Difference between revisions of "CSC231 Homework 6 Solution"

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(New page: <code><pre> ;;; hw6.asm ;;; Amy Gray ;;; This program reads a bmp file into memory and decodes a secret ;;; message hidden in the pixels via steganography ;;; %include "mytools2.inc" ; f...)
 
 
(3 intermediate revisions by the same user not shown)
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<code><pre>
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<onlydft>
 +
<source lang="asm">
 
;;; hw6.asm
 
;;; hw6.asm
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;;; Amy Gray
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;;; Ellysha Raelen Recto
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;;; This program reads a bmp file into memory and decodes a secret
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;;;
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;;; message hidden in the pixels via steganography
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;;; This program written from the skeleton program, makegreyBmp.asm, written
 +
;;; by D. Thiebaut. This program reads a bmp file and reads the last bit of
 +
;;; each byte. The bytes read contains a secret msg which is then displayed.
 
;;;  
 
;;;  
 
%include "mytools2.inc" ; for basic printing to the screen
 
%include "mytools2.inc" ; for basic printing to the screen
 
%include "fileio2.inc" ; for basic file operations
 
%include "fileio2.inc" ; for basic file operations
−
+
 
%assign SYS_EXIT 1
 
%assign SYS_EXIT 1
 
%assign MAXPIXEL 5000000
 
%assign MAXPIXEL 5000000
 +
 +
%assign SYS_WRITE 4
 +
%assign STDOUT 1
  
 
%assign RED            2
 
%assign RED            2
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errorMsg db    "Error accessing file!",0x0A
 
errorMsg db    "Error accessing file!",0x0A
 
errorLen equ    $-errorMsg
 
errorLen equ    $-errorMsg
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message  db 0 ;will be used to store decoded byte
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msg db 0
  
 
section .bss
 
section .bss
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filename resb  255 ; reserve 255 chars for filename
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filename resb  255   ; reserve 255 chars for filename
 
handle  resd 1 ; file descriptor
 
handle  resd 1 ; file descriptor
 
noRead resd 1 ; contains number of bytes read
 
noRead resd 1 ; contains number of bytes read
 
lWidth resd 1 ; the line width rounded to the nearest
 
lWidth resd 1 ; the line width rounded to the nearest
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       ; dword boundary lWidth=(((width-1)/4)+1)*4
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       ; dword boundary lWidth=(((width-1)/4)+1)*4
  
 
;;; ----------------------------------------------------------
 
;;; ----------------------------------------------------------
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cmp al,0
 
cmp al,0
 
jne copy
 
jne copy
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+
 
 
;;; --- open file ---
 
;;; --- open file ---
 
         openFile filename, [handle], errorMsg, errorLen
 
         openFile filename, [handle], errorMsg, errorLen
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;;; -------------------------------------------------------------
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;; -----------------------------------
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;;; mainloop: Extract message hidden in the image's pixels. Goes
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;; add your byte-examining loop  below!
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;;;     through eight bytes from the image and stores the
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        ;; you can access the bytes of the pixels as  byte[ebx+pixelBuf]
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;;; least significant bit from in a new byte, which is
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        ;; and increment ebx in the loop to access successive bytes
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;;; a character from the message. The message ends with
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;; -----------------------------------
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;;; a zero, so each newly formed byte is checked to see
 
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;;; if it is equal to zero.  If yes, the program jumps
 
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;;; to the end.  If no, the program prints the character
 
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;;; registers modified: eax, ebx, ecx, edx
 
−
;;; -------------------------------------------------------------
 
−
 
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add ebx, 7 ;moves to eighth byte
 
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;bits are stored backwards from 7 to 0
 
−
 
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;; for some reason, when 'jmp mainloop' is called below, the program
 
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;; was jumping back to 'add ebx, 7' and adding 7 to ebx (causing everything
 
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;; to go wrong).  'add ebx, 0' is put in as a placeholder.  The program
 
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;; jumps back to here, does nothing and continues on to mainloop
 
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add ebx, 0
 
  
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mainloop:
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;; -----------------------------------
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call decode1byte
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;; mainFor: main section for reading secret bytes
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cmp byte[message], 0
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        ;; written by: Ellysha Raelen Recto
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je theEnd
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        ;; registers modified: none
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;; -----------------------------------
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push ebx ;ebx needs to be saved for accessing pixels
 
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mov ecx, message
 
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mov eax, SYS_WRITE
 
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mov ebx, STDOUT
 
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mov edx, 1
 
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int 0x80
 
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pop ebx ;restore ebx
 
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jmp mainloop ;repeat
 
−
 
−
 
 
−
 
 
−
 
  
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mainFor:
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call decode1byte ; calls method to decode 1 byte of secret msg
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cmp dl, 0 ; if 1 byte is zero, end of secret msg
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je exit ; go to exit if end of msg is found
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call printByte ; print byte
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jmp mainFor ; decode more bytes if end of msg isnt found
  
 
;; -----------------------------------
 
;; -----------------------------------
 
;; end of pixel loop area
 
;; end of pixel loop area
 
;; -----------------------------------
 
;; -----------------------------------
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theEnd:
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;;; --- position file pointer back to beginning of file ---
 
;;; --- position file pointer back to beginning of file ---
 
  seek0 [handle]
 
  seek0 [handle]
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;;; --- close file ---
 
;;; --- close file ---
 
close [handle]
 
close [handle]
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+
 
 
;;; --- exit() ---
 
;;; --- exit() ---
 
exit:
 
exit:
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ret 8
 
ret 8
  
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;;; -------------------------------------------------------------
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;;; --------------------------------------------------------
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;;; decode1byte: decodes 1 byte of secret msg, first bit of the byte
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;;; decode1byte: runs through a series of eight bytes
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;;; is from the last bit of first 8th byte of image
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;;; and extracts the last bit from each.
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;;; registers modified: edx, ecx, eax, ebx
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;;; Starts at the eighth bit and works backward
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;;; -------------------------------------------------------------
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;;; Each extracted bit is stored in edx as
 
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;;; a new byte. This byte is transferred into
 
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;;; the variable 'message'
 
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;;; registers modified: ebx
 
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;;; -------------------------------------------------------
 
−
 
 
decode1byte:
 
decode1byte:
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push edx
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mov dl, 0
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push eax
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mov ecx,7 ; reads first 7 bits
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push ecx
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for: call read1bit ; call method to read 1 bit with shifting
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dec ebx ; goes to next bit
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mov edx, 0 ;will hold secret byte
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loop for
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mov eax, 0 ;will hold byte to extract from
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mov al, byte[ebx+pixelBuf+7] ; reads last bit
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mov ecx, 7
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and al, 1
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or dl, al
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add ebx, 15
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ret
  
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;; the loop runs 7 times.  At the end of each run, the
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;;; -------------------------------------------------------------
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;; secret byte is shifted to the left to make room for
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;;; printByte: prints 1 byte of secret msg
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;; the next new bit. The eighth bit is then extracted
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;;; registers modified: edx, ecx, eax, ebx
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;; outside the loop, so that no shift happens.  An
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;;; -------------------------------------------------------------
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;; eighth shift would cause the first bit to be lost
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printByte:
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;; and an extraneous zero to appear at the end.
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pushad ; pushes registers to stack
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forbyte:
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mov byte[msg], dl ; moves byte read from image to msg
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mov al, byte[ebx+pixelBuf] ;byte to extract from
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mov eax, SYS_WRITE
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and al, 1 ;clear all but least significant bit
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mov ebx, STDOUT
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or dl, al ;add new bit to dl
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mov ecx, msg ; prints msg
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shl dl, 1 ;make room for next bit
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mov edx, 1
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dec ebx ;move to next byte
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int 0x80
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loop forbyte
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popad ; retrieves values of registers from stack
 +
ret
  
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mov al, byte[ebx+pixelBuf] ;get 8th bit
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;;; -------------------------------------------------------------
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and al, 1
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;;; read1bit: used to read a bit if it's one of the first 7 bits of
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or dl, al
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;;; a byte from the image
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mov byte[message], dl ;store decoded byte in 'message'
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;;; registers modified: edx, eax
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;;; -------------------------------------------------------------
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read1bit:
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mov al, byte[ebx+pixelBuf+7]  
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and al, 1 ; get the last bit of byte from image
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or dl, al ; saves the values of the bits from
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; the bytes of already read to dl
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shl dl, 1 ; shifts bits to make room for next bit
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; to be read
 +
ret
  
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add ebx, 15 ;jumps to next set of eight bytes
 
−
 
−
pop ecx
 
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pop eax
 
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pop edx
 
−
 
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ret
 
  
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</pre></code>
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</source>
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</onlydft>
 +
[[Category:CSC231]][[Category:nasm]]

Latest revision as of 13:39, 20 September 2014


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