly betw

een a pair of jondos

– A thought: authentication between sensor nodes in a sensor net?


Anonymous Publishing 1/5

– The problem: How to anonymously publish on the Web?

– The current WWW architecture provides little support for anonymous publishing.

– For example: The URL identifies the Web server where the resource is located.

– Several attempts:• JANUS and the rewebber service• TAZ servers and the rewebber network• Publius


Anonymous Publishing 1/5

JANUS– The Rewebber service provides anonymity services fo

r both browsers and Web servers.– The Rewebber services actss as an anonymizing HTT

P server. anonymous browsing– To support anonymous publishing, the Rewebber serv

ice makes use of encrypted URLs that are part of nested URLs.

– e.g., http://proxy.ABC.edu/http://www.dcsl.net/sample.htm http://proxy.ABC.edu/url_encrypted/rxmy2198za

– The anonymizing proxy server takes care of decryption and encryption of the URLs.


Cookies 1/5

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Privacy & Anonymity in the WWW