Patent Description:
When establishing cryptographically secured, that is, encrypted, communications between two parties, it is necessary to arrange for the parties to share an encryption key in case symmetric encryption is used. In symmetric encryption, the same key is used for both encrypting and decrypting, whereas in public-key encryption, a public key is used for encrypting and an associated private key is used for decrypting. As the public key may be sent over an unsecured channel, establishing secured communications using public key encryption is easy, albeit public-key based cryptosystems are computationally more intensive than symmetric-key cryptosystems.

In general, a shared secret known to both parties may be used as a symmetric encryption key, or in deriving a symmetric encryption key. Such a shared secret may be established using out-of-band communications, such as a courier, or by using a cryptographic key exchange over a non-trusted communication channel. A Diffie-Hellman key exchange is an example of a cryptographic key exchange which may be used to establish a shared secret between two parties over a non-trusted communication channel. <CIT> discloses techniques for encrypting a data signal. A chirp signal is transmitted within a frequency band of interest from which a group delay variation (GDV) is determined, from which an encryption key is generated.

According to some aspects, there is provided the subject-matter of the independent claims. Some embodiments are defined in the dependent claims. The embodiments, examples and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention.

According to a first aspect of the present disclosure, there is provided an apparatus as claimed in claim <NUM>.

According to a second aspect of the present disclosure, there is provided a method as claimed in claim <NUM>.

According to a third aspect of the present disclosure, there is provided an apparatus as claimed in claim <NUM>.

Methods are described herein which employ wireless communication channel definition information to derive a shared secret between communication parties. In particular, channel definition information of a wireless communication channel other than one used in communication may be employed. The channel definition information may comprise a channel frequency response, for example, or a channel impulse response, or a parameter related to, or derivable from a model of said channels. For example, a channel definition may be measured concerning a wireless communication channel in a first frequency band and a channel definition may be obtained concerning a wireless communication channel on a second frequency band. The channel definition on the second frequency band may be either measured, or it may be calculated using information on correlation between communication channels on the first and second frequency bands. The channel definition obtained on the second frequency band may then be used as keying material, or to generate the keying material, optionally together with the channel information on the first frequency band. This provides a benefit in terms of communication security, as the first and second frequency bands differ in frequency, and the keying material obtained using the channel definition of the second frequency band, which is on a higher frequency, enhances the randomness, or security, of the keying material as it relates to a location of the user equipment device. It is noted that the term randomness is used herein as a description of information that at least appears to exhibit random-like properties. Thus random may be, in practice, pseudo-random. Wireless communication channels on a higher frequency vary faster as a function of place than do channels on lower frequencies. In addition, a potential eavesdropper would need to know the characteristics of the higher frequency channel to compromise security of the herein disclosed system. If that information is agreed only between the intended transmitter and receivers, an additional layer of security results.

<FIG> illustrates an example system in accordance with at least some embodiments of the present invention. The example of <FIG> is a cellular system, but the herein disclosed methods are not limited to being applied in a cellular context. <FIG> illustrates a base station <NUM>, which is configured to operate in accordance with a cellular communication standard, such as long term evolution, LTE, or fifth generation, <NUM>, also known as New Radio, NR, both as specified by the <NUM>rd generation partnership project, 3GPP. Where a non-cellular system is used, an access node, such as access point, corresponding to base station <NUM> may be configured in accordance with a non-cellular communication standard such as wireless local area network, WLAN, or worldwide interoperability for microwave access, WiMAX, for example.

Base station <NUM> is coupled with a core network node <NUM> via link <NUM>, which may comprise a wire-line connection, for example. Core network node <NUM> may comprise a mobility management entity, MME, a serving gateway, S-GW, or an access and mobility management function, AMF, for example. The core network may comprise a gateway <NUM>, connected to core network node <NUM> via link <NUM>. Gateway <NUM> enables communication with further networks <NUM>, via inter-network link <NUM>. In non-cellular systems, core network node <NUM> and gateway <NUM> may be absent, with the access node corresponding to base station <NUM> being directly connected to further networks, for example. Link <NUM> and inter-network link <NUM> may be wire-line links, for example.

Further, in the illustrated example situation, base station <NUM> is in wireless radio communication with user equipments, UEs <NUM> and <NUM>. Each UE may comprise, for example, a smartphone, feature phone, tablet or laptop computer, Internet-of-Things, IoT, node, smart wearable or a connected car connectivity module, for example. Naturally, separate UEs need not be of a same type. Wireless communication channel <NUM> connects base station <NUM> with UE <NUM>, and wireless communication channel <NUM> connects base station <NUM> with UE <NUM>.

Radio communication between UEs and their serving base station may be encrypted to enhance privacy of communicated information. Where a multi-layer protocol stack is used, different layers of the stack may use different encryption mechanisms. In general, a physical layer is the lowest layer in a protocol stack. Data encrypted in a physical layer encryption mechanism may be decrypted in the base station and UE, respectively, for uplink and downlink communication. In some cases, separate application-layer encryption may be used on top of, and in addition to, physical-layer encryption to obtain end-to-end encryption between communication endpoints.

Encryption mechanisms for the physical layer benefit from being quick and light-weight to implement, since the physical layer needs to be established every time communications are initialized from a UE toward the network. For example some IoT-type UEs may transmit only short packets, wherefore a complex key agreement protocol would add, in relative terms, a lot of overhead, reducing also the energy efficiency of the communication.

Physical-layer key generation, PKG, establishes cryptographic keys from highly correlated measurements of wireless channels, which relies on reciprocal channel characteristics between the uplink and downlink - or similarly between any two communication parties, including sidelink or direct device-to-device communication. Within PKG, secure key generation may depend on three principles: channel reciprocity, spatial decorrelation and temporal variations. Channel reciprocity indicates that the same, or similar, channel characteristics can be observed at both ends of the same communication channel, which forms the basis for generating cryptographic keying material for encryption key generation. In case of bidirectional communication, for time-division duplexing, TDD, systems, both the uplink and downlink are in the same carrier frequency band, and the channel responses obtained by a base station and a UE are reciprocal, generating the same or similar channel definition information. However, for frequency division duplexing, FDD, systems, the uplink and downlink transmit using different carrier frequencies, and the uplink and downlink experience dis-identical fading behaviour. Key generation for these FDD-based systems will provide information-theoretically secure keys for them, and hence it is preferable. Examples of FDD key generation mechanisms are described in [<NUM>] and [<NUM>]. In FDD, it should also be recognized that the UE and base station need not determine exactly the same channel in detail, and long as they determine the same channel definition information, which may be coarser in nature than the specific channel estimation result. For example, the obstacles and distance are the same in UL and DL, even if the used frequency may differ somewhat, enabling determination of sufficiently similar channel definition information, that a shared secret may be achieved based on it.

Herein are described methods for key generation where the inherent randomness of fading wireless radio channels between two parties is utilized in a novel way to generate keys. <FIG> illustrates phases of a process of generating cryptographic keying material for encryption key derivation.

<FIG> illustrates an example process in accordance with at least some embodiments of the present invention. In the process of <FIG>, initially, in phase <NUM>, channel probing is performed to measure a channel definition of a wireless communication channel. Then, phase <NUM>, randomness is extracted from results of the channel probing of phase <NUM>. After phase <NUM>, the process advances to phase <NUM>, where channel probing results of a wireless communication channel on a second frequency band, different and of a higher frequency range than that used in phase <NUM>, are obtained and randomness is extracted from these results. In optional phase <NUM>, quantization is applied to the output of phase <NUM>, and in phase <NUM> the obtained cryptographic keying material is formatted for input to phase <NUM>. In phase <NUM>, at least one encryption key is derived from the keying material. The second frequency band may have a lower bound at a higher frequency than an upper bound of the first frequency band. In other words, the lowest-frequency end of the second frequency band may be a higher frequency than a highest-frequency end of the first frequency band.

Key derivation may rely on exploiting channel definition similarities between a first and a second frequency band. The first and second frequency bands may be, for example, on frequency range <NUM>, FR1, and frequency range <NUM>, FR2, respectively, as defined by 3GPP. Therein FR1 is defined as under <NUM>, and FR2 is defined as being of at least <NUM>. In general a frequency of a second wireless communication channel, used in addition to a first wireless communication channel, may have a centre frequency of at least two, three or four times the centre frequency of the first wireless communication channel.

As noted above, a channel definition of a wireless communication channel may comprise a channel frequency response or a channel impulse response, for example. A frequency response of a channel spans the same bandwidth as the stimulus signal measured on the wireless communication channel to determine the frequency response, and the frequency response is a discrete fourier transform of the channel impulse response. The impulse response is the response of the wireless communication channel to a brief input signal, revealing e.g. reflections in the channel. It is noted that other definitions of channel are also viable. For example, a channel may be defined also as a mixture of its temporal and frequency domain characteristics. Mappings may exist between different channel definitions, for example, it may be possible to obtain a second channel definition from a first channel definition using a suitable mapping. For example, time of arrival, ToA, of the second channel definition may be obtained from ToA of the first channel definition. A channel impulse response of a second channel definition may, in some embodiments, be calculated from a first channel definition channel impulse response using a mapping. For example, the mapping may comprise a correlation function, employ location-based information, and/or be a location-specific mapping function which enables an accurate obtaining of the second channel definition information from the first channel definition information.

In terms of <FIG>, two UEs, <NUM> and <NUM>, are located in two different locations and a base station, or access node, covers both locations using a transmission on the first frequency band, which may lie in FR1, for example. The lower location correlation of the higher-frequency second frequency band, which may lie in FR2, for example, is used in both locations to generate two different keys in each UE for its physical layer encryption.

The herein described process leverages the assumption that base station <NUM> and UEs can, or could, work with both the first and the second frequency band, for example on FR1 and FR2, as described above. Initially, UEs <NUM> and <NUM> receive data packets over wireless communication channels <NUM> and <NUM>, respectively, on the first frequency band. In this phase, channel estimation procedures are performed by the UEs on the first frequency band to derive channel definitions. Based on the distance between UEs <NUM> and <NUM>, the degree of similarity in channel definition data varies. In general lower frequencies are associated with longer wavelengths, wherefore the channel changes slower as a function of location than with channels on higher frequencies. For encryption key derivation, channel definition information obtained from the first frequency band may be converted into channel definition information of the second frequency band by exploiting a previously determined spatial/time correlation function between wireless communication channels on the first and second frequency bands at each terminal location. An encryption key, for example for use on the physical layer of a protocol stack, may be obtained from the channel definition information of a wireless communication channel on the second frequency band. In comparison with channel definition information of wireless communication channels on the first frequency band, channels definition information of wireless communication channels on the second frequency band exhibits less spatial correlation with respect to different locations due to its higher frequency, wherefore it is almost certain to have significantly different derived encryption keys in each UE.

Alternatively to converting the channel definition information from the first frequency band to the second frequency band using the previously determined correlation, the UEs may be configured to measure the channel definition information on the second frequency band. To enable this, the base station may be configured to transmit a signal on the second frequency band for the UE to measure.

In general, in 3GPP networks FR1 links are used for coverage and cell-edge scenarios, whereas FR2 is more preferable for high-capacity deployments and load balancing purposes. From this perspective, different priorities may be assigned to each frequency range, and UEs may carry out radio resource management, RRM, measurements when there is a measurement gap for other frequencies.

It has been observed that there exists high correlation in key multipath statistics between FR1 and FR2, such that angle of arrival, direction of arrival, time of arrival and angle of departure in both line-of-sight and non-line-of-sight scenarios [<NUM>]. This correlation has been used for optimal transmit/receive beam-pair selection in configuration of FR2 links and base station discovery. Further, a deep learning-based approach for beam selection was proposed in [<NUM>] where sub-<NUM> link information is utilized in a deep-learning artificial neural network, enabling a reduction in beam sweeping overhead by <NUM>%. In most frameworks, however, FR1 and FR2 connections are considered separate, standalone solutions. These documents thus describe ways of determining the correlation between channel definitions of wireless communication channels on different frequency bands. In other words, these documents describe ways of exploiting the features of different wireless communications on different frequency bands for generating cryptographic keys. In at least some embodiments of the present disclosure, availability of multiple communication channels is utilized for providing encryption between two communication parties. The encryption may be obtained in transition between time, frequency and location-based features of two different communication channels.

To derive the encryption key, for example for the physical layer, UEs <NUM> and <NUM> of <FIG> estimate their respective channel definitions, such as channel state information, CSI, or channel frequency response, from receiving data frames from base station <NUM> via respective first communication channels on the first frequency band, which is on a lower frequency than the second frequency band. If a network controller transmits a signal x(t) to UEs <NUM> and <NUM> on the first frequency band, the received signals Y at these UEs may be defined, respectively, as:
<MAT>
<MAT>
where H<NUM>,<NUM>(f) and H<NUM>,<NUM>(f) denote the respective channel frequency responses, and W<NUM> and W<NUM> are the respective zero-mean additive white Gaussian noises, AWGN. The channel frequency responses, H<NUM>,<NUM> and H<NUM>,<NUM>, are estimated in the respective UEs by decoding a signal field of the received packets. Once the channel estimation procedure is completed for H<NUM>,<NUM> at UE <NUM>, Ĥ<NUM>,<NUM> may be obtained. Then based on the mapping capability/availability at UE <NUM>, UE <NUM> can generate Ĥ<NUM>,<NUM>. For example, a mapping capability based on a correlation between H<NUM>,<NUM> and H<NUM>,<NUM> may yield the following relation between Ĥ<NUM>,<NUM> and Ĥ<NUM>,<NUM>:
<MAT>.

) is the known or learned mapping function between the wireless communication channels, more specifically the channel definitions of the communication channels, at UE <NUM>. Then, UE <NUM> may follow the key generation procedure given in <FIG>. A similar procedure can be followed in UE <NUM>, after estimating Ĥ<NUM>,<NUM>, UE <NUM> can either utilize generate a channel definition on the second frequency band based on its own correlation, such that
<MAT>.

Note that in the context of higher frequencies, in general f(. ) are different functions since UEs <NUM> and <NUM> are located at a distance greater than the coherence distance of wireless communication channels on the second frequency band, e.g. FR2. In other words, each UE may select a function, such as f(. ), based on its location. To this end, each UE may store a plurality of functions and a mapping from a set of UE locations to the set of functions, to enable selecting the correct function for the current UE location. The UE may obtain the functions by storing them based on its earlier measurements while moving about, and/or the UE may receive such functions from other network elements. For example, UEs moving in a coverage area of a cell may determine the functions and report them to a base station controlling the cell, and the base station may then share the function(s) to other UEs as they enter the cell, and/or based on need, for example based on where in the cell the other UE(s) happen to be. In some cases, instead of using a function, the UE may have previously stored information which defines the channel definition information for the second-frequency band channel. The base station may be used to share such channel definition information between UEs as is described above for the functions. Then, UE <NUM> may follow the key derivation procedures given in <FIG>, where the second-frequency band channel definition information is utilized.

Then UE <NUM> can transmit its signal back to base station <NUM>, or another device, after deriving an encryption key from Ĥ<NUM>,<NUM> and UE <NUM> should not be successful in obtaining this secret key due to the decorrelated wireless communication channels in the second frequency band.

The herein described encryption key derivation mechanism provides several advantages. In detail, cases are addressed where the channel coherence time is short such that reciprocity between uplink and downlink is weak. That is, when base station <NUM> cannot apply precoding to the transmitted signal based on uplink channel estimates. In cases where the eavesdropper is co-located, or almost so, with UE <NUM> or UE <NUM>, he will observe the channel measurements that are highly correlated, such that the communication is vulnerable to attacks. By using the proposed mechanism, utilizing the higher-frequency second frequency band can produce different keys due to narrower spatial correlation.

A further advantage lies in static devices. Many IoT devices, such as unattended devices, may be stationary and the wireless communication channel randomness is limited for this reason. From this aspect, generating the key by utilizing the higher-frequency second frequency band can produce different keys due to narrower spatial correlation.

The herein disclosed method is also suitable for devices that likely cannot support a full encryption key-based security protocol, such as one knows from current 3GPP standards. Such devices may include passive IoT energy-harvesting devices that may have very limited battery storage, or even no electrical energy storage at all. In such cases the physical layer security may be sufficient, or even the only option.

<FIG> illustrates an example apparatus capable of supporting at least some embodiments of the present invention. Illustrated is device <NUM>, which may comprise, for example, a mobile communication device such as a UE or, in applicable parts, a base station of <FIG>. Comprised in device <NUM> is processor <NUM>, which may comprise, for example, a single- or multi-core processor wherein a single-core processor comprises one processing core and a multi-core processor comprises more than one processing core. Processor <NUM> may comprise, in general, a control device. Processor <NUM> may comprise more than one processor. When processor <NUM> comprises more than one processor, device <NUM> may be a distributed device wherein processing of tasks takes place in more than one physical unit. Processor <NUM> may be a control device. A processing core may comprise, for example, a Cortex-A8 processing core manufactured by ARM Holdings or a Zen processing core designed by Advanced Micro Devices Corporation. Processor <NUM> may comprise at least one Qualcomm Snapdragon and/or Intel Atom processor. Processor <NUM> may comprise at least one application-specific integrated circuit, ASIC. Processor <NUM> may comprise at least one field-programmable gate array, FPGA. Processor <NUM> may be means for performing method steps in device <NUM>, such as determining, deriving, using, communicating, encrypting and decrypting. Processor <NUM> may be configured, at least in part by computer instructions, to perform actions.

A processor may comprise circuitry, or be constituted as circuitry or circuitries, the circuitry or circuitries being configured to perform phases of methods in accordance with embodiments described herein. As used in this application, the term "circuitry" may refer to one or more or all of the following: (a) hardware-only circuit implementations, such as implementations in only analogue and/or digital circuitry, and (b) combinations of hardware circuits and software, such as, as applicable: (i) a combination of analogue and/or digital hardware circuit(s) with software/firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or base station, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

Memory <NUM> may be non-transitory. The term "non-transitory", as used herein, is a limitation of the medium itself (that is, tangible, not a signal) as opposed to a limitation on data storage persistency (for example, RAM vs. ROM).

Device <NUM> may comprise a transmitter <NUM>. Device <NUM> may comprise a receiver <NUM>. Transmitter <NUM> and receiver <NUM> may be configured to transmit and receive, respectively, information in accordance with at least one cellular or non-cellular standard. Transmitter <NUM> may comprise more than one transmitter. Receiver <NUM> may comprise more than one receiver. Transmitter <NUM> and/or receiver <NUM> may be configured to operate in accordance with global system for mobile communication, GSM, wideband code division multiple access, WCDMA, <NUM>, long term evolution, LTE, IS-<NUM>, wireless local area network, WLAN, Ethernet and/or worldwide interoperability for microwave access, WiMAX, standards, for example.

Device <NUM> may comprise user interface, UI, <NUM>. UI <NUM> may comprise at least one of a display, a keyboard, a touchscreen, a vibrator arranged to signal to a user by causing device <NUM> to vibrate, a speaker and a microphone. A user may be able to operate device <NUM> via UI <NUM>, for example to accept incoming telephone calls, to originate telephone calls or video calls, to browse the Internet, to manage digital files stored in memory <NUM> or on a cloud accessible via transmitter <NUM> and receiver <NUM>, or via NFC transceiver <NUM>, and/or to play games.

Device <NUM> may comprise or be arranged to accept a user identity module <NUM>. User identity module <NUM> may comprise, for example, a subscriber identity module, SIM, card installable in device <NUM>. A user identity module <NUM> may comprise information identifying a subscription of a user of device <NUM>. A user identity module <NUM> may comprise cryptographic information usable to verify the identity of a user of device <NUM> and/or to facilitate encryption of communicated information and billing of the user of device <NUM> for communication effected via device <NUM>.

Device <NUM> may comprise further devices not illustrated in <FIG>. For example, where device <NUM> comprises a smartphone, it may comprise at least one digital camera. Some devices <NUM> may comprise a back-facing camera and a front-facing camera, wherein the back-facing camera may be intended for digital photography and the front-facing camera for video telephony. Device <NUM> may comprise a fingerprint sensor arranged to authenticate, at least in part, a user of device <NUM>. In some embodiments, device <NUM> lacks at least one device described above. For example, some devices <NUM> may lack a NFC transceiver <NUM> and/or user identity module <NUM>.

Processor <NUM>, memory <NUM>, transmitter <NUM>, receiver <NUM>, NFC transceiver <NUM>, UI <NUM> and/or user identity module <NUM> may be interconnected by electrical leads internal to device <NUM> in a multitude of different ways. For example, each of the aforementioned devices may be separately connected to a master bus internal to device <NUM>, to allow for the devices to exchange information. However, as the skilled person will appreciate, this is only one example and depending on the embodiment various ways of interconnecting at least two of the aforementioned devices may be selected without departing from the scope of the present invention.

<FIG> illustrates signalling in accordance with at least some embodiments of the present invention. On the vertical axes are disposed, from the left to the right, base station <NUM>, UE <NUM> and UE <NUM> of <FIG>. Time advances from the top toward the bottom.

In phases <NUM> and <NUM>, respectively, UEs <NUM> and <NUM> determine to initiate connectivity toward the network. They transmit an initial message to base station <NUM> in phases <NUM> and <NUM>, respectively, informing base station <NUM> of their intent to initiate connectivity.

To initialize physical-layer encryption between the UEs and base station <NUM>, the UEs and base station <NUM> will use characteristics of wireless communication channels between the base station and each UE, separately. As the UEs are not in the same location, they will have distinct wireless communication channels connecting them to the base station.

In phase <NUM>, UE <NUM> receives a signal, or signals, from base station <NUM>. Likewise in phase <NUM>, UE <NUM> receives a signal, or signals, from base station <NUM>. The signal, or signals, received in the UEs may be the same ones, in case they are broadcasted from base station <NUM>, or they may be separate signals addressed separately to the two UEs. In phase <NUM>, UE <NUM> measures a channel definition of a wireless channel connecting it to base station <NUM>, and in phase <NUM> UE <NUM> does the same with a wireless channel connecting UE <NUM> to base station <NUM>. Thus both UEs will be in possession of channel definitions describing wireless communication channels they have with base station <NUM>. As the communication channels are not the same, the channel definitions are not the same either, however if the UEs are close to each other, the channel definitions may have some similarities.

In phase <NUM>, UE <NUM> derives channel definition information of a second wireless communication channel between itself and base station <NUM>, the second wireless communication channel being on a frequency band that is higher than the frequency band used in phase <NUM>. As described herein above, this may comprise measuring the channel definition information of the second wireless communication channel, or obtaining this channel definition by using a pre-determined correlation function and the channel definition measured in phase <NUM>. In phase <NUM>, UE <NUM> does likewise with its second wireless communication channel, either measuring the channel definition of the second wireless communication channel connecting UE <NUM> to base station <NUM> on the second frequency band, or deriving it using a pre-determined correlation function.

Both UEs use the channel definition of their respective wireless communication channels with base station <NUM> on the second frequency band to derive keying material and respective encryption keys. Since the second frequency band is on a higher frequency, the channel definitions on that frequency band resemble each other less than those on the first frequency band, resulting in keying material and encryption keys which exhibit higher randomness between the UEs, than in the case that only channel definitions regarding wireless communication channels on the first frequency band would have been used.

Finally, in phase <NUM> UE <NUM> communicates with base station <NUM> using a protocol stack, such that the encryption key derived in phase <NUM> using keying material obtained from the channel definition of the communication channel on the second frequency band is used. UE <NUM> does likewise in phase <NUM>, using the encryption key it derived in phase <NUM>, using keying material obtained from the channel definition of the communication channel on the second frequency band is used.

<FIG> is a flow graph of a method in accordance with at least some embodiments of the present invention. The phases of the illustrated method may be performed in UE <NUM> or base station <NUM>, for example, or in a control device configured to control the functioning thereof, when installed therein.

Phase <NUM> comprises determining, in an apparatus, a first channel definition describing a first wireless communication channel between the apparatus and a second apparatus, the first wireless communication channel being on a first frequency band. Phase <NUM> comprises determining a second channel definition describing a second wireless communication channel between the apparatus and the second apparatus, the second wireless communication channel being on a second frequency band which has a lower bound at a higher frequency than an upper bound of the first frequency band. Finally, phase <NUM> comprises deriving an encryption key at least partly based on the second channel definition, and using the encryption key to encrypt information before it is transmitted to the second apparatus over the first wireless communication channel. Also the first channel definition may be used in the deriving of the encryption key. The upper bound of the first frequency band may be six gigahertz while the lower bound of the second frequency band may be twenty-four gigahertz.

At least some embodiments of the present invention find industrial application in securing wireless communications.

Claim 1:
An apparatus comprising means for:
- determining a first channel definition comprising a channel frequency response or a channel impulse response, the first channel definition describing a first wireless communication channel between the apparatus and a second apparatus, the first wireless communication channel being on a first frequency band, from data packets received on the first frequency band;
- measuring a second channel definition comprising a channel frequency response or a channel impulse response describing a second wireless communication channel between the apparatus and the second apparatus, the second wireless communication channel being on a second frequency band which has a lower bound at a higher frequency than an upper bound of the first frequency band, using a signal received on the second frequency band, wherein the upper bound of the first frequency band is less than ten gigahertz, and the lower bound of the second frequency band is more than twenty gigahertz, and
- deriving an encryption key at least partly based on the second channel definition, and using the encryption key to encrypt information before it is transmitted to the second apparatus over the first wireless communication channel.