CSC231 Homework 6

Steganography

This homework is due on Wed. Oct 29th, at 11:59 p.m. + 1min.

It deals with steganography, and specifically the problem of decoding an images where the least significant bit of each byte is used to store one eighth of an ascii character.

An example will illustrate the concept. Assume that the first 16 bytes of a BMP image file are as follows:

         A2 
         B2 
         C7 
         FF 
         00 
         20 
         31 
         30 

         AA 
         AA 
         AB 
         AB
         00 
         20 
         21 
         21 

Let's rewrite the same 16 bytes, but this time we write next to each byte its least significant bit as a separate entity:

         A2 0
         B2 0
         C7 1
         FF 1
         00 0
         20 0
         31 1
         30 0

         AA 0
         AA 0
         AB 1
         AB 1
         00 0
         20 0
         21 1
         21 1

You notice that when the byte contains an even number, the LSB is 0, and 1 otherwise.

Now, let's group these 16 least significant bits as 2 bytes. We get 00110010 00110011, or 0x32 0x33. We recognize here the ASCII codes for the character '2' and for the character '3'. We have in effect "hidden" the string "23" in the first 16 bytes of the image, and we have done so in such a way that the image still looks like an original when viewed by a human being. This is because the human eye cannot notice a 1-bit change in a color coded in 8 bits.

Of course, we assumed here that the MSB of the ASCII charcter is the first one that gets stored in the pixel bytes, but it could just as well be the LSB. You have to figure out how the secret message gets encoded into the pixel bytes... Your assignment is to write an "extractor" program that will take the name of a BMP file on the command line, load the image in memory, and extract from the least significant bits of the bytes forming the image a series of ASCII bytes that it will then print on the screen.

As is the convention for character strings in C, the secret message is terminated by a 0-byte, i.e. a byte whose 8 bits are 0 (as we did in class on Wed. Oct 22nd).

Use the following files as a skeleton for working with BMP files:

• makegreyBmp.asm
• mytools2.inc
• fileio2.inc

To open a bmp file and process it with makegreyBmp.asm, proceed as follows (assume the image file is called mountains.bmp):

             nasm -f elf -F stabs makegreyBmp.asm
             ld -o makegreyBmp makegreyBmp.o

             makegreyBmp  mountains.bmp

Once the program returns to the Linux pompt, your mountains.bmp file will have been modified and transformed into a (dark) black and white image.

Here are 4 images that contain secret information that your program should be able to decode:

• media:CSC231_secret1.bmp
• media:CSC231_secret2.bmp
• media:CSC231_secret3.bmp
• media:CSC231_secret4.bmp

Note that the first three images look absolutely identical, but all 3 contain different messages.

Once your program extracts a string that looks like English text, and no strange characters, you're done! Nothing else to submit except your program! Submit your program (called hw6.asm) as follows

     submit hw6 hw6.asm