CS 351: Introduction to Cyber Security XXXXXXXXXXAssignment 1 1) A...

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CS 351: Introduction to Cyber Security
XXXXXXXXXXAssignment 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)
2) 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
it 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.

) Gina forges Roger's signature on a letter and sends the letter to Rachel.

c) Rhonda registers the domain name “AddisonWesley.com" 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 Ma
y, and Ma
y opened the
file, and sends her PIN to Henry. Henry now can have access to Ma
y's
computer.
Vashu Patel
DO NOT USE CHATGPT. WE HAVE TRACKERS
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)
5) 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)
6) 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.

assignment-1-hsznyq3t.pdf

Aditi · Accepted Answer

Solution
1.
To break an affine cypher, we must first identify the encryption function's two unknown values: the multiplicative key (a) and indeed the additive key (b) (b). This may be accomplished by comparing the frequency distribution of letters in the ciphertext to the frequency distribution of letters in the English language.
To begin, we determine the frequency of the each word in the ciphertext:
'B' is the most common letter.
'U' is the second most common letter.
Additional letters: less common
The frequency of each letter in the English language is then calculated:
'E' is the most often used letter.
'T' is the second most common letter.
Additional letters: less common
We may guess that 'B' and 'U' correlate to 'E' and 'T,' respectively. This is because 'E' and 'T' are the most common letters in English, whereas 'B' and 'U' are the most common letters in the ciphertext. As a result, we may construct the following equation system:
'B' = a * 'E' + b (mod 26) (mod 26)
'U' = a * 'T' + b (mod 26) (mod 26)
This set of equations may be resolved using modular arithmetic. Then, utilising the assumption that the letters A, B,..., Z are equal to 0, 1, and 25, respectively, we convert the letters B, U, and T to their corresponding numerical values of 0 and 19, 4 and 19, and 4 and 19, respectively:
0 = 4a + b (mod 26) (mod 26)
19 = 19a + b (mod 26) (mod 26)
To get the values of "a" and "b," we can solve this system of equations. Eliminating "b" by deducting the first equation from the second is one approach to accomplish this:
19 - 0 = (19 - 4) (19 - 4)
a (mod 26) (mod 26)
15 = 15a (mod 26) (mod 26)
The modular inverse of 15 mod 26 is 7, which we can calculate since 15 and 26 are coprime.
15a = 15 * 7 * a (mod 26) (mod 26)
15a = 105a (mod 26) (mod 26)
a = 105a (mod 26) (mod 26)
a = 5 (mod 26) (mod 26)
We can use either of the original equations to calculate "b" now that we know what "a" is worth. Using the first equation as an illustration:
0 = 4 * 5 + b (mod 26) (mod 26)
b = 22 (mod 26) (mod 26)
b = -4 (mod 26) (mod 26)
b = 22 (because we want a positive number) (since we want a positive value)
As a result, the additive key (b) is 22 and the multiplicative key (a) is 5. We employ the affine cipher's inverse function, which is: to decipher the ciphertext.
x = (a^-1 * (y - b)) (mod 26)
Hence, a = 3 and b =15
2.
Indeed, an attacker may listen in on the conversation and get the key by doing the following: If A and B are partners, and K is a key of size N.
1. A generates random string R
2.  Calculates S : = K XOR R
3. send S to B
4. B calculates S1:= S XOR K
5. sends S1 to A
6. A validates whether R is equal to S1 or not?
As a result,

CS 351: Introduction to Cyber Security XXXXXXXXXXAssignment 1 1) A ciphertext has been generated with an affine cipher. The most frequent letter of the...

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