Cyber Security Fundamentals Honor Pledge Worksheet

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Name: Date: Honor Pledge: I pledge that the only resources I used to complete this homework were the course textbook, the course slides, or my personal course notes. I did not confer with any other student, consult any solutions manuals, printed or on-line, nor did I do any Internet searches to solve any problems for this homework assignment. The solutions below are my own work. Student Signature___________________ Cyber Security Fundamentals – CHAPTER 2 HOMEWORK 75 points total 2.A [20] Suppose that 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? Hint: Try the above using a small 4 bit key and random 4 bit string. Solution: Cyber Security Fundamentals Chapter 2 Homework (B) 1 2.B [35] This problem uses a real-world example of a symmetric cipher, from an old U.S. Special Forces manual (public domain). The document, filename Special Forces.pdf, is on Isidore in the Homework Templates folder. a. Using the two keys (memory words) network security and cryptographic, encrypt the following message. (That is, perform a DOUBLE transposition of the message below with the first transposition using the key (memory word) network security and the second transposition using the key (memory word) cryptographic.) Midway upon the journey of our life I found myself within a forest dark, For the straightforward pathway had been lost. Skip redundant letters and excess letters in the memory words and ignore spaces and punctuation in the message. Show your work or no credit will be given. Note: The message is from the Dante’s Divine Comedy. b. Decrypt the ciphertext. Show your work or no credit will be given. c. Comment on when it would be appropriate to use this technique and what its advantages are. Solution: Cyber Security Fundamentals Chapter 2 Homework (B) 2 2.C [20] Suppose H(m) is a collision-resistant hash function that maps a message of arbitrary bit length into an n-bit hash value. Is it true that, for all messages x, x′ with x ≠ x′, we have H(x) ≠ H(x′)? Explain your answer. Hint: Work out a problem for arbitrary message lengths for a n=3-bit hash value, then generalize your solution. Solution: Cyber Security Fundamentals Chapter 2 Homework (B) 3
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Explanation & Answer

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1
Name:

Date:

Honor Pledge: I pledge that the only resources I used to complete this homework were the course
textbook, the course slides, or my personal course notes. I did not confer with any other student,
consult any solutions manuals, printed or on-line, nor did I do any Internet searches to solve any
problems for this homework assignment. The solutions below are my own work.

Student Signature___________________
2.A Solution
Sure, an intruder can listen in on a conversation and figure out the key in the following
manner: Assume A and B are two parties, and K defines a key of length N.
i)

A random string R is generated by partner A

ii)

S is calculated using the formula K XOR R.

iii)

The calculated S, is then sent to partner B.

iv)

Partner B then computes the S1 using the formula SXOR K.

v)

The calculated S1 is then sent to partner A.

vi)

Validation is done by partner A to determine if R is equal to S1.

As a result, an attacker who overhears the dialogue will possess both S and S1. As a result,
he/she can compute the key as K = S1 XOR S.
2.B Solution
a)

Encryption
Message: Midway upon the journey of our life I found myself within a forest dark, For the

straightforward pathway had been lost.

2
Keys: n...


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