Demystifying AES Key Expansion: A Deep Dive into Secure Cryptography

In the world of cryptography, AES (Advanced Encryption Standard) stands tall as one of the most widely used encryption algorithms, trusted for its robust security features. At the heart of AES lies a critical process known as key expansion, which plays a pivotal role in generating the round keys used for encryption and decryption. In this blog post, we will delve into the intricacies of AES key expansion, unraveling its significance and exploring the mechanisms behind it.

Understanding AES Encryption

Before delving into key expansion, let's grasp the fundamentals of AES encryption briefly. AES operates on fixed block sizes of 128 bits, with key lengths of 128, 192, or 256 bits. It employs a substitution-permutation network (SPN) structure, dividing the encryption process into several rounds. Each round consists of four main operations: SubBytes, ShiftRows, MixColumns, and AddRoundKey.

The Role of Key Expansion

Key expansion is the process of transforming a relatively short key into a larger set of round keys, which are used in each round of the AES encryption and decryption process. This expansion ensures that the encryption algorithm remains secure and resistant to various cryptographic attacks, such as brute force and differential cryptanalysis.

The Key Expansion Algorithm Process with Algorithm Source Code

import 'dart:io';

import 'list_data.dart';

var rConColumn = [];
var round;
Data data = Data();

void main() {
  round = 03;
  var currentRoundHexKey = [
    ['47', '40', 'A3', '4C'],
    ['37', 'D4', '70', '9F'],
    ['94', 'E4', '3A', '42'],
    ['ED', 'A5', 'AE', 'BC'],
  ];

  int keyRowCount = currentRoundHexKey.length; //total row in current Round key
  int keyColumnCount =
      currentRoundHexKey[0].length; //total column in current Round key

  var sBoxProcessingColumn = []; //currently working column before Sbox
  var afterSBoxProcessingColumn = []; //after processing sbox column
  var nxtRoundHexKey = []; //next Round key or final result

  int n = keyColumnCount - 1;

//loop for finding column before processing sbox
  for (int i = 0; i < keyRowCount;) {
    i++;
    if (i == keyColumnCount) {
      i = 0;
      sBoxProcessingColumn.add(currentRoundHexKey[i][n]);
      break;
    }
    sBoxProcessingColumn.add(currentRoundHexKey[i][n]);
  }

//loop for finding column after processing sbox
  for (int i = 0; i < sBoxProcessingColumn.length; i++) {
    String firstChar = sBoxProcessingColumn[i].substring(0, 1);
    String secondChar = sBoxProcessingColumn[i].substring(1);

    int row = int.parse(firstChar, radix: 16);
    int colmn = int.parse(secondChar, radix: 16);

    String value = data.s_Box[row][colmn];
    afterSBoxProcessingColumn.add(value);
  }

  rConTableValue(round);
  String? result;

  for (int i = 0; i < keyRowCount; i++) {
    nxtRoundHexKey.add([]);
  }

//loop for xor operation
  for (int i = 0; i < keyColumnCount; i++) {
    if (i == 0) {
      for (int j = 0; j < keyRowCount; j++) {
        String result1 =
            xorHex(afterSBoxProcessingColumn[j], currentRoundHexKey[j][i]);
        result = xorHex(result1, rConColumn[j]);
        nxtRoundHexKey[i].add(result);
      }
    } else {
      for (int j = 0; j < keyRowCount; j++) {
        result = xorHex(nxtRoundHexKey[i - 1][j], currentRoundHexKey[j][i]);
        nxtRoundHexKey[i].add(result);
      }
    }
  }
  //Next Round Key Output
  print('Round ${round + 1} AES Key : ');
  for (int i = 0; i < keyRowCount; i++) {
    for (int j = 0; j < keyColumnCount; j++) {
      stdout.write('${nxtRoundHexKey[j][i]}  ');
    }
    print('');
  }
}

//function for complete rcon table
void rConTableValue(int round) {
  rConColumn.add(data.rCon_table[round]);
  rConColumn.add('00');
  rConColumn.add('00');
  rConColumn.add('00');
}

//function for Xor operation between two string
String xorHex(String hex1, String hex2) {
  int int1 = int.parse(hex1, radix: 16);
  int int2 = int.parse(hex2, radix: 16);

  int result = int1 ^ int2;

  String hexResult =
      result.toRadixString(16); // Convert result back to hexadecimal

  return hexResult.toUpperCase();
}

class Data{
  var rCon_table = [
  '01', '02', '04', '08', '10', '20', '40', '80', '1B', '36', '6C', 'D8', 'AB', '4D'
  ];

var s_Box = [
  [
    "63", "7C", "77", "7B", "F2", "6B", "6F", "C5", "30", "01", "67", "2B", "FE", "D7", "AB", "76",
  ],
  [
    "CA", "82", "C9", "7D", "FA", "59", "47", "F0", "AD", "D4", "A2", "AF", "9C", "A4", "72", "C0",
  ],
  [
    "B7", "FD", "93", "26", "36", "3F", "F7", "CC", "34", "A5", "E5", "F1", "71", "D8", "31", "15",
  ],
  [
    "04", "C7", "23", "C3", "18", "96", "05", "9A", "07", "12", "80", "E2", "EB", "27", "B2", "75",
  ],
  [
    "09", "83", "2C", "1A", "1B", "6E", "5A", "A0", "52", "3B", "D6", "B3", "29", "E3", "2F", "84",
  ],
  [
    "53", "D1", "00", "ED", "20", "FC", "B1", "5B", "6A", "CB", "BE", "39", "4A", "4C", "58", "CF",
  ],
  [
    "D0", "EF", "AA", "FB", "43", "4D", "33", "85", "45", "F9", "02", "7F", "50", "3C", "9F", "A8",
  ],
  [
    "51", "A3", "40", "8F", "92", "9D", "38", "F5", "BC", "B6", "DA", "21", "10", "FF", "F3", "D2",
  ],
  [
    "CD", "0C", "13", "EC", "5F", "97", "44", "17", "C4", "A7", "7E", "3D", "64", "5D", "19", "73",
  ],
  [
    "60", "81", "4F", "DC", "22", "2A", "90", "88", "46", "EE", "B8", "14", "DE", "5E", "0B", "DB",
  ],
  [
    "E0", "32", "3A", "0A", "49", "06", "24", "5C", "C2", "D3", "AC", "62", "91", "95", "E4", "79",
  ],
  [
    "E7", "C8", "37", "6D", "8D", "D5", "4E", "A9", "6C", "56", "F4", "EA", "65", "7A", "AE", "08",
  ],
  [
    "BA", "78", "25", "2E", "1C", "A6", "B4", "C6", "E8", "DD", "74", "1F", "4B", "BD", "8B", "8A",
  ],
  [
    "70", "3E", "B5", "66", "48", "03", "F6", "0E", "61", "35", "57", "B9", "86", "C1", "1D", "9E",
  ],
  [
    "E1", "F8", "98", "11", "69", "D9", "8E", "94", "9B", "1E", "87", "E9", "CE", "55", "28", "DF",
  ],
  [
    "8C", "A1", "89", "0D", "BF", "E6", "42", "68", "41", "99", "2D", "0F", "B0", "54", "BB", "16",
  ],
];

}