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Questions and Answers
What is the Feistel Cipher?
What is the Feistel Cipher?
The Feistel Cipher structure uses a different algorithm for encryption and decryption.
The Feistel Cipher structure uses a different algorithm for encryption and decryption.
False
What is the difficult part of designing a Feistel Cipher?
What is the difficult part of designing a Feistel Cipher?
Selection of round function 'f'
The number of rounds in a Feistel Cipher depends on the desired _______________ from the system.
The number of rounds in a Feistel Cipher depends on the desired _______________ from the system.
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What happens to the subkeys in the decryption process?
What happens to the subkeys in the decryption process?
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The final swapping of 'L' and 'R' in the last step of the Feistel Cipher is optional.
The final swapping of 'L' and 'R' in the last step of the Feistel Cipher is optional.
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What is the tradeoff in increasing the number of rounds in a Feistel Cipher?
What is the tradeoff in increasing the number of rounds in a Feistel Cipher?
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The encryption process uses the Feistel structure consisting of multiple rounds of processing of the _______________, each round consisting of a 'substitution' step followed by a permutation step.
The encryption process uses the Feistel structure consisting of multiple rounds of processing of the _______________, each round consisting of a 'substitution' step followed by a permutation step.
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Match the following components of the Feistel Cipher structure with their descriptions:
Match the following components of the Feistel Cipher structure with their descriptions:
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DES is not an example of a Feistel Cipher.
DES is not an example of a Feistel Cipher.
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Study Notes
Feistel Cipher
- A design model for block ciphers, not a specific scheme
- Many block ciphers, including DES, are derived from the Feistel Cipher
Encryption Process
- Uses the Feistel structure with multiple rounds of processing
- Each round consists of a substitution step followed by a permutation step
- The Feistel structure is shown in an illustration
Decryption Process
- Similar to the encryption process, but with a ciphertext block as input
- The process is identical, except for the subkeys used in reverse order
- The final swapping of 'L' and 'R' is essential for decryption
Round Function 'f'
- The difficult part of designing a Feistel Cipher
- Needs to have several important properties to be unbreakable, but these are beyond the scope of our discussion
Number of Rounds
- Depends on the desired security and efficiency–security tradeoff
- More rounds provide more security, but also make the encryption and decryption processes slower
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Description
Understand the design model of Feistel Cipher, its application in block ciphers, and the encryption process involved.
