CS 161 Computer Security
Fall 2005 Joseph/Tygar/Vazirani/WagnerMT 2 Soln
Problem 1. [Firewalls and Network Threats] (30 points)
List and explain three network threats that a firewall does not protect against. (If a threat only applies to
certain types of firewalls, then explain why this is the case.)
Sample threats: (1) Attacks against open ports, such as buffer overrun attacks against unblocked
services; (2) Malicious code or attacks carried in email or web traffic (many firewalls do not scan
or examine email and web payloads); (3) Attacks on the firewall itself (e.g., trying to penetrate the
firewall code by exploiting a buffer overflow in the firewall’s packet parsing code); (4) Internal attacks
by malicious insiders; (5) Attacks from compromised internal machines against other internal machines
(e.g., a laptop becomes infected with a worm, which tries to infect other inside hosts)—applies to
perimeter firewalls; (6) Attacks from compromised machines which have a VPN or other tunnel through
the firewall—applies to perimeter firewalls; (7) Denial of service attacks against the network link or the
firewall itself.
Grading: 10 point per threat, broken down as 5 points for the threat and 5 points for the explanation.
Problem 2. [Zero-Knowledge Proofs] (20 points)
Let (N,e)be Alice’s RSA public-key and (N,d)be her private key. Suppose that Bob claims to have
a signed message from Alice: he claims to have s=mdmod Nfor some particular mmod N(which he
reveals). Bob wishes to prove to Charlie that he has this signed message, without revealing any information
about s. The following are the first two steps in a protocol by which Bob can provide a zero-knowledge
proof of knowledge about s:
• Bob selects a random number rmod Nand computes t=remod N. He sends tmod Nto Charlie.
• Charlie randomly chooses one of two challenges: I) He asks Bob to send him Alice’s signature on t,
namely tdmod N. II) He asks Bob to send him Alice’s signature on m·t, namely (m·t)dmod N.
1. Fill in the last two steps of the protocol. i.e. how does Bob respond to each challenge. And what
should Charlie do to check each response.
• Bob sends I) r or II) r ·smod N, according to Charlie’s challenge.
• Charlie checks that I) re=tmod N. II) (r·s)e=t·mmod N.
Grading: 8 points, broken down as 2+2 for what Bob sends (cases I+II) and 2+2 for what Charlie
checks. No credit for telling Bob to send tdmod N or (m·t)dmod N (Bob doesn’t know d).
CS 161, Fall 2005, MT 2 Soln 1
