High-Payload Image Steganography Using Two-Way Block Matching

High-Payload Image Steganography Using

Two-Way Block Matching

IEEE Signal Processing Letters, vol. 13 no.3, March 2006

Ran-Zan Wang and Yeh-Shun Chenspeaker: 李惠龍

Outline

Introduction Proposed scheme Experimental results Conclusion

Introduction

Embedding algorithm

Extraction algorithm

Message

Cover medi a

Stego medi a

Message

Key Key

Introduction

Substitution system Least-significant-bit (LSB): utilize some

mapping rules to embed the message in certain LSB planes of the cover image

8 7 6 5 4 3 1122

LSBMSB

Proposed scheme

CO

OCandECand

IM

OB+OBIFFEB+EDIFF

eind

EB+ECand

oind

OB+OCand

Choose a highest similarity to original IM block

Embed EB/OB, eind/oind, parameters and not-well-match blocks

Proposed scheme

OCand and ECand blocks (amount 2t-1,respectively): Generate Cand images by replacing the q LSB of CO wit

h its (q+1) to 2q LSB. Divide Cand image into blocks of size mxn Two difference blocks PD(D) and ND(D), D={dij}

Use a threshold z and φ to choose OCand and ECand blocks (assign PD(D) to OCand and ND(D) to ECand)

njmidnd

njmidpd

ijDij

Dijij

1,1,

1,1,

Dist(Dc, Dj)< φ, c-z≤ j <cc

Proposed scheme

IM blocks OBr or EBr is defined to be the corresponding odd/even in

teger closest to μr.

rr

rrr EDIFFEB

ODIFFOBB

njmiEBbediffEDIFF

njmiOBbodiffODIFF

rijijr

rijijr

1,1},{}{

1,1},{}{

Proposed scheme

IM blocks

100 98

95 102

μr=99

EBr

98 98

98 98

2 0

-3 4+

EDIFFrBr

rr

rrr EDIFFEB

ODIFFOBB

100 98

95 102

μr=99

OBr

99 99

99 99

EX:

-1 1

-4 3+

ODIFFrBr

Proposed scheme

Find indices (oind and eind)

120)},,(min{arg

120)},,(min{argt

jrr

tjrr

jECandEDIFFDisteind

jOCandODIFFDistoind

r

r

eindr

oindr

r ECandEB

OCandOBB

),(),( reindrroindr BECandEBDistBOCandOBDistrr

otherwise

rr

rrr EDIFFEB

ODIFFOBB

njmi

ijij spnm

SPDist1,1

2)(1

),(

Embed OBr and oindr or EBr and eindr

Proposed scheme

Sufficiently large error Dist(ODIFF, OCand) or Dist(EDIFF, ECand) contain

large errors. Directly embed these block in the CO. The number of not-well-matched blocks:

r

r

eindr

oindr

r ECandEB

OCandOBB

rr

rrr EDIFFEB

ODIFFOBB

knmnmwh

tkwhq IMIMcoco

)(

Proposed scheme

Embedding scheme Hop method Key: embedding location encode by Huffman coding scheme Embedded data:

Bases, indices, and not-well matched blocks

(stego-image) Parameters: him, wim, k, q, t, z, m, n, and φ Huffman table

Proposed scheme

Extraction scheme Extract parameters, Huffman table, and stego-ima

ge Decode stego-image to obtain the bases, indeius

and not-well-matched blocks Generate odd candidate blocks and even candida

te blocks from stego-image Construct IM.

Proposed scheme

Construct IM. If indi≠2t-1

Base is odd: Baseindi+ the indith odd candidate bloc

k

Base is even: Baseindi+ the indith even candidate bl

ock If indi=2t-1

Take not-well-matched block

r

r

eindr

oindr

r ECandEB

OCandOBB

Experimental Results

Parameters: q=2, t=16, z=3, φ=32, and a block size: 4x4

37.93dB (Stego and Cover)

32.41dB (extract and origin)

Conclusion

Propose a high-payload image steganography method

High quality by two-way block-matching and hop scheme