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Questions and Answers
What is the main difference between block ciphers and stream ciphers?
What is the main difference between block ciphers and stream ciphers?
- Block ciphers produce ciphertext blocks of equal length, while stream ciphers produce variable length ciphertext.
- Block ciphers require a table of 2^64 entries for a 64-bit block, while stream ciphers do not require tables.
- Block ciphers use Feistel Cipher Structure, while stream ciphers are based on DES.
- Block ciphers process messages in blocks, while stream ciphers process messages a bit or byte at a time. (correct)
Why is a Feistel Cipher Structure needed in block ciphers?
Why is a Feistel Cipher Structure needed in block ciphers?
- To increase the speed of encryption in block ciphers.
- To provide variable key sizes for secure encryption.
- To eliminate the need for substitution tables in block ciphers.
- To efficiently recover messages by decrypting ciphertext. (correct)
How many different plaintext blocks exist for a n-bit block in block ciphers?
How many different plaintext blocks exist for a n-bit block in block ciphers?
- $2^{n-1}$
- $n!$
- $n^2$
- $2^n$ (correct)
Which cryptographic algorithms provide both secrecy and authentication services?
Which cryptographic algorithms provide both secrecy and authentication services?
What is the typical size of a block used in block ciphers?
What is the typical size of a block used in block ciphers?
What is the main concept behind the Feistel cipher structure?
What is the main concept behind the Feistel cipher structure?
What is the purpose of the confusion and diffusion principles in cipher design, as suggested by Claude Shannon?
What is the purpose of the confusion and diffusion principles in cipher design, as suggested by Claude Shannon?
What is the primary advantage of increasing the key size in a Feistel cipher design?
What is the primary advantage of increasing the key size in a Feistel cipher design?
Which of the following statements about the Data Encryption Standard (DES) is true?
Which of the following statements about the Data Encryption Standard (DES) is true?
What is the primary concern in the design of the round function in a Feistel cipher?
What is the primary concern in the design of the round function in a Feistel cipher?
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Study Notes
Block Ciphers
- Process messages in blocks, each of which is then en/decrypted
- Typically use blocks of 64 or 128 bits
- Most symmetric block ciphers are based on a Feistel Cipher Structure
Feistel Cipher Structure
- Devised by Horst Feistel
- Based on concept of invertible product cipher
- Partitions input block into two halves
- Processes through multiple rounds which perform a substitution on left data half based on round function of right half and subkey, then has a permutation swapping halves
- Implements Shannon’s substitution-permutation network concept
Feistel Cipher Design Principles
- Block size: increasing size improves security, but slows cipher
- Key size: increasing size improves security, makes exhaustive key searching harder, but may slow cipher
- Number of rounds: increasing number improves security, but slows cipher (typical 16)
- Subkey generation: greater complexity can make analysis harder, but slows cipher
- Round function: greater complexity can make analysis harder, but slows cipher
- Fast software en/decryption and ease of analysis are more recent concerns for practical use and testing
Data Encryption Standard (DES)
- Most widely used block cipher in the world
- Adopted in 1977 by NBS (National Bureau of Standards) as FIPS (Federal Information Processing Standard) PUB 46
- Encrypts 64-bit data using 56-bit key
- Has widespread use, but has considerable controversy over its security
DES History
- IBM developed Lucifer cipher (1971) by a team led by Feistel
- Used 64-bit data blocks with 128-bit key
- Redeveloped as a commercial cipher with input from NSA and others
- In 1973, NBS issued a request for proposals for a national cipher standard
- IBM submitted DES, which was selected as the standard
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