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Chapter 23: Problem 18

Give an example of an encryption algorithm and explain how it works.

Short Answer

Expert verified
AES (Advanced Encryption Standard) is a symmetric encryption algorithm. It starts with a key expansion, then proceeds with rounds that include four steps: SubBytes, ShiftRows, MixColumns, and AddRoundKey. The final round includes only three steps - SubBytes, ShiftRows, and AddRoundKey.

Step by step solution

01

Choose Encryption Algorithm

For this exercise, let's choose AES (Advanced Encryption Standard) as the encryption algorithm to explain.
02

AES Overview

AES is a symmetric encryption algorithm. This means the same key is used for both encrypting and decrypting the data. AES operates on blocks of data and each block is considered as a two-dimensional array. The size of these blocks is 128 bits, and they can use keys of 128, 192, or 256 bits.
03

Key Expansion

Before beginning the encryption process, AES performs a key expansion routine that turns the input key (of either 128, 192, or 256 bits) into several round keys.
04

Round 0 - AddRoundKey

Before any main rounds of transformation start, the AddRoundKey step happens. It is simple bitwise XOR operation between the input block and the first round key.
05

Main Rounds

Each main round includes four steps: SubBytes, ShiftRows, MixColumns, and AddRoundKey. SubBytes step substitutes each byte in the block by another byte according to a substitution box (S-Box). ShiftRows step shifts each row of the block to the left a specific number of times. MixColumns step mixes the four bytes in each column. AddRoundKey step is similar to the one in Round 0.
06

Final Round

In the last round, only three steps are executed - SubBytes, ShiftRows, and AddRoundKey.

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