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Substitution-Permutation Ciphers and S-P Networks

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Substitution-Permutation Ciphers and S-P Networks

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Questions and Answers

What is the purpose of the Key Schedule algorithm in DES?

To perform the initial and final permutations on the plaintext
To perform the expansion permutation used in the Feistel function
To implement the substitution boxes (S-boxes) used in the Feistel function
To generate the 16 subkeys used in the 16 rounds of DES encryption (correct)

In the DES decryption process, which operation is performed first?

Perform the 16 rounds of the Feistel network in reverse order (correct)
Apply the final permutation
Swap the left and right halves of the ciphertext block
Apply the initial permutation

What is the purpose of the Avalanche Effect in DES?

To ensure that the same plaintext always produces the same ciphertext
To ensure that a small change in the plaintext results in a large change in the ciphertext (correct)
To ensure that the key schedule algorithm generates unique subkeys for each round
To ensure that the encryption and decryption processes are computationally efficient

What is the purpose of the Feistel design in DES?

To allow the same algorithm to be used for both encryption and decryption, with only a change in the order of the subkeys (B) Signup and view all the answers

What was one of the main factors that led to the eventual replacement of DES with more secure algorithms like AES?

The small key size of DES (56 bits), which made it vulnerable to brute-force attacks as computing power increased (C) Signup and view all the answers

In the DES encryption process, what is the purpose of the initial and final permutations?

To increase the diffusion and confusion of the plaintext and ciphertext (B) Signup and view all the answers

What is the purpose of the expansion permutation in the Feistel function of DES?

To expand the 32-bit half-block to 48 bits, allowing each bit to be used in multiple S-box calculations (C) Signup and view all the answers

Which of the following is a weakness of the Electronic Codebook (ECB) mode of operation in DES?

Repetitions in the plaintext are reflected in the ciphertext, making it vulnerable to code-book analysis (B) Signup and view all the answers

What is the purpose of the Initialization Vector (IV) in the Cipher Block Chaining (CBC) mode of operation in DES?

To provide a unique starting value for the chaining process, preventing patterns in the ciphertext (D) Signup and view all the answers

Which of the following is a strength of the Cipher Feedback (CFB) mode of operation in DES?

The encryption process is computationally efficient for streaming data (D) Signup and view all the answers