## NPTEL Foundations of Cryptography Week 4 Assignment Answers 2024

1. Which of the following is/are incorrect about modes of operation?

- A sequence of n messages messages need at least n calls to a secure PRF for encryption
- Stateful variant of Counter mode is secure whereas that of OFB mode is insecure
- Cipher-text computation is parallelizable in OFB mode
- The length of plain text should be a multiple of block length in CBC mode where as plaintext of any length can be encrypted in OFB mode

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2. Which of the following statement(s) is/are incorrect?

- S-boxes are used to produce confusion in an SPN
- A 3-round Feistel network is secure whereas a 2-round one is not
- Diffusion is used to produce avalanche effect in an SPN
- An r-round Subtitution-Permutation network has r rounds of key-mixing, substitution and permutation

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3.

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4. Select the correct option(s) from the following:

- Assuming all block cipher constructions use secure PRFs, this implies they all are CPA-secure
- CTR and OFB modes can be viewed as stream-ciphers
- When a single bit of ciphertext gets corrupted while transmission from sender to receiver, the entire recovered plaintext is incorrect in CBC mode
- Assuming secure PRFs are used in their construction, the stateless version of OFB-mode encryption is CPA-secure while the stateful version is not

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5.

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