Steganography - hide a message inside a bitmap image

[Wayne Diamond]

discussion - http://www.powerbasic.com/support/pb...ad.php?t=19322

theory, shmeory...
introducing steganography to powerbasic ...

while cryptography is the art of concealing the contents of a message,
steganography is the art of concealing the actual message itself. almost
any media data transport can be used, such as audio, video, images, even
plain text! i've chosen to use bitmaps for this demo because they are of
a very simple format and very easy to work with compared to advanced
formats such as gif and jpeg.

this is steganography at its most basic level, and the idea is very simple...
for every byte of plaintext, store each bit in 8 different bytes in the target,
modifying only the least significant bit each time.
for example, suppose you want to embed the letter "a" (&b01100001) in a bmp.
first, take one byte from the source data (ie. the bitmap image data). now
replace its least significant bit with the first bit from the letter a (0).
then take the second byte from the source data, and replace its least significant
bit with the second bit from the letter a (1). repeat until all 8 bits have
been embedded (this will require 8 bytes of source data, so if you have 8000
bytes of source data you'll be able to embed up to 1000 bytes of plaintext).
because only the least significant bit is being modified, the changes to the
data are extremely subtle. for example, if a character in the original source
data is "aa", it will either stay as "aa" or become "ab", depending on the bit
being embedded. this subtlety is the reason why the naked eye can't see any
alterations in the image after a message has been embedded in it.


demo...
this simple demo allows you to embed a message of up to 255 characters in a
bitmap image. the before-and-after images will visually look identical to the
naked eye, giving no indication that the image has been altered in any way.

note that to keep things simple, this demo features no cryptography or security
whatsoever, but it is easy to encrypt a message before embedding it using
steganographic techniques.

after running this demo, use a graphics viewer to have a look at the before and
after images - they should look virtually identical to your naked eye. then have
a look at the files using a hex editor, and enjoy the subtle changes

'#########################################################
#compile exe
'ready-to-compile in pbcc
'just delete waitkey$ and replace stdout with msgbox to compile in pbdll/pbwin
'modify the pbmain function so that it points at a valid bmp before compiling.
  
'based on this bmp specification:
' http://www.wotsit.org/filestore/bmpfrmat.zip 
type bitmap_header
 widentifier as word
 dwfilesize as dword
 dwreserved as dword
 dwdataoffset as dword
 dwheadersize as dword
 dwwidth as dword
 dwheight as dword
 wplanes as word
 wbitsperpixel as word
 dwcompression as dword
 dwdatasize as dword
 dwhresolution as dword
 dwvresolution as dword
 dwcolors as dword
 dwimportantcolors as dword
end type
 
'#########################################################
sub savemessage(sfile as string, splaintext as string)
on error resume next
local f as long, char as byte, char2 as byte, x as byte, datapos as long, tmp as long
local bmp as bitmap_header, sdata as string, stext as string
if len(splaintext) > 255 then
    stdout "maximum message length in this version is 255 characters"
    exit sub
end if
stext = chr$(len(splaintext)) & splaintext  'first byte of stext = len of stext
f = freefile
open sfile for binary access read lock shared as #f
 get #f, 1, bmp
 sdata = space$(bmp.dwdatasize)
 get #f, bmp.dwdataoffset, sdata  'fill sdata with the data from dwdataoffset
close #f
stdout "this file can hold a maximum of " & trim$(str$(cint(bmp.dwdatasize / 8))) & " plaintext characters"
datapos = 1
for f = 1 to len(stext)
    char = asc(mid$(stext, f, 1))
    for x = 0 to 7
     char2 = asc(mid$(sdata, datapos,1))
     tmp = bit(char, x)
     if tmp = 0 then
        bit reset char2, 0
     else
        bit set char2, 0
     end if
     mid$(sdata, datapos,1) = chr$(char2)
     datapos = datapos + 1
    next x
next f
f = freefile
open sfile for binary access write lock shared as #f
 put #f, bmp.dwdataoffset, sdata
close #f
end sub

'#########################################################
sub showmessage (sfile as string)
on error resume next
local f as long, i as long, sdata as string, sout as string
local tmp as long, datapos as long, char as byte, char2 as byte, bmp as bitmap_header
f = freefile
open sfile for binary access read lock shared as #f
 get #f, 1, bmp
 sdata = space$(bmp.dwdatasize)
 get #f, bmp.dwdataoffset, sdata  'fill sdata with the data from dwdataoffset
close #f
char = 0
for i = 1 to 8
 char = asc(mid$(sdata, i, 1))
 tmp = bit(char, 0)
 if tmp = 0 then
     bit reset char2, i - 1
 else
     bit set char2, i - 1
 end if
next i
stdout "message size = " & str$(char2)
sout = space$(char2)
datapos = 9
for i = 1 to char2
 char2 = 0
 for f = 0 to 7
  char = asc(mid$(sdata, datapos, 1))
  tmp = bit(char, 0)
  if tmp = 0 then
      bit reset char2, f
  else
      bit set char2, f
  end if
  datapos = datapos + 1
 next f
 mid$(sout, i, 1) = chr$(char2)
next i
stdout "extracted message = " & sout
end sub
 
'#########################################################
function pbmain() as long
on error resume next
'create the bitmap to use for encoding/decoding messages
kill "d:\temp\steg.bmp"
filecopy "d:\winnt\coffee bean.bmp", "d:\temp\steg.bmp"
 
'## encode a message
savemessage "d:\temp\steg.bmp", "my test message"
 
'## decode the message
showmessage "d:\temp\steg.bmp"
waitkey$
end function

stir well and enjoy!


------------------
the powerbasic crypto archives




[this message has been edited by wayne diamond (edited september 09, 2002).]

[Peter Lameijn]

Wayne,

Your method works on some bitmaps, but not all. Lots of bitmaps have dwDataSize set to zero, which is
valid for uncompressed files. Also when you use a 16 color bitmap, result can be quite colorful...
(With changing the LSBit, you don't actually change the color value of the pixel, but it's INDEX value
in the color table used. This colortable can be set to any sequence, so it's possible that entry 1 is black
and entry 2 is red; would give quite colored results )

Another way is using the color table to store text. The only thing is that you're
restricted to he max. number of colors for the text length. This does not change
any colors..

'==============================================================================
' Embed text message in bitmap file.
' Length of text depends on number of colors used
' Peter Lameijn.
'==============================================================================
#Compile Exe
#Include "win32api.inc"
 
'------------------------------------------------------------------------------
Function WriteBmpText(BmpFile As String, sPlainText As String) As Long
  Local BMFH As BITMAPFILEHEADER, hFile As Dword, dStr As String, CtLen As Dword, Cnt As Long
  hFile = FreeFile
  Open BmpFile For Binary As #hFile
    Get$ #hFile, 14, dStr
    Poke$ VarPtr(BMFH), dStr
    Get$ #hFile, 40, dStr
    CtLen = (BMFH.bfOffBits - 14 - 40) \ 4
    Print "Max. number of characters to embed is" + Str$(CtLen)
    If Len(sPlainText) <= CtLen Then
      For Cnt = 1 To Len(sPlainText)
        Get$ #hFile, 3, dStr
        Put$ #hFile, Mid$(sPlainText, Cnt, 1)
      Next
      Function = 1
    End If
  Close #hFile
End Function
 
'------------------------------------------------------------------------------
Function ReadBmpText (BmpFile As String) As String
  Local BMFH As BITMAPFILEHEADER, hFile As Dword, dStr As String, Cnt As Long, Text As String
  hFile = FreeFile
  Open BmpFile For Binary As hFile
  Get$ #hFile, 14, dStr
  Poke$ VarPtr(BMFH), dStr
  Get$ #hFile, 40, dStr
  For Cnt = 1 To ((BMFH.bfOffBits - 54) \ 4)
    Get$ #hFile, 4, dStr
    If Mid$(dStr,4,1) = Chr$(0) Then Exit For
    Text = Text + Mid$(dStr,4,1)
  Next
  Close #hFile
  Function = Text
End Function
 
'------------------------------------------------------------------------------
Function PbMain() As Long
  Local dStr As String
If Dir$("c:\temp\textbmp.bmp") <> "" Then Kill "c:\temp\textbmp.bmp"
FileCopy "c:\winnt\winnt256.bmp", "c:\temp\textbmp.bmp"
 
WriteBmpText "c:\temp\textbmp.bmp", "Embedded test..."
dStr =  ReadBmpText  ("c:\temp\textbmp.bmp")
Print "Retrieved text: " ; dStr
WaitKey$
End Function

------------------
Peter.
mailtoeterL@pabx-group.com.nospampeterL@pabx-group.com.nospam</A>