# Direct Spread Spectrum Technology for Data Hiding in Audio +
## 1 Introduction  
Various techniques are used to conceal information [1, 2, 3]. The most common of them are cryptography techniques [4, 5, 6]. In this case, meaningful messages are converted into meaningless, noise-like forms, i.e., this becomes useless data for an unauthorized user (who does not know the secret cryptographic key). Thus, cryptography hides the semantic content of the transmitted data, whereas the very existence of messages is not hidden [3, 7]. Another approach is to use steganographic techniques [3, 8]. In this case, informational messages are hidden inside other data (also called containers, covers, carriers, etc.). Carriers can be presented, for example, in the form of various multimedia files (images, video, audio, texts, etc.) [9]. A range of applications exist with multimedia data, so the transfer of video or audio files on the internet does not cause any suspicion. At the same time, only the one who has the secret steganographic key knows that an informational message is hidden in these covers. It is similar to a suitcase with a double bottom; the second bottom is reliably disguised, and it is difficult to detect a hidden message without a secret key. Thus, steganography hides the fact that messages exist [2, 8]. The present paper discusses steganographic information hiding techniques in which audio files act as multimedia containers. Hiding data in audio has great prospects. Firstly, such data are significantly popular on the internet. For example, we can use audio messages in messengers and social networks as covers. At the same time, the amount of hidden data can be quite large. In addition, there is a problem with copyright protection of audio content on the internet. Hiding digital watermarks (with a copyright label) in audio containers can be an effective solution. Thus, hiding data in audio is really very relevant and may have a commercial implementation. In our experiments, we used the simplest audio signals recorded in the digital WAV format [10; 11]. However, all our results and suggestions can be interpreted for a more general case with other audio container formats. In order to conceal information, this work employs a special technology traditionally used in radio communication systems [12; 13; 14; 15]. This is a direct spreading of the spectrum [16], which implements wideband modulation with pseudo-random sequences (chip codes) [17]. The correlation properties of chip codes guarantee the absence of mutual interference in the communication channel [18]. As a result, the radio communication system acquires numerous useful properties: noise immunity, high bandwidth and subscriber capacity, absence of mutual interference, environmental friendliness of communication, and many others [19]. Direct spread spectrum technology is implemented in many industry standards and applications, for example:  
* Code division multiple access communication systems (IS-95, CDMA2000, WCDMA, DS-CDMA, TD-CDMA, TD-SCDMA, etc. );
* Global satellite navigation systems (American GPS, European Galileo, and Russian GLONASS);
* Family of wireless network protocols Wi-Fi (family of standards IEEE 802.11), and much more. Direct spread spectrum technology can also be used in steganography [20; 21; 22]. In particular, in one of the first works [21] in this area, it was proposed to interpret the carrier as noise in the communication channel. In this case, the task of hiding information in multimedia files is equivalent to transmitting useful signals over communication channels with natural additive noise. Currently, a large number of studies on steganography and the direct spreading of the spectrum have already been conducted. Various researchers studied this issue in relation to various computer applications. It turns out that the main characteristics of reliability and safety depend on the properties of the spreading sequences. In particular, in [23; 24], we investigated various ways of generating chip codes in relation to cover images.