Sample midterm
1. (3 pts) Define what is vulnerability and what is exploit. What effect can exploit
have on a system?
Vulnerability is any defect/bug in the system design, specification, implementation,
or configuration that, when exploited leads to undesired behavior.
Exploit is a set of actions that exercises the vulnerability.
Crash application or OS, allow unauthorized access, slow down the application
2. (5 pts) Explain what is tragedy of commons. Why do we say that most problems
in cyber security suffer from tragedy of commons phenomenon?
Tragedy of commons is the phenomenon where good of everyone depends on
everybody’s actions. If a few parties misbehave they benefit from this. If a lot of
parties misbehave everyone suffers.
Because often our security depends on security of others, or on their behavior.
3. (4 pts) Describe how a homophonic cipher works. Then describe how a
polymorphic (polyalphabetic) cipher works.
Homophonic cipher maps each symbol of alphabet into multiple symbols in cipher
alphabet – more frequent symbols are mapped into more symbols in cipher
alphabet. When encrypting we choose one of the output symbols at random.
We have multiple mappings, each mapping is used for one or a few symbols to
encrypt them. Each mapping maps one symbol of plaintext alphabet to one symbol
in ciphertext alphabet.
4. (10 pts) Define what is a block cipher and what is a stream cipher. What modes
of operation exist for a block cipher (it is enough to just list the modes).
Block cipher encrypts text in blocks – one block of input leads to one block of output.
(aka polygram cipher).
Stream cipher encrypts text character by character (bit by bit).
Two modes were covered in class: ECB and CBC.
5. (10 pts) Explain how linear feedback shift registers work
We initialize with value between 1 and 2n-1 where n is number of bits in LFSR
We combine some bits from register using XOR and tap sequence = one bit that goes
to the left of the register
One bit is pushed out on the right and it becomes one bit of the key
6. (8 pts) Alice wants to send message M to Bob. How can she ensure the following
properties, using asymmetric cryptography:
a. (2 pts) Confidentiality
Encrypt M with Bob’s public key
b. (2 pts) Integrity but not confidentiality
Send M and encrypt it with Alice’s private key
OR
Send M and send H(M) encrypted with Alice’s private key (cheaper)
c. (2 pts) Confidentiality and integrity
Send M encrypted with Bob’s public key, send H(M) encrypted with
Alice’s private key
d. (2 pts) Confidentiality, integrity and non-repudiation
same as c.