Extracting text from images

In the previous recipe, we saw how to hide text in the RGBA values of an image. This recipe will let us extract that data out.

How to do it…

We saw in the previous recipe that we split up a characters byte into 8 bits and spread them over the LSBs of two pixels. Here's that diagram again as a refresher:

The following is the script that will do the extraction:

from PIL import Image
from itertools import izip

def get_pixel_pairs(iterable):
    a = iter(iterable)
    return izip(a, a)


def get_LSB(value):
    if value & 1 == 0:
        return '0'
    else:
        return '1'

def extract_message(carrier):
    c_image = Image.open(carrier)
    pixel_list = list(c_image.getdata())
    message = ""

    for pix1, pix2 in get_pixel_pairs(pixel_list):
        message_byte = "0b"
        for p in pix1:
            message_byte += get_LSB(p)

        for p in pix2:
            message_byte += get_LSB(p)

        if message_byte == "0b00000000":
            break

        message += chr(int(message_byte,2))
    return message

print extract_message('messagehidden.png')

How it works…

First, we import the Image module from PIL; we also import the izip module from itertools. The izip module will be used to return pairs of pixels:

from PIL import Image
from itertools import izip

Next, we create two helper functions. The get_pixel_pairs function takes in our pixel list and returns the pairs back; as each message character was split over two pixels, this makes extraction easier. The other helper function get_LSB will take in an R, G, B, or A value and use a bit mask to get the LSB value and return it in a string format:

def get_pixel_pairs(iterable):
    a = iter(iterable)
    return izip(a, a)


def get_LSB(value):
    if value & 1 == 0:
        return '0'
    else:
        return '1'

Next, we have our main extract_message function. This takes in the filename of our carrier image:

def extract_message(carrier):

We then create an image object from the filename passed in and then create an array of pixels from the image data. We also create an empty string called message; this will hold our extracted text:

c_image = Image.open(carrier)
pixel_list = list(c_image.getdata())
message = ""

Next, we create a for loop that will iterate over all of the pixel pairs returned using our helper function get_pixel_pairs; we set the returned pairs to pix1 and pix2:

for pix1, pix2 in get_pixel_pairs(pixel_list):

The next part of code that we will create is a string variable that will hold our binary string. Python knows that it'll be the binary representation of a string by the 0b prefix. We then iterate over the RGBA values in each pixel (pix1 and pix2) and pass that value to our helper function, get_LSB, the value that's returned is appended onto our binary string:

message_byte = "0b"
for p in pix1:
    message_byte += get_LSB(p)
for p in pix2:
    message_byte += get_LSB(p)

When the preceding code runs, we will get a string representation of the binary for the character that was hidden. The string will look like this 0b01100111, we placed a stop character at the end of the message that was hidden that will be 0x00, when this is outputted by the extraction part we need to break out of the for loop as we know we have hit the end of the hidden text. The next part does that check for us:

if message_byte == "0b00000000":
            break

If it's not our stop byte, then we can convert the byte to its original character and append it onto the end of our message string:

message += chr(int(message_byte,2))

All that's left to do is return the complete message string back from the function.

There's more…

Now that we have our hide and extract functions, we can put them together into a class that we will use for the next recipe. We will add a check to test if the class has been used by another or if it is being run on its own. The whole script looks like the following. The hide and extract functions have been modified slightly to accept an image URL; this script will be used in the C2 example in Chapter 8, Payloads and Shells:

#!/usr/bin/env python

import sys
import urllib
import cStringIO

from optparse import OptionParser
from PIL import Image
from itertools import izip

def get_pixel_pairs(iterable):
    a = iter(iterable)
    return izip(a, a)

def set_LSB(value, bit):
    if bit == '0':
        value = value & 254
    else:
        value = value | 1
    return value

def get_LSB(value):
    if value & 1 == 0:
        return '0'
    else:
        return '1'

def extract_message(carrier, from_url=False):
    if from_url:
        f = cStringIO.StringIO(urllib.urlopen(carrier).read())
        c_image = Image.open(f)
    else:
        c_image = Image.open(carrier)

    pixel_list = list(c_image.getdata())
    message = ""

    for pix1, pix2 in get_pixel_pairs(pixel_list):
        message_byte = "0b"
        for p in pix1:
            message_byte += get_LSB(p)

        for p in pix2:
            message_byte += get_LSB(p)
            
        if message_byte == "0b00000000":
            break

        message += chr(int(message_byte,2))
    return message

def hide_message(carrier, message, outfile, from_url=False):
    message += chr(0)
    if from_url:
        f = cStringIO.StringIO(urllib.urlopen(carrier).read())
        c_image = Image.open(f)
    else:
        c_image = Image.open(carrier)
        
    c_image = c_image.convert('RGBA')

    out = Image.new(c_image.mode, c_image.size)
    width, height = c_image.size
    pixList = list(c_image.getdata())
    newArray = []

    for i in range(len(message)):
        charInt = ord(message[i])
        cb = str(bin(charInt))[2:].zfill(8)
        pix1 = pixList[i*2]
        pix2 = pixList[(i*2)+1]
        newpix1 = []
        newpix2 = []

        for j in range(0,4):
            newpix1.append(set_LSB(pix1[j], cb[j]))
            newpix2.append(set_LSB(pix2[j], cb[j+4]))

        newArray.append(tuple(newpix1))
        newArray.append(tuple(newpix2))

    newArray.extend(pixList[len(message)*2:])

    out.putdata(newArray)
    out.save(outfile)
    return outfile   


if __name__ == "__main__":


    usage = "usage: %prog [options] arg1 arg2"
    parser = OptionParser(usage=usage)
    parser.add_option("-c", "--carrier", dest="carrier",
                help="The filename of the image used as the carrier.",
                metavar="FILE")
    parser.add_option("-m", "--message", dest="message",
                help="The text to be hidden.",
                metavar="FILE")
    parser.add_option("-o", "--output", dest="output",
                help="The filename the output file.",
                metavar="FILE")
    parser.add_option("-e", "--extract",
                action="store_true", dest="extract", default=False,
                help="Extract hidden message from carrier and save to output filename.")
    parser.add_option("-u", "--url",
                action="store_true", dest="from_url", default=False,
                help="Extract hidden message from carrier and save to output filename.")

    (options, args) = parser.parse_args()
    if len(sys.argv) == 1:
        print "TEST MODE\nHide Function Test Starting ..."
        print hide_message('carrier.png', 'The quick brown fox jumps over the lazy dogs back.', 'messagehidden.png')
        print "Hide test passed, testing message extraction ..."
        print extract_message('messagehidden.png')
    else:
        if options.extract == True:
            if options.carrier is None:
                parser.error("a carrier filename -c is required for extraction")
            else:
                print extract_message(options.carrier, options.from_url)
        else:
            if options.carrier is None or options.message is None or options.output is None:
                parser.error("a carrier filename -c, message filename -m and output filename -o are required for steg")
            else:
                hide_message(options.carrier, options.message, options.output, options.from_url)

Previous Chapter

Hiding text in images

Next Chapter

Enabling command and control using steganography

Table of Contents for Python: Penetration Testing for Developers

Extracting text from images

How to do it…

How it works…

There's more…

Table of Contents for
Python: Penetration Testing for Developers