Simple steganography in Python

hide_data_in_bmp.py

import sys
import string

def main():
	if(len(sys.argv) > 2):
		encrypt()
	else:
		decrypt()
	

def encrypt():
	with open(sys.argv[1],'rb') as bmp_file:
		bmp = bmp_file.read()
	with open(sys.argv[2],'rb') as to_hide_file:
		to_hide = to_hide_file.read()

	hide_data(bmp,to_hide)

def decrypt():
	with open(sys.argv[1],'rb') as bmp_file:
		bmp = bmp_file.read()

	start_offset = bmp[10] #The byte at position 10 tells us where the color data starts

	#Deconstruct each byte and get its final bit
	bits = []
	for i in range(start_offset,len(bmp)):
		bits.append(nth_bit_present(bmp[i],0))

	#Combine our bit array into bytes
	out_bytes = []
	for i in range(0,len(bits),8): #Take each 8-bit chunk
		if(len(bits) - i > 8):
			out_bytes.append(bits_to_byte(bits[i:i+8]))


	#Convert bytes to characters
	out = []
	for b in out_bytes:
		out.append(chr(b))

	print(''.join(out))


def hide_data(bmp,data):
	start_offset = bmp[10] #The byte at position 10 tells us where the color data starts

	bmpa = bytearray(bmp)
	data_array = data_to_bits(data)

	#We need to make sure there is enough space to hide our data
	assert len(data_array) < len(bmpa) + start_offset
	
	for i in range(len(data_array)):
		bmpa[i + start_offset] = set_final_bit(bmpa[i + start_offset],data_array[i])

	write_hidden_bmp(bmpa)

def data_to_bits(data):
	bits = []
	for i in range(len(data)):
		#A byte can at max be 8 digits long, i.e. 0b11111111 = 255
		#We start at the left most bit (position 7) and work down to 0
		for j in range(7,-1,-1):
			#Create the logic array of bits for our data
			bits.append(nth_bit_present(data[i],j))

	return bits

def set_final_bit(my_byte,ends_in_one):
	new_byte = 0
	if ends_in_one:
		if(nth_bit_present(my_byte,0)):
			new_byte = my_byte #No modification needed, it already ends in one
		else:
			new_byte = my_byte + 1
	else:
		if(nth_bit_present(my_byte,0)):
			new_byte = my_byte - 1
		else:
			new_byte = my_byte #No modification needed, it already ends in zero
	return new_byte



def nth_bit_present(my_byte,n):
	#Bitwise check to see what the nth bit is
	#If we get anything other than 0, it is TRUE else FALSE
	return (my_byte & (1 << n)) != 0

def bits_to_byte(bits):
	assert len(bits) == 8
	new_byte = 0
	for i in range(8):
		if bits[i]==True:
			#This bit==1 and the "position" we are at in the byte is 7-i
			#Bitwise OR will insert a 1 a this position
			new_byte |= 1 << 7 - i
		else:
			#This bit==0 and the "position" we are at in the byte is 7-i
			#Bitwise OR will insert a 0 a this position
			new_byte |= 0 << 7 - i
	return new_byte

def write_hidden_bmp(bytes_to_write):
	with open(sys.argv[1].replace('.bmp','_hidden.bmp'),'wb') as out:
		out.write(bytes_to_write)

if __name__=="__main__":
	main()

LICENSE.txt

The MIT License (MIT)

Copyright (c) 2014 Phillip Johnson

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.