Loop Help

Hi I am working on following data hiding algorithm in images using Matlab

Example: Suppose the cipher text to be sent is: 11001011 01111010 10101010 10011001 01010101. This data is five bytes. So n=5 and p=2. Suppose the different bytes of the digital image are A,B,C,D,E etc. From table-3 we can see that in byte A of the carrier file we embeded the data bits 11 in 6th and 8th bit locations, and next value of p becomes 0. We embed the next data bits 00 into byte B in 6th and 7th bit locations, next value of p becomes 1. Now we embed the next two bits 10 in C in 7th and 8th bit locations and so on.

Carrier FileByte  Operation          location      IndexVariable,P
  Byte A           Embed (11)         6th and 8th       2
  Byte B           Embed (00)         6th and 7th       0
  Byte C           Embed (10)         7th and 8th       1
  Byte D           Embed (11)         6th and 8th       2
  Byte E           Embed (01)         6th and 7th       0
  Byte F           Embed (11)         7th and 8th       1
  Byte G           Embed (10)         6th and 8th       2
  Byte H           Embed (10)         6th and 7th       0
  Byte I           Embed (10)         7th and 8th       1
  Byte J           Embed (10)         6th and 8th       2
  Byte K           Embed (10)         6th and 7th       0
  Byte L           Embed (10)         7th and 8th       1
so on………

I have used following logic-

I have converted image into row format and storing the ciphertext length in 7th&8th bits of first four pixles

using below logic in attached file for this

pv=v(1,1);
pb=dec2bin(pv,8);
pb(7)=b(1);
pb(8)=b(2);
pv=bin2dec(pb);
v(1,1)=pv;

pv=v(1,2);
pb=dec2bin(pv,8);
pb(7)=b(3);
pb(8)=b(4);
pv=bin2dec(pb);
v(1,2)=pv;

pv=v(1,3);
pb=dec2bin(pv,8);
pb(7)=b(5);
pb(8)=b(6);
pv=bin2dec(pb);
v(1,3)=pv;

pv=v(1,4);
pb=dec2bin(pv,8);
pb(7)=b(7);
pb(8)=b(8);
pv=bin2dec(pb);
v(1,4)=pv;

Now I want to store bits of my ciphertext following way

Say ciphertext is 11001011 01111010 10101010 10011001 01010101

This data is five bytes. So n=5 therefore p=2 ((Mod(n,3))

 Carrier FileByte  Operation          location      IndexVariable,P
    (1,5)           Embed (11)         6th and 8th       2
    (1,6)           Embed (00)         6th and 7th       0
    (1,7)           Embed (10)         7th and 8th       1
    (1,8)           Embed (11)         6th and 8th       2
    (1,9)           Embed (01)         6th and 7th       0
    (1,10)          Embed (11)         7th and 8th       1
    (1,11)          Embed (10)         6th and 8th       2
    (1,12)          Embed (10)         6th and 7th       0
    (1,13)          Embed (10)         7th and 8th       1
    (1,14)          Embed (10)         6th and 8th       2
    (1,15)          Embed (10)         6th and 7th       0
    (1,16)          Embed (10)         7th and 8th       1
--so on

But i am not getting how to implement above logic..

I am calculating index variable and based on its value taking locations as fixed..in this case p=2 so embedding data in each 6th and 8th bit locations of next pixels which is not as per above table.. I am struggling in framing above logic in Matlab where bit locations and index variable are changing.

my code as below. pls provide me direction

Thanks in Advance

my code--

global l ciphertxt; 

[f p]=uigetfile('*.*','Open Cover Image');
fp=[p f];

im=imread(fp);
[r c n]=size(im);
if n==3
    im=rgb2gray(im);
end

figure;
imshow(im)
title('Input Image')

v=reshape(im,1,r*c);

b=dec2bin(l,8);

pv=v(1,1);
pb=dec2bin(pv,8);
pb(7)=b(1);
pb(8)=b(2);
pv=bin2dec(pb);
v(1,1)=pv;

pv=v(1,2);
pb=dec2bin(pv,8);
pb(7)=b(3);
pb(8)=b(4);
pv=bin2dec(pb);
v(1,2)=pv;

pv=v(1,3);
pb=dec2bin(pv,8);
pb(7)=b(5);
pb(8)=b(6);
pv=bin2dec(pb);
v(1,3)=pv;

pv=v(1,4);
pb=dec2bin(pv,8);
pb(7)=b(7);
pb(8)=b(8);
pv=bin2dec(pb);
v(1,4)=pv;

l=length(ciphertxt);

for i=1:l

      t=ciphertxt(i); 
      n=abs(t);
      b1=dec2bin(n,8);

      p=mod(l,3);
    
    %%%%%%%%%here i want my index variable and bit locations dynamic to store data after 4th pixel as below
    %%Carrier FileByte  Operation          location      IndexVariable,P
      %%%(1,5)           Embed (11)         6th and 8th       2
      %%(1,6)           Embed (00)         6th and 7th       0
      %%(1,7)           Embed (10)         7th and 8th       1
      %%(1,8)           Embed (11)         6th and 8th       2
    %%%%%%%%%

      if p==0

         pv=v(1,(i-1)*4+5);
         b2=dec2bin(pv,8);
         b2(6)=b1(1);
         b2(7)=b1(2);
         nn=bin2dec(b2);
         v(1,(i-1)*4+5)=nn;

         pv=v(1,(i-1)*4+6);
         b2=dec2bin(pv,8);
         b2(6)=b1(3);
         b2(7)=b1(4);
         nn=bin2dec(b2);
         v(1,(i-1)*4+6)=nn;

         pv=v(1,(i-1)*4+7);
         b2=dec2bin(pv,8);
         b2(6)=b1(5);
         b2(7)=b1(6);
         nn=bin2dec(b2);
         v(1,(i-1)*4+7)=nn;

         pv=v(1,(i-1)*4+8);
         b2=dec2bin(pv,8);
         b2(6)=b1(7);
         b2(7)=b1(8);
         nn=bin2dec(b2);
         v(1,(i-1)*4+8)=nn;

      end

      if p==1

         pv=v(1,(i-1)*4+5);
         b2=dec2bin(pv,8);
         b2(7)=b1(1);
         b2(8)=b1(2);
         nn=bin2dec(b2);
         v(1,(i-1)*4+5)=nn;

         pv=v(1,(i-1)*4+6);
         b2=dec2bin(pv,8);
         b2(7)=b1(3);
         b2(8)=b1(4);
         nn=bin2dec(b2);
         v(1,(i-1)*4+6)=nn;

         pv=v(1,(i-1)*4+7);
         b2=dec2bin(pv,8);
         b2(7)=b1(5);
         b2(8)=b1(6);
         nn=bin2dec(b2);
         v(1,(i-1)*4+7)=nn;

         pv=v(1,(i-1)*4+8);
         b2=dec2bin(pv,8);
         b2(7)=b1(7);
         b2(8)=b1(8);
         nn=bin2dec(b2);
         v(1,(i-1)*4+8)=nn;

      end

      if p==2

         pv=v(1,(i-1)*4+5);
         b2=dec2bin(pv,8);
         b2(6)=b1(1);
         b2(8)=b1(2);
         nn=bin2dec(b2);
         v(1,(i-1)*4+5)=nn;

         pv=v(1,(i-1)*4+6);
         b2=dec2bin(pv,8);
         b2(6)=b1(3);
         b2(8)=b1(4);
         nn=bin2dec(b2);
         v(1,(i-1)*4+6)=nn;

         pv=v(1,(i-1)*4+7);
         b2=dec2bin(pv,8);
         b2(6)=b1(5);
         b2(8)=b1(6);
         nn=bin2dec(b2);
         v(1,(i-1)*4+7)=nn;

         pv=v(1,(i-1)*4+8);
         b2=dec2bin(pv,8);
         b2(6)=b1(7);
         b2(8)=b1(8);
         nn=bin2dec(b2);
         v(1,(i-1)*4+8)=nn;

      end

end

steg=uint8(reshape(v,r,c));
figure;
imshow(steg)
title('Stego Image')

[f p]=uiputfile('*.bmp','Save Stego Image');
fp=[p f];

imwrite(steg,fp);

[Nil]

anyone pls help me out..

[Jan]

I do not understand the question. What is "above logic" in "But i am not getting how to implement above logic"?

[Nil]

Hi Jan Simon..

I have converted image into row format and storing the ciphertext length in 7th&8th bits of first four pixles and in next pixles based on index variable value, I will store bits of cipher text

if p=0 then embed data in 6th and 7th bits

is p=1 then embed data in 7th and 8th bits

if p=2 then embed data in 6th and 8th bits

Example-

Say ciphertext is 11001011 01111010 10101010 10011001 01010101

This data is five bytes. So n=5 therefore p=2 ((Mod(n,3))

Carrier FileByte Operation location IndexVariable,P

(1,5) Embed (11) 6th and 8th 2

(1,6) Embed (00) 6th and 7th 0

(1,7) Embed (10) 7th and 8th 1

(1,8) Embed (11) 6th and 8th 2

(1,9) Embed (01) 6th and 7th 0

(1,10) Embed (11) 7th and 8th 1

(1,11) Embed (10) 6th and 8th 2

(1,12) Embed (10) 6th and 7th 0

(1,13) Embed (10) 7th and 8th 1

(1,14) Embed (10) 6th and 8th 2

(1,15) Embed (10) 6th and 7th 0

(1,16) Embed (10) 7th and 8th 1

--so on