10 Questions
What is the main advantage of using a Feistel cipher structure?
It reduces the size of the code or circuitry required to implement the cipher.
What is the purpose of the concept of 'Diffusion' in the Feistel cipher?
To ensure that a single bit change in the plaintext results in a completely different ciphertext.
What is the purpose of the concept of 'Confusion' in the Feistel cipher?
To ensure that a single bit change in the key results in a completely different ciphertext.
What is the purpose of creating 16 subkeys, each 48-bits long, in the DES algorithm?
To increase the complexity of the key schedule.
Which of the following is NOT a key property of the Feistel cipher structure?
It requires a large number of rounds to be secure.
What is the purpose of the PC-1 permutation table?
To generate the 56-bit key used in the DES algorithm
What is the value of $C_4$ in the solution?
1011 0000 1110 0010 0010 1000 1000
What is the value of $D_4$ in the solution?
0010 0010 0010 1101 1111 1110 0010
What is the final value of the 56-bit key $K_4$?
0010 1010 1011 0101 0000 0000 1010 1100 1101 1110 0011 1000
What is the purpose of the circular left shift operation on the key in the DES algorithm?
To generate the 16 round keys used in the DES algorithm
Study Notes
Feistel Cipher
- A symmetric structure used in constructing block ciphers, also known as a Feistel network.
- Offers the advantage of similar encryption and decryption operations, requiring only a reversal of the key schedule.
- Reduces the size of the code or circuitry required to implement the cipher by nearly half.
Feistel's Proposal
- Utilizes a cipher that alternates between substitutions and permutations.
- Implements diffusion and confusion to hide relationships between the ciphertext and plaintext, and the ciphertext and encryption key.
Diffusion
- Hides the relationship between the ciphertext and plaintext by making each symbol in the ciphertext dependent on some or all symbols in the plaintext.
- A single change in the plaintext will result in multiple changes in the ciphertext.
Confusion
- Hides the relationship between the ciphertext and encryption key by making most or all bits in the ciphertext change when a single bit in the key is changed.
Data Encryption Standard (DES)
- A block cipher that uses the Feistel structure.
How DES Works
- The 64-bit key is permuted according to the PC-1 table to create 16 subkeys, each 48-bits long.
- The subkeys are used in the encryption and decryption process.
Example: Key Generation
- A hexadecimal key K = 133457799BBCDFF1 is converted to a binary key.
- The binary key is permuted according to the PC-1 table to create 16 subkeys.
- Each subkey is 48-bits long.
Learn about the symmetric structure used in the construction of block ciphers known as a Feistel network. Discover how encryption and decryption operations are similar, requiring a reversal of the key schedule. Explore Feistel's proposal of alternating substitutions and permutations in ciphers.
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