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1. Isolated and Ensemble Audio Preprocessing Methods for Detecting Adversarial Examples against Automatic Speech Recognition

   Rajaratnam, Krishan; Shah, Kunal; Kalita, Jugal (November 2018, Conference on Computational Linguistics and Speech Processing (ROCLING))

   An adversarial attack is an exploitative process in which minute alterations are made to natural inputs, causing the inputs to be misclassified by neural models. In the field of speech recognition, this has become an issue of increasing significance. Although adversarial attacks were originally introduced in computer vision, they have since infiltrated the realm of speech recognition. In 2017, a genetic attack was shown to be quite potent against the Speech Commands Model. Limited-vocabulary speech classifiers, such as the Speech Commands Model, are used in a variety of applications, particularly in telephony; as such, adversarial examples produced by this attack pose as a major security threat. This paper explores various methods of detecting these adversarial examples with combinations of audio preprocessing. One particular combined defense incorporating compressions, speech coding, filtering, and audio panning was shown to be quite effective against the attack on the Speech Commands Model, detecting audio adversarial examples with 93.5% precision and 91.2% recall. 

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2. Detecting Adversarial Audio via Activation Quantization Error

   https://doi.org/10.1109/IJCNN48605.2020.9206986  

   Liu, Heng; Ditzler, Gregory (July 2020, International Joint Conference on Neural Networks (IJCNN))

   The robustness and vulnerability of Deep Neural Networks (DNN) are quickly becoming a critical area of interest since these models are in widespread use across real-world applications (i.e., image and audio analysis, recommendation system, natural language analysis, etc.). A DNN's vulnerability is exploited by an adversary to generate data to attack the model; however, the majority of adversarial data generators have focused on image domains with far fewer work on audio domains. More recently, audio analysis models were shown to be vulnerable to adversarial audio examples (e.g., speech command classification, automatic speech recognition, etc.). Thus, one urgent open problem is to detect adversarial audio reliably. In this contribution, we incorporate a separate and yet related DNN technique to detect adversarial audio, namely model quantization. Then we propose an algorithm to detect adversarial audio by using a DNN's quantization error. Specifically, we demonstrate that adversarial audio typically exhibits a larger activation quantization error than benign audio. The quantization error is measured using character error rates. We use the difference in errors to discriminate adversarial audio. Experiments with three the-state-of-the-art audio attack algorithms against the DeepSpeech model show our detection algorithm achieved high accuracy on the Mozilla dataset. 

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3. Adversarial Audio Attacks that Evade Temporal Dependency

   https://doi.org/10.1109/SSCI47803.2020.9308597  

   Liu, Heng; Ditzler, Gregory (December 2020, IEEE Symposium Series on Computational Intelligence (SSCI))

   As the real-world applications (image segmentation, speech recognition, machine translation, etc.) are increasingly adopting Deep Neural Networks (DNNs), DNN's vulnerabilities in a malicious environment have become an increasingly important research topic in adversarial machine learning. Adversarial machine learning (AML) focuses on exploring vulnerabilities and defensive techniques for machine learning models. Recent work has shown that most adversarial audio generation methods fail to consider audios' temporal dependency (TD) (i.e., adversarial audios exhibit weaker TD than benign audios). As a result, the adversarial audios are easily detectable by examining their TD. Therefore, one area of interest in the audio AML community is to develop a novel attack that evades a TD-based detection model. In this contribution, we revisit the LSTM model for audio transcription and propose a new audio attack algorithm that evades the TD-based detection by explicitly controlling the TD in generated adversarial audios. The experimental results show that the detectability of our adversarial audio is significantly reduced compared to the state-of-the-art audio attack algorithms. Furthermore, experiments also show that our adversarial audios remain nearly indistinguishable from benign audios with only negligible perturbation magnitude. 

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4. CommanderGabble: A Universal Attack Against ASR Systems Leveraging Fast Speech

   https://doi.org/10.1145/3485832.3485892  

   Zhang, Zhaohe; Yang, Edwin; Fang, Song (December 2021, Annual Computer Security Applications Conference (ACSAC))

   Automatic Speech Recognition (ASR) systems are widely used in various online transcription services and personal digital assistants. Emerging lines of research have demonstrated that ASR systems are vulnerable to hidden voice commands, i.e., audio that can be recognized by ASRs but not by humans. Such attacks, however, often either highly depend on white-box knowledge of a specific machine learning model or require special hardware to construct the adversarial audio. This paper proposes a new model-agnostic and easily-constructed attack, called CommanderGabble, which uses fast speech to camouflage voice commands. Both humans and ASR systems often misinterpret fast speech, and such misinterpretation can be exploited to launch hidden voice command attacks. Specifically, by carefully manipulating the phonetic structure of a target voice command, ASRs can be caused to derive a hidden meaning from the manipulated, high-speed version. We implement the discovered attacks both over-the-wire and over-the-air, and conduct a suite of experiments to demonstrate their efficacy against 7 practical ASR systems. Our experimental results show that the over-the-wire attacks can disguise as many as 96 out of 100 tested voice commands into adversarial ones, and that the over-the-air attacks are consistently successful for all 18 chosen commands in multiple real-world scenarios. 

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5. Compression to the Rescue: Defending from Adversarial Attacks Across Modalities

   Das, Nilaksh; Shanbhogue, Madhuri; Chen, Shang-Tse; Hohman, Fred; Li, Siwei; Chen, Li; Kounavis, Michael E.; Chau, Duen Horng (August 2018, ACM SIGKDD Conference on Knowledge Discovery and Data Mining)

   Research in the upcoming field of adversarial ML has revealed that machine learning, especially deep learning, is highly vulnerable to imperceptible adversarial perturbations, both in the domain of vision as well as speech. This has induced an urgent need to devise fast and practical approaches to secure deep learning models from adversarial attacks, so that they can be safely deployed in real-world applications. In this showcase, we put forth the idea of compression as a viable solution to defend against adversarial attacks across modalities. Since most of these attacks depend on the gradient of the model to craft an adversarial instance, compression, which is usually non-differentiable, denies a useful gradient to the attacker. In the vision domain we have JPEG compression, and in the audio domain we have MP3 compression and AMR encoding -- all widely adopted techniques that have very fast implementations on most platforms, and can be feasibly leveraged as defenses. We will show the effectiveness of these techniques against adversarial attacks through live demonstrations, both for vision as well as speech. These demonstrations would include real-time computation of adversarial perturbations for images and audio, as well as interactive application of compression for defense. We would invite and encourage the audience to experiment with their own images and audio samples during the demonstrations. This work was undertaken jointly by researchers from Georgia Institute of Technology and Intel Corporation. 

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