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Introduction to Cyber Security

CS 351: Introduction to Cyber Security
Assignment 1
1) A ciphertext has been generated with an affine cipher. The most
frequent letter of the ciphertext is ‘B’, and the second most frequent letter
of the ciphertext is ‘U’. Break this code.
(10 pts)
Suppose someone suggests the following way to confirm that the two
of you are both in possession of the same secret key. You create a random
bit string the length of the key, XOR it with the key, and send the result
over the channel. Your partner XORs the incoming block with the key
(which should be the same as your key) and sends it back. You check, and
if what you receive is your original random string, you have verified that
your partner has the same secret key, yet neither of you has ever
transmitted the key. Is there a flaw in this scheme?
(15 pts)
3) Classify each of the following as a violation of confidentiality, of
integrity, of availability, or of some combination thereof, and explain a bit
why you think so.
(20 pts)
a) Carol changes the amount of Angelo’s check from $100 to $1,000.
b) Gina forges Roger’s signature on a letter and sends the letter to Rachel.
c) Rhonda registers the domain name “” and refuses
to let the publishing house buy or use that domain name.
d) Jonah and Peter were original acquaintance, Jonah even told Peter his credit
card number and information. However, Peter made a mischief, and inform the
credit card company to cancel the card.
e) Henry sends a doc containing a Trojan horse to Marry, and Marry opened the
file, and sends her PIN to Henry. Henry now can have access to Marry’s

CS 351: Introduction to Cyber Security
4) A well designed hash function normally has multiple properties, including
collision resistance, which means it is very hard for one to find a pair of inputs
x≠y, such that h(x) = h(y). Could we just use a collision resistant function to do
encryption? i.e., Enc(k,m)=f(k,m), where f is a collision resistant function.
(15 pts)
Why Diffie-Helman key exchange algorithm alone is not secure against man
in the middle attacks, please show step by step. How can we improve security
and make sure that the algorithm is secure against MITM attacks? (Draw a
schema for both problems and explain the steps)
(20 pts)
We know that a digital signature is for the purpose of ensuring data
integrity and authenticity.
(20 points)
a) Checksum adds all the bits of the message (or blocks), is checksum a good
way to construct a digital signature scheme? How about a hash function, i.e,
Sign(M)= h(M)?
b) If we use a hash to generate a signature in a more complicated way as
follows Sign(k,m) = σ = h(k) XOR m XOR h(m), and m, σ will be sent along.
Would this be a secure signature? Briefly explain.

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