SE2350726A1 - Determining when a portable key device is on a front side or back side based on sound signals - Google Patents

Abstract

ABSTRACT It is provided a method for determining when a portable key device (2, 2a, 2b) is located on a front side (14) or on a back side (16) in relation to a door (15). The method comprises: obtaining (42) a first sound signal of a first sound path (35a) from a speaker to a microphone, wherein one of the speaker and the microphone is mounted on the front side (14); obtaining (44) a second sound signal of a second sound path (35b) from a speaker to a microphone, wherein one of the speaker and the microphone is mounted on the back side (16); and determining (50) that the portable key device is located on the front side (14) or on the back side (16) based on which one of the first sound signal and the second sound signal is stronger.

Description

DETERMINING WHEN A PORTABLE KEY DEVICE IS ON A FRONT SIDE OR BACK SIDE BASED ON SOUND SIGNALS TECHNICAL FIELD

[0001] The present disclosure relates to the field of determining when a portable key device is on a front side or back side of a door, and in particular to determining when a portable key device is on a front side or back side based on sound signals, e.g. ultrasound.

BACKGROUND

[0002] Locks and keys are evolving from the traditional pure mechanical locks. These days, there are wireless interfaces for electronic locks, e.g. by interacting with a portable key device. For instance, Radio Frequency Identification (RFID) or Near-Field Communication (NFC) has been used as the wireless interface.

[0003] When RFID is used, the user needs to present the portable key device in close proximity to a reader connected to the lock. Hence, RFID typically requires a user to actively take out the portable key device (e.g. RFID card or NFC enabled smartphone) and bring it to be within centimetres of the lock. This is inconvenient and potentially insecure, since it is easier for an attacker to steal the portable key device when it is exposed and presented to the lock for the access control.

[0004] By using other short-range wireless communication with slightly longer communication range, e.g. Bluetooth Low Energy (BLE), Bluetooth or Ultra High Frequency (UHF), the portable key device can stay in a pocket or handbag and still be used for access control, sometimes denoted seamless access. However, the increased range also causes new problems. One problem that occurs if the lock unlocks whenever an authorised portable key device is within range, is that when a person on the inside of an electronic lock walks past the electronic lock, the electronic lock could unintentionally be unlocked, and anyone could gain access to the restricted physical space.

[0005] By determining the position of the portable key device in relation to the lock, it can be determined if the portable key device (and thus the user) is on the inside or the outside. In this way, the automatic access control can still be used, but a person walking past on the inside will not trigger the unlock procedure.

SUMMARY

[0006] One object is to improve how it is determined when a portable key device is on a front side or back side.

[0007] According to a first aspect, it is provided a method for determining when a portable key device is located on a front side or on a back side in relation to a door secured by an electronic lock. The method is performed by a location determiner and comprises: obtaining a first sound signal of a first sound path from a speaker to a microphone, wherein one of the speaker and the microphone, of the first sound path, is mounted on the front side of the door and the other forms part of the portable key device; obtaining a second sound signal of a second sound path from a speaker to a microphone, wherein one of the speaker and the microphone, of the second sound path, is mounted on the back side of the door and the other forms part of the portable key device; determining which one of the first sound signal and the second sound signal is stronger; and determining that the portable key device is located on the front side or on the back side in relation to the door based on which one of the first sound signal and the second sound signal is stronger.

[0008] The first sound path may be a path from a first speaker mounted on the front side to a microphone of the portable key device, in which case the second sound path is a path from a second speaker mounted on the back side to the microphone of the portable key device.

[0009] The first sound path may be a path from a device speaker of the portable key device to a first microphone mounted on the front side, in which case the second sound path is a path from the device speaker to a second microphone mounted on the back side.

[0010] The first sound signal and the second sound signal may be ultrasound signals. 3

[0011] The method may further comprise: authenticating the portable key device.

[0012] The method may further comprise: demodulating at least one of the first sound signal and/ second sound signal, yielding a demodulated sound signal. [0013] The method further comprise: verifying the demodulated sound signal.

[0014] The verifying the demodulated signal may comprise verifying the demodulated sound signal against data obtained during the authenticating.

[0015] According to a second aspect, it is provided a location determiner for determining when a portable key device is located on a front side or on a back side in relation to a door secured by an electronic lock. The location determiner comprises: processing circuitry; and a memory circuitry storing instructions that, when executed by the processing circuitry, cause the location determiner to: obtain a first sound signal of a first sound path from a speaker to a microphone, wherein one of the speaker and the microphone, of the first sound path, is mounted on the front side of the door and the other forms part of the portable key device; obtain a second sound signal of a second sound path from a speaker to a microphone, wherein one of the speaker and the microphone, of the second sound path, is mounted on the back side of the door and the other forms part of the portable key device; determine which one of the first sound signal and the second sound signal is stronger; and determine that the portable key device is located on the front side or on the back side in relation to the door based on which one of the first sound signal and the second sound signal is stronger.

[0016] According to a third aspect, it is provided a computer program for determining when a portable key device is located on a front side or on a back side in relation to a door secured by an electronic lock. The computer program comprises computer program code which, when executed on a location determiner causes the location determiner to: obtain a first sound signal of a first sound path from a speaker to a microphone, wherein one of the speaker and the microphone, of the first sound path, is mounted on the front side of the door and the other forms part of the portable key device; obtain a second sound signal of a second sound path from a speaker to a microphone, wherein one of the speaker and the microphone, of the second sound path, 4 is mounted on the back side of the door and the other forms part of the portable key device; determine which one of the first sound signal and the second sound signal is stronger; and determine that the portable key device is located on the front side or on the back side in relation to the door based on which one of the first sound signal and the second sound signal is stronger.

[0017] According to a fourth aspect, it is provided a computer program product comprising a computer program according to the third aspect and a computer readable means comprising non-transitory memory in which the computer program is stored.

[0018] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/ an /the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Aspects and embodiments are now described, by way of example, with refer- ence to the accompanying drawings, in which:

[0020] Fig 1 is a schematic diagram showing an electronics access control system being an environment in which embodiments presented herein can be applied;

[0021] Figs 2A-C are schematic diagrams illustrating embodiments of where the location determiner can be implemented;

[0022] Figs 3A-D are schematic top views of the environment of Fig 1, illustrating two embodiments for how sound paths are provided;

[0023] Fig 4 is a flow chart illustrating embodiments of methods for determining whether a portable key device is located on a front side or on a back side in relation to a door secured by an electronic lock;

[0024] Fig 5 is a schematic diagram illustrating components of the location determiner of Fig 1; and

[0025] Fig 6 shows one example of a computer program product comprising computer readable means.

DETAILED DESCRIPTION

[0026] The aspects of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. These aspects may, however, be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and to fully convey the scope of all aspects of invention to those skilled in the art. Like numbers refer to like elements throughout the description.

[0027] According to embodiments presented herein, sound signals, such as ultrasound, are used to determine which side of a door that a portable key device is located. The signal strengths of sound signals of two separate sound paths between mounted sound equipment (speakers or microphones) and a portable key device are compared where one (and only one) of the sound paths is line-of-sight. By comparing the signal strengths of the sound signals of the two sound paths, it can be determined which one represents the line-of-sight sound path, whereby it can be determined on which side that the portable key device is located. By using sound as the communication medium, local communication is implied. Furthermore, when ultrasound is used, the sound signalling is unnoticeable for any people in the vicinity.

[0028] Fig 1 is a schematic diagram showing an electronics access control system 10 being an environment in which embodiments presented herein can be applied. Access to a back side 16 of a door (and an electronic lock 12) is restricted by a door 15, which is selectively unlockable. The door 15 stands between the back side 16 and a front side 14. The back side 16 is restricted physical space and the front side 14 of the door 15 is an accessible physical space. However, note that the front side 14 can be a restricted physical space in itself, but in relation to this particular door 15, the front side 14 is accessible. In other words, the back side 16 is inside the door 15 and the front side 14 is outside the door 15. The door 15 can equally well be implemented as be, gate, hatch, window, etc. Optionally, a handle 17 or doorknob is provided to allow opening of the door 15, once unlocked. Alternatively or additionally, the door 15 is provided in the form of a revolving door or sliding door arrangement that can be operated using a motor and other components of a door opener/ door closer, in which case the user does not need to physically interact with the door to open it. In order to unlock or lock the door 15, an electronic lock 12 is provided. The electronic lock 12 can be in an unlocked state or locked state. The door 15 is provided in a surrounding fixed structure 11, such as a wall, fence, etc.

[0029] In the vicinity of the electronic lock 12, it is mounted at least two speakers and/ or one or at least two microphones. In particular, there can be one outside speaker 30a fixedly mounted on the front side 14 of the door 15 and the surrounding fixed structure 11, and one inside speaker 30b (not shown here, see Figs gA-B) mounted on the back side 16 of the door 15 and the surrounding fixed structure 11. Additionally or alternatively, there is one outside microphone 33a fixedly mounted on the front side 14 of the door 15 and the surrounding fixed structure 11, and one inside microphone 33b (not shown here, see Figs 3C-D) mounted on the back side 16 of the door 15 and the surrounding fixed structure 11.

[0030] Portable key devices 2a-b are provided in this scenario, where a first user 4a carries a first portable key device 2a and a second user 4b carrier a second portable key device 2b. The portable key device 2a-b can be carried or worn by the users and may be implemented as a smart phone, wearable device, key fob, etc. In this example, the first user 4a and the first portable key device 2a are located on the front side 14 of the location determiner 1 (i.e. outside the door 15 and the electronic lock 12) and the second user 4b and the second portable key device 2b are located on back side 16 of the location determiner (i.e. inside the door 15 and the electronic lock 12).

[0031] Depending on the location of the portable key device 2a, b, there can be line- of-sight for a sound path between a speaker/microphone mounted by the electronic lock 12 and corresponding microphone/ speaker of the portable key device 2a-b. In each sound path, there is speaker and a microphone. In the example of Fig 1, there is line of sight for a sound path between the outside speaker 30a/ outside microphone 33a and the 7 first portable key device 2a (on the outside 14), but not line-of-sight for a sound path between the outside speaker 30a/ outside microphone 33a and the second portable key device 2b (on the inside 16). Line-of-sight is to be interpreted as line of sight in terms of sound signal propagation. In other words, there may be a thin fabric cover or similar protecting any microphone or speaker. Furthermore, there can be relatively small objects around which sound signals can pass.

[0032] The front side 14 can be defined as the outside of the door, being a side of the door that provides a line-of-sight sound path with an outside portable key device and the back side 16 can be defined as the opposite side, i.e. the inside.

[0033] The location determiner 1 is used to determine when any portable key devices, such as a first portable key device 2a and a second portable key device 2b in the vicinity of the electronic lock 12 is located on the front side 14 or the back side 16 of the location determiner 1. This determination is based on sound signal strength obtained using hardware (speakers/microphones) that is mounted on both the front side 14 and the back side 16, for a sound path to / from the portable key device 2.

[0034] The electronic lock 12 is able to receive and send signals from /to portable key devices 2a-b over a communication channel which may be a short-range wireless interface, e.g. Bluetooth, BLE, UHF, ZigBee, any of the IEEE 802.11 standards, any of the IEEE 802.15 standards, etc. Optionally, the electronic lock 12 comprises a separate unit, also known as an access control reader, for communicating with the portable key devices 2a-b and evaluating access. The portable key devices can comprise digital cryptographic keys for the electronic authentication.

[0035] When the access control by the electronic lock 12 results in granted access, the electronic lock 12 is set in an unlocked state. When the electronic lock 12 is in the unlocked state, the barrier 15 can be opened and when the electronic lock 12 is in a locked state, the barrier 15 cannot be opened. In this way, access to the back side 16 of the location determiner 1 is controlled by the electronic lock 12. It is to be noted that the electronic lock 12 can be mounted in the fixed structure 11 by the physical barrier 15 (as shown) or in the physical barrier 15 itself (not shown). 8

[0036] The electronic lock 12 can perform an access control for any portable key device 2a-b presented to it. However, according to embodiments presented herein, access is only be granted after the portable key device 2a is determined to be on the front side 14 of the location determiner. The reason for this is that if access is granted by the electronic lock 12 whenever a portable key device is within communicable range, the second portable key device 2b (in the restricted physical space 16, i.e. on the inside) can result in the electronic lock 12 unlocking when the second user 4b walks by, without intending to unlock the electronic lock 12 to open the barrier. An unauthorised person on the front side 14 could then open the barrier 15 and gain access to the restricted physical space 16.

[0037] Figs 2A-C are schematic diagrams illustrating embodiments of where the location determiner 1 can be implemented.

[0038] In Fig 2A, the location determiner 1 is shown implemented in the electronic lock 12. The electronic lock 12 is thus the host device for the location determiner 1 in this implementation.

[0039] In Fig 2B, the location determiner 1 is shown implemented in a portable key device 2, e.g. being any one of the portable key devices 2a-b of Fig 1. The portable key device 2 is thus the host device for the location determiner 1 in this implementation.

[0040] In Fig 2C, the location determiner 1 is shown as implemented as a stand- alone device. The location determiner 1 thus does not have a host device in this implementation.

[0041] Figs 3A-D are schematic top views of the environment of Fig 1, illustrating two embodiments for how sound paths are provided. Both embodiments are based on providing a structure such that there are two speakers or two microphones respectively mounted on either side of the door, and a microphone or speaker provided in a portable key device 2, e.g. being any one of the portable key devices 2a-b of Fig 1.

[0042] Looking now to Fig 3A, there is a first speaker 30a mounted on the front side 14 and a second speaker 30b mounted on the back side 16. The speakers 30a-b can be mounted to the surrounding fixed structure 11 and/ or to the door 15. The portable key 9 device 2 comprises a microphone 32 that is capable of capturing sound signals coming from the speakers 30a-b, at least (for the purposes of embodiments presented herein) from one that is in line-of-sight and within a range that allows the sound to be captured.

[0043] When the portable key device 2 is on the front side 14, as illustrated in Fig 3A, there is line-of-sight on a first sound path 35a between the microphone 32 of the portable key device 2 and the first speaker 30a, but no line-of-sight on a second sound path 35b between the microphone 32 of the portable key device 2 and the second speaker 30b.

[0044] Conversely, as illustrated in Fig 3B, when the portable key device 2 is on the back side 16, there is a line-of-sight on the second sound path 35b between the microphone 32 of the portable key device 2 and the second speaker 30b, but no line-of- sight on the first sound path 35b between the microphone 32 of the portable key device 2 and the second speaker 30b.

[0045] Sound signals are captured both for the first sound path 35a and the second sound path 35b. The sound paths 35a-b can be distinguished from each other as described in more detail below.

[0046] The location determiner 1 determines which one of the sound signals that is stronger. When there is a significant difference in sound signal strength, i.e. more than a threshold difference, the sound path with the stronger sound signal strength is considered to indicate the side that exhibits a line-of-sight sound path to the portable key device 2. The portable key device 2 is then determined to be on the side that has the line-of-sight sound path.

[0047] Looking now to Fig 3C, there is here a first microphone 33a mounted on the front side 14 and a second microphone 33b mounted on the back side 16. The microphones can be mounted to the surrounding fixed structure 11 and/ or to the door 15. The portable key device 2 comprises a speaker 31 that is capable of transmitting sound to the microphones 33a-b, to at least (for the purposes of embodiments presented herein) speakers 30a-b, from one that is in line-of-sight and within a range that allows the sound to be captured. 1O

[0048] Fig 3A illustrates the situation when the portable key device 2 is located on the front side 14 and the first sound path 35a exhibits line-of-sight and the second sound path 35b is not line-of-sight. Conversely, Fig 3B illustrates the situation when the portable key device 2 is located on the back side 16 and the second sound path 35b exhibits line-of-sight and the first sound path 35a is not line-of-sight.

[0049] In analogy with the situation illustrated by Figs 3A-D, the line-of-sight sound path can be determined by comparing sound signal strengths, and the portable key device 2 can be determined to be on the side that exhibits line-of-sight.

[0050] Fig 4 is a flow chart illustrating embodiments of methods for determining whether a portable key device is located on a front side 14 or on a back side 16 in relation to a door 15 secured by an electronic lock 12. These embodiments are based on sound signals. The sound signals can be ultrasound signals, in which case the sound signals are unnoticeable for any people in the vicinity. The ultrasound can e.g. be in the range of 18-20 kHz, which modern smartphones are capable of transmitting and/ or capturing.

[0051] In an optional authenticate step 40, the location determiner 1 authenticates the portable key device. This can e.g. be performed over a local wireless connection, such as Bluetooth, BLE, UHF, Wi-Fi, etc., that implicitly (or explicitly) indicates that the portable key device 2 is in the vicinity of the electronic lock 12, whereby it is feasible to determine side of the portable key device 2 based on sound signals.

[0052] In an obtain first sound signal step 42, the location determiner 1 obtains a first sound signal of a first sound path 35a from a speaker to a microphone. One (and only one) of the speaker and the microphone, of the first sound path 35a, is mounted on the front side 14 of the door 15. The other (of the speaker and the microphone of the first sound path 35a) forms part of the portable key device. In this way, the first sound path 35a is line-of-sight when the portable key device 2 is on the front side 14 of the door 15.

[0053] In an obtain second sound signal step 44, the location determiner 1 obtains a second sound signal of a second sound path 35b from a speaker to a microphone. One (and only one) of the speaker and the microphone, of the second sound path 35b, is 11 mounted on the back side 16 of the door 15. The other (of the speaker and the microphone of the first sound path 35a) forms part of the portable key device. In this way, the second sound path 35a is line-of-sight when the portable key device 2 is on the back side 16 of the door 15.

[0054] The first sound signal and the second sound signal are based on analogue sound signals that are sampled and converted to digital sound signals using an analogue-to-digital (A/ D) converter. The strength of each one of the first sound signal and the second sound signal can be obtained by evaluating the amplitude of the digital signal in the digital domain, or by measuring signal strength in the analogue domain and converting the measured signal strength to a digital equivalent. It is to be note that in steps 42 and 44, either one of the first sound signal and the second sound signal can be of such low strength that the captured sound signal is indistinguishable from ambient noise.

[0055] In one embodiment, as illustrated in Figs 3A-B and described above, the first sound path 35a is a path from a first speaker 30a mounted on the front side 14 to a microphone 32 of the portable key device 2. In this case, the second sound path 35b is a path from a second speaker 30b mounted on the back side 16 to the microphone 32 of the portable key device.

[0056] Alternatively, as illustrated in Figs 3C-D and described above, the first sound path 35a is a path from a device speaker 31 of the portable key device 2 to a first microphone 33a mounted on the front side 14, and the second sound path 35b is a path from the device speaker 31 to a second microphone 33b mounted on the back side 16.

[0057] The two sound paths 35a-b can be distinguished from each other e.g. based on different timing or modulation of the sounds coming from the two speakers 30a-b. For instance, sounds transmitted from the two speakers 30a-b or received by the two microphones 33a-b can be scheduled at different times. Alternatively or additionally, sounds from for the two sound paths 35a-b can be transmitted at different frequencies, or can be modulated differently. The modulation can be any suitable sound modulation, including time-based modulation and/ or frequency-based modulation. 12

[0058] In an optional demodulate step 45, the location determiner 1 demodulates at least one of the first sound signal and/ second sound signal, yielding a demodulated sound signal.

[0059] In an optional verifg the demodulated sound signal step 46, the location determiner 1 verifies the demodulated sound signal. In one embodiment, this comprises verifying the demodulated sound signal against data obtained during the authentication. In one embodiment, the verification is comparison against a pre-defined time and/ or frequency pattern that can be associated with the speaker carrying device (either mounted by the electronic lock 12 or the portable key device 2).

[0060] Using the verification, an additional layer of authentication is achieved, using a separate communication medium. Hence, even if an attacker somehow were to trick the authentication to approve access, the attacker would also need to be able to trick or circumvent the sound-based verification, which greatly increases the hurdle of an attacker to gain physical access to the back side 16.

[0061] In a determine stronger sound signal step 48, the location determiner 1 determines which one of the first sound signal and the second sound signal is stronger.

[0062] In a determine side step 50, the location determiner 1 determines that the portable key device is located on the front side 14 or on the back side 16 in relation to the door 15 based on which one of the first sound signal and the second sound signal is stronger. When the first sound signal is stronger, this indicates that there is line-of-sight on the front side 14 over the first sound path, i.e. the portable key device 2 is on the front side 14. When the second sound signal is stronger, this indicates that there is line- of-sight on the back side 16 over the second sound path, i.e. the portable key device 2 is on the back side 16.

[0063] Optionally, after the determine side step 50, there is a separate intent determination, e.g. based on a time-of-flight (ToF) sensor, image sensor, lidar, etc.

[0064] Fig 5 is a schematic diagram illustrating components of the location determiner 1 of Fig 1. It is to be noted that when the location determiner 1 is implemented in a host device, one or more of the mentioned components can be shared 13 with the host device. Processing circuitry 60 is provided using any combination of one or more of a suitable central processing unit (CPU), graphics processing unit (GPU), multiprocessor, neural processing unit (NPU), microcontroller, digital signal processor (DSP), etc., capable of executing software instructions 67 stored in memory circuitry 64, which can thus be a computer program product. The processing circuitry 60 could alternatively be implemented using an application specific integrated circuit (ASIC), field programmable gate array (FPGA), etc. The processing circuitry 60 can be configured to execute the method described with reference to Fig 4 above.

[0065] The memory circuitry 64 can be any combination of random-access memory (RAM) and/ or read-only memory (ROM). The memory circuitry 64 also comprises non- transitory persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid-state memory or even remotely mounted memory.

[0066] A data memory 66 is also provided for reading and/ or storing data during execution of software instructions in the processing circuitry 60. The data memory 66 can be any combination of RAM and/ or ROM.

[0067] Fig 6 shows one example of a computer program product 90 comprising computer readable means. On this computer readable means, a computer program 91 can be stored in a non-transitory memory. The computer program can cause processing circuitry to execute a method according to embodiments described herein. In this example, the computer program product 90 is in the form of a removable solid-state memory, e.g. a Universal Serial Bus (USB) drive. As explained above, the computer program product could also be embodied in a memory of a device, such as the computer program product 64 of Fig 5. While the computer program 91 is here schematically shown as a section of the removable solid-state memory, the computer program can be stored in any way which is suitable for the computer program product, such as another type of removable solid-state memory, or an optical disc, such as a CD (compact disc), a DVD (digital versatile disc) or a Blu-Ray disc.

[0068] The aspects of the present disclosure have mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled 14 in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims. Thus, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims.

Claims (11)

1. A method for determining when a portable key device (2, 2a, 2b) is located on a front side (14) or on a back side (16) in relation to a door (15) secured by an electronic lock (12), the method being performed in a location determiner (1) and comprising: obtaining (42) a first sound signal of a first sound path (35a) from a speaker to a microphone, wherein one of the speaker and the microphone, of the first sound path (35a), is mounted on the front side (14) of the door (15) and the other forms part of the portable key device (2, 2a, 2b); obtaining (44) a second sound signal of a second sound path (35b) from a speaker to a microphone, wherein one of the speaker and the microphone, of the second sound path (35b), is mounted on the back side (16) of the door (15) and the other forms part of the portable key device (2, 2a, 2b); determining (48) which one of the first sound signal and the second sound signal is stronger; and determining (50) that the portable key device is located on the front side (14) or on the back side (16) in relation to the door (15) based on which one of the first sound signal and the second sound signal is stronger.

2. The method according to claim 1, wherein the first sound path (35a) is a path from a first speaker (3oa) mounted on the front side (14) to a microphone (32) of the portable key device (2), and the second sound path (35b) is a path from a second speaker (3ob) mounted on the back side (16) to the microphone (32) of the portable key device.

3. The method according to claim 1, wherein the first sound path (35a) is a path from a device speaker (31) of the portable key device (2) to a first microphone (33a) mounted on the front side (14), and the second sound path (35b) is a path from the device speaker (31) to a second microphone (33b) mounted on the back side (16).

4. The method according to any one of the preceding claims, wherein the first sound signal and the second sound signal are ultrasound signals.

5. The method according to any one of the preceding claims, further comprising: authenticating (40) the portable key device (2a, 2b).

6. The method according to any one of the preceding claims, further comprising: demodulating (45) at least one of the first sound signal and/ second sound signal, yielding a demodulated sound signal.

7. The method according to claim 6, further comprising: verifying (46) the demodulated sound signal.

8. The method according to claim 7, when dependent on claim 5, wherein the verifying (46) the demodulated signal comprises verifying the demodulated sound signal against data obtained during the authenticating.

9. A location determiner (1) for determining when a portable key device (2a, 2b) is located on a front side (14) or on a back side (16) in relation to a door (15) secured by an electronic lock (12), the location determiner (1) comprising: processing circuitry (6o); and a memory circuitry (64) storing instructions (67) that, when executed by the processing circuitry, cause the location determiner (1) to: obtain a first sound signal of a first sound path (35a) from a speaker to a microphone, wherein one of the speaker and the microphone, of the first sound path (35a), is mounted on the front side (14) of the door (15) and the other forms part of the portable key device (2, 2a, 2b); obtain a second sound signal of a second sound path (35b) from a speaker to a microphone, wherein one of the speaker and the microphone, of the second sound path (35b), is mounted on the back side (16) of the door (15) and the other forms part of the portable key device (2, 2a, 2b); determine which one of the first sound signal and the second sound signal is stronger; and determine that the portable key device is located on the front side (14) or on the back side (16) in relation to the door (15) based on which one of the first sound signal and the second sound signal is stronger. 1o.

A computer program (67, 91) for determining when a portable key device (2a, 2b) is located on a front side (14) or on a back side (16) in relation to a door (15) secured by an electronic lock (12), the computer program comprising computer program code which, when executed on a location determiner (1) causes the location determiner (1) to:obtain a first sound signal of a first sound path (35a) from a speaker to a microphone, wherein one of the speaker and the microphone, of the first sound path (35a), is mounted on the front side (14) of the door (15) and the other forms part of the portable key device (2, 2a, 2b); obtain a second sound signal of a second sound path (35b) from a speaker to a microphone, wherein one of the speaker and the microphone, of the second sound path (35b), is mounted on the back side (16) of the door (15) and the other forms part of the portable key device (2, 2a, 2b); determine which one of the first sound signal and the second sound signal is stronger; and determine that the portable key device is located on the front side (14) or on the back side (16) in relation to the door (15) based on which one of the first sound signal and the second sound signal is stronger.

11. A computer program product (64, 90) comprising a computer program according to claim 10 and a computer readable means comprising non-transitory memory in which the computer program is stored.