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Questions and Answers
What is the main advantage of modern block ciphers over stream ciphers?
What is the main advantage of modern block ciphers over stream ciphers?
- They are faster to encrypt data.
- They are simpler to implement.
- They can process messages byte by byte.
- They provide better security through confusion and diffusion. (correct)
Which component is NOT a fundamental part of Shannon's substitution-permutation (S-P) networks?
Which component is NOT a fundamental part of Shannon's substitution-permutation (S-P) networks?
- Diffusion
- Confusion
- Permutation
- Encryption key (correct)
How does a Feistel cipher structure achieve encryption?
How does a Feistel cipher structure achieve encryption?
- By partitioning input into halves and processing them through multiple rounds. (correct)
- Through linear transformation only.
- By using a single round for encryption.
- By encrypting the entire block at once.
What is the role of substitution (S-box) in modern block ciphers?
What is the role of substitution (S-box) in modern block ciphers?
What does increasing the block size in a block cipher generally do?
What does increasing the block size in a block cipher generally do?
In the context of ciphers, what does 'diffusion' refer to?
In the context of ciphers, what does 'diffusion' refer to?
What aspect does 'confusion' focus on in encryption processes?
What aspect does 'confusion' focus on in encryption processes?
In block ciphers, which feature is primarily responsible for statistical obscurity?
In block ciphers, which feature is primarily responsible for statistical obscurity?
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Study Notes
Block Ciphers
- Block Ciphers encipher and decipher messages in blocks, which are often 64 bits or larger
- Block ciphers are more commonly used than stream ciphers
- Block ciphers are a type of symmetric cryptographic algorithm
Stream Ciphers
- Stream ciphers encipher and decipher messages one bit or byte at a time
Block Cipher Principles
- Most block ciphers use the Feistel cipher structure
- Modern block ciphers are based on the principles of the Substitution-Permutation (S-P) network, which was developed by Claude Shannon
- Shannon developed the concept of S-P networks as modern substitution-transposition product ciphers
- S-P networks are the basis of modern block ciphers
- The S-P Network uses the two primitive operations of substitution and permutation
- Substitution is performed by a substitution box (S-box)
- Permutation is performed by a permutation box (P-box)
- S-P networks provide confusion and diffusion of the message
Confusion and Diffusion
- A cipher needs to remove the statistical properties of the original message
- A one-time pad can completely obscure the statistical properties of a message
- Shannon suggested combining substitution and permutation to achieve confusion and diffusion
- Diffusion distributes the statistical structure of the plaintext over the ciphertext
- Confusion makes the relationship between the ciphertext and the key as complex as possible
Feistel Cipher Structure
- The Feistel cipher is based on invertible product ciphers
- The Feistel cipher partitions the input block into two halves
- The Feistel cipher uses several rounds to encrypt the input block
- The Feistel cipher performs a substitution to the left half of the block, which is based on a round function of the right half and a subkey
- The Feistel cipher permutes the two halves of the block, which is known as a swap
- The Feistel cipher implements Shannon’s substitution-permutation network concept
Feistel Cipher Design Principles
- The size of the block can influence the security, a larger block size can improve security but it will also slow the cipher
- The size of the key can influence the security, a larger key size can improve security but it may also make it more difficult to implement the cipher
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