Microsoft PowerPoint - Steganography by cgq15394

VIEWS: 237 PAGES: 53

hidden-letter pdf

More Info
                        that is
               Hiding information
      into pictures and other media

                  Prof.Dr. Mihály Tóth

                        Seminar #1 in HTI   1
October 2003
  Phenomena of Steganography
   Carrying (or Covering) media
     • Which may be picture, video, sound file,
       radio communication, even the
       structure of a File system.
     • The carry medium ought to look
   The hidden information (or the
   information to hide) called Stego
   medium or Stego message.
     • Which may be open message, but may
       be encrypted one as well.
December 03         Seminar #1 in HTI         2
The Basic Types of Steganography
   Substitutional Steganography
    • Some elements of the redundant covering
      medium are substituted by the elements of
      the stego medium.
   Selecting Steganography
    • Some elements of the covering medium are
      selected to carry the hidden information.
    • The relevant information is hidden in a
      narrow-band region of a wide band noise.
      The hiding (narrow) band is changed by
      time to time. (Radio Communication.)

December 03         Seminar #1 in HTI         3
The Basic Types of Steganography
   Constructional Steganography
    • The Stego message is made similar
      to the structure of the covering

December 03       Seminar #1 in HTI       4
          Some characteristics (1)
   The most simple hiding methods (like
   the substitution) cannot tolerate the
   edition of the covering medium
    • Compression, picture edition,
      resizing, printing, copying, etc..
   On the other hand it should be a basic
   requirement, that the hidden
   information ought to survive such
   changes of the covering medium

December 03        Seminar #1 in HTI    5
          Some characteristics (2)
   The applied method strongly depends
    • The characteristic of the covering
      medium and
    • The goal of the hiding
   There exist cases when we do not
   want the message being completely
   hidden (watermark).

December 03         Seminar #1 in HTI      6
          Some characteristics (3)
   There are some cases when hiding
   has one common feature with
   encryption, for recovering hidden
   information one needs a KEY.
   Nowadays Steganography is applied
   mostly to put copy right information
   to the media sold (This is known as
   electronic watermarking.)

December 03        Seminar #1 in HTI      7
      Ancient methods for hiding (1)
       Aineas Taktikosz :Poliokretika
         (360 A.D. )
              – Every nth letter of a plaintext had to be
                read, or nth letters of the words.
              – Passing through a thread on holes of a
                pottery disc whose holes are meant
                different letters agreed in advance.
              – Letters on a certain page of a book
                signed by hardly visible dots. It is
                applicable for newspapers or letters as
              – Cutting the hair of a slave and writing
                the message to his bald head skin.
                (Herodotus about 450 A.D.)

December 03                  Seminar #1 in HTI              8
      Ancient methods for hiding (2)
       –Sewing into clothes, shues, collar of a dog,
        rein of a horse, or anything else.

      –Writing it onto a blown bladder of a cow, then
       compressing the bladder
      –Writing between the rows of an innocent text
       by invisible ink.
   Naturally our ancestors applied
   secret writings as well. They used
   both substitution and permutation of
   letters and other codes for light
December 03                Seminar #1 in HTI            9
       Hiding methods in WW 2
    In WW2 the hardly visible dots still were
    used to denote the selected letters in a
    newspaper, for example.
    Many soldiers tried to send home hidden
    messages that way, but the method was
    well known by the censors. (If they cannot
    discover the hiding method, they mixed the words
    at least or put the stamp in an other position,
    The encrypton of messages, however,
    acquired more and more importance.

December 03          Seminar #1 in HTI           10
  History of securing information
   In ancient times hiding was preferred
   rather then encryption.
   Since the middle centuries
   encryption (vs. Decryption) play
   more and more important role.
   Nowadays cryptographic methods
   are preferred more then hiding
   (when securing messages).

December 03      Seminar #1 in HTI     11
        Goals of hiding nowadays
   Hiding Copy-right into media.
   Identifying message in a hidden way
    • Identifying the sender or owner
    • Identifying the individual addressee
    • Hiding commercial information
   In common name:
   electronic watermarking

December 03         Seminar #1 in HTI        12
                Hiding message in text
          Denotating some way the letters of the
          carrier text to hide a secret message in
          it. For example
              • Hardly visible dots below or above the
                selected letters or marking them by
                invisible ink. (Applied from ancient times
                up to WW. II.
              • Hardly visible dislocation of the selected
              • Very little modification of the shapes of
                selected fonts.
              • An example of dislocation of letters is
                shown in the next slide.

December 03                 Seminar #1 in HTI                13
December 03   Seminar #1 in HTI   14

     Methods when the hidden information is
     carried by the appearance of the text,
     (not only the text file.)
     Such a way the printed or copied text also
     carries the hidden information. (E.g. a
     confidential circular letter.)
     This makes difficult to select out the
     appropriate letters by a computer.
     The printing/copying/scanning processes
     bring additional problems to revoke the
     hidden text.
December 03         Seminar #1 in HTI        15
 Marking the selected letters in a
 covering text by an other way.
   Each font in the text occupies a standard
   That area is not necessarily the same for each
   font. For example the letter m is generally
   wider then the letter i. (Proportional vs. fix
   font size.)
   Any letter could be moved or modified a bit
   within its standard font area. Such way
   particular letters of a carrier text could be
   The marked letters can be selected out of the
   text by an appropriate program. (The text
   could be photocopied as well.)
December 03         Seminar #1 in HTI          16
        Moving the font out of its standard area

                                          Done by Mr. Árpád
                                          Horváth assistant

December 03           Seminar #1 in HTI                       17
              Moving the selected fonts
   Text processors of Wysiwyg (what you see is
   what you get) generally cannot do that.
   (Word, WordPerfect, Desktop publishing
   softwares, like Adobe’s PageMaker,
   „Programmable” text processors
   (like LateX) are able to do.
   Such application needs a separate selecting
   program which is able to accept picture files
   as well. (E.g. scanned or copied texts). This
   is not a simple job and the addressee must
   have it in advance.
   The character recogniser software loose such
   hidden information.
December 03          Seminar #1 in HTI      18
              Modifying fonts

  Increasing the lenght of the vertical lines
  in some characters (like b,d,h,k,l,p,q) with
  a small amount is not prominent.
  This also may carry information.
  Arabic writing allows many decorative
  variations of the letters in a text
  Which may be used to mark selected
  Any proposition?

December 03        Seminar #1 in HTI         19
         Methods for hiding coded
          information into a text
   At the ends of rows we may insert exra
   spaces. Each space at a row-end may
   represent a single bit. (Max 3 spaces, for
   more then 3 could be conspicuous.)
   In an A4 page there are 50 rows and
   altogether 50x3 bit= 150 bits (less then 20
   bytes) could be hidden such a way. This
   method can resist compression.
   One has to read the first letters of words
   preceded by double spaces.
   Modifying the font sizes or spaces.
December 03        Seminar #1 in HTI        20
  Modifying the structure of the text
   Synonims of some words can be used
   instead of the original word, or
   changing the linguistic structure of the
   text (applicable in most cases)
   Both of the above can be used to hide
   Revocation the hidden information the
   original text (like a key) is needed.
   Very small amount of information can be
   hidden that way. (Goal: identification)
December 03        Seminar #1 in HTI          21
 Hiding information into pictures
1.    If the picture (graphics) was
      made directly for that purpose
      See the example in the next
2.    In so called cover pictures. (We
      will come back to that.)
      Using covering pictures is the most
      frequent method of hiding
December 03        Seminar #1 in HTI        22
December 03   Seminar #1 in HTI   23
   Hiding in covering pictures (1)
   Applicability depends
   upon the way of
   representing colors.
   It cannot be applied in
   case of color palette.
   In case of the colors
   represented by 3 color
   bytes it is applicable,
   Very vulnerable

                       Neighboring color coordinates may
                       denote very different colors.
December 03           Seminar #1 in HTI                24
   Hiding in covering pictures (2)
    The color of each pixel) is defined by 3 bytes.
    (There are a couple of methods defining color.
    This is just one of them.)
    The different colors for a pixel can be as many as
    2^(8*3)=16 777 216.
    Though human eye is very sensitive, such an extreme
    variety of colors cannot be distinguished. The color-
    change of a pixel is not visible if the LSBs of the color bytes
    are changed. (In fact the average number of changes affect
    half of the color bytes only)
    The hidden information is carried by the LSBs of
    the color bytes.
              Color information that belongs to each pixel of a picture

December 03                    Seminar #1 in HTI                          25
                                           A rejtett üzenet bitjei
 Changes of the three basic (additive)
   colors (Sat:240, Lum: 120/112)
         In the first row the LSB changed by 1 bit
         In the second row the change is 1 bit at the
         16th positional value.

December 03             Seminar #1 in HTI               26
  In an other coordinata system:
                                         Perhaps some very
                                         little change of
       255    255   255
       239    239   239

                                         some colors are
                                         visible in the
                                         second row of the
                                         previous slide. It
                                         happened on 256
                                         grade scale of the
                                         basic colors.
                                         The luminescence
          0     0     0
                                         is also changed
December 03
                                         from 120 to 112. 27
                          Seminar #1 in HTI
              Visual experiences
   Changing the LSBs of the basic colors
   cannot be detected by a visual way, at least
   not on a computer screen. Changing,
   however, the last four bits of a color byte
   could be detected visually.
   The reason for that is in the physiological
   characteristic of the human eyes.
   The black and white and gray-scale sensing
   abilities of the human eyes are much better,
   but in spite of that the gray-scale pictures
   are more suitable for hiding information in
   Changing LSBs the average byte change is
   half of the total number of color bytes.
December 03        Seminar #1 in HTI        28
        How much information could be
            hidden in a picture?
    In a Kodak format CD picture the maximum
    number of pixels are 2048 x 3072 pixel. (
    19 MB) A single digital photo is able to hide
    2,3 MB (12 %) information such way that it
    does not cause any visible change of the
    I am going to demonstrate it by a program
    developed by a student of mine, Mr. Péter
    Teleki. It has been developed exactly for
    experimental and demonstration purpose
    and it operates very fast. Instead describing
    the program I am to show some of its

December 03          Seminar #1 in HTI          29
    Demonstrating how to hide information
        in a bitmap format picture
    In the following slides I am to show two
    pictures in each slide.
    The left picture is the one in which there is
    no hidden information while the right one
    contains a text of 4862 bytes in the LSBs
    of the pixels.
    To hide that big amount of information we
    used the LSBs of all three color bytes of
    the three basic colors.

December 03         Seminar #1 in HTI          30
LSB hiding in 24 bit bmp picture
 The right picture contains 4862 hidden letters

December 03           Seminar #1 in HTI           31
      Sizes of picture files shown
              without hidden

Bitmap        308 614 bytes              308 614 bytes

JPG           21 795 bytes               21 952 bytes

December 03          Seminar #1 in HTI                   32
              Four bit hiding
   The following slides demonstrate the
   case when the second half (i.e. four
   bits) of the color bytes are used for
   hiding. In those cases the changing
   of the picture can be sensed visually
   Exceptionally on the bigger areas of
   the same or similarly colored parts of
   the picture.

December 03       Seminar #1 in HTI     33
         Steganography (cont.)

          Hiding by 1 bit                   Hiding by 4 bits

December 03             Seminar #1 in HTI                      34
              Steganography (cont.)
   Though the hiding method shown is
   very sensitive to the editing of the
   covering picture, it has a
   considerable advantage too:
   It does not increase the size of the
   covering file
   for it utilizes its redundancy
   (As it was shown by bitmap images
   of Kandó sculpture)

December 03          Seminar #1 in HTI    35
              Steganography (cont.)
   Steganography covers even the existence of any
   hidden message.
   One disadvantage is that quite an amount of
   covering information is needed to hide whatever
   we want to. At least 8 to 10 times more if we
   really want it to be non detectable.
   Nevertheless if the fact of the hiding becomes
   evident then the entire business is virtually
   useless, if we had not encrypted the information in
   The really big disadvantage is that editing or
   compressing the picture do catastrophic damage
   to the hidden information.

December 03           Seminar #1 in HTI             36
     The electronic watermarking
   It has at least two meanings as follows
   There are
    • visible and
    • non visible e-watermarks.
   The problem is that visibility as such is
   very subjective.
   We may talk about hardly visible and
   very much visible e-watermarks as well.

December 03          Seminar #1 in HTI         37
   This is a hiding technology which is
   suitable to produce both
     • Non-detectable e-watermarks and
     • e-watermarks of different grade of
   The carrier medium is modified a
   suitable way by the information to be
   the watermark and the colors of the
   pixels below the mask are modified.
   The visibility depends upon the amount
   of modification.

December 03         Seminar #1 in HTI       38
       Demonstrating masking (1)

(18 957

December 03     Seminar #1 in HTI   39
       Demonstrating masking (2)
   To the
   (19 296
   bytes =
   + 1,8%

December 03     Seminar #1 in HTI               40
              Applying masking
   The location of masked stego
   information within the picture’s area
   is NOT indifferent.
   Essentially the same technology is
   applied in case of video tapes or
   DVDs, but
    • the watermark is put different locations
      on the subsequent slides
    • there is no watermark in EVERY slides of
      the movie.
December 03        Seminar #1 in HTI        41
      DCT: Discrete Cosine Transformation

   The JPEG technology
   divides the picture
   into 8x8 pixels mat-
   rices, then it performs
   a diagonal scanning
   and quantize the
   pixels’ difference to
   the average pixel
   color of the 8 by 8
   matrix. When quanti-
   zing it applies a modi-
                                          Scanning the 8 by 8 pixels matrix
   fied version of FFT.

December 03           Seminar #1 in HTI                                       42
  DCT: Discrete Cosine Transformation

   The floating point results got from
   the scanning and quantizing can be
   truncated either to downward or
   upward correlating the bit values to
   be hidden.
   This technology is applied mostly to
   JPEG compressions.

December 03      Seminar #1 in HTI        43
      The DCT can be a tool as well
            to reveal hiding
   For the close relation between DFT and
   DCT the last one could be considered as a
   kind of discrete frequency spectrum.
   That spectrum is the one that changes
   significantly if there is any hidden
   information in a given picture.
   In fact, the size of the picture file is also
   increases when it contains hidden
   It does not recover the hidden information
   just detects its existence. (We’ll
   demonstrate it later.)
December 03         Seminar #1 in HTI          44
          The Spread Spectrum
    Its principle was invented at the end of the
    Second WW along with the RADAR
    Such a communication applies a very wide
    band spectrum, but
    At a time a narrow band of that spectrum
    carries valuable information only, while in all
    the other part of the spectrum noise is
    In addition the relevant narrow band is
    changed by time to time following a pattern
    that has been agreed upon between the
    communicating partners.HTI
December 03            Seminar #1 in                45
              Hiding information into
                 sound tracks (1)
    The carrier is a digitized audio information.
    In case of digital sound CDs the sampling
    frequency standard is 44 kHz and
    the quantizing resolution is at least 12 bits.
    That yields an amplitude scale of 4096 levels, in
    which changing the LSB means a 0.025 % relative
    modification which is essentially a distortion.
    Such an extremely little distortion cannot be
    perceived by human ear.
    In case of silent parts of a sound track it still may
    be overheard.

December 03            Seminar #1 in HTI              46
              Hiding information into
                 sound tracks(2)
     Both the hiding and the recovering
     technology is essentially the same as
     it was shown in case of covering
     Naturally it needs computer and
     appropriate program.
     Some commercial studio software are
     prepared in advance to insert

December 03         Seminar #1 in HTI   47
               Summary (1)
   The meaning of Steganography is
   hiding information and the related
   There is a principal difference between
   Steganography and Encryption,
   however they can meet at some points
   They can be applied together, i.e.
   encrypted information can be hidden in
   addition. (As it will be demonstrated soon.)
December 03         Seminar #1 in HTI         48
                Summary (2)
   To hide something a covering
   medium is always needed. (Picture,
   sound track, text or even the
   structure of a file system, etc.)
   The covering medium must be
   redundant, otherwise the hidden
   information could be detected easily.
   (This is the conception of hiding into noise.)

December 03            Seminar #1 in HTI            49
              Summary (3)
   The technology of hiding should
   match the nature of the medium.
   The hidden information should not be
   lost, if the carrying medium is
   edited, modified, formatted, re-sized,
   compressed or printed.
   That’s a difficult task to realize.

December 03      Seminar #1 in HTI     50
              Summary (4)
   It’s an expectation as well, that the
   fact of hidden information should be
   impossible to detect by other then
   the addressee.
   On the other hand security services
   should have methods to detect such
   information. At least its existence.
   Realizing a good trade-off there are
   different technologies.

December 03      Seminar #1 in HTI         51
              Summary (5)
   Nowadays the most popular application
   of Steganography is hiding copy rights
   and other commercial information.
   Such kind of hidden information is
   known as e-watermark.
   The e-watermark is not always
   invisible. There are cases when it is
   made deliberately strikingly visible. E.g.
   in case of trial versions of software.
December 03        Seminar #1 in HTI       52
      Now a demonstration of a
     commercial stego software
          will commence!

                      Seminar #1 in HTI     53
October 2003

To top