A Secure AMR Fixed Codebook Steganographic Scheme Based on Pulse Distribution Model

A Secure AMR Fixed Codebook Steganographic Scheme Based on Pulse Distribution Model

Abstract: Adaptive multi-rate (AMR), a popular audio compression standard, is widely used in mobile communication and mobile Internet applications and has become a novel carrier for hiding information. To improve the statistical security, this paper presents a steganographic scheme in the AMR fixed codebook (FCB) domain based on the pulse distribution model (PDMAFS), which is obtained from the distribution characteristics of the FCB value in the cover audio. The pulse positions in stego audio are controlled by message encoding and random masking to make the statistical distribution of the FCB parameters close to that of the cover audio.

The experimental results show that the statistical security of the proposed scheme is better than that of the existing schemes. Furthermore, the hiding capacity is maintained compared with the existing schemes. The average hiding capacity can reach 2.06 kbps at an audio compression rate of 12.2 kbps and the auditory concealment is good. To the best of our knowledge, this is the first secure AMR FCB steganographic scheme that improves the statistical security based on the distribution model of the cover audio. This scheme can be extended to other audio

compression codecs under the principle of Algebraic Code Excited Linear Prediction (ACELP), such as G.723.1 and G.729.

Existing system: In Miao, three types of steganalytic features were extracted to represent the relationship between the pulses in the same track, including transition probability of the Markov, conditional and joint entropy. Ren found that the probability of the same pulse position (SPP) in the same track is higher in the stego audio than in the cover audio; therefore, they proposed to extract SPP as the steganalytic feature.

Because the existing steganographic methods failed to maintain the distribution characteristics of the cover, both schemes in and have good detection accuracy for AMR FCB steganographic methods. Although the scheme proposed in improved the statistical security of AMR FCB steganography, its embedding capacity is reduced. In this paper, an AMR FCB steganographic scheme that is based on the pulse distribution model (PDM-AFS) is proposed.

Proposed system: An AMR FCB steganographic scheme (PDM-AFS) that is based on the pulse distribution pattern of the cover AMR audio is proposed. The scheme improves the randomness of pulse positions by message encoding and random masking to make the statistical distribution of the FCB parameters in the stego audio close to that of the cover audio. Thus, the statistical security is substantially improved. Moreover, the proposed scheme maintains an embedding capacity as high as that of, and higher than that of the rest of this paper is organized as follows.

Advantages: Because the existing steganographic methods failed to maintain the distribution characteristics of the cover, both schemes in have good detection accuracy for AMR FCB steganographic methods.

Although the scheme proposed in improved the statistical security of AMR FCB steganography, its embedding capacity is reduced. In this paper, an AMR FCB steganographic scheme that is based on the pulse distribution model (PDM-AFS) is proposed.

Disadvantages: The function of the encoding phase is to encode the secret message as a specific structure to solve the problem that the second pulse may be the same as the first pulse after the embedding operation.

The function of the randomization is to make the tag information shown sufficiently random to maintain the distribution characteristics of the stego audio close to those of the cover audio. After these processes, the scrambling operation scrambles the embedded messages by the unit of the pulse index to ensure the security of the embedded message.

Modules: Secret Message Processing Module: The rights part a flowchart of the secret message processing module. This module consists of four main parts: preprocessing, encoding, randomization and scrambling. In the preprocessing phase, the original message is compressed and encrypted to make it more secure and random. In the encoding phase, the preprocessed message is encoded as the structure shown. The function of the encoding phase is to encode the secret message as a specific structure to solve the problem that the second pulse may be the same as the first pulse after the embedding operation. The function of the randomization is to make the tag information shown in Fig 8 sufficiently random to maintain the distribution characteristics of the stego audio close to those of the cover audio. After these processes, the scrambling operation scrambles the embedded messages by the unit of the pulse index to ensure the security of the embedded message.

Encoding: In the proposed scheme, the second pulse is calculated according to the first pulse position and the secret message; therefore, in some cases, the specific secret message will make the second pulse equal to the first pulse, which will lead to a change in the SPP feature. For example, in the 12.2 kbps mode, the secret message �000� will make the second pulse equal to the first pulse. To solve this problem, the second pulse should not be changed in these cases. Tag information is attached to the secret message to represent it. The secret message is encoded as the structure

shown. Twelve bits of the encoded message are treated as an encoding unit that includes two components: 9 bits of the secret message and 3 bits of the tag information. The secret message is the message after preprocessing, which is divided into 3 sets of 3-bit message, where each 3-bit message is denoted as msg.

A Secure Steganographic Scheme Pdm-Afs: The target for designing a secure steganographic scheme is to make the statistical distribution of the stego audio as close as possible to that of the cover audio.

According to the analysis above, this paper proposes an AMR FCB steganographic scheme based on the pulse distribution model (PDM-AFS). The main aims of the proposed scheme focus on three goals: 1) to achieve maximum hiding capacity; 2) to ensure that the SPP value in each track is not changed after embedding; and 3) to ensure that the two pulses in the same track are as random as possible to make the Markov transition probability feature flat. To achieve the first goal, the whole pulse index value, rather than part of it, is used as the embedding domain because the pulse index value in the FCB represents the position of the pulse selected as the nonzero pulse.

Relative embedding bitrate: The Markov transition probability feature of the cover and stego audio. The two axes in the horizontal plane represent the first pulse position and the second pulse position in the same track. The vertical axis is the Markov transition probabilities between the two pulse positions. In this experiment, the steganographic schemes are Geiser, Miao and A FAat a relative embedding bitrate (EBR) of 100%, where EBR is the ratio of the actual embedding message length to the maximum embedding length. As shown in Fig 2, for the cover audio, the diagonal value of the Markov transition probability is substantially lower than that of the other position pairs, and the Markov transition probability of the non-diagonal is nearly uniform. However, for the stego audio generated at EBR of 100%, regardless of the type of steganographic scheme, the phenomenon wherein the diagonal value is low and the non-diagonal value is flat no longer exists. The reason for this difference is that in the AMR encoding procedure, the two pulse positions are chosen by the FCB search procedure, which prefers to select different FCB pulse

positions in each track to improve the coding efficiency. However, the operation of embedding changes the distribution pattern.

Interleaved single – pulse permutation: In the AMR codec, the fixed codebook that represents the innovation vectors of each sub-frame is implemented by the algebraic codebook, the structure of which is based on the interleaved single-pulse permutation (ISPP) design. In the fixed codebook, there can be only Nc nonzero pulses in each code excitation vector Ck. To satisfy certain algebraic structures and bit allocation requirements, these nonzero pulse amplitudes and positions are limited. Different coding modes have different distributions. Considering the 12.2 kbps encoding mode as an example, there are 10 nonzero pulses encoded as excitation vectors, the magnitude of which is either +1 or 1. In each frame, there are four sub-frames, each sub-frame has 40 pulse positions which are assigned to five different tracks. There are two nonzero pulse positions in each track, which are selected by the FCB search procedure. The pulses of each track in the algebraic codebook are shown in Table I.