(Spring 2012 - Wagner and Paxson):
A. Consider a blogging site that allows users to post content they author
and also make comments on each other's posts.
1. Discuss how a worm could propagate within such a site. What would
you expect the progression of the infection to look like, in terms
of infections present at a given time?
2. Suppose the goal is to make the blogging site immune to the problem
of worms. Explain the steps you could take, and the benefits and
costs of each.
3. Suppose the costs of prevention are viewed as too high, or its
efficacy to uncertain. Sketch a detector for identifying that
such a worm is spreading.
4. For your detection mechanism, qualitatively assess its properties in
terms of false negatives and false positives.
5. Suppose you have concrete values for false positive and false negative
rate. How does the Base Rate Fallacy come into consideration when
6. Suppose you have two mechanisms for detecting such a worm. The first
analyzes the timing of the activity of individual users. The second
assesses the prevalence of particular strings of content as seen
globally across the site.
Discuss the issues that arise when assessing which of these detectors
B. Google has a cluster of thousands of machines. They want to log
security-relevant events that occur on those machines: e.g., each
command that an operator types into a root shell, each time a user
resets their Gmail password, and so on.
1. Describe a possible design to accomodate this -- it doesn't need
to be fancy -- and describe what security properties it does and
2. Now let's say we want to store all those audit log entries in the
cloud, on some server, but we don't want to trust that server in
the cloud. What can we do? Suggest a scheme, and describe what
security properties it does and doesn't provide.
(Fall 2011 - D. Song and Paxson):
1. Consider the problem of defending a web site from a DDoS attack.
a) An attacker sends a stream of spoofed SYN packets to port 80 of the
web server. Discuss two techniques that could be employed to find the
hosts sending the flood. How effectively would they work in
b) For the same attack, discuss techniques a site can use by itself
(i.e., not requiring cooperation by the broader network) to defend
against the attack. How well do these approaches work, and what are
c) Suppose that during the flood an ISP upstream of yours activates a
monitoring box that inspects packets heading to your site and uses
anomaly detection to identify and block likely spoofed packets.
If they do this without your permission, have they violated the
d) Now suppose that the attacker launches a DDoS attack using non-spoofed
packets. To what degree does that change the opportunities available to
the attacker? What about for the defenses you mentioned?
2. Sketch the problem of buffer overflow attacks. Discuss the range of
defenses and their pros and cons.
3. This question concerns TLS.
a) Sketch an attack on TLS.
[ultimately, the examination for this sub-question drove towards
the problem of stolen certs / compromised CAs.]
b) How might we detect that a given TLS session is using a stolen
certificate or stems from a compromised CA?
c) How effective is such detection, in terms of false positives and
d) Sketch, to the degree that you can, how DNSSEC works. (NOTE: we
recognize that DNSSEC was not on the syllabus, and sketch it for
students as needed.)
e) Suppose that instead of using CA's, clients retrieve public keys
for use in TLS via DNSSEC queries. Compare the properties of this
approach with how TLS works today.
(Fall 2008 - Tygar & Wagner):
1. Bounce message spam typically occurs when a spammer sends many spam
emails with the From: line containing a forged email address -- say,
to make it appear that the spam emails were from me. Many of those
spam emails will be undeliverable or will be destined to an invalid
email message, and thus a bounce message will be sent back to the
email address listed in the From: line -- i.e., back to me. Consequently
when a spammer sends one million spam emails with my email address in
the From: line, my inbox may become clogged with thousands of bounce
messages for emails I never sent. This is a nuisance.
(a) Design a mechanism to protect me against bounce message spam.
I don't want to see nuisance bounce messages, but I always want to
see bounce messages for emails that I do send.
(b) What are the privacy implications?
(c) How can we minimize the storage requirements?
(c) New requirement: I want to read and send email from several
different email clients, such as my laptop and my cellphone. I want
to configure them once, but I want to minimize the amount of state
that they must save and avoid the need for my clients to exchange
data. Augment your scheme to provide this property.
2. Name as many ways as you can think of that a user Alice on the
Berkeley email system could prevent me from reading my email for
the next 24 hours.
3. The FastTrack system is an automated system for toll payment.
The authorities send you a battery-powered transponder that you
put in the car; when you drive over the Bay bridge, their equipment
interrogate your transponder wirelessly and then bill you monthly.
(a) Describe the security goals that such a system ought to provide.
(b) How well does the current system meet these goals?
(c) Sketch how you would design the system, if you wanted to
ensure that all of these goals were met.