Assembly for x86 processors

In the following C++ program (encode.cpp which calls into an inline asm code) where an encrypted file is created by the command

encode clear1.txt encoded

Encryption code [0-255]? 0

Reading clear1.txt and creating encoded

Press any key to continue....

The contents of clear1.txt is a text file containing 2 lines of content:

1234

abcd

Under a hex editor it can be seen to have 12 bytes as its this content:

31 32 33 34 0D 0A 61 62 63 64 0D 0A

(0D 0A are the new line characters in windows)

The .cpp file is here:

#include

#include

using namespace std;

void main( int argcount, char *args[] )

{

if( argcount < 3 ) {

count << "Usage: encode infile outfile" << end1; return;

}

const int BUFSIZE = 2000;

char buffer[BUFSIZE];

unsigned int count;

unsigned char encryptCode;

cout << "Encryption code [0-255] ? ";

cin >> encryptCode;

ifstream infile( args[1], ios : : binary );

ofstream outfile( args[2], ios : : binary );

cout << "Reading" << args[1] << " and creating " << args[2] << end1;

while (!infile.eof() )

{

infile.read(buffer, BUFFSIZE );

count = infile.gcount();

___asm {

lea esi, buffer

mov ecx, count

mov al, encryptCode

L1:

xor [esi] , al

inc esi

Loop L1

} // asm

outfile.write (buffer, count) ;

}

}

If 0 (zero) is used as the encryption code (0 is ASII CODE 30H), What does the contents of the file (encoded) look like? Show the contents (12 bytes) of encoded here (in hex):

What command would you run on the command line to decrypt the encoded file? What key must you use?