CSCI283172 Fall 2006

1
Privacy, Anonymity

• CSCI283-172 Fall 2006
• GWU

2
Cookies

• Post-it notes for the web (typically 4KB)
• Small files maintained on users hard disk,
  readable only by the site that created them (up
  to 20 per site)
• Used to
• Preserve state information about a transaction
• identify you when you return to a web site so you
  dont have to remember a password
• help web sites understand how people use them
• Cookies can be harmful
• used to profile users and track their activities
  without their knowledge, especially across web
  sites
• Can be disabled
• To learn about cookies, see Cookie Central

3
How DoubleClick Works
4
Privacy

• We take privacy in our daily lives for granted
• In the internet that is not the case
• Examples
• Pentium III chip serial numbers
• Read via software (ActiveX or Applets)
• Helps track a user over the web
• After pressure from privacy activists Intel
  decided to turn it off by default
• Could be turned on by software
• Not in later chips

5
Privacy

• Cookies
• Used to keep a track of the sites you visit
• double-click and other advertising agencies are
  main employers of cookies
• Carnivore sniffer
• Employed by the FBI
• Sniffs email packages, installed at ISP or in LAN
• Emails can be scanned in real time
• Supposed to obtain warrant to do this
• You could encrypt your message

6
Platform for Privacy Preferences (P3P)

• P3P
• Developed by World Wide Web Consortium
• Protocol allowing users to interrogate websites
  about privacy
• P3P-enabled site posts machine-readable privacy
  policy summary (IBM P3P editor)
• User sets up his privacy preferences in his
  browser
• Users browser examines the summary does not
  allow access to non-compliant sites
• Compliance is voluntary. Validator available.
• For more info see http//www.w3.org/P3P/

7
Using P3P on your Web site

• Formulate privacy policy
• Translate privacy policy into P3P format
• Use a policy generator tool
• Place P3P policy on web site
• One policy for entire site or multiple policies
  for different parts of the site
• Associate policy with web resources
• Place P3P policy reference file (which identifies
  location of relevant policy file) at well-known
  location on server
• Configure server to insert P3P header with link
  to P3P policy reference file or
• Insert link to P3P policy reference file in HTML
  content

8
A Simple HTTP transaction
WebServer
SOURCE W3.ORG
9
Transaction with P3P 1.0
WebServer
SOURCE W3.ORG
10
The P3P vocabulary

• Who is collecting data?
• What data is collected?
• For what purpose will data be used?
• Is there an ability to opt-in or opt-out of some
  data uses?
• Who are the data recipients (anyone beyond the
  data collector)?

• To what information does the data collector
  provide access?
• What is the data retention policy?
• How will disputes about the policy be resolved?
• Where is the human-readable privacy policy?

11
Transparency
http//www.att.com/accessatt/

• P3P clients can check a privacy policy each time
  it changes
• P3P clients can check privacy policies on all
  objects in a web page, including ads and
  invisible images

http//adforce.imgis.com/?adlink2685231146ADF
ORCE
12
Ways to Achieve Privacy

• Encryption
• Privacy of content
• Compromised end nodes could expose everything
• CPO (chief Privacy Officer) post in companies

• Anonymity
• Privacy of connection
• Privacy of identifier

13
Why Anonymity?

• A report by the American Association for the
  Advancement of Science (AAAS) found that
• Anonymous communication online is a morally
  neutral technology.
• Anonymous communication should be regarded as a
  strong human right in the U.S. it is a
  constitutional right (2nd amend.).

14
Why Anonymity?

• The Internet provides previously inconceivable
  opportunities for gathering info about YOU!
• Anonymous communication would provide ability for
  spamming, deception, and fraud.
• In reality, most anonymous protocols require
  cooperation of recipient.
• For good people provides privacy over the net,
  allows anon tips for police and journalists,
  whistle-blowing, discussion groups.

15
What is Anonymity?

• Anonymus
• of unknown authorship or origin, lacking
  individuality, distinction, or recognizability
  ltthe anonymous faces in the crowdgt
• Merriam-Webster's Collegiate Dictionary
• Anonymity does not mean that you cannot be
  identified.
• Anonymity means that you are indistinguishable
  from some particular group The likelihood that
  you are the originator of a message is reduced.

16
Terminology

• Terminology proposed by Pfitzman and Kohntopp
• Last Modification June 17, 2001
• Anonymity is the state of being not identifiable
  within a set of subjects, the anonymity set.
• i.e. A sender will be anonymous among the set of
  possible senders, the same argument goes for the
  recipient.
• The attacker never forgets anything so the
  anonymity set never increases for the attacker
  but decreases or has no change.
• But if misinformation is used one would be able
  to introduce uncertainty into the system thus
  increase the anonymity set.

17
Unlinkability

• Unlinkability of two or more items (e.g.,
  subjects, messages, events, actions, ...) means
  that within this system, these items are no more
  and no less related than they are related
  concerning the a-priori knowledge.
• e.g.
• Sender/Receiver (Anonymous Delivery)
• Merchant/Buyer (Anonymous Authentication or
  electronic cash)

18
Anonymous Delivery
19
Types of Anonymity

• Pfitzman and Waidner discuss 3 types of
  anonymity
• Sender Anonymity
• Receiver Anonymity
• Unlinkability of Sender and Receiver

20
Levels of Anonymity

• The probability of x being the initiator
• The degree of anonymity

Ref. Shields, C. and Levine, B.N. 2000. A
protocol for Anonymous Communication Over the
Internet.
21
Informal Definition

• Absolute Privacy means that the attacker has no
  way to distinguish the situation in which a
  potential sender actually sent communication and
  those in which it did not.
• Beyond Suspicion means that the attacker can not
  distinguish between a set of possible senders.

22
Informal Definition

• Probable Innocence if in the attackers point of
  view, the sender appears no more likely to be the
  originator
• Possible Innocence from the attackers point of
  view if there is a nontrivial probability that
  the real sender is someone else.

23
Informal Definition

• Exposed if from the attackers point of view there
  is a high probability about who the sender is.
• Provably Exposed if the attacker can identify the
  identity of the sender and prove it to everyone
  else.

24
Single Proxy Approach (Trusted Third Party)

• Anonymizer.com, Lucent personalized web
  assistant.
• Connections between initiator and responder using
  a proxy.
• Must trust the proxy!

P
I
R
25
Pseudonymity tools
26
Mix-net

• Untraceable electronic mail, return addresses,
  and digital pseudonyms
• Chaum 1981
• Two Assumptions
• No correlation between a set of sealed and
  unsealed items
• Anyone may learn the origin, destination and
  representation of all messages and may inject,
  remove, or modify messages

27
Mix-net

• Mix A computer node which will process each mail
  before it is delivered
• Simple case
• R is a nonce, M the message, and K public key

28
Mix-net

• What happens if B needs to reply
• We need an untraceable return address
• So A includes the following with the message sent
• Its return address K1(R1, A)
• Also a public key generated for this occasion Ka
• R is used for sealing by each mix
• A has all the R(s) since he generated them
• Ex.

29
Chaum Mixes (1981)
Sender
Mix C
C
kB
Mix A
C
kB
Mix B
Sender routes message randomly through network
of Mixes, using layered public-key encryption.
30
Crowds
31
Dining Cryptographer (DC) MIX - Chaum

• n cryptographers at dinner
• Waiter says bill has been paid either by one of
  them, or the NSA
• How to determine if one of them paid while not
  knowing which one?
• Each flips a coin and shows it to the
  cryptographer on the left thus each
  cryptographer sees two coins
• Each then announces whether the two he saw were
  same or different unless he paid, in which case
  he lies
• If they are all following the rules, the number
  of different will be even if none of them paid,
  odd if one paid
• Vulnerable to cheating cryptographers
• Used for anonymous broadcast

32
Anonymous Authentication

• Unlinkable
• Electronic cash
• Electronic Voting
• One Solution
• Homomorphic encryption E(a b) E(a) E(b)
• Is homomorphic encryption insecure?