The Game Theory of Phishing
By: Jeremy Clark
May 1, 2007
By all measures, the amount of internet fraud is rising. Morgan
Keegan reports the number of new phishing sites increased in its order
of magnitude from 4,367 in October 2005 to 37,444 in October 2006. And
phishing is not the only source of online fraud, the number of victims
of identity theft is growing as well.
In response to the escalation of phishing attacks, a plethora of
anti-phishing tools have been unleashed—Firefox extensions, IE
toolbars, and psychedelic colour-shifting borders for your browser, as
well as, perhaps more sensibly, blacklists of known phishing sites
including a list maintained by web titan Google. Of course, these tools
only work in so far as users take the time to install them and learn
how to use them. On the latter point, news on the usability of security
front is equally despairing. A user study conducted by Rachna Dhamija
(Harvard), J. D. Tygar (Berkley), and Marti Hearst (Berkley), presented
last year at the Conference on Human Factors in Computer Science, had
participants evaluate 20 websites—7 legitimate, 13 fraudulent—and
differentiate between them. The best phishing site fooled over 90% of
the participants, with many users reasoning that page’s nice layout and
animated graphics were a sure sign of its legitimacy. Numerous other
usability studies have examined the effectiveness of various
anti-phishing technologies, and its typical to hear them described as
unintuitive at best and unusable at worst (not to mention an eyesore).
All of this brings us to the magnificent architecture of some of
Ottawa’s oldest banks. With their tall pillars, imposing lobbies,
marble floors, and brass railings, bank architecture showcases
impressive work by great architects like John M. Lyle. (Okay, pardon
the non sequitur. I assure you I am going somewhere with
this). What is perhaps most intriguing about bank architecture is the
reason for the notable buildings. Why exactly were banks so impressive
and what happened? There is an easy answer: the magnificent designs
were a consequence of competition (an answer easy enough to be
articulated in The Canadian Encyclopedia). The problem with
this answer is that it does not adequately explain why bank buildings
have become less and less impressive over the past century while there
is still substantial competition, nor does it explain why there was not
a similar architectural arms race in hardware stores, feed mills, or
other competitive industries.
A better answer comes from the work of economist Michael Spencer on
asymmetric information and signaling theory (for which he shared the
2001 Nobel prize). Before the days of governmental oversight and a
banking oligopoly, there existed the threat that the new bank that
opened up down the street might be a fraud with crooks planning to run
off with your money. By building impressive buildings, legitimate banks
sent a signal of quality to customers that fraudulent banks could not
afford to send. An expensive building assured potential customers that
the bank was planning on long-term establishment and was committed to
high standards of service.
These types of scenarios are called signaling games in game theory.
A basic signaling game has two participants, a sender and a receiver.
The sender knows something about herself (called her type) that is not
observable to the receiver. The sender’s objective is to signify her
type in a signal that differentiates her from other senders of
different types, and to provoke an appropriate response from the
recipient. Examples of signals include the education level of a job
applicant, a full-page advertisement in the New York Times, or the
striking blue-green plumage of a peacock.
The problem of phishing and fraudulent websites is also a signaling
game, where legitimate websites need to find the online equivalent of
an impressive building to signal their type to users. The problem is
that the most obvious parallel to the offline world—an impressive
website—is completely inadequate. Whether or not the bank customers of
lore worked out the game theory of their situation, the signal worked
because customers naturally gravitated towards banks with nice
buildings. Once the signal became common, most customers did not need
an education campaign in how to differentiate between legitimate and
fraudulent banks to make the correct choice. In other words, their
ulterior motives led them to the right decision. As the user study
mention above indicates, this natural instinct is still instilled in
modern internet users. When presented with an impressive website with
fancy graphics and a cutting edge layout, a significant proportion of
users conclude that is a signal of its legitimacy. While designing the
kind of full-featured websites banks commonly use does cost a small
fortune, the problem lays in the fact that all this hard work can be
copied effortlessly. Phishing is thus a twofold problem: (1) we do not
have a good signal, and (2) the signal that users naturally look for is
not good.
It may be possible to address the second through user education if
only we could solve the first. One potential signal might be website
seals offered by watchdog organizations like TRUSTe and BBBOnLine.
Benjamin Edelman of Harvard empirically studied websites baring these
seals. He found that while a BBBOnLine seal slightly increased the
probability of the site being trustworthy (but not enough to be an
adequate signal), a TRUSTe seal actually decreased the probability that
is was trustworthy. That is to say, a site with no seal at all is more
likely to be trustworthy than one with a TRUSTe seal. Thus the seal not
only fails as an adequate signal, it actually results in adverse
selection. In the same paper, presented last year at the Workshop on
the Economics of Information Security, Edelman also found that search
engine advertisements are more than twice as likely to be untrustworthy
as the accompanying search results—another display of adverse
selection.
Perhaps a more promising area of third party accreditation is
through website certificate authorities. The largest certificate
issuers are, respectively, Verisign, GeoTrust, Comodo, GoDaddy, and
Entrust. Until recently, a certificate from any of these authorities
evoked the same response in browsers—a padlock being displayed—despite
the fact that the verification process varies radically from authority
to authority. Recently, however, Microsoft has agreed to implement a
new, tiered approach to displaying certificate indicators. In new
versions of Internet Explorer, the address bar will display a red
toolbar if the site is a suspected phishing site, yellow if the site
has a traditional certificate, and green if it has an extended
validation (EV) certificate (and as always, white for no certificate).
Receiving an EV certificate requires an extensive investigation process
that will likely catch any fraudulent attempts at certification.
EV certificates have the potential to be an adequate signal. However
this is only half of the problem, as the other half is getting users to
recognize the signal and act accordingly. Time will tell if the EV
process is extensive enough to demarcate legitimate companies from
fraudulent ones, and if users will adapt to recognizing and
understanding the implications of the signal. In the meanwhile,
economic game theory still dictates that one way a company can signal
its legitimacy is by spending more money than a fraudulent one could
afford. In my opinion, nothing would say quality like an SSL
certificate that costs a million dollars, turns the IE address bar
sparkling gold, and puts a dollar sign over the lock. Anyone want to
help me start MilliSign?
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