(403) Data Encryption Standard

Chapter 1: Intro

• The Data Encryption Standard (DES) is a widely used symmetric encryption scheme.

• Published in 1977 and standardized in 1979.

Chapter 2: DES

Key and Ciphertext

• DES utilizes a key size of 64 bits (8 bytes).

• Out of these, one parity bit is allocated for each byte, making the effective key length 56 bits.

• The ciphertext generated by DES is also a 64-bit block.

Structure of DES

• DES operates through 16 rounds of operations.

• From the original 56-bit key, it generates 16 subkeys—one for each round.

• Decryption follows a similar process as encryption but uses the subkeys in reverse order.

  • Example: In decryption, the first round uses subkey 16, the second round uses subkey 15, and so forth.

Principles of Encryption

• Diffusion is a critical principle in encryption, generally achieved through permutation.

• In DES, permutation alters the positions of bits within the data.

• Each round of DES has consistent operations using different per-round keys, inputting ciphertext from the previous round and outputting text for the next round.

Chapter 3: Decryption

Decryption Process

• The input data is divided into two halves: the left half and the right half.

• The output's left half is simply the input's right half.

• The output's right half is derived from the input's left half combined with the result of a specific function, known as the mangler function.

The Mangler Function

• The input to the mangler function is a 32-bit right half, which is expanded to 48 bits.

• This 48-bit value is then combined with the 48-bit round key.

• S-boxes are employed to substitute the expanded 48-bit value back into a 32-bit value.

Algebraic Representation

• DES operations in a round can be represented algebraically:

  • The left output half corresponds to the input's right half.

  • The right output half results from combining the left input half with the output of the mangler function, which takes the right input half and the round key.