Automatic speaker verification (ASV) systems use a playback detector to filter out playback attacks and ensure verification reliability. Since current playback detection models are almost always trained using genuine and played-back speech, it may be possible to degrade their performance by transforming the acoustic characteristics of the played-back speech close to that of the genuine speech. One way to do this is to enhance speech "stolen" from the target speaker before playback. We tested the effectiveness of a playback attack using this method by using the speech enhancement generative adversarial network to transform acoustic characteristics. Experimental results showed that use of this "enhanced stolen speech" method significantly increases the equal error rates for the baseline used in the ASVspoof 2017 challenge and for a light convolutional neural network-based method. The results also showed that its use degrades the performance of a Gaussian mixture model-universal background model-based ASV system. This type of attack is thus an urgent problem needing to be solved.
@cite_14: Automatic speaker verification (ASV) systems are subject to various kinds of malicious attacks. Replay, voice conversion and speech synthesis attacks drastically degrade the performance of a standard ASV system by increasing its false acceptance rates. This issue raised a high level of interest in the speech research community where the possible voice spoofing attacks and their related countermeasures have been investigated. However, much less effort has been devoted in creating realistic and diverse spoofing attack databases that foster researchers to correctly evaluate their countermeasures against attacks. The existing studies are not complete in terms of types of attacks, and often difficult to reproduce because of unavailability of public databases. In this paper we introduce the voice spoofing data-set of AVspoof, a public audio-visual spoofing database. AVspoof includes ten realistic spoofing threats generated using replay, speech synthesis and voice conversion. In addition, we provide a set of experimental results that show the effect of such attacks on current state-of-the-art ASV systems.
@cite_13: This tutorial provides an overview of the basic theory of hidden Markov models (HMMs) as originated by L.E. Baum and T. Petrie (1966) and gives practical details on methods of implementation of the theory along with a description of selected applications of the theory to distinct problems in speech recognition. Results from a number of original sources are combined to provide a single source of acquiring the background required to pursue further this area of research. The author first reviews the theory of discrete Markov chains and shows how the concept of hidden states, where the observation is a probabilistic function of the state, can be used effectively. The theory is illustrated with two simple examples, namely coin-tossing, and the classic balls-in-urns system. Three fundamental problems of HMMs are noted and several practical techniques for solving these problems are given. The various types of HMMs that have been studied, including ergodic as well as left-right models, are described. >
@cite_8: This paper reports on some ways to try to deceive a state-of-the-art speaker verification (SV) system. In order to evaluate the risk in SV systems one has to take into account the possible intentional impostors who know whom they are attacking. We defined a worst case scenario where the impostor has extensive knowledge about the person to deceive as well as the system to attack. In this framework we tested our SV system against concatenated client speech, re-synthesis of the client speech and diphone synthesis of the client.
Pioneering work on playback attacks was reported by Lindberg and Blomberg in 1999 @cite_8 . They pre-recorded the numbers one to ten of two speakers and then concatenated various combinations of them to attack a hidden Markov model (HMM) @cite_13 -based text-dependent ASV system. They demonstrated considerable increase in both the EER and false acceptance rate (FAR) compared to verification without attacks. More recently, investigated the effect of playback attacks against ASV systems and also achieved a large increase in FAR @cite_14 . Compared to these conventional playback attacks, our method further degrades the performance of playback spoofing CMs by enhancing the speech.