# Spreading-Based Voice Encryption by Means of OVSF Codes
## 1 Introduction  
Lately, with the advance of technology, the volume of information processed daily has grown exponentially. In most cases, the information transmitted becomes important, especially in communication systems where the content of the information is confidential and can be valuable. Therefore, the security of the information has become a necessity when transmitting private information. In recent years, some authors have proposed new methodologies aimed at providing a level of data security and integrity. The purpose of transmitting information under any secure communication method is to provide a level of privacy over the data, which means limiting access by a third party or unauthorized people [1, 2]. In this regard, encryption is a security mechanism used to provide confidentiality and protection to a system, guaranteeing reliability and protection over the information to be transmitted. Accordingly, different techniques have been implemented based on the codification of voice signals in order to obtain an undecipherable audio signal whose characteristics are totally different with respect to the original one, including security, confidentiality, and integrity aspects [3]. Some encryption algorithms such as DES (data encryption standard), AES (advanced encryption standard), Blowfish, and RSA (Rivest, Shamir y Adleman) are standard algorithms frequently used for the encryption of different data types, such as voice [2, 4]. Methods based on scrambling systems are also commonly used to modify the information of an input signal. For example, using a key or seed value to generate a variety of coded signals provides high reliability [5]. Chaotic maps contribute an essential part in the encryption of voice signals and algorithms that are implemented and use chaoticmaps are effective and safe [6]. For example, the ADPCM (adaptive differential pulse code modulation) coding and the generation of logistic maps for transformation into binary words are used to perform substitution operations on audio signals with sensitive information [7]. Algorithms based on Henen chaotic maps and chaotic economic map have also been proposed, to create independent sequences with each other, adding more sensitivity and complexity to the scheme [8; 9]. Another approach of dual-channel audio encryption is based on chaotic systems with changeable multi-scroll, in order to generate one time-key reliant on the file's hash value, and transforming the original speech samples position in order to make brute-force attack impossible [10]. In addition, voice encryption in communication systems can be used for multichannel digital audio signals, according to the technique called binaural cue coding (BCC). In this case, the audio signal is broken down into frequency bands to extract the most important information and the use of pseudo-random sources generates a binary flow that allows the mixing of information to transmit an encoded audio signal [11]. Finally, an encryption technique for audio signals based on the Collatz conjecture has been proposed in [12]. The authors use a coding process using variable length codes based on the Collatz conjecture. In order to increase the level of security and uncertainty of the output signal, there is a spread process through a coding based on the Collatz conjecture, ensuring that samples of the modified signal are uncorrelated with the original one. As a result, an audio signal with similar characteristics to a noise signal is transmitted. Since the used codes are a variable length, the process has a high computational cost, mainly in the recovering stage. According to the above, this document presents an improved methodology aimed at encrypting the original information of a voice signal. In this case, the principal goal is to transform the original content of a voice signal to an unintelligible signal by third parties through the spread and scrambling of the input samples. For the spreading process, we use OVSF-based (orthogonal variable spreading factor) codes, previously randomized.