Claude Shannon and Substitution-Permutation Ciphers

Cipher Block Chaining (CBC) vs Electronic Codebook (ECB)

• CBC mode enhances security by using an initialization vector (IV) to ensure identical plaintext blocks encrypt to different ciphertexts, unlike ECB which uses fixed block encryption.

Feistel Cipher Structure in DES

• A key design principle of Feistel ciphers is to allow reuse of the same round function for both encryption and decryption.
• Decryption in a Feistel cipher involves the same steps as encryption but processes the subkeys in reverse order.
• The Data Encryption Standard (DES) employs a key size of 56 bits.

Security Attacks and Vulnerabilities

• Timing attacks can exploit the variable time taken for cipher calculations, depending on input values.
• The DES key schedule is crucial for generating subkeys used during encryption rounds.

DES Internal Mechanisms

• In a Feistel structure, the right half of the input block is updated by performing operations involving the left half and the round function.
• The Initial Permutation (IP) is designed to rearrange the input bits to enhance diffusion before the main encryption process.
• The DES round function F(R, K) comprises expansion, substitution (S-boxes), and permutation, but does not include key scheduling as a component.

DES S-boxes and Key Schedule

• The 'autoclaving' feature in S-boxes allows the output to depend on both the input and the key, adding non-linearity.
• The purpose of the DES key schedule is to create multiple subkeys from the initial key for use throughout the rounds of encryption.

Controversies and Security Features

• The 56-bit key size of DES has sparked controversy due to its susceptibility to brute-force attacks, especially as computational power increases.
• The 'avalanche effect' refers to a desirable property wherein a small change in plaintext or key results in significant changes in the ciphertext.

Motivation and Limitations of DES Variants

• The Feistel cipher structure was motivated by the need for a reversible yet secure encryption method that simplifies both encryption and decryption.
• Double-DES is discouraged due to its vulnerability to meet-in-the-middle attacks, which reduce its effective security.
• In comparison, Three-Key Triple-DES has greater security due to the independence of all three keys, unlike Two-Key Triple-DES which reuses one key.

Properties and Design Principles of Feistel Ciphers

• A key property allowing efficient decryption in Feistel ciphers is the symmetric structure, which processes round functions in reverse.
• A principal design principle of Feistel ciphers is to maintain a balance between security and performance through multiple rounds of processing.
• The subkey generation process differs between single-DES and Triple-DES, with Triple-DES producing more subkeys due to multiple keys being utilized.

Learn about Claude Shannon's introduction of the idea of substitution-permutation (S-P) networks in 1949, which formed the basis of modern block ciphers. Discover how S-P networks are based on the two primitive cryptographic operations: substitution and permutation.