Implementation of the Koch-Zhao steganographic method in Ruby

This article will consider the most preferred steganographic method of hiding information in images, both from the point of view of resistance to various types of attacks, and from the point of view of maintaining acceptable image quality.

The Koch-Zhao algorithm for embedding information uses the frequency domain of the container and consists in the relative replacement of the values ​​of the coefficients of the discrete cosine transform (DCT). The image is divided into blocks of 8 × 8 pixels and DCT is applied to each block. Each block is suitable for recording one bit of information.

So the hiding algorithm will be as follows:

- iterate the image with a double array in increments of 8
- at each iteration we create a temporary array of 8x8 pixels, each element of which will be a set of three pixel colors.
- apply DCT to this array, and get an array of coefficients of size 8x8
- select 2 coefficients and calculate their difference modulo
- if the difference is less than or equal to 25, then assign the first coefficient a positive value of the second + constant, or the same but with a minus sign (this called bit transfer)
- if the difference is less than or equal to -25, then the same actions are only for the second coefficient.
- then the reverse DCT is applied to transfer our coefficients back to the spatial domain.
- then copy the new color values ​​to the image.

The implementation of the DCT and ODKP functions in the Ruby language is as follows:

def dct(dct,arr)
  s=0
  (0..7).each do |i|
    (0..7).each do |j|
      temp = 0.0
      (0..7).each do |x|
        (0..7).each do |y|
          temp = temp + $cos_t[i][x]*$cos_t[j][y]*arr[x][y][:blue]
        end
      end
      dct[i][j] = $e[i][j]*temp
    end
  end
  return dct
end
def idct(dct,arr)
  (0..7).each do |i|
    (0..7).each do |j|
      temp = 0
      (0..7).each do |x|
        (0..7).each do |y|
          temp += dct[x][y]*$cos_t[x][i]*$cos_t[y][j]*$e[x][y]
          arr[i][j][:blue] = (temp > 255) ? 255 : (temp < 0 ) ? 0 : temp.round;
        end
      end
    end
  end
  return arr
end

$ cos_t and $ e are global array variables with values ​​already populated.

It should be noted that the method is quite resistant to image distortion, even to its significant change, but is not applicable to hide large amounts of data.

Full code of the script that implements hiding / reading information:

require 'RMagick'
include Magick
$cos_t = [            [1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0],
                      [0.9807853, 0.8314696, 0.5555702, 0.1950903,
                       -0.1950903,-0.5555702,-0.8314696,-0.9807853],
                       [0.9238795, 0.3826834,-0.3826834,-0.9238795,
                        -0.9238795,-0.3826834, 0.3826834, 0.9238795],
                        [0.8314696,-0.1950903,-0.9807853,-0.5555702,
                         0.5555702, 0.9807853, 0.1950903,-0.8314696],
                         [0.7071068,-0.7071068,-0.7071068, 0.7071068,
                          0.7071068,-0.7071068,-0.7071068, 0.7071068],
                          [0.5555702,-0.9807853, 0.1950903, 0.8314696,
                           -0.8314696,-0.1950903, 0.9807853,-0.5555702],
                           [0.3826834,-0.9238795, 0.9238795,-0.3826834,
                            -0.3826834, 0.9238795,-0.9238795, 0.3826834],
                            [0.1950903,-0.5555702, 0.8314696,-0.9807853,
                             0.9807853,-0.8314696, 0.5555702,-0.1950903] ]
$e = [             [0.125, 0.176777777, 0.176777777, 0.176777777,
                    0.176777777, 0.176777777, 0.176777777, 0.176777777],
                    [0.176777777, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25],
                    [0.176777777, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25],
                    [0.176777777, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25],
                    [0.176777777, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25],
                    [0.176777777, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25],
                    [0.176777777, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25],
                    [0.176777777, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25, 0.25]];
def insert_message(image,text)
  i = image.copy
  dct = Array.new(8).map!{Array.new(8)}
  k = 0
  l = 0
  s=0
  temp = Array.new(8).map!{Array.new(8)}
  (0..(image.columns - 1)).step(8) do |i|
    (0..(image.rows - 1)).step(8) do |j|
      break if l >= text.length*8
      (0..7).each do |x|
        (0..7).each do |y|
          temp[x][y] = { 
            :red=>image.pixel_color(j+x,i+y).red,
            :green=>image.pixel_color(j+x,i+y).green,
            :blue=>image.pixel_color(j+x,i+y).blue
          }
        end
      end 
      dct = dct(dct,temp)
      k = dct[3][4].abs - dct[4][3].abs
      if get_bit(text,l)
        if k<=25
          dct[3][4] = (dct[3][4]>=0) ? dct[4][3].abs + 150 : -1*(dct[4][3].abs + 150)
        end
      else
        if k>=-25
          dct[4][3] = (dct[4][3]>=0) ? dct[3][4].abs + 150 : -1*(dct[3][4].abs + 150)
        end
      end
      xt  = temp.clone
      temp = idct(dct,temp)
      (0..7).each do |x|
        (0..7).each do |y|
          pixel = Pixel.new(temp[x][y][:red],temp[x][y][:green],temp[x][y][:blue])
          image.pixel_color(j+x,i+y,pixel)
        end
      end
      l=l+1
    end
    break if l >= text.length
  end
  image.write('cr.bmp')
  return image
end
def read_message(image)
  k = 0
  s=0
  out = []
  l=0
  a=''
  p=0
  b=0
  dct = Array.new(8).map!{Array.new(8)}
  temp = Array.new(8).map!{Array.new(8)}
  (0..(image.columns - 1)).step(8) do |i|
    (0..(image.rows - 1)).step(8) do |j|
      (0..7).each do |x|
        (0..7).each do |y|
          temp[x][y] = { 
            :red=>image.pixel_color(j+x,i+y).red,
            :green=>image.pixel_color(j+x,i+y).green,
            :blue=>image.pixel_color(j+x,i+y).blue
          }
        end
      end
      l=l+1
      dct = dct(dct,temp)
      k = dct[3][4].abs-dct[4][3].abs
      if k>=25
        a=1
      elsif k<=-25
        a=0
      else
        a=-1
        break
      end
      b |= a << p
      if p==7
        out.push(b.chr)
        b=0
      end
      p=(p<7) ? p+1 : 0
    end
    if a==-1
      break
    end
  end
  return out.join
end
def dct(dct,arr)
  s=0
  (0..7).each do |i|
    (0..7).each do |j|
      temp = 0.0
      (0..7).each do |x|
        (0..7).each do |y|
          temp = temp + $cos_t[i][x]*$cos_t[j][y]*arr[x][y][:blue]
        end
      end
      dct[i][j] = $e[i][j]*temp
    end
  end
  return dct
end
def idct(dct,arr)
  (0..7).each do |i|
    (0..7).each do |j|
      temp = 0
      (0..7).each do |x|
        (0..7).each do |y|
          temp += dct[x][y]*$cos_t[x][i]*$cos_t[y][j]*$e[x][y]
          arr[i][j][:blue] = (temp > 255) ? 255 : (temp < 0 ) ? 0 : temp.round;
        end
      end
    end
  end
  return arr
end
def get_bit(str,pos)
  return true if  str[pos/8].ord & (1 << pos % 8) > 0
  return false if  str[pos/8].ord & (1 << pos % 8) <= 0
end
image = Magick::Image.read('src.bmp').first
insert_message(image,"qweqwe")
image = Magick::Image.read('dst.bmp').first
puts read_message image