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**NPTEL** **Foundations Of Cryptography** **Assignment 1 Answers 2023**

**1. Which of the following is/are valid condition(s) for a perfectly-secure encryption scheme?**

Answer:-

**2. Consider an instance of shift cipher with the probability distribution over the message space as follows: P[M=a] = 0.3, P[M=b] = 0.3, P[M=c] = 0.4. What is the probability that the ciphertext is ‘D’?**

a. 2/13

b. 1/26

c. 3/26

d. Nothing can be said regarding the probability that the ciphertext is “D”

Answer:-

3. **Which of the following statement(s) is/are necessarily correct? **

**I**) Any encryption scheme with key length less than message length can not be a perfectly-secure scheme. **II**) Given the one-time pad scheme, the scheme is no longer perfectly-secure if on any subsequent invocation of the key generation algorithm, an earlier key gets re-generated.

a. **I**-only

b. **II**-only

c. Both **I** and **II**

d. Neither **I** nor **II**

Answer:-

**4. A malicious adversary is best modeled in which of the following adversarial models?**

a. CPA

b. COA

c. KPA

d. CCA

Answer:-

**5. Assume you are given a 300 character encrypted message, encrypted in Vigenère cryptosystem, in which you know the plaintext word CRYPTOGRAPHY occurs exactly two times, and we know that the ciphertext sequence GTMJKGKTOJYQ is the encryption of CRYPTOGRAPHY. The first occurrence starts at character position 18 and second at character position 246 (we start counting from 1). What is the length of the key used for encryption ?**

a. 5

b. 6

c. 7

d. 8

Answer:-

## About Foundations of Cryptography

The course provides the basic paradigm and principles of modern cryptography. The focus of this course will be on definitions and constructions of various cryptographic objects. We will try to understand what security properties are desirable in such objects, how to formally define these properties, and how to design objects that satisfy the definitions. The aim is that at the end of this course, the students are able to understand a significant portion of current cryptography research papers and standards.

## CRITERIA TO GET A CERTIFICATE

Average assignment score = 25% of the average of best 8 assignments out of the total 12 assignments given in the course.

Exam score = 75% of the proctored certification exam score out of 100

Final score = Average assignment score + Exam score**YOU WILL BE ELIGIBLE FOR A CERTIFICATE ONLY IF THE AVERAGE ASSIGNMENT SCORE >=10/25 AND EXAM SCORE >= 30/75. If one of the 2 criteria is not met, you will not get the certificate even if the Final score >= 40/100.**

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