## NPTEL Foundations of Cryptography Week 2 Assignment Answers 2024

1.

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2. Which of the following is(are) not true about PRG?

- A PRG is a randomized algorithm
- A computationally unbounded adversary can’t determine the seed
- PRG is used to encrypt long messages
- We can construct a PRG, which is secure against a computationally bounded adversary

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3. What do we mean by the term “efficient” adversary?

- Adversary is very capable with almost infinite resources
- Running time of adversary is upper bounded by some exponential in security parameter
- Adversary performs its job effectively and efficiently
- Running time of adversary is necessarily upper bounded by some polynomial in security parameter

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4. Which of the following is(are) correct?

- 3−√n is a negligible function
- Distinguishing ability of the adversary is defined as its ability to distinguish between an encryption of 1 versus an encryption of 0 with probability much better than half, over a message space of {0,1}
- While using a PRG, same key can be used for every instance of encryption
- n−log(n) is a negligible function

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5. Difference between perfect security and semantic security involves one or more of the following:

- Bounds on how long the adversary can run the experiment
- In the respective indistinguishability experiment, adversary has the exact same probability of success
- Bounds on computational capabilities of the adversary
- None of the above

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