Block Ciphers and Data Encryption

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Questions and Answers

In stream ciphers, what characteristic of the keystream makes the cipher unbreakable, assuming no other vulnerabilities?

The keystream is shorter than the message.
The keystream is generated using a complex algorithm.
The keystream is transmitted securely along with the ciphertext.
The keystream is as long as the plaintext bit stream and is randomly generated. (correct)

What practical consideration necessitates the use of an algorithmic procedure for a bit-stream generator in stream ciphers?

To enable both users to produce the keystream from a shared key. (correct)
To allow for easier distribution of the keystream.
To speed up the encryption process.
To ensure the keystream is truly random.

Which of the following is a main characteristic that distinguishes a block cipher from a stream cipher?

Block ciphers encrypt data block by block, whereas stream ciphers encrypt data bit by bit. (correct)
Block ciphers are used in network applications, while stream ciphers are used in file encryption.
Block ciphers encrypt data bit by bit, whereas stream ciphers encrypt data block by block.
Block ciphers use a symmetric encryption key, and stream ciphers use an asymmetric encryption key.

In the context of block ciphers, what is the primary function of substitutions and permutations as proposed by Claude Shannon and implemented by Feistel?

To introduce confusion and diffusion, respectively, thus thwarting statistical analysis. (B) Signup and view all the answers

In cryptography, what is 'diffusion' intended to achieve?

To ensure each plaintext digit affects the value of many ciphertext digits. (A) Signup and view all the answers

In the context of cryptography, what does 'confusion' primarily aim to achieve?

Making the relationship between the statistics of the ciphertext and the encryption key as complex as possible. (A) Signup and view all the answers

In a Feistel cipher structure, what is the function of the 'F' function?

It performs the substitution, contributing to the cipher's confusion. (C) Signup and view all the answers

What is the role of permutation in each round of a Feistel cipher?

To diffuse the effect of each bit in the plaintext through the ciphertext. (B) Signup and view all the answers

Which statement accurately describes the impact of a larger block size on the security and speed of a Feistel cipher?

Larger block sizes increase security but reduce encryption/decryption speed. (A) Signup and view all the answers

Why is a single round of a Feistel cipher considered inadequate for security?

Because a single round does not provide sufficient mixing and confusion to withstand cryptanalysis. (B) Signup and view all the answers

Which criterion should be considered when determining the number of rounds in a block cipher design?

The number of rounds should be chosen so that known cryptanalytic efforts require more effort than a brute-force key search attack. (B) Signup and view all the answers

In the design of the 'F' function in a Feistel cipher, what is the importance of nonlinearity?

The more nonlinear the 'F' function, the more difficult any type of cryptanalysis becomes. (A) Signup and view all the answers

In the context of the design of the 'F' function in a Feistel cipher, what do the Strict Avalanche Criterion (SAC) and Bit Independence Criterion (BIC) aim to strengthen?

The effectiveness of the confusion function. (D) Signup and view all the answers

What is the purpose of the key schedule algorithm in a Feistel block cipher?

To generate one subkey for each round from the main key. (B) Signup and view all the answers

What is the key consideration when selecting subkeys in a Feistel cipher?

Subkeys should maximize the difficulty of deducing individual subkeys and the main key. (C) Signup and view all the answers

For Data Encryption Standard (DES), which of the following is true?

Data are encrypted in 64-bit blocks using a 56-bit key. (D) Signup and view all the answers

What is the role of the combination of a left circular shift and a permutation in DES?

To produce a 48-bit subkey for each round. (A) Signup and view all the answers

What is the function of the Expansion P-box in DES?

To expand the 32 bits from the right half of the data to 48 bits. (A) Signup and view all the answers

What is the purpose of the S-boxes in DES?

To introduce non-linearity and confusion into the cipher. (D) Signup and view all the answers

What is one potential consequence of the avalanche effect in DES?

Small changes in the plaintext or key result in significant changes in the ciphertext. (A) Signup and view all the answers

What security risk is associated with the Data Encryption Standard (DES) due to its key size?

The key size is vulnerable to brute-force attacks due to its limited length. (C) Signup and view all the answers

What is a "timing attack" in the context of cryptography?

An attack that exploits the varying amounts of time an encryption or decryption algorithm takes to process different inputs. (D) Signup and view all the answers

Compared to more powerful symmetric ciphers like AES, how successful are timing attacks against DES?

Timing attacks have been mostly ineffective against DES and more powerful symmetric ciphers. (A) Signup and view all the answers

Which of the following is a recommended minimum security goal for the key schedule in a block cipher?

The key schedule should guarantee both the Strict Avalanche Criterion and Bit Independence Criterion for key/ciphertext relationships. (A) Signup and view all the answers

In the context of a Feistel cipher, what best describes the impact of increasing the number of rounds on the cipher's resistance to cryptanalysis?

It makes it more difficult to perform cryptanalysis, enhancing the security of the cipher. (B) Signup and view all the answers

In a Feistel cipher, what is the benefit of using multiple rounds?

Multiple rounds offer increasing security for any encryption cipher. (C) Signup and view all the answers

How does the 'avalanche effect influence the robustness of a cryptographic algorithm?

By changing the output so that no pattern can be recognized, improving the algorithm's resistance to brute force attacks or cryptanalysis. (E) Signup and view all the answers

Once the algorithm itself has been concisely and clearly explained, it is easier to analyze that algorithm for:

Cryptanalytic vulnerabilites (E) Signup and view all the answers

When would the speed of encryption be a factor of concern?

In encrypting in applications or utility functions, and so would preciude hardware implementation. (E) Signup and view all the answers

Flashcards

Stream Cipher

Encrypts data one bit or byte at a time, using an algorithm to generate a keystream.

Block Cipher

A cryptographic method where a block of plaintext is transformed into a ciphertext block of equal length.

Feistel Cipher

An encryption process that divides the plaintext into blocks and processes them through multiple rounds.