Hi, I currently have an task to complete on Assembly.
I have been supplid the following code
#include <iostream> //for cin >> and cout <<
#include <conio.h> //for _kbhit
using namespace std;
#define dollarchar '$'
const int maxchars = 5; // you may alter this but it should be at least 5!
//----------------------------- Subroutines C++ ----------------------------------------------------------
void get_char (char& a_character) // check your ASCII tables to help make sense of this
{ cin >> a_character;
while ((a_character < '0' | a_character > 'z') && (a_character != dollarchar))
{ cout << "Alphanumeric characters only please > ";
cin >> a_character;
}
}
void show_char (char a_character)
{
cout << a_character ;
}
void show_string (char a_string[], int upto_here)
{ //cout << "\n\n";
int i;
for (i=0; i<upto_here; i++)
cout << a_string [i];
cout << "\n\n";
}
//----------------------------- End of Subroutines -------------------------------------------------------------
int main()
{
int charcount;
char EKey; // Encryption key,
char SChars[maxchars], EChars[maxchars], DChars[maxchars]; // Source, Encrypted, Decrypted character string
char s_char, e_char, d_char; // source, encrypted, decrypted char
cout << "\nPlease enter your Encryption Key (EKey) letter then <enter>: ";
cin >> EKey;
cout << "\nNow enter upto " << maxchars << " alphanumeric characters and press enter:\n\n";
charcount = 0;
get_char (s_char); // Get a character from user
while ((charcount < maxchars) && (s_char != dollarchar))
{
SChars [charcount] = s_char; // Store in the source chars array
__asm { // call the encrypt subroutine
push eax // save register values on stack to be safe
push ecx
movsx ecx,s_char // enregister the source character and encryption key
movsx eax,EKey
call encrypt11 // encrypt (s_char,e_char,EKey);
mov e_char,al // only need lower half of EAX to return result to e_char
pop ecx // restore original register values from stack
pop eax
}
EChars [charcount] = e_char; // Store in the encrypted chars array
get_char (s_char); // Get next character from user
charcount++; // Maintain count of characters entered
}
cout << "Original source string = ";
show_string (SChars, charcount);
cout << "Encrypted string = ";
show_string (EChars, charcount);
while ( !_kbhit()); //hold the screen until a key is pressed
return (0);
//----------------------------- ASM Subroutines ------------------------------------------------------
__asm {
// Encrypt subroutine. You should paste in the encryption routine you've been allocated from BB and
// overwrite this initial version. Leave on the appended number but change the 'call' above to use the
// correct 'encryptnn' label.
// Inputs: register EAX = Encryption Key value, and ECX = the character to be encrypted.
// Outputs: register EAX = the encrypted value of the source character.
Encrypt11:
push edx
push ecx
ror al,1
ror al,1
ror al,1
mov edx,eax
pop eax
sub eax,0x02
xor eax,edx
rol al,1
rol al,1
rol al,1
pop edx
ret
}
//----------------------------- End of ASM Subroutines -----------------------------------------------
} // end of whole programFirst of all I need to figure out what the code after Encrpyt11 does.
Second, I need to implement the standard call method. I need to alter the program so that it adopts the accepted C++ standard calling procedure (i.e. ‘_stdcall’) for passing parameters into the subroutine. This uses the stack and the EBP register rather than the general purpose registers.
And thirdly, I need to add a decryption subroutine written in assembler that reverses the encryption. Also, add C++ code to put the decrypted char (d_char) into the appropriate array (DChars, already declared in the C++ program).
I've been looking at some guides and tutorials and stuff online but struggling with all three tasks. Any help would be greatly appreciated.