### Get All Weeks Cryptography and Information Theory Quiz Answers

This course combines cryptography (the techniques for protecting information from unauthorized access) and information theory (the study of information coding and transfer). More specifically, the course studies cryptography from the information-theoretical perspective and discusses the concepts such as entropy and attacker knowledge capabilities, e.g., Kerckhoff’s Principle.

It also contrasts information-theoretic security and computational security to highlight the different trains of thought that drive the cryptographic algorithmic construction and the security analyses.

### Cryptography and Information Theory Quiz Answers

### Week 1 Quiz Answers

#### Quiz 1: Kerckhoff’s Principle and Security by Obscurity

Q1. Which of the following violate Kerckhoff’s principle? Select all that are applicable.

**The company password is written using an invisible ink, and only the people within the company know the fact that invisible ink is used.**- The attacker has the copy of the system.
**A company states that its system is “secure” because it uses confidential and proprietary protocols.**- The algorithms for popular symmetric ciphers such as DES and AES are publicly known.

Q2. Using the cryptography jargon, when Alice wants to communicate to Bob, who holds the key among Alice, Bob, and Eve? Select all that are applicable.

- Eve
**Bob****Alice**

Q3. Which of the following is the least similar to Kerckhoff’s Principle?

- “Attacker knows the system”
- Shannon Maxim
**Security by Obscurity**- Open design

Q4. Solve the following steganographic message. The grading is space- and case-sensitive so, in your answer, use all lower case letters and skip all punctuation marks, i.e., your answer should only be comprised only of spaces, numbers, and lower-case letters.

Dear George,

Greetings to all at Oxford. Many thanks for your

letter and for the Summer examination package.

All Entry Forms and Fees Forms should be ready

for final despatch to the Syndicate by Friday

20th or at the very latest, I’m told. by the 21st.

Admin has improved here, though there’s room

for improvement still; just give us all two or three

more years and we’ll really show your! Please

don’t let these wretched 16+ proposals destroy

your basis O and A pattern. Certainly this

sort of change, if implemented immediately,

would bring chaos.

Sincerely yours.

Answer: your package ready friday 21st room three phase please destroy this immediately

Q5. Which of the followings are examples of steganography?

**Touching different parts of your faces (where different parts indicate different messages) while being engaged in another activity**- Broadcasting your password to another person and hoping that the person forgot of your password by now
**Invisible ink****Embedding another message on the least significant bits on images (so that they are indistinguishable in human eyes)****Blinking based on Morse code while saying something else**- Using passwords for authentication

### Week 2 Quiz Answers

#### Quiz 1: Throwing a Dice

Q1. There is a typical dice (with each side showing distinct numbers between 1 and 6). In each event, you throw the dice and observe the number. What is the information entropy for one event in bits?

2.58

Q2. (This problem builds on the previous problem. )

There is a typical dice (with each side showing distinct numbers between 1 and 6). In each event, you throw the dice and observe the number. What is the information entropy for *five* event in bits?

13

Q3. (This problem builds on the previous problem. )

Now suppose you modify the dice so that the side which originally showed the number 6 becomes a 5 (i.e., the numbers 1, 2, 3, 4 occupy one side each, and 5 occupies two sides). What is the information entropy for one event?

2.25

#### Quiz 2: Entropy: Balls in a Bin

Q1. There are five red balls, three yellow balls, and four green balls in a bin. In each event, you pick one ball from the bin and observe the color of the ball. The balls are only distinguishable by their colors. After observation, you put the ball back into the bin.

What is the probability of choosing a red ball in an event? Express your answer upto three decimal digits, e.g., 0.352.

Enter answer here

Q2. As was in the previous question (the same setup with the previous question), there are five red balls, three yellow balls, and four green balls in a bin. In each event, you pick one ball from the bin and observe the color of the ball. The balls are only distinguishable with their colors. After observation, you put the ball back into the bin.

What is the information entropy for one event (pick ball once) in bits? Express your answer upto three decimal digits.

Enter answer here

Q3. As was in the previous question (the same setup with the previous question), there are five red balls, three yellow balls, and four green balls in a bin. In each event, you pick one ball from the bin and observe the color of the ball. The balls are only distinguishable with their colors. After observation, you put the ball back into the bin.

What is the information entropy for five events in bits?

Enter answer here

Q4. As was in the previous question (the same setup with the previous question), there are five red balls, three yellow balls, and four green balls in a bin. In each event, you pick one ball from the bin and observe the color of the ball. The balls are only distinguishable with their colors. After observation, you put the ball back into the bin.

You can re-color or re-paint one ball of your choice to any of the three colors (red, yellow, green).

Which ball would you re-color and to which color in order to maximize the entropy?

- Re-color a yellow ball to green
- Re-color a red ball to green
- Re-color a green ball to red
- Re-color a red ball to yellow
- Re-color a green ball to yellow
- Re-color a yellow ball to red

Q5. Now, the bin contains 12 green balls. In each event, you pick one ball from the bin and observe the color of the ball. The balls are only distinguishable with their colors. After observation, you put the ball back into the bin.

What is the entropy value for five events in bits?

Enter answer here

### Week 3 Quiz Answers

#### Quiz 1: Brute-Force Attack, Cryptanalysis, and One-Time Pad

Q1. True or False: Brute-force attack assumes that the attacker can distinguish between the correct and the incorrect key after the decryption trial. For example, the incorrect key can yield gibberish while the correct key yields a meaningful message.

- False
**True**

Q2. When the key is 10-bits long, how many decryptions do the brute-force attacker try in the worst case (this is the worst case in the Alice-Bob perspective and corresponds to the case when the attacker gets lucky)?

1

Q3. When the key is 10-bits long, how many decryptions do the brute-force attacker try on average?

512

Q4. When the key is 64-bits long, how long does it take in days for an attacker to brute-force search/attack for the key on average? The attacker can perform 10 trillion (10^13) decryptions per second, given the attacker processor and the encryption/decryption algorithm.

10.67

Q5. True or False: Known-plaintext attack is a stronger threat than ciphertext-only attack because the attacker uses the known plaintext-ciphertext pairs to more efficiently find the key that is being used by Alice and Bob.

- False
**True**

Q6. Is the following claim (that your friend generated a true one-time pad for perfect secrecy) true or false?

A friend wants to use a one-time pad to encrypt a data of size 1 kilobyte/KB and ensure perfect secrecy. He generates a key that is of size 1KB by taking an 1B-long seed and using a deterministic algorithm to produce a 1KB-long key. The friend uses a secret algorithm, which he does not tell anybody, and the output key passes the pseudo-random test published by US NIST.

**False**- True

Q7. True or False: One-time pad is popularly used for systems that require secure data protection.

- True
**False**

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