A Fragile Watermarking Scheme Using Prediction Modes for H.264/AVC Content Authentication

Nasir Mehmood Department of Computer Science Quaid-i-Azam University, 45320

Islamabad, Pakistan nasir.chandio@gmail.com

Mubashar Mushtaq Department of Computer Science Quaid-i-Azam University, 45320

Islamabad, Pakistan mubashar@ qau.edu.pk

Abstract— Digital watermarking has been proposed as an effective scheme for copyright protection and content authentication. Fragile video watermarking is a type of video watermarking in which embedded watermark in the digital signal is destroyed when the signal is tempered or manipulated illegally. Authenticity of video streams can be proven by embedding fragile watermarks. We propose a fragile watermarking scheme for H.264/AVC video authentication. The scheme embeds fragile watermark in intra 4x4 (I4x4) prediction modes in the intra (I) frames only during encoding. The watermark is generated from features that are robust to recompression and is embedded into the prediction modes according to defined mapping rules. Experimental results show that our proposed scheme is sensitive to recompression and has a very low effect on the video quality and bitrate. Watermark can be embedded during encoding stage and authentication is verified during decoding process.

Keywords- Authentication, Fragile Watermarking, H.264/AVC, Prediction Modes

I. INTRODUCTION The development of video tempering and video editing

softwares have raised the need for the development of mechanisms to protect owner’s copyrights and to prove authenticity of the multimedia data such as audio, images and video [3]. An authentication system is required to prove that the multimedia data is authentic and has not been tempered or edited. Digital watermarking deals with hiding secret information such as owner’s signature in the multimedia data to prove ownership and authenticity of the data. Two types of watermarking schemes have been proposed in literature, robust watermarking schemes and fragile watermarking schemes [4][9][13]. Robust watermarks cannot be removed by malicious users by modifying the watermarked data without degrading the quality of the signal severely. Fragile watermark is destroyed when host data is tempered [1] [2]. Fragile watermarks are sensitive to tempering and editing therefore they are used to prove authenticity of the multimedia data. Video content authentication schemes have applications in medical images and judiciary where a video content has to be proven to be authentic.

H.264/AVC is the recent video compression standard developed by ISO and ITU-T jointly. It has better compression ratios with good video quality compared to other compression standards like MPEG2 and MPEG4 part 2 [11].

In this paper, we propose a fragile video watermarking scheme for H.264/AVC video content authentication. We embed fragile watermark during encoding stage. The proposed scheme works in two stages. In the first stage, we generate watermark using robust features and in the second stage, we embed watermark in the H.264/AVC video stream that can further be used in different prospective applications. Our proposed scheme is sensitive to recompression and has very low effect on the video quality and bitrate. Experimental results show feasibility of the proposed scheme without affecting the overall Quality of Service (QoS) for the existing video streams.

Digital video watermarking has caught tremendous attentions during the last few years and different schemes have been proposed for video authenticity and copyright protection. A hybrid watermarking scheme for H.264/AVC is proposed in [1] that embeds robust watermarks in DCT coefficients of intra frames and fragile watermarks in the motion vectors of P frames. In this scheme robust watermarks are used for copyright protection and fragile watermarks are used for content authentication. Both watermarks have to be fed externally i.e. they are not generated from the contents of the video stream.

The rest of the paper is organized as follows. Section II presents related work. Section III presents proposed method. Section IV presents experimental results and finally conclusion and future perspective is presented in section V.

II. RELATED WORK In [2] authors have proposed a semi-fragile watermarking

scheme for H.264/AVC authentication. In this scheme the watermark is embedded by modulating quantized AC coefficients of I frames. Disadvantage of this scheme is that watermark is not generated from the contents of the video stream. Another, watermarking scheme proposed in [3] is also

7th IEEE Workshop on Security in Communication Networks 2012 SICK 2012, Clearwater

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fragile and embeds watermarks in the motion vector. In this scheme effect on the video quality is negligible but effect on bitrate is high.

Data hiding algorithms have been proposed that are based on prediction modes in H.264/AVC [6-7]. The watermarking scheme proposed in [10] embeds watermarks in IPCM macroblocks. It uses the concept of differential energy on IPCM macroblocks. Our proposed algorithm for H.264/AVC video authentication is based on embedding watermark in the prediction modes. However, the watermarks are generated from the quantized DCT coefficients that results in more robust watermarks. The generated watermarks can be used for the video authentication more efficiently. Furthermore proposed scheme is secure against malicious attacks. We have used pseudo random sequence to encrypt the watermark.

III. PROPOSED METHOD Our proposed method works in two stages. In the first

stage watermark is generated using robust features (DCT coefficients) and in the second stage watermark is embedded into the intra frame. We modify intra 4x4 prediction modes in the intra frames only to embed watermark. Numbers of non-zero quantized transform coefficients of the macroblocks in corresponding frames during successive re-encoding are highly correlated. Watermark to be embedded in the current intra frame is generated from number of non zero quantized transform coefficients of all the macroblocks in all previous frames till the last intra frame. We use coefficients of luminance component (Y) only to generate watermark. During decoding, the two watermarks, one generated from non-zero quantized transform coefficients and the other extracted from 4x4 intra prediction modes, are compared to validate the authenticity of the video stream.

A. Watermark Generation Let N be the size of the frame in number of macroblocks

and n be the intra period, we number the frames from 0 to n-1 from intra frame to the last P frame before the next intra frame.

1) We define energy eijk of kth 4x4 luma block in the jth macroblock in the ith frame, to be the number of non-zero quantized transform coefficients in that 4x4 block. We calculate energy Eij of the jth macroblock in the ith frame as given in the following equation.

∑

=

=16

1kijkij eE

(1)

2) We calculate accumulated energy Ωj of the j macroblock as follows.

ij

n

ij E∑

−

=

=Ω1

0

(2)

3) We generate watermark wj corresponding to the jth macroblock as follows

⎩⎨⎧ Ω>Ω

=pj

jotherwise

ifw

10

(3)

Where p= (j+d)%N and d can be set to any suitable integral value. In our experiments using QCIF sequences we have used d=50.

4) For security, we generated a pseudo random sequence pj with key k using linear feedback shift register and encrypted the watermark as follows:

jjj wpW ⊕= )1&( (4)

where ⊕ is the XOR operator.

B. Watermark Embedding The watermark W generated in the previous subsection is

embedded in the prediction modes of only 4x4 intra blocks of the intra frame. Not all the 16 blocks are modified for embedding. Only prediction mode of one block in a macroblock is used for watermarking embedding. We have used prediction mode of 4th 4x4 block in the 1st 8x8 block in a macroblock to embed the watermark. The 4x4 block selection for watermarking embedding is done randomly. All other prediction modes are left intact. The macroblock whose prediction mode is selected by encoder as I16 is not watermarked at all because it may increase bitrate and/or decrease the overall video quality. The prediction mode of the selected block is forced to be odd or even according as the watermark is 1 or 0 respectively. The map

8,7,6,5,4,3,2,1,01,0: →f (5)

is defined as:

pbf =)( (6)

Where b is the watermark bit and p is 4x4 prediction mode with minimum cost such that bp =1&

We modify 4x4 prediction modes to embed the watermark bit during encoding process according to the mapping function f defined in (6).

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C. Watermark Extraction and Authentication During decoding process following steps are performed

for watermark extraction and authentication:

1) The Watermark w is generated from the nonzero quantized transform coefficients as described in previous subsection.

2) The embedded Watermark W is extracted from each macroblock whose type is I4x4. Prediction mode of the 4th 4x4 block in the 1st 8x8 block in a macroblock is used to extract the watermark. Inverse mapping of f in (6) is used to extract the watermark. The inverse mapping g of f

1,08,7,6,5,4,3,2,1,0: →g (7)

is defined as

1&)( ppg = (8)

Where p is the prediction mode of the 4th 4x4 block in the 1st 8x8 block in a macroblock. Once the 4x4 prediction mode of the block is decoded, the watermark bit is extracted according to the map g defined in (8).

3) Decrypted watermark ω is calculated using the same pseudo random sequence pj based on the key k presented in Eq. 4 that was used for encryption, as follows

jj )1&( Wpj ⊕=ω (9)

Where ⊕ is the XOR operator. 4) In the final stage, the two watermarks w and ω are

compared to validate the authenticity of video stream. We calculate ρ, the ratio between numbers of matching watermarks bit to the total number of extracted watermarks ω. to check similarity of the two watermarks. If ρ is equal to 1, generated and extracted watermarks are equal and if ρ is less than 1 then the two watermarks are not equal and authentication of video stream is failed.

IV. EXPERIMENTAL RESULTS We have used H.264 reference software JM8.6 [5] for our

experiments. Fifty frames of five QCIF video sequences are used to get statistics of the proposed scheme during the experiments. Frame rate is 30 fps and intra period is 15. B frames are not used in our experiments. Fix Quantization parameter (QP) 24, 28 and 32 is used for initial encoding when watermarking is embedded in the sequence. Then, we decode and re-encode the watermarked sequences to check fragility of the watermark. In case of active rate control, effect of the proposed scheme on video quality is also examined for different output bitrates.

A. Quality and Imperceptibility Analysis Table I shows the PSNR values of luminance for

watermarked and unwatermarked video sequences for different quantization parameters (QP). The PSNR values obtained after watermarking is much closer to those obtained without watermarking the video sequences. Pvar is defined as follows

YYVAR PSNRRPSNP −′= (10)

The average value of Pvar is positive for QP 24 and 32,

meaning that our proposed scheme has no impact on the video quality in case of QP 24 and 32. For QP 28, average value of Pvar is -0.01. The scheme has negligible impact in case of QP 28. Our proposed scheme has minimal effect on the video quality. Decoded unwatermarked (upper) and watermarked (lower) intra frames, encoded with QP=28, are shown in fig. 2. The unwatermarked and watermarked decoded frames are very similar and one cannot find difference between the two. Hence our proposed scheme is truly imperceptible. Fig. 1 shows the comparisons of SNRY values of all the fifty frames watermarked and unwatermarked video sequences for QP=28. We can see that there is a slight difference in the SNRY values but this difference is very small. Table II shows Y-PSNR values for active rate control corresponding to different output bitrates. Fig. 3 shows Y-PSNR values for active rate control for output bitrates. Y-PSNR values are very close in case of active rate control as well.

B. Bitrate Analysis Table III shows bitrates of watermarked and un-

watermarked video sequences. Rvar shows the percentage increase in bitrate that is defined as follows

100var ×−′

=R

RRR (11)

Where R and R' are the bitrates generated by original encoder and modified encoder respectively.

On the average minimum bitrate increase is 0.52% in case of QP=28 and maximum average increase is 1.02% in case of QP=32. Comparing different sequences, maximum increase in bitrate is for Claire (1.06%, 0.91%, 1.6% for QP 24, 28, 32 respectively). Overall maximum, minimum and average bitrate increase is 1.6% (Claire, QP=32), -0.20% (suzie, QP=32) and 0.69% respectively. The bitrates for different video sequences are show in fig. 4. We see that the bitrates generated for unwatermarked and watermarked video sequences are very close.

C. Watermark Fragility Analysis For comparing the generated watermark and extracted

watermark, we define ρ as follows:

1016

Figure 2. Decoded unwatermarked (upper) and watermarked (lower) frames (QP=28)

37.8

38

38.2

38.4

38.6

38.8

39

39.2

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Frame Number

Y-SN

R (d

B)

OriginalWatermarked

36

36.5

37

37.5

38

38.5

39

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Frame Number

Y-SN

R (d

B)

OriginalWatermarked

(a) Akiyo (b) Carphone

38.5

39

39.5

40

40.5

41

41.5

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Frame Number

Y-SN

R (d

B)

OriginalWatermarked

35.8

36

36.2

36.4

36.6

36.8

37

37.2

37.4

37.6

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Frame Number

Y-SN

R (d

B)

OriginalWatermarked

(c) Claire (d) Foreman

35.5

36

36.5

37

37.5

38

38.5

39

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Frame Number

Y-SN

R (d

B)

OriginalWatermarked

(e) Suzie

Figure 1. Y-SNR values of all the 50 frames for QP=28

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TABLE I Y-PSNR VALUES CORRESPONDING TO DIFFERENT QP (PSNRY FOR UNWATERMARKED VIDEO, PSNR'Y FOR WATERMARKED VIDEO)

Sequence QP=24 QP=28 QP=32

PSNRY PSNR'Y Pvar PSNRY PSNR'Y Pvar PSNRY PSNR'Y Pvar

Akiyo 41.43 41.45 0.02 38.68 38.67 -0.01 35.81 35.90 0.09

Carphone 40.32 40.33 0.01 37.58 37.55 -0.03 34.87 34.87 0.00

Claire 42.89 42.90 0.01 40.15 40.13 -0.02 37.34 37.39 0.05

Foreman 39.37 39.35 -0.02 36.78 36.79 0.01 34.09 34.08 -0.01

Suzie 39.85 39.85 0.00 37.37 37.35 -0.02 35.01 34.98 -0.03

Average 40.77 40.78 0.01 38.11 38.10 -0.01 35.42 35.44 0.02

4141.5

4242.5

4343.5

4444.5

4545.5

4646.5

4747.5

48

100 150 200 250 300

Bitrate (kbps)

Y-P

SNR

(dB

)

OriginalWatermarked

3636.5

3737.5

3838.5

3939.5

4040.5

4141.5

4242.5

100 150 200 250 300

Bitrate (kbps)

Y-PS

NR

(dB

)

OriginalWatermarked

(a) Akiyo (b) Carphone

43

43.5

44

44.5

45

45.5

46

46.5

47

47.5

48

48.5

49

100 150 200 250 300

Bitrate (kbps)

Y-PS

NR

(dB

)

OriginalWatermarked

3434.5

3535.5

3636.5

3737.5

3838.5

3939.5

4040.5

100 150 200 250 300

Bitrate

Y-PS

NR

(dB

)

OriginalWatermarked

(c) Claire (d) Foreman

37.5

38

38.5

39

39.5

40

40.5

41

41.5

42

42.5

100 150 200 250 300

Bitrate

Y-PS

NR

(dB

)

OriginalWatermarked

(e) Suzie

Figure 3. Y-PSNR values for active rate control

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TABLE II Y-PSNR VALUES CORRESPONDING TO DIFFERENT OUTPUT BITRATES (PSNRY FOR UNWATERMARKED VIDEO, PSNR'Y FOR WATERMARKED VIDEO)

Sequence Bitrate (kbps)

100 150 200 250 300

PSNRY PSNR'Y PSNRY PSNR'

Y PSNRY PSNR'Y PSNRY PSNR'

Y PSNRY PSNR'Y

Akiyo 41.50 41.48 43.71 43.70 45.33 45.31 46.57 46.57 47.61 47.62

Carphone 36.62 36.67 38.59 38.60 39.96 39.93 41.04 41.05 41.98 41.98

Claire 43.49 43.49 45.61 45.62 47.03 47.03 48.03 48.01 48.77 48.75

Foreman 34.54 34.54 36.79 36.80 38.21 38.18 39.27 39.23 40.22 40.19

Suzie 37.88 37.84 39.27 39.29 40.43 40.40 41.34 41.36 42.18 42.18

Average 38.81 38.80 40.79 40.80 42.19 42.17 43.25 43.24 44.15 44.14

TABLE III BITRATE (KBPS) CORRESPONDING TO DIFFERENT QUANTIZATION PARAMETERS (R FOR UNWATERMARKED VIDEO, R’ FOR WATERMARKED VIDEO)

Sequence QP=24 QP=28 QP=32

R R' Rvar(%) R R' Rvar(%) R R' Rvar(%)

Akiyo 98.16 98.94 0.79 64.43 64.73 0.47 43.34 43.95 1.41

Carphone 208.20 209.16 0.46 125.48 126.08 0.48 75.42 76.58 1.54

Claire 86.27 87.19 1.06 53.88 54.37 0.91 34.94 35.50 1.60

Foreman 249.23 249.70 0.19 158.00 158.17 0.11 99.77 100.52 0.75

Suzie 171.40 171.64 0.14 95.71 96.33 0.65 56.41 56.30 -0.20

Average 162.65 163.33 0.53 99.50 99.94 0.52 61.98 62.57 1.02

0

50

100

150

200

250

300

Akiyo Carphone Claire Foreman Suzie

Sequence

Bitr

ate

(kbp

s)

OriginalWatermarked

0

20

40

60

80

100

120

140

160

180

Akiyo Carphone Claire Foreman Suzie

Sequence

Bitr

ate

(kbp

s)

OriginalWatermarked

(a) QP=24 (b) QP=28

0

20

40

60

80

100

120

Akiyo Carphone Claire Foreman Suzie

Sequence

Bitr

ate

(kbp

s)

OriginalWatermarked

(c) QP=32

Figure 4. Bitrates (kbps) of watermarked and unwatermarked video sequences

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TABLE IV Ρ VALUES CORRESPONDING TO RECOMPRESSION ATTACKS

Sequence Initial QP

No Attack

Recompression with QP Avg. 24 25 26 27 28 29 30 31 32

Akiyo

24 1 0.66 0.57 0.53 0.53 0.53 0.50 0.45 0.47 0.51 0.53

28 1 0.59 0.63 0.57 0.57 0.57 0.51 0.52 0.50 0.42 0.54

32 1 0.64 0.63 0.62 0.60 0.60 0.59 0.57 0.56 0.53 0.59

Avg. 1 0.63 0.61 0.57 0.57 0.57 0.53 0.51 0.51 0.49 0.55

Carphone

24 1 0.61 0.55 0.53 0.51 0.50 0.45 0.49 0.46 0.47 0.51

28 1 0.54 0.57 0.57 0.57 0.50 0.53 0.50 0.48 0.50 0.53

32 1 0.55 0.55 0.58 0.56 0.50 0.54 0.51 0.51 0.50 0.53

Avg. 1 0.57 0.55 0.56 0.55 0.50 0.50 0.50 0.48 0.49 0.52

Claire

24 1 0.46 0.44 0.40 0.41 0.37 0.37 0.38 0.38 0.40 0.40

28 1 0.41 0.45 0.44 0.47 0.41 0.38 0.38 0.40 0.41 0.42

32 1 0.49 0.48 0.51 0.46 0.42 0.47 0.45 0.49 0.45 0.47

Avg. 1 0.45 0.46 0.45 0.45 0.40 0.40 0.40 0.43 0.42 0.43

Foreman

24 1 0.64 0.60 0.60 0.55 0.52 0.58 0.57 0.57 0.57 0.58

28 1 0.63 0.66 0.60 0.62 0.63 0.57 0.58 0.53 0.57 0.60

32 1 0.64 0.62 0.67 0.63 0.64 0.61 0.63 0.61 0.60 0.63

Avg. 1 0.64 0.63 0.62 0.60 0.60 0.59 0.59 0.57 0.58 0.60

Suzie 24 1 0.67 0.59 0.58 0.58 0.61 0.58 0.55 0.57 0.55 0.59

28 1 0.64 0.62 0.65 0.66 0.59 0.59 0.58 0.56 0.55 0.60

0

0.2

0.4

0.6

0.8

1

NoAttack

24 25 26 27 28 29 30 31 32

Recompression QP

ρAkiyoCarphoneClaireForemanSuzie

0

0.2

0.4

0.6

0.8

1

NoAttack

24 25 26 27 28 29 30 31 32

Recompression QP

ρ

AkiyoCarphoneClaireForemanSuzie

(a) Initial QP=24 (b) Initial QP=28

0

0.2

0.4

0.6

0.8

1

NoAttack

24 25 26 27 28 29 30 31 32

Recompression QP

ρ

AkiyoCarphoneClaireForemanSuzie

0

0.2

0.4

0.6

0.8

1

NoAttack

24 25 26 27 28 29 30 31 32

Recompression QP

ρ

AkiyoCarphoneClaireForemanSuzie

(c) Initial QP=32 (d) Average over QP=24,28,32

Figure 5. ρ values corresponding to recompression attacks

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32 1 0.64 0.65 0.65 0.65 0.65 0.60 0.60 0.65 0.59 0.63

Avg. 1 0.65 0.62 0.63 0.63 0.62 0.59 0.58 0.60 0.56 0.61

Avg. 1 0.59 0.57 0.57 0.56 0.54 0.52 0.52 0.52 0.51 0.54

||

),(||

1

ω

ωδρ

ω

∑== j

jjw

(12)

We have embedded watermark in intra frames only and we have 3 intra frames in each video sequence. For each video sequence average ρ over all the three intra frames is calculated and is shown in table IV. When no attack is made on the video, the value of ρ is 1 for each video sequence. We have reencoded the watermarked video sequences with different quantization steps from 24 to 32. The corresponding ρ values are shown in table IV. In case of no attack value of ρ is equal to 1. In case of recompression attack all the ρ values are significantly less than 1. Comparing different recompression QP values, minimum average value of ρ is 0.51 in case of QP=32 and maximum is 0.59 in the case of QP=24. If we compare video sequences, minimum average value of ρ is 0.43 in case of Claire video sequence and maximum is 0.61 in case of Suzie. Overall average of ρ is 0.54. Fig. 5 shows average values of ρ over three intra frames for the five video sequences. We can see that in case of no attack the value of ρ is 1 and in case of recompression attack, they are significantly less than 1.

Comparing different recompression QP values, minimum average value of ρ is 0.51 in case of QP=32 and maximum is 0.59 in the case of QP=24. If we compare video sequences, minimum average value of ρ is 0.43 in case of Claire video sequence and maximum is 0.61 in case of Suzie. Overall average of ρ is 0.54. Fig. 5 shows average values of ρ for the five video sequences corresponding to no attack and recompression attack with QP ranging from 24 to 32. We can see that in case of no attack the value of ρ is 1 and in case of recompression attack, they are significantly less than 1.

V. CONCLUSION AND FUTURE PERSPECTIVE We have proposed a fragile watermarking scheme for

H.264/AVC that is sensitive to recompression. The watermark is embedded into the prediction modes of intra 4x4 blocks in intra frames. The 9 intra 4x4 prediction modes are divided into two groups, each group representing 0 and 1 respectively. In each group, we selected the prediction mode which has lowest cost. Experimental results show that the proposed watermarking scheme is fragile and the embedded watermark is sensitive to recompression. Thus, the proposed scheme is highly viable to authenticate the originality of video streams. Furthermore our experimental results show that the proposed scheme is imperceptible and has very small and negligible effect on the video quality. The effect on the

bitrate increase due to watermarking is also very small and negligible. Furthermore our proposed scheme is content based, that is, the watermark to be embedded in the prediction modes is generated from the content itself. In future, we aim to extend the proposed scheme for main profile of the H.264/AVC. We will also add some results regarding security of our proposed scheme.

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