Detection of replay attack

Abstract

In order to detect a replay attack on a voice biometrics system, a first signal from received sound is generated at a first microphone, and a second signal from the received sound is generated at a second microphone. The first and second signals are used to determine a location of an apparent source of the received sound. It is determined that the received sound may result from a replay attack if the apparent source of the received sound is diffuse.

Description

TECHNICAL FIELD

Embodiments described herein relate to methods and devices for detecting a replay attack on a voice biometrics system.

BACKGROUND

Voice biometrics systems are becoming widely used. In such a system, a user trains the system by providing samples of their speech during an enrolment phase. In subsequent use, the system is able to discriminate between the enrolled user and non-registered speakers. Voice biometrics systems can in principle be used to control access to a wide range of services and systems.

One way for a malicious party to attempt to defeat a voice biometrics system is to obtain a recording of the enrolled user's speech, and to play back the recording in an attempt to impersonate the enrolled user and to gain access to services that are intended to be restricted to the enrolled user.

This is referred to as a replay attack, or as a spoofing attack.

SUMMARY

According to an aspect of the present invention, there is provided a method of detecting a replay attack on a voice biometrics system, the method comprising: generating a first signal from received sound at a first microphone;

• • generating a second signal from the received sound at a second microphone;
  • using the first and second signals to determine a location of an apparent source of the received sound; and
  • determining that the received sound may result from a replay attack if the apparent source of the received sound is diffuse.

According to another aspect of the present invention, there is provided a system for detecting a replay attack on a voice biometrics system, the system being configured for performing the method.

According to an aspect of the present invention, there is provided a detecting a replay attack on a speech recognition system, for example a voice biometrics system, the method comprising:

• • generating a first signal from received sound at a first microphone;
  • generating a second signal from the received sound at a second microphone;
  • determining a correlation function based on a correlation between the first signal and the second signal;
  • calculating a width of a central lobe of the determined correlation function; and
  • determining that the received sound may result from a replay attack if the width of the central lobe of the determined correlation function exceeds a threshold value.

According to another aspect of the present invention, there is provided a system for detecting a replay attack on a speech recognition system, for example a voice biometrics system, the system being configured for performing the method.

According to an aspect of the present invention, there is provided a method of detecting a replay attack on a voice biometrics system, the method comprising:

• • generating a first signal from received sound at a first microphone, wherein the first signal has a first component at a first frequency and a second component at a second frequency, and wherein the first frequency is higher than the second frequency;
  • generating a second signal from the received sound at a second microphone, wherein the second signal has a first component at the first frequency and a second component at the second frequency;
  • determining a first correlation function based on a correlation between the first component of the first signal and the first component of the second signal;
  • calculating a width of a central lobe of the determined first correlation function;
  • determining a second correlation function based on a correlation between the second component of the first signal and the second component of the second signal;
  • calculating a width of a central lobe of the determined second correlation function; and
  • determining that the received sound may result from a replay attack if the width of the central lobe of the determined second correlation function exceeds the width of the central lobe of the determined first correlation function by more than a threshold value.

According to another aspect of the present invention, there is provided a system for detecting a replay attack on a voice biometrics system, the system being configured for performing the method.

According to another aspect of the present invention, there is provided a device comprising a system according to any previous aspect. The device may comprise a mobile telephone, an audio player, a video player, a mobile computing platform, a games device, a remote controller device, a toy, a machine, or a home automation controller or a domestic appliance.

According to another aspect of the present invention, there is provided a computer program product, comprising a computer-readable tangible medium, and instructions for performing a method according to the first, second, or third aspect.

According to another aspect of the present invention, there is provided a non-transitory computer readable storage medium having computer-executable instructions stored thereon that, when executed by processor circuitry, cause the processor circuitry to perform a method according to the first, second, or third aspect.

According to another aspect of the present invention, there is provided a device comprising the non-transitory computer readable storage medium according to the previous aspect. The device may comprise a mobile telephone, an audio player, a video player, a mobile computing platform, a games device, a remote controller device, a toy, a machine, or a home automation controller or a domestic appliance.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the present invention, and to show how it may be put into effect, reference will now be made to the accompanying drawings, in which:—

FIG. 1 illustrates a smartphone;

FIG. 2 is a schematic diagram, illustrating the form of the smartphone;

FIG. 3 illustrates a first situation in which a replay attack is being performed;

FIG. 4 illustrates a second situation in which a replay attack is being performed;

FIG. 5 illustrates sound transmission in the arrangement of FIG. 4;

FIG. 6 is a flow chart illustrating a method;

FIG. 7 is a block diagram illustrating a system for performing the method of FIG. 6;

FIG. 8 illustrates a first result of performing the method of FIG. 6; and

FIG. 9 illustrates a second result of performing the method of FIG. 6.

DETAILED DESCRIPTION OF EMBODIMENTS

The description below sets forth example embodiments according to this disclosure. Further example embodiments and implementations will be apparent to those having ordinary skill in the art. Further, those having ordinary skill in the art will recognize that various equivalent techniques may be applied in lieu of, or in conjunction with, the embodiments discussed below, and all such equivalents should be deemed as being encompassed by the present disclosure.

The methods described herein can be implemented in a wide range of devices and systems, for example a mobile telephone, an audio player, a video player, a mobile computing platform, a games device, a remote controller device, a toy, a machine, or a home automation controller or a domestic appliance. However, for ease of explanation of one embodiment, an illustrative example will be described, in which the implementation occurs in a smartphone.

FIG. 1 illustrates a smartphone 10, having microphones 12, 12a, and 12b for detecting ambient sounds. In this example, the microphone 12 is of course used for detecting the speech of a user who is holding the smartphone 10, while the microphones 12a, 12b are provided on the upper part of the sides of the smartphone 10, and are therefore not clearly visible in FIG. 1.

FIG. 2 is a schematic diagram, illustrating the form of the smartphone 10.

Specifically, FIG. 2 shows various interconnected components of the smartphone 10. It will be appreciated that the smartphone 10 will in practice contain many other components, but the following description is sufficient for an understanding of the present invention.

Thus, FIG. 2 shows the microphone 12 mentioned above. In certain embodiments, the smartphone 10 is provided with multiple microphones 12, 12a, 12b, etc.

FIG. 2 also shows a memory 14, which may in practice be provided as a single component or as multiple components. The memory 14 is provided for storing data and program instructions.

FIG. 2 also shows a processor 16, which again may in practice be provided as a single component or as multiple components. For example, one component of the processor 16 may be an applications processor of the smartphone 10.

FIG. 2 also shows a transceiver 18, which is provided for allowing the smartphone 10 to communicate with external networks. For example, the transceiver 18 may include circuitry for establishing an internet connection either over a WiFi local area network or over a cellular network.

FIG. 2 also shows audio processing circuitry 20, for performing operations on the audio signals detected by the microphone 12 as required. For example, the audio processing circuitry 20 may filter the audio signals or perform other signal processing operations.

FIG. 2 also shows at least one sensor 22. In embodiments of the present invention, the sensor is a magnetic field sensor for detecting a magnetic field. For example, the sensor 22 may be a Hall effect sensor, that is able to provide separate measurements of the magnet field strength in three orthogonal directions. Further examples of sensors which may be used may comprise gyro sensors, accelerometers, or software-based sensors operable to determine phone orientation, wherein such software-based sensors may operate in combination with software programs such as the FaceTime™ system provided by Apple, Inc.

In this embodiment, the smartphone 10 is provided with voice biometric functionality, and with control functionality. Thus, the smartphone 10 is able to perform various functions in response to spoken commands from an enrolled user. The biometric functionality is able to distinguish between spoken commands from the enrolled user, and the same commands when spoken by a different person. Thus, certain embodiments of the invention relate to operation of a smartphone or another portable electronic device with some sort of voice operability, for example a tablet or laptop computer, a games console, a home control system, a home entertainment system, an in-vehicle entertainment system, a domestic appliance, or the like, in which the voice biometric functionality is performed in the device that is intended to carry out the spoken command. Certain other embodiments relate to systems in which the voice biometric functionality is performed on a smartphone or other device, which then transmits the commands to a separate device if the voice biometric functionality is able to confirm that the speaker was the enrolled user.

In some embodiments, while voice biometric functionality is performed on the smartphone 10 or other device that is located close to the user, the spoken commands are transmitted using the transceiver 18 to a remote speech recognition system, which determines the meaning of the spoken commands. For example, the speech recognition system may be located on one or more remote servers in a cloud computing environment. Signals based on the meaning of the spoken commands are then returned to the smartphone 10 or other local device. In other embodiments, the speech recognition system is also provided on the smartphone 10.

One attempt to deceive a voice biometric system or an automatic speech recognition system is to play a recording of an enrolled user's voice in a so-called replay or spoof attack.

The method is therefore described herein with reference to one example, where it is desirable to detect when sound is being played through a loudspeaker, rather than being generated by a human speaker. However, the method is equally applicable to other situations in which it is useful to detect whether sound is coming from a point source or a more diffuse source. One such example might be when it is desired to detect when the sound received by an automatic speech recognition system has been generated by a loudspeaker.

FIG. 3 shows an example of a situation in which a replay attack is being performed. Thus, in FIG. 3, the smartphone 10 is provided with voice biometric functionality. In this example, the smartphone 10 is in the possession, at least temporarily, of an attacker, who has another smartphone 30. The smartphone 30 has been used to record the voice of the enrolled user of the smartphone 10. The smartphone 30 is brought close to the microphone inlet 12 of the smartphone 10, and the recording of the enrolled user's voice is played back. If the voice biometric system is unable to detect that the enrolled user's voice that it detects is a recording, the attacker will gain access to one or more services that are intended to be accessible only by the enrolled user.

It is known that smartphones, such as the smartphone 30, are typically provided with loudspeakers that are of relatively low quality due to size constraints. Thus, the recording of an enrolled user's voice played back through such a loudspeaker will not be a perfect match with the user's voice, and this fact can be used to identify replay attacks. For example, loudspeakers may have certain frequency characteristics, and if these frequency characteristics can be detected in a speech signal that is received by the voice biometrics system, it may be considered that the speech signal has resulted from a replay attack.

FIG. 4 shows a second example of a situation in which a replay attack is being performed, in an attempt to overcome the method of detection described above. Thus, in FIG. 4, the smartphone 10 is provided with voice biometric functionality. Again, in this example, the smartphone 10 is in the possession, at least temporarily, of an attacker, who has another smartphone 40. The smartphone 40 has been used to record the voice of the enrolled user of the smartphone 10.

In this example, the smartphone 40 is connected to a high quality loudspeaker 150. Then, the smartphone 10 is positioned close to the loudspeaker 150, and the recording of the enrolled user's voice is played back through the loudspeaker 150. As before, if the voice biometric system is unable to detect that the enrolled user's voice that it detects is a recording, the attacker will gain access to one or more services that are intended to be accessible only by the enrolled user.

In this example, the loudspeaker 150 may be of high enough quality that the recording of the enrolled user's voice played back through the loudspeaker will not be reliably distinguishable from the user's voice, and so the audio features of the speech signal cannot be used to identify the replay attack.

In this example, the loudspeaker 150 is an electrostatic loudspeaker such as a Quad® ESL, or a balanced mode radiator loudspeaker, or a bending mode or bending wave loudspeaker, or any other type of flat panel loudspeaker.

One feature of many such loudspeakers is that the apparent source of the sound is not at one point, but is diffuse, i.e. distributed over the loudspeaker.

FIG. 5 shows a typical arrangement, in which the loudspeaker device 150 is being used to replay speech that is detected by the smartphone 10. Thus, FIG. 5 shows sounds from a point 152 in the lower part of the loudspeaker 150 reaching the microphone 12 that is located at the bottom end of the smartphone 10, and also reaching the microphones 12a and 12b that are located at the top end of the smartphone 10. FIG. 5 also shows sounds from a point 154 in the upper part of the loudspeaker 150 reaching the microphone 12 that is located at the bottom end of the smartphone 10, and also reaching the microphones 12a and 12b that are located at the top end of the smartphone 10.

It can therefore be seen from FIG. 5 that, as seen from the smartphone 10, the sounds that it detects come from a highly diffuse source.

This contrasts with the situation where a human is speaking, when the sounds come from a relatively small area, with the human mouth for example having a maximum jaw range of motion (ROM) or maximum mouth opening (MMO) of about 5-8 cm.

The appreciation of this fact is used in the method described herein.

FIG. 6 is a flow chart, illustrating a method of detecting a replay attack on a voice biometrics system, and FIG. 7 is a block diagram illustrating functional blocks in the voice biometrics system.

Specifically, in FIG. 6, in step 170, a first signal is generated at a first microphone. For example, the first microphone may be the microphone 12 that is located at the bottom end of the smartphone 10. This microphone generates a signal in response to the received sound. Similarly, in step 172, a second signal is generated at a second microphone. For example, the second microphone may be one of the microphones 12a that is located at the upper end of the smartphone 10. Again, this microphone generates a signal in response to the received sound.

In the case of a smartphone, the first and second microphones may be spaced apart by a distance in the range of 5-20 cm.

As part of the biometric operation, the first and second signals are passed to a feature extraction block 190, which extracts features of the audio signals in a known manner. In one example, the features of the audio signals may be Mel Frequency Cepstral Coefficients (MFCCs). These features are passed to a model comparison block 192, where they are compared with the corresponding features extracted from the user's speech during an enrolment process. Based on the comparison, it is determined whether the detected speech is the speech of the enrolled user.

Meanwhile, the first and second signals are also passed to a location information derivation block 194.

In step 174, the location information derivation block 194 uses the first and second signals to determine a location of an apparent source of the received sound.

More specifically, in one example, in step 176, the location information derivation block 194 performs a correlation operation on the first signal and the second signal, and determines a correlation function.

The correlation operation determines a value for the cross-correlation Rxy between the first signal and the second signal, for a range of time offsets. Thus, in this example, the first and second signals are responding to the same received sound. However, the value of the correlation depends on the position of the source of the received sound. For example, if the sound arrives at the first microphone before the second microphone, the signals will need to be offset relative to each other in one direction to achieve a match between them. This results in a high value in the correlation function with an offset in that direction. If the sound arrives at the second microphone before the first microphone, the signals will need to be offset relative to each other in the other direction to achieve a match between them. This results in a high value in the correlation function with an offset in that other direction.

This assumes that the source of the received sound is a point source. However, in reality, the source has a finite width, and so the overall correlation function that is calculated is the integral, across the whole width of the source, of these correlations between received sounds coming from point sources.

Specifically, for any point in the finite width of the source of the sound, the times of flight of the sound from that point to the two microphones can be calculated as τ and τ_P. The difference between these two times will depend on the angle at which sound from that source meets the plane containing the two microphones. If the source of the sound is a diffuse source, extending from −w_O to w_O in the direction of the width, then the correlation result is the integral of the correlations across the width of the source:

$R\left(\tau ,{\tau }_P\right)=\frac{1}{2\pi }{\int }_{-w_o}^{w_o}{e}^{\mathrm{iw}\left(\tau -{\tau }_P\right)}\mathrm{dw}$ $\mathrm{Thus}\text{:}$ $R\left(\tau ,{\tau }_P\right)=\frac{w_o}{2\pi }\sin c\left(w_o\left(\tau -{\tau }_P\right)\right)$

The width of central lobe of this function therefore depends on the width of the source of the sound.

By making an assumption about the likely distance of the loudspeaker 150 from the smartphone 10, in the situation illustrated in FIG. 5 (for example, that it is likely that the attacker would place the smartphone between 0.10 and 1.0 metres from the loudspeaker), a suitable threshold value can be set. This threshold can represent the maximum width of the central lobe that would be expected, if the source of the sound were in fact a human mouth. If the width of the central lobe exceeds this threshold value, it can be determined that the source of the sound is likely to be a loudspeaker.

FIG. 8 illustrates an example situation where the sound is arriving at the first and second microphones from a narrow source that is located equidistantly from the first and second microphones. Thus, the peak in the central lobe of the correlation function Rxy is relatively sharp, with a width W₁ between the points at which the central lobe of the correlation function reaches zero, and is located at zero offset.

By contrast, FIG. 9 illustrates an example situation where the sound is arriving at the first and second microphones from a diffuse source, similar to that shown in FIG. 5. Thus, some parts of the source (for example the point 152 in the lower part of the loudspeaker 150) are closer to the microphone 12 that is located at the bottom end of the smartphone 10 than to the microphone 12a that is located at the top end of the smartphone 10.

Thus, sound from the point 152, and other similar points, arrives at the microphone 12 before it arrives at the microphone 12a. Therefore, as discussed above, sounds from these points result in a high value in the correlation function with an offset in one particular direction.

Conversely, other parts of the source (for example the point 154 in the upper part of the loudspeaker 150) are closer to the microphone 12a that is located at the top end of the smartphone 10 than to the microphone 12 that is located at the bottom end of the smartphone 10.

Thus, sound from the point 154, and other similar points, arrives at the microphone 12a before it arrives at the microphone 12. Therefore, as discussed above, sounds from these points result in a high value in the correlation function with an offset in the opposite direction to the sounds from points such as the point 152.

Thus, in FIG. 9, the peak in the central lobe of the correlation function Rxy is much less sharp than in FIG. 8, with a width W2 between the points at which the central lobe of the correlation function reaches zero.

In step 178 of the process shown in FIG. 6, this width of the central lobe of the correlation function is calculated.

In step 180 of the process shown in FIG. 6, this calculated width of the central lobe of the correlation function is passed to a determination block 196, and it is determined that the received sound may result from a replay attack if the apparent source of the received sound is diffuse. For example, the apparent source of the received sound may be considered to be diffuse if it is larger than a human mouth, for example if it exceeds a diameter of 5 cm.

Thus, as shown at step 182, it is determined that the received sound may result from a replay attack if the width of the central lobe of the correlation function exceeds a threshold value. That threshold value may be chosen so that the width of the central lobe of the correlation function exceeds the threshold value if the source of the received sound exceeds a diameter of about 5-8 cm.

In some embodiments, information may be obtained about the usage mode of the smartphone. For example, information may be obtained about the distance of the smartphone from the source of the received sound, for example using an ultrasound or optical proximity detection function. The threshold value may then be set based on the distance of the smartphone from the source of the received sound.

If it is determined that the received sound may result from a replay attack, an output flag or signal is sent to a further function in the voice biometric system. For example, the output of the model comparison block 192 may be halted, or may be altered so that a subsequent processing block gives less weight (or no weight at all) to an output indicating that the voice was that of the enrolled speaker.

In the example above, signals from two microphones were used to determine whether the source of the received sound is diffuse. In other examples, signals from three or more microphones may be cross-correlated (for example cross-correlated against each other in pairs) to obtain more information about the spatial diversity of the source of the detected sound.

In the examples given above, the signals from two microphones were used to determine whether the source of the received sound is diffuse. A further development of this is based on the recognition that, for certain loudspeakers at least, the idea may be extended by noting that the apparent width of the loudspeaker will vary with frequency. More specifically, the loudspeaker will appear to be wider at low frequencies than at higher frequencies.

In order to take advantage of this, the location information derivation block 194 includes two or more band-pass filters, for extracting respective frequency bands of the received signal. The method described above is then performed separately on these two frequency bands. More specifically, the first microphone generates a first signal from received sound, wherein the first signal has a first component at a first frequency and a second component at a second frequency, and wherein the first frequency is higher than the second frequency. The second microphone generates a second signal from the received sound.

Then, a first correlation function is determined based on a correlation between the first component of the first signal and the first component of the second signal. The width of a central lobe of that first correlation function is calculated. A second correlation function is determined based on a correlation between the second component of the first signal and the second component of the second signal. The width of a central lobe of that second correlation function is calculated.

The two widths are then compared, and it is determined that the received sound may have been generated by a loudspeaker, and for example may result from a replay attack, if the width of the central lobe of the determined second correlation function exceeds the width of the central lobe of the determined first correlation function by more than a threshold value.

There are therefore disclosed methods and systems that can be used for detecting a possible replay attack.

The skilled person will recognise that some aspects of the above-described apparatus and methods may be embodied as processor control code, for example on a non-volatile carrier medium such as a disk, CD- or DVD-ROM, programmed memory such as read only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. For many applications, embodiments of the invention will be implemented on a DSP (Digital Signal Processor), ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array). Thus the code may comprise conventional program code or microcode or, for example code for setting up or controlling an ASIC or FPGA. The code may also comprise code for dynamically configuring re-configurable apparatus such as re-programmable logic gate arrays. Similarly the code may comprise code for a hardware description language such as Verilog™ or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate, the code may be distributed between a plurality of coupled components in communication with one another. Where appropriate, the embodiments may also be implemented using code running on a field-(re)programmable analogue array or similar device in order to configure analogue hardware.

Note that as used herein the term module shall be used to refer to a functional unit or block which may be implemented at least partly by dedicated hardware components such as custom defined circuitry and/or at least partly be implemented by one or more software processors or appropriate code running on a suitable general purpose processor or the like. A module may itself comprise other modules or functional units. A module may be provided by multiple components or sub-modules which need not be co-located and could be provided on different integrated circuits and/or running on different processors.

Embodiments may be implemented in a host device, especially a portable and/or battery powered host device such as a mobile computing device, for example, a laptop or tablet computer, a games console, a remote control device, a home automation controller or a domestic appliance including a domestic temperature or lighting control system, a toy, a machine such as a robot, an audio player, a video player, or a mobile telephone for example a smartphone.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference numerals or labels in the claims shall not be construed so as to limit their scope.

Claims

1. A method of detecting a replay attack on a speech recognition system, for example a voice biometrics system, the method comprising: generating a first signal from received sound at a first microphone;generating a second signal from the received sound at a second microphone;determining a correlation function based on a correlation between the first signal and the second signal;calculating a width of a central lobe of the determined correlation function; anddetermining that the received sound may result from a replay attack if the width of the central lobe of the determined correlation function exceeds a threshold value.

2. A method according to claim 1, comprising: obtaining information about a distance of a sound source from the first and second microphones; andsetting the threshold value based on the distance.

3. A method according to claim 2, wherein obtaining information about the distance of a sound source from the first and second microphones comprises: determining a usage mode of a device comprising the first and second microphones.

4. A method according to claim 1, wherein the first and second microphones are spaced apart by a distance of 5-20 cm.

5. A system for detecting a replay attack in a speaker recognition system, the system comprising an input for receiving a speech signal from at least a first microphone and a second microphone, and comprising a processor, and being configured to implement the method according to claim 1.

6. A device comprising a system as claimed in claim 5.

7. A device as claimed in claim 6, wherein the device comprises a mobile telephone, an audio player, a video player, a mobile computing platform, a games device, a remote controller device, a toy, a machine, or a home automation controller or a domestic appliance.