Question 4 The following describes a toy block cipher. The block size is 8 bits

ID: 3605914 • Letter: Q

Question

Question 4

The following describes a toy block cipher. The block size is 8 bits and the key size is 16 bits. A

round will consist of the following.

1. The input is XORed with the rst 8 bits of the round subkey

2. The input, considered as an element of GF(28) with irreducible P(x) = x8 + x4 + x3 + x + 1, is squared.

3. The left 4 bits of the block are swapped with the right 4 bits of the block. For example 0x4A becomes 0xA4.

There are two rounds. The key schedule is the following:

1. During the rst round, the subkey is the rst 8 bits of the key.

2. During the second round, the subkey is the second 8 bits of the key.

Question 4 part a

Use the above cipher with key k = 0x1111 to encrypt the block B = 0x88.

Answer here

Question 4 part b

Use this block cipher to encrypt the plaintext x =0x4A01 in OFB mode with IV =0x22 and key

k =0x4444. Please show all intermediate steps.

include "FlashDrive.h"
using namespace std;
using namespace cs52;

void main( )
{
cs52::FlashDrive drive1( 10, 0, false );
cs52::FlashDrive drive2( 20, 0, false );

drive1.plugIn( );
drive1.formatDrive( );
drive1.writeData( 5 );
drive1.pullOut( );

drive2.plugIn( );
drive2.formatDrive( );
drive2.writeData( 1 );
drive2.pullOut( );

cs52::FlashDrive combined = drive1 + drive2;
cout << "this drive's filled to " << combined.getUsed( ) << endl;

cs52::FlashDrive other = combined – drive1;
cout << "the other cup's filled to " << other.getUsed( ) << endl;

if (combined > other) {
cout << "looks like combined is bigger..." << endl;
}
else {
cout << "looks like other is bigger..." << endl;
}

if (drive2 > other) {
cout << "looks like drive2 is bigger..." << endl;
}
else {
cout << "looks like other is bigger..." << endl;
}

if (drive2 < drive1) {
cout << "looks like drive2 is smaller..." << endl;
}
else {
cout << "looks like drive1 is smaller..." << endl;
}
}

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