The heart of this cipher is the DES function, f. The DES function applies a 48-bit key to the rightmost 32 bits to produce a 32-bit output. Expansion Permutation Box ? Since right input is 32-bit and round key is a 48-bit, we first need to expand right input to 48 bits. Permutation logic is graphically depicted in the following illustration ?, In cryptography, a permutation box (or P -box) is a method of bit-shuffling used to permute or transpose bits across S-boxes inputs, retaining diffusion while transposing. [1] An example of a 64-bit expansion P -box which spreads the input S-boxes to as many output S-boxes as possible.
9.3 THE DES S-BOXES, P -BOX, AND INITIAL PERMUTATION (IP) Tables 9.4 to 9.11 specify the seven DES S-boxes, each with a 6-bit input (x 0, x 1, x 2, x 3, x 4, x 5, x 6) and a 4-bit output (y 0, y 1, y 2, y 3) each table contains 4 rows and 15 columns, where. Bits (x 0, x 6) identify a row in the table, andbits (x 1, x 2, x 3, x 4) identify a column in the table.TABLE 9.4 DES S-Box S[0], can be defined mathematically, DES uses Table 6.2 to define this P -box. Table 6 6Table 6.6 Expansion PExpansion P -box tablebox table 6.16. 6.2.2 Continue Whitener (XOR) After the expansion permutation, DES uses the XOR operation on the expanded right section and the round key. Note that both the right section and the key are 48-, Data Encryption Standard – Tutorialspoint, Lecture 4 Data Encryption Standard (DES), Permutation box – Wikipedia
