Effect of PN Sequence on Message Bits in Digital
Watermarking
Dinesh Dhaka / Kamini Sharma/Rakshita Chauhan
ECE, RCEW, Jaipur
Abstract: In this ‘e’ era (internet era) where we use internet for various purposes such as file transfer, uploading
pictures on social sites etc. security of data is major concern. There are various ways by which we can securely
transmit our data. The major areas in the field of data security are Cryptography, Steganography and Digital
Watermarking. Digital Watermarking is gaining a lot of attention nowadays. Digital watermarking is the process
of embedding data (stream of bits ) in a file, file here can be in image, text, audio, video form. Digital
watermarking are of various types depending on its visibility, domain in which we are using etc. and it also find
its application in various field. Here, we are using digital watermarking for data hiding purpose and to ensure its
security we are using spread spectrum concept in LSB technique. The technique is analyzed through its
imperceptibility and effect on its value by increasing number of bits in pseudo noise sequence is also observed.
Keywords: Digital Watermarking, LSB Technique, Pseudo Noise Sequence, Spread Spectrum.
Introduction
Digital Watermarking is a process which is mainly use for copyright protection, though it has application in other fields
also such as fingerprinting, data hiding, broadcast monitoring, medical safety etc. Digital Watermarking is basically a
process of data embedding in a cover file. The cover file is called as ‘Asset’ and data to be embedded is called as
‘Message’. Digital watermarking can be in visible form or it can be invisible depending on human visibility. Digital
watermarking can also be classified on the basis of domain such as spatial domain watermarking (LSB), spectrum domain
watermarking(DCT,DWT), hybrid domain watermarking(DCT-DWT)[1,2]. The spectrum domain watermarking is
another type of digital watermarking which can be use in both spatial and spectral domain. Here, we are using spread
spectrum in spatial domain. Thus, we are here mainly concerned with LSB technique using direct sequence spread
spectrum to hide stream of bit which is here any number which user want to transmit secretively such as any date
(ddmmyy format) or mobile number etc.
Digital Watermarking
On HumanVisibility
Visible
Invisible
On Data
Domain
Robustness
Image Text Audio Video Spatial Spectral Hybrid Robust Fragile Semi Fragile
Figure 1. Classification of Digital Watermarking
Spatial Domain Watermarking
The most common technique use for watermarking in spatial domain is Least Significant Bit technique. LSB technique works
on the principal of pixel modification[4]. In this technique first location of pixels in cover image is found using luminance
value, edge detection technique etc. That is we can find block with highest luminance value or block with non edges to
embed data as they contain least information and hence embedding data in them will has negligible effect on cover file[3].
The data bit is embed in last bit of pixel(8-bit pixel value) as shown in figure 2. of cover file that is asset at sender’s end. The
cover file which is here image is then called as watermarked image. On receiver’s side the data bit is extracted from each
pixel in which it is embed. Thus, here we can say that there must present a key which posses knowledge of data hiding
location and is govern by sender and receiver only. Therefore, in watermarking system the main components are asset,
message to be embed and key (known to sender and receiver only).
1
0
1
0
0
1
0
1
0
0
0
0
Insertion of message bit in last bit of
asset pixel
8-bit pixel value of asset
1
0
1
0
0
1
0
1
Modified pixel value of asset
Figure 2. LSB Technique
Direct Sequence Spread Spectrum
Digital watermarking ca be implemented using direct spread spectrum sequence technique in spatial domain. In direct
sequence spread spectrum along with message, key we need one more component that is pseudo noise sequence (PN
Sequence). In direct sequence spread spectrum (DSSS) each bit in the original signal is represented by multiple bits using
sequence of bits called as PN sequence .
Original Signal
Spread Signal
PN Sequence
Figure 3. Direct Sequence Spread Spectrum
A. PN sequence are random number generator which are initialized with particular ‘seed’. The knowledge of seed is
possessed by sender and receiver only. The PN property is useful for watermarking because it makes it difficult for
an attacker to estimate the watermark from marked media. In addition, with properly chosen PN signals, even if the
attacker can perfectly estimate some small segments of the watermark, it is not possible to determine the rest of the
mark.
In DSSS, XOR function is mainly used. The Exclusive-or function between the original digital signal and PN sequence can
achieve required results.
Example:
Sender’s Side:
Receiver’s Side:
PN Sequence
10100001
Received Signal
01010001
Input
10
PN Sequence
10100001
Output
PN Sequence XOR Input = 01010001
Signal
11110000 = 10
Thus, this is the concept of direct sequence spread spectrum which we can use in digital watermarking as this process ensures
more security.
LSB Using DSSS
In LSB using DSSS the sequence obtained after XORring is embed in last bit of selected image pixels. At receiver ed the
last bit from selected pixels are extracted which are further XORed using PN sequence saved as ‘seed’ to get original
message.
Let selected image pixels be :
10111000
10001010
00110100 10101000
01010010
01110010 11100001 10010011
And let the resultant bit be 01010001 as shown in above example then resultant pixels after embedding:
10111000
10001011
00110100 10101001
01010010
01110010 11100001 10010011
Here, in this paper we will see effect of spreading bits on watermarking. The effect is analyzed using PSNR values and MSE
values.
Original Image
Watermarked Image
Watermark
Message in Binary form
Message = ‘8888340000’
Random PN Sequence
Figure. 4 Generation of Digital Watermarking using DSSS
Table 1: Comparison in values using different bits in PN sequence per original bit
Values
2-Bit
3-Bit
4-Bit
MSE
0.0272
0.0480
0.0636
PSNR(dB)
63.7792
61.3194
60.0995
Conclusion/Results
In this paper we have seen use of LSB technique using direct sequence spread spectrum by varying number of bits in PN
sequence. We observed that there is not much difference in values thus for ensuring more security we can use more number
of bits also.
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