palettes and gif - university of surrey...colour palettes the palette in matlab loading a palette...

Palettes and GIFCSM25 Secure Information Hiding

Dr Hans Georg Schaathun

University of Surrey

Spring 2007

Dr Hans Georg Schaathun Palettes and GIF Spring 2007 1 / 30

Learning outcomes

Learn how images are represented using a paletteGet an overview of hiding techniques in GIF et c.Understand how grayscale steganography can be adapted forpalette images


Colour palettes The image format

Outline

1 Colour palettesThe image formatThe palette in MatlabSteganography in the palette

2 Steganography in the pixmapWhat about LSB?Palette sorting

3 Steganalysis of palette images



The colour palette

The palette is a table of coloursTypically 256 for imagesSometimes 8 or 16 for screen viewEach colour has an indexThe palette can be stored as

array of (say) 256 entrieseach entry: RGB colour. . . using 3 × 8 bits

0

1

2

3

4

5

6...

...255



Using the colour paletteThe GIF file

The GIF file format uses the colour palette. . . in the spatial domainLike the grayscale image,

GIF is a matrix, Ientry (x , y) defines the colour of pixel (x , y)

however Ix ,y is the index of a colour in the paletteA GIF file has to store both

A paletteThe pixel matrix (pixmap)



Which is best?GIF? JPEG? ‘true colour’?

GIF saves space compared to ‘true colour’Perfect representation of selected coloursPerfect for drawings and computer graphics

where only selected colours are usedJPEG is better for photos

Colours are ‘truer’can ‘blur’, losing contrast and detail


Colour palettes The palette in Matlab

Outline






Loading a palette image

Remember to store the palette[pixmap,palette] = imread ( ’picture.gif’ )imshow ( pixmap, palette )otherwise, treated as a weird grayscale image

You can work on palette and pixmap separately

>> [X,P] = imread ( ’krinslogo.gif’ ) ;>> whos X PName Size Bytes Class AttributesP 256x3 6144 doubleX 120x90 10800 uint8



ExercisePreliminaries

Suppose a GIF file uses a 8-bit palette. How much space doesthis GIF file save compared to a 24-bit true-colour image?Consider a 64 × 64 image and a 512 × 768 image. Give theanswer as a percentage.How do you generate a random binary message (bit string) inmatlab?Look up in the help system and find the necessary commands togenerate a random N binary (logical) matrix.


Colour palettes Steganography in the palette

Outline






A simple idea

Data hiding is possiblein the paletteleaving the pixmap unalteredHow? Any ideas?

Two basic ideaschanging colourschanging order

Main disadvantage: low capacity



Changing colours

Minor changes to palette coloursas if they were pixels in ‘true colour’only difference: there are only (say) 256 entries

For instance, LSBCapacity 3 × 256 bits (= 768 bits)



Changing order

Define a canonical colour orderi.e. obtained by a deterministic sorting algorithmalways same order, regardless of input order

Assign colour indices according to canonical orderFor each message, assign a permutation P on {1, 2, 3, . . . , N} (Nis palette size)Shuffle the palette using the permutation of the messageDecoder can find canonical order (deterministic)

and hence deduce P which maps to the message.N! = 1 · 2 · 3 · . . . · N possible messages

Approx. 1684 bits for an 8-bit palette



Large-scale data hiding

pixmap is main part of palette image fileto hide a lot of data, the pixmap must be usedthe palette is small

can cover a correspondingly small amount of data


Steganography in the pixmap What about LSB?

Outline






First attempt: LSB

We still have a pixmap as beforeLogical to attempt LSB

in the colour indices in selected pixels

What happens?



ExerciseLSB test

Take any GIF image from the web. Some test GIF images can befound on the module web pages.

Make stego-images by embedding random binary messages oflength 10%, 30%, 50% and 75% of capacity in the least significantbit.View the images. What does it look like?Extract the message again. Do you get the same message back?

Hint: Reuse functions from previous exercises.



What it looks like



So, what do we do?

Green areas, are not too badVisible, but still green

Brown is terribleIt is suddenly green

Why is this?Neighbour colour in the palette, can be

Similaror far away

Somehow, we have to change to a similar colour


Steganography in the pixmap Palette sorting

Outline






Three possible solutions

Colour reductionColour sortingBeyond sorting



Reduce the paletteAn ancient solution

Reduce colour depth by 50%

256 colours → 128 coloursDuplicate each colour (or use a slight variation of it)

Such that colour i and colour i + 1 are identical (or almost)

Now LSB embedding is hardly perceptible visuallyHow do you steganalyse it?



SortingAnother obvious one

Sort the palette before embeddingConsecutive colours similar

Unsort before shippingSorted palette may be suspectThe original order should look innocent

Many different sort ordersSort by luminenceTravelling salesman (EzStego)

Let (ri , bi , gi) be RGB of colour iMinimise

P255i=0[(ri − ri+1)

2 + (bi − bi+1)2 + (gi − gi+1)

2]



Abandon sortingaccording to Jessica Fridrich

Input: Image I, Palette C = {c1, c2, . . . , cL}, Message ~mOutput: Image J

for each bit b of ~mchoose a pixel Ix ,yif Ix ,y mod 2 6= b, then

find ci ∈ C such that ci mod 2 = band ||ci − Ix ,y ||2 is minimised

Ix ,y := ciend if

end for



Sort order and colour difference

What is the distance ||c1 − c2||2 between colours?Euclidean: |R1 − R2|2 + |G1 − G2|2 + |B1 − B2|2Ignore chrominence: |Y1 − Y2|

No obvious choice.Same problem for sorting and ‘abandon sorting’


Steganalysis of palette images

General principles

General approaches in the spatial domainVisual steganalysisStatistical steganalysis

. . . similar to other representations in the spatial domainStructural steganalysis

Consider the file formatDoes it look ‘normal’?



Reading

Core Reading

David Wayner: Disappearing Cryptography Chapter 17



Structural steganalysis

Structural steganalysis considers file format et c.Palette structure can be significant

Duplicate colours?Unusual palette order?

General structural artifacts (not limited to palette images)Resolution requirements; palette size; et c.Bad or limited software can create artifacts



General, Spatial Approaches

Most attacks on spatial, grayscale images applyχ2/pairs of valuespairs analysisRS analysis

Statistics may varyparameters may need tuningreasoning is the same

Let’s explore it in excersises.



ExerciseSteganalysis

Take the stego-images created in the previous message, and plot theχ2 statistic as a function of N. What do you observe?

RemarkExtra, challenging exercises on the exercise sheet on the web page.


palettes and gif - university of surrey...colour palettes the palette in matlab loading a palette...

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