Patent Publication Number: US-2022215083-A1

Title: Hearing device system, devices and method of creating a trusted bond between a hearing device and a user application

Description:
RELATED APPLICATION DATA 
     This application is a continuation of U.S. patent application Ser. No. 15/813,135 filed on Nov. 14, 2017, pending, which claims priority to, and the benefit of, European Patent Application No. 16202918.5, filed on Dec. 8, 2016, pending. The entire disclosures of the above applications are expressly incorporated by reference herein. 
    
    
     FIELD 
     The present disclosure relates to a hearing device system comprising a hearing device and a user accessory device. In particular, the present disclosure relates to methods and devices for creating a trusted bond between entities of a hearing device system. 
     BACKGROUND 
     Wireless communication to and from different entities of a hearing device system has been increasing in continuation of the developments within wireless communication technology. However, the new technologies entail new challenges for the hearing aid manufacturers in order to secure communication in a hearing device system. Wireless communication interfaces of a hearing device system desirably use an open standard-based interface. However, this poses many challenges in terms of security. 
     SUMMARY 
     There is a need for apparatus, devices and methods for providing easy, efficient, and secure pairing of a user application and a hearing device. 
     Accordingly, a method, performed in a user application, of creating a trusted bond between a hearing device and the user application is disclosed, wherein the method comprises: obtaining first authentication material; transmitting a first authentication request comprising a first authentication type identifier and/or first authentication data to the hearing device; receiving an authentication response comprising an authentication key identifier; storing an authentication key and the authentication key identifier, wherein the authentication key is optionally based on the first authentication material; and connecting the user application to the hearing device using the authentication key and the authentication key identifier. 
     Further, a method, performed in a hearing device, of creating a trusted bond between the hearing device and a user application is disclosed, wherein the method comprises: optionally transmitting security data of the hearing device, wherein the security data are indicative of an authentication type applied in the hearing device; transmitting first authentication material; receiving a first authentication request comprising a first authentication type identifier and/or first authentication data; verifying the first authentication data; and determining and storing an authentication key if verifying the first authentication data is successful. 
     Also, a user application configured to perform methods performed in a user application described herein is provided. A hearing device configured to perform methods performed in a hearing device described herein is provided. 
     A user application for a user accessory device of a hearing device system comprising a hearing device is provided, the user accessory device comprising a processing unit; a memory unit; and an interface, wherein the user application when running on the user accessory device is configured to: obtain first authentication material; transmit a first authentication request comprising a first authentication type identifier and/or first authentication data to the hearing device; receive an authentication response comprising an authentication key identifier; store an authentication key and the authentication key identifier, wherein the authentication key is based on the first authentication material; and connect the user application to the hearing device using the authentication key and the authentication key identifier. 
     Even further, a hearing device is provided, the hearing device comprising: a processing unit; a memory unit; and an interface, wherein the hearing device is configured to: transmit first authentication material; receive a first authentication request comprising a first authentication type identifier and/or first authentication data; verify the first authentication data; and determine and store an authentication key if verifying the first authentication data is successful. 
     It is an important advantage of the present disclosure that a secure and easy pairing between a user application and a hearing device is provided, at least after an initial pairing. Further, the present disclosure allows for a plurality of ways to authenticate a user application towards a hearing device, in turn providing increased design freedom for a dispenser. 
     The present methods, applications and devices enables a dispenser, the hearing device manufacturer and even the hearing device user to select a suitable initial method of pairing or creating a trusted bond from a plurality of ways to creating a trusted bond between a hearing device and a user application. 
     It is an important advantage that the authentication key is not transmitted between the user application and the hearing device, which increases the security level in hearing device system communication. 
     A method of establishing a connection with a hearing device, includes: obtaining first authentication material; transmitting a first authentication request comprising a first authentication type identifier and first authentication data to the hearing device; receiving an authentication response comprising an authentication key identifier; storing an authentication key and the authentication key identifier, wherein the authentication key is based on the first authentication material; and establishing a connection with the hearing device using the authentication key and the authentication key identifier. 
     Optionally, the first authentication material comprises a hearing device identifier and a first hearing device challenge value. 
     Optionally, the method further includes determining a first common secret based on the first authentication material. 
     Optionally, the method further includes determining an application session key, and calculating the first authentication data based on the application session key. 
     Optionally, the method further includes verifying the authentication response; wherein the act of storing the authentication key and the authentication key identifier and/or the act of establishing the connection with the hearing device using the authentication key and the authentication key identifier, is performed if the act of verifying the authentication response results in a successful verification. 
     Optionally, the method further includes calculating the authentication key based on the first authentication material. 
     Optionally, the method further includes obtaining security data from the hearing device, and wherein the first authentication type identifier is based on the security data. 
     Optionally, if the security data is indicative of a primary authentication type, the method further comprises: receiving an authentication accept message; obtaining second authentication material comprising a hearing device challenge value; determining second authentication data based on the second authentication material; and transmitting a second authentication request comprising a second authentication type identifier and the second authentication data to the hearing device. 
     Optionally, if the security data is indicative of a secondary authentication type, the method further comprises obtaining a passcode, and creating the first authentication request based on the passcode. 
     Optionally, the method further includes obtaining a public key of the hearing device, and wherein the authentication key is based on the public key of the hearing device. 
     A method performed by a hearing device includes: transmitting security data of the hearing device, wherein the security data is indicative of an authentication type applied in the hearing device; transmitting first authentication material; receiving a first authentication request comprising a first authentication type identifier and first authentication data; verifying the first authentication data; and determining and storing an authentication key if the first authentication data is successfully verified. 
     Optionally, if the security data is indicative of a primary authentication type and if the first authentication data is successfully verified, the method further comprises: transmitting an authentication accept message; transmitting second authentication material comprising a second hearing device challenge value; receiving a second authentication request comprising a second authentication type identifier and second authentication data; verifying the second authentication data; and transmitting an authentication response comprising an authentication key identifier indicative of the authentication key in the hearing device if the second authentication data is successfully verified. 
     Optionally, the method further includes: starting a timer if the first authentication data is successfully verified; and deleting or overwriting the authentication key if the timer reaches a timer threshold. 
     Optionally, if the security data is indicative of a secondary authentication type, the method further comprises: retrieving a passcode; wherein the first authentication data is verified based on the passcode. 
     A program product includes a set of instructions, an execution of which by a processing unit of a user accessory device will cause the user accessory device to perform a method, the method comprising: obtaining first authentication material; transmitting a first authentication request comprising a first authentication type identifier and first authentication data to a hearing device; receiving an authentication response comprising an authentication key identifier; storing an authentication key and the authentication key identifier, wherein the authentication key is based on the first authentication material; and establishing a connection with the hearing device using the authentication key and the authentication key identifier. 
     A hearing device includes: a processing unit; and a memory unit coupled to the processing unit; wherein the processing unit of the hearing device is configured to: transmit first authentication material; receive a first authentication request comprising a first authentication type identifier and first authentication data; verify the first authentication data; and determine and store an authentication key if the first authentication data is successfully verified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the attached drawings, in which: 
         FIG. 1  schematically illustrates a hearing system, 
         FIG. 2  shows an exemplary signaling diagram of signaling between a user application and a hearing device, 
         FIG. 3  shows an exemplary signaling diagram of signaling between a user application and a hearing device, 
         FIG. 4  is a flow diagram of an exemplary method, 
         FIG. 5  is a flow diagram of an exemplary method, 
         FIG. 6  is a flow diagram of an exemplary method, 
         FIG. 7  is a flow diagram of an exemplary method, 
         FIG. 8  is a flow diagram of an exemplary method, and 
         FIG. 9  schematically illustrates an exemplary hearing device. 
     
    
    
     DETAILED DESCRIPTION 
     Various exemplary embodiments and details are described hereinafter, with reference to the figures when relevant. It should be noted that the figures may or may not be drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the invention or as a limitation on the scope of the invention. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described. 
     The present disclosure relates to improved security in hearing system communication, and in particular for communication between a user accessory device having a user application installed thereon and a hearing device. The present disclosure relates to effective hearing system communication that is robust against replay attacks, unauthorized access, battery exhaustion attacks, and man-in-the-middle attacks. 
     The present method is intended for use in an initial authentication when a user application for the first time wants to create a connection to a hearing device. It is an important advantage that subsequent pairing is secure and efficient, e.g. can be performed without user interaction. 
     As used herein the term “identifier” refers to a piece of data that is used for identifying, such as for categorizing, and/or uniquely identifying. The identifier may be in a form of a word, a number, a letter, a symbol, a list, an array, or any combination thereof. For example, the identifier as a number may be in the form of an integer, such as unsigned integer, uint, with a length of e.g. 8 bits, 16 bits, 32 bits, or more, such as an array of unsigned integers. An identifier may have a length of several bytes. For example, a hearing device identifier may have a length of 20 bytes. 
     As used herein static string may be a string of characters, such as from 4 to 16 characters. 
     The hashing function HASH used herein may be any suitable hashing function, such as SHA-1, SHA-2, SHA-3, MD5 or other hashing functions. 
     The user accessory device comprises a memory unit and an interface respectively connected to a processing unit. The memory unit may include removable and non-removable data storage units including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), etc. The memory unit has a user application stored thereon. The interface comprises an antenna and a wireless transceiver, e.g. configured for wireless communication at frequencies in the range from 2.4 to 2.5 GHz. The interface may be configured for communication, such as wireless communication, with the hearing device comprising an antenna and a wireless transceiver. 
     The present disclosure relates to easy and secure pairing between a user accessory device and a hearing device. The user accessory device forms an accessory device to the hearing device. The hearing device may be a hearing aid, e.g. of the behind-the-ear (BTE) type, in-the-ear (ITE) type, in-the-canal (ITC) type, receiver-in-canal (RIC) type or receiver-in-the-ear (RITE) type. Typically, the hearing device and the user accessory device is in possession of and controlled by the hearing device user. 
     The methods and devices create a trusted bond between a hearing device and the user application, thus reducing the complexity of subsequent authentication procedure between the user application and the hearing device. 
     The method performed in a user application comprises obtaining first authentication material. In one or more exemplary methods, obtaining first authentication material may comprise receiving first authentication material from a hearing device, e.g. in response to the user application transmitting a read message to the hearing device. The first authentication material may comprise a hearing device identifier and/or a first hearing device challenge value. 
     The method performed in a user application may comprise determining the first authentication data based on the first authentication material. The method performed in a user application may comprise determining a first common secret based on the first authentication material. The first common secret may be based on application key material. The first authentication data may be based on the first common secret. 
     The method performed in a user application comprises transmitting a first authentication request comprising a first authentication type identifier and/or first authentication data to the hearing device. The first authentication type identifier is indicative of the type of authentication used in the present authentication. Use of a first authentication type identifier facilitates the use of different initial authentication schemes, e.g. as determined by the hearing device. 
     The method performed in a user application comprises receiving an authentication response comprising an authentication key identifier. The authentication response may comprise hearing device authentication data. The authentication response may comprise an authentication status identifier indicative of the authentication status. The method performed in a user application may comprise verifying the authentication response, e.g. by verifying the hearing device authentication data and/or the authentication status identifier. Verifying the hearing device authentication data may comprise determining a hearing device session key, e.g. based on the first common secret and/or a static string, and verifying the hearing device authentication data based on the hearing device session key. 
     The hearing device session key HD_SK may be given as: 
       HD_SK=HASH(CS_1,HD_SK_STRING), 
     wherein HASH is a hashing function, CS_1 is the first common secret and HD_SK_STRING is a static string. 
     The method performed in a user application comprises storing an authentication key and the authentication key identifier, wherein the authentication key is based on the first authentication material, optionally if verifying the hearing device authentication data is successful. 
     The method performed in a user application may comprise verifying the authentication response. One or more of the acts of storing authentication key and authentication key identifier and/or connecting the user application to the hearing device using the authentication key and the authentication key identifier may be performed if verifying the authentication response is successful. 
     Also, the method performed in a user application comprises connecting the user application to the hearing device using the authentication key and the authentication key identifier, optionally if verifying the hearing device authentication data/authentication response is successful. Connecting the user application to the hearing device may comprise transmitting a first authentication request comprising the authentication key identifier, first authentication data based on the stored authentication key as keying material stored in the user application, and optionally a first authentication type identifier indicative of a tertiary authentication type. 
     The first authentication data may be based on the first common secret. The method performed in a user application may comprise, e.g. as part of determining the first authentication data, determining an application session key, e.g. based on the first common secret and/or a static string, and calculating the first authentication data based on the application session key. 
     The first common secret CS_1 may, e.g. if first authentication type identifier is indicative of primary authentication type, be given as: 
       CS_1=HASH(HD_KEY,HD_CHALLENGE_1), 
     wherein HASH is a hashing function, HD_KEY is a hearing device key, e.g. based on the hearing device identifier, and HD_CHALLENGE_1 is the first hearing device challenge value. 
     The hearing device key HD_KEY may be given as: 
       HD_KEY=HASH(HD_ID,APP_KEY), 
     wherein HASH is a hashing function, HD_ID is the hearing device identifier, and APP_KEY is keying material stored in the user application. The hearing device key is also found or stored in the hearing device, thus enabling the hearing device to calculate the first common secret. 
     The application session key APP_SK may be given as: 
       APP_SK=HASH(CS_1,APP_SK_STRING), 
     wherein HASH is a hashing function, CS_1 is the first common secret and APP_SK_STRING is a static string. 
     APP_SK_STRING may be different from or the same as HD_SK_STRING. Thus, application session key and hearing device session key may be the same or different keys. 
     The first authentication data AD_1 may be given as: 
       AD_1=AES_COUNTER(APP_SK,APP_S_STRING), 
     wherein AES_COUNTER is encryption with AES in counter mode, APP_SK is the application session key and APP_S_STRING is a static string. Thus, the first authentication data may be generated by encrypting a static string with the application session key based on the first common secret. 
     The first authentication data AD_1 may be generated by applying a hashing function to the application session key and/or a static string. 
     The method performed in a user application may comprise determining or calculating the authentication key based on the first authentication material. The authentication key may be based on the first common secret. 
     The authentication key AUTH_KEY may be given as: 
       AUTH_KEY=HASH(CS_1,AUTH_KEY_STRING), 
     wherein HASH is a hashing function, CS_1 is the first common secret and AUTH_KEY_STRING is a static string. 
     The method performed in a user application may comprise obtaining security data from the hearing device. The first authentication type identifier may be based on the security data. Thereby is allowed a user application and a hearing device to apply different types of authentication, e.g. as selected during fitting of the hearing device. The security data from the hearing device may comprise a type identifier indicative of the authentication to be applied in the user application/accepted by the hearing device. The security data may comprise a keying material identifier enabling the user application to verify if the user application supports communication to the hearing device. 
     The security data may be indicative of a primary authentication type. The method performed in a user application may comprise optionally receiving an authentication accept message; obtaining second authentication material, e.g. comprising a second hearing device challenge value and/or the hearing device identifier, optionally in response to receiving the authentication accept message; determining second authentication data based on the second authentication material; and transmitting a second authentication request comprising a second authentication type identifier and/or the second authentication data to the hearing device, optionally if the security data are indicative of a primary authentication type. The method performed in a user application may, optionally if the security data are indicative of a primary authentication type, comprise performing a primary authentication scheme. 
     The second authentication data may be based on a second common secret. The method performed in a user application may comprise, e.g. as part of determining the second authentication data, determining a secondary application session key, e.g. based on the second common secret and/or a static string, and calculating the second authentication data based on the application session key. 
     The second authentication data AD_2 may be given as: 
       AD_2=AES_COUNTER(APP_SK_2,APP_S_STRING), 
     wherein AES_COUNTER is encryption with AES in counter mode, APP_SK_2 is a secondary application session key and APP_S_STRING is a static string. Thus, the second authentication data may be generated by an encrypting a static string with a secondary application session key, e.g. based on a second common secret. 
     The second authentication data AD_2 may be generated by applying a hashing function to the secondary application session key and/or a static string. 
     The second common secret CS_2 may be given as: 
       CS_2=HASH(HD_KEY,HD_CHALLENGE_2), 
     wherein HASH is a hashing function, HD_KEY is the hearing device key, e.g. based on the hearing device identifier, and HD_CHALLENGE_2 is the second hearing device challenge value. 
     The secondary application session key APP_SK_2 may be given as: 
       APP_SK_2=HASH(CS_2,APP_SK_STRING), 
     wherein HASH is a hashing function, CS_2 is the second common secret and APP_SK_STRING is a static string. 
     Verifying the hearing device authentication data may comprise determining a secondary hearing device session key, e.g. based on the second common secret and/or a static string, and verifying the hearing device authentication data based on the secondary hearing device session key. 
     A secondary hearing device session key HD_SK_2 may be given as: 
       HD_SK_2=HASH(CS_2,HD_SK_STRING), 
     wherein HASH is a hashing function, CS_2 is the second common secret and HD_SK_STRING is a static string. 
     The security data may be indicative of a secondary authentication type. The method performed in a user application may, optionally if the security data are indicative of a secondary authentication type, comprise performing a second authentication scheme different from the first authentication scheme. The method performed in a user application may, optionally if the security data are indicative of a secondary authentication type, comprise obtaining a passcode from a user of the user application and creating the first authentication request, e.g. first authentication data, based on the passcode. Obtaining a passcode from a user of the user application may comprise receiving the passcode via a user interface of a user accessory device running the user application. The authentication key may be based on the passcode. 
     The first common secret CS_1 may, e.g. if first authentication type identifier is indicative of secondary authentication type, be given as: 
       CS_1=HASH(HASH(HD_KEY,HD_CHALLENGE_1),PASSCODE_INFO), 
     wherein HASH is a hashing function, HD_KEY is a hearing device key, e.g. based on the hearing device identifier, HD_CHALLENGE_1 is the first hearing device challenge value, and PASSCODE_INFO is based on the passcode entered by the user. The passcode info PASSCODE_INFO may be given as: 
       PASSCODE_INFO=HASH(HD_ID,PASSCODE_STR), 
     wherein HASH is a hashing function, HD_ID is the hearing device identifier, and PASSCODE_STR is a string of characters with the passcode entered by the user. 
     The method performed in a user application may comprise obtaining a public key of the hearing device. The authentication key and/or the first authentication data may be based on the public key of the hearing device. The first common secret may be based on the public key of the hearing device. The public key of the hearing device may be a Diffie-Hellman public key. The method performed in a user application may comprise transmitting a public key of the user application to the hearing device. The first common secret may be based on the public key of the hearing device and the public key of the user application. Basing the first authentication data on public keys of the hearing device and the user application may protect the passcode. 
     The first common secret CS_1 may, e.g. if first authentication type identifier is indicative of primary authentication type, be given as: 
       CS_1=HASH(HASH(HD_KEY,HD_CHALLENGE_1),DH_SECRET), 
     wherein HASH is a hashing function, HD_KEY is a hearing device key, e.g. based on the hearing device identifier, HD_CHALLENGE_1 is the first hearing device challenge value, and DH_SECRET is the common secret of the Diffie-Hellman algorithm derived from the public keys of the hearing device and the user application. 
     The second common secret CS_2 may, e.g. if first authentication type identifier is indicative of primary authentication type, be given as: 
       CS_2=HASH(HASH(HD_KEY,HD_CHALLENGE_2),AUTH_KEY), 
     wherein HASH is a hashing function, HD_CHALLENGE_2 is the second hearing device challenge value, and AUTH_KEY is the authentication key stored in the temporary memory and given as: 
       AUTH_KEY=HASH(CS_1,AUTH_KEY_STRING). 
     The first common secret CS_1 may, e.g. if first authentication type identifier is indicative of secondary authentication type, be given as: 
       CS_1=HASH(HASH(HASH(HD_KEY,HD_CHALLENGE_1),PASSCODE_INFO),DH_SECRET) 
     wherein HASH is a hashing function, HD_KEY is a hearing device key, e.g. based on the hearing device identifier, HD_CHALLENGE_1 is the first hearing device challenge value, PASSCODE_INFO is based on the passcode entered by the user, and DH_SECRET is the common secret of the Diffie-Hellman algorithm derived from the public keys of the hearing device and the user application. The passcode info PASSCODE_INFO may be given as: 
       PASSCODE_INFO=HASH(HD_ID,PASSCODE_STR), 
     wherein HASH is a hashing function, HD_ID is the hearing device identifier, and PASSCODE_STR is a string of characters with the passcode entered by the user. 
     The present disclosure also provides a method, performed in a hearing device, of creating a trusted bond between the hearing device and a user application. 
     The method performed in a hearing device may comprise transmitting security data of the hearing device, wherein the security data are optionally indicative of an authentication type applied in the hearing device. 
     The method performed in a hearing device may comprise connecting the user application to the hearing device using the authentication key and the authentication key identifier. 
     The method performed in a hearing device comprises transmitting first authentication material. The first authentication material may comprise a hearing device identifier and/or a first hearing device challenge value. The method performed in a hearing device may comprise retrieving the hearing device identifier from a memory unit and/or generating the first hearing device challenge value, e.g. as a random or pseudo-random value. The method performed in a hearing device may comprise storing the first hearing device challenge value in the hearing device. 
     The method performed in a hearing device comprises receiving a first authentication request comprising a first authentication type identifier and/or first authentication data. The first authentication type identifier is indicative of which type of authentication is being used by the user application. 
     The method performed in a hearing device comprises verifying the first authentication data. Verifying the first authentication data in the hearing device may be based on the first authentication material, e.g. hearing device identifier and/or first hearing device challenge value. 
     The method performed in a hearing device comprises determining and storing an authentication key, e.g. in a temporary memory and/or in authentication key storage in memory unit of the hearing device, if verifying the first authentication data is successful. 
     The security data may be indicative of a primary authentication type. The method performed in a hearing device may, if the security data is indicative of a primary authentication type and/or verifying the first authentication data is successful, comprise transmitting an authentication accept message; transmitting second authentication material comprising a second hearing device challenge value, optionally in response to receiving a read message from the user application; and receiving a second authentication request comprising a second authentication type identifier and/or second authentication data. 
     The method performed in a hearing device may comprise verifying the second authentication data and transmitting an authentication response comprising an authentication key identifier indicative of an authentication key in the hearing device, optionally if verifying the second authentication data is successful. Verifying the second authentication data in the hearing device may be based on the second authentication material, e.g. hearing device identifier and/or second hearing device challenge value. 
     The authentication response may comprise hearing device authentication data. The method performed in a hearing device may comprise determining the hearing device authentication data based on a common secret, such as first common secret and/or second common secret depending on authentication type. 
     The hearing device authentication data HD_AD may be given as: 
       HD_AD=AES_COUNTER(HD_SK,HD_S_STRING), 
     wherein AES_COUNTER is encryption with AES in counter mode, HD_SK is the hearing device session key and HD_S_STRING is a static string. Thus, the hearing device authentication data may be generated by encrypting a static string with the hearing device session key based on the first common secret. 
     The hearing device authentication data HD_AD may be given as: 
       HD_AD=AES_COUNTER(HD_SK_2,HD_S_STRING), 
     wherein AES_COUNTER is encryption with AES in counter mode, HD_SK_2 is the secondary hearing device session key and HD_S_STRING is a static string. Thus, the hearing device authentication data may be generated by encrypting a static string with the secondary hearing device session key based on the second common secret. 
     The hearing device authentication data may be generated by applying a hashing function to the hearing device session key, the secondary hearing device session key, and/or a static string. 
     The method performed in a hearing device may comprise storing the authentication key in authentication key storage in memory unit of the hearing device, optionally if verifying the first or second authentication data is successful. The authentication key storage may comprise or have room for a plurality of authentication keys used by the hearing device for authenticating communication with different user applications or other entities. Each authentication key stored in the authentication key storage has a corresponding authentication key identifier. 
     The method performed in a hearing device may comprise starting a timer if verifying the first authentication data is successful. The method performed in a hearing device may comprise deleting or overwriting the authentication key, e.g. in the temporary memory, if the timer reaches a timer threshold. The method performed in a hearing device may comprise stopping and/or resetting the timer, optionally in response to a successful verification of the second authentication data. The method performed in a hearing device may comprise stopping and/or resetting the timer in response to receiving a read message from the user application or in response to receiving the second authentication request from the user application. 
     The method performed in a hearing device may comprise deleting or overwriting the authentication key, e.g. in the temporary memory, if verifying the second authentication data fails. 
     The security data may be indicative of a secondary authentication type. The method performed in a hearing device may, optionally if the security data are indicative of a secondary authentication type, comprise retrieving a passcode, e.g. from the memory unit of the hearing device; verifying the first authentication data based on the passcode; and transmitting an authentication response comprising an authentication key identifier indicative of an authentication key in the hearing device, if verifying the first authentication data is successful. The method performed in a hearing device may, e.g. if the security data are indicative of a secondary authentication type, comprise determining and storing the authentication key, e.g. in authentication key storage in a memory unit of the hearing device, e.g. if verifying the first authentication data is successful. 
     The method performed in a hearing device may comprise connecting the user application to the hearing device using the authentication key and the authentication key identifier. Connecting the user application to the hearing device may comprise receiving a first authentication request comprising the authentication key identifier and optionally a first authentication type identifier indicative of a tertiary authentication type. 
     Features described in relation to the method performed in a user application may also be applied in the method performed in a hearing device. In particular, calculation of common secrets for verification of authentication data are preferably performed the same way in both the hearing device and the user application. 
     The present disclosure also relates to a user application for a user accessory device. The user accessory device may be a smartphone, a smartwatch or a tablet computer. The user application is, when installed on the user accessory device, configured to create a trusted bond between a hearing device and the user application. 
     The present methods and devices enables simple and secure connections between a hearing device and a user application after an initial creation of a trusted bond as disclosed herein. Accordingly, processing-heavy authentication procedures may be avoided or at least heavily reduced after the trusted bond has been created. 
     Namely, the methods, user applications and hearing devices disclosed herein enable hearing system communication that is robust against security threats, vulnerabilities and attacks by implementing appropriate safeguards and countermeasures, such as security mechanisms, to protect against threats and attacks. 
     Throughout, the same reference numerals are used for identical or corresponding parts. 
       FIG. 1  shows an exemplary hearing system. The hearing system  2  comprises a server device  4  and a hearing device system  6  comprising a hearing device  8  and a user accessory device  10 . The user accessory device  10  may be a smartphone configured to wirelessly communicate with the hearing device  8 . A user application  12  is installed on the user accessory device  10 . The user application may be for controlling the hearing device  8  and/or assisting a hearing device user. In one or more exemplary user applications, the user application  12  is configured to transfer firmware and/or hearing device settings to the hearing device. In one or more exemplary user applications, the user application  12  is configured to control operating parameters, such as volume, program, etc., of the hearing device 
     The hearing device  8  may be configured to compensate for hearing loss of a user of the hearing device  8 . The hearing device  8  is configured to configured to communicate with the user accessory device  10 /user application  12 , e.g. using a wireless and/or wired first communication link  20 . The first communication link  20  may be a single hop communication link or a multi-hop communication link. The first communication link  20  may be carried over a short-range communication system, such as Bluetooth, Bluetooth low energy, IEEE 802.11 and/or ZigBee. 
     The hearing device  8  comprises an interface including an antenna  24  and a radio transceiver  26  coupled to the antenna  24  for receiving/transmitting wireless communication including first communication link  20 . The hearing device  8  comprises a set of microphones comprising a first microphone  28  and optionally a second microphone  30  for provision of respective first and second microphone input signals. The hearing device  8  may be a single-microphone hearing device. The hearing device  8  comprises a memory unit (not shown) connected to the processing unit  32 . The hearing device  8  comprises a processing unit  32  connected to the transceiver  26  and microphones  28 ,  30  for receiving and processing input signals. The processing unit  32  is configured to compensate for a hearing loss of a user based on hearing device settings and to provide an electrical output signal based on the input signals. A receiver  34  converts the electrical output signal to an audio output signal to be directed towards an eardrum of the hearing device user. 
     The user accessory device  10  comprises a processing unit  36 , a memory unit  38 , an interface  40 . The user application  12  is installed in the memory unit  38  of the user accessory device  10  and, when running on the user accessory device, configured to obtain first authentication material, e.g. with obtain module  202   a ; transmit a first authentication request comprising a first authentication type identifier and first authentication data to the hearing device, e.g. with transmit module  204   a ; receive an authentication response comprising an authentication key identifier, e.g. with receive module  206   a ; store an authentication key and the authentication key identifier, e.g. with storing module  208   a , wherein the authentication key is based on the first authentication material; and connect the user application to the hearing device using the authentication key and the authentication key identifier, e.g. with connecting module  210   a . The connecting module  210   a  may also be used for connecting with the server device, e.g. in via second communication link  22 . 
       FIG. 2  shows an exemplary signaling diagram  100  between a user application  12  and a hearing device  8  illustrating exemplary methods of creating a trusted bond between a hearing device and a user application according to a primary authentication type. 
     The user application  12  obtains first authentication material including a hearing device identifier HD_ID and a first hearing device challenge value HD_CHALLENGE_1 by transmitting a read message  102  to the hearing device. The hearing device generates the first hearing device challenge value, stores the first hearing device challenge value in the memory unit of the hearing device, and transmits the first authentication material  104  to the user application in a read response message  106  in response to receiving the read message from the user application. 
     Further, the user application and the hearing device exchange public keys for the Diffie-Hellmann algorithm by key exchange signalling  107 . In the primary authentication type, the key exchange signalling may be omitted. 
     The user application  12  determines first authentication data AD_1 and transmits a first authentication type identifier AUT_T_ID and the first authentication data AD_1 to the hearing device in a first authentication request  108 . The first authentication type identifier AUT_T_ID is in first authentication request  108  indicative of a first message in a primary authentication type. The first authentication data AD_1 are given by: 
       AD_1=AES_COUNTER(APP_SK,APP_S_STRING), 
     wherein AES_COUNTER is encryption with AES in counter mode and APP_S_STRING is a static string. 
     APP_SK is an application session key and given as: 
       APP_SK=HASH(CS_1,APP_SK_STRING), 
     wherein HASH is a hashing function, such as SHA-2, and APP_SK_STRING is a static string. 
     CS_1 is a first common secret and is given as: 
       CS_1=HASH(HASH(HD_KEY,HD_CHALLENGE_1),DH_SECRET), 
     wherein HASH is a hashing function, such as SHA-2, HD_KEY is a hearing device key, HD_CHALLENGE_1 is the first hearing device challenge value, and DH_SECRET is the common secret of the Diffie-Hellman algorithm derived from the public keys of the hearing device and the user application during key exchange signalling  107 . The two hashing functions used for determining CS_1 may be the same or different hashing function. 
     HD_KEY is a hearing device key based on the hearing device identifier and given as: 
       HD_KEY=HASH(HD_ID,APP_KEY), 
     wherein HASH is a hashing function, HD_ID is the hearing device identifier, and APP_KEY is keying material stored in the user application. 
     The hearing device  8  receives the first authentication request  108  and verifies the first authentication data AD_1 by comparing AD_1 with authentication data calculated in the hearing device. If the hearing device  8  successfully verifies the first authentication data, and optionally the hearing device accepts the primary authentication type indicated by the first authentication type identifier, the hearing device calculates an authentication key, stores the authentication key in a temporary memory, transmits an authentication accept message  110  and starts a timer. 
     Assuming the hearing device is rebooted before the timer reaches a time threshold, the user application, after receipt of the authentication accept message, detects that the hearing device is visible again and transmits a read message  112  to the hearing device for obtaining second authentication material. The hearing device generates a second hearing device challenge value HD_CHALLENGE_2, stores the second hearing device challenge value in the memory unit of the hearing device, and transmits the second authentication material  114  to the user application in a read response message  116  in response to receiving the read message  112  from the user application. 
     Upon receipt of the read response message  116 , the user application  12  determines second authentication data AD_2 and transmits a first authentication type identifier AUT_T_ID and the second authentication data AD_2 to the hearing device in a second authentication request  118 . The first authentication type identifier AUT_T_ID is in second authentication request  118  indicative of a second message in a primary authentication type. The second authentication data AD_2 are given by: 
       AD_2=AES_COUNTER(APP_SK_2,APP_S_STRING), 
     wherein AES_COUNTER is encryption with AES in counter mode and APP_S_STRING is a static string. 
     APP_SK_2 is a secondary application session key and given as: 
       APP_SK_2=HASH(CS_2,APP_SK_STRING), 
     wherein HASH is a hashing function, such as SHA-2, and APP_SK_STRING is a static string. 
     CS_2 is a second common secret and is given as: 
       CS_2=HASH(HASH(HD_KEY,HD_CHALLENGE_2),AUTH_KEY), 
     wherein HASH, such as SHA-2, is a hashing function, HD_CHALLENGE_2 is the second hearing device challenge value, and AUTH_KEY is the authentication key given as: 
       AUTH_KEY=HASH(CS_1,AUTH_KEY_STRING). 
     HD_KEY is again a hearing device key based on the hearing device identifier and given as: 
       HD_KEY=HASH(HD_ID,APP_KEY), 
     wherein HASH is a hashing function, such as SHA-2, HD_ID is the hearing device identifier, and APP_KEY is keying material stored in the user application. Again, the same or different hashing functions may be used in different steps for determining the secondary application session key. 
     The hearing device  8  receives the second authentication request  118  and verifies the second authentication request/second authentication data AD_2 by comparing AD_2 with authentication data calculated in the hearing device. If the timer has reached a timer threshold, the hearing device deletes or overwrites the authentication key, thus preventing connection using the authentication key at a later point in time. 
     If the hearing device  8  successfully verifies the second authentication data, the hearing device stores the authentication key from temporary memory in authentication key storage in memory unit of the hearing device with an associated authentication key identifier, determines hearing device authentication data based on the second common secret, and transmits an authentication response  120  comprising hearing device authentication data HD_AD and the authentication key identifier AUTH_KEY_ID to the user application  12 . 
     The hearing device authentication data HD_AD is given as: 
       HD_AD=AES_COUNTER(HD_SK_2,HD_S_STRING), 
     wherein AES_COUNTER is encryption with AES in counter mode, HD_SK_2 is the secondary hearing device session key based on the second common secret CS_2, and HD_S_STRING is a static string. 
     The secondary hearing device session key HD_SK_2 is given as: 
       HD_SK_2=HASH(CS_2,HD_SK_STRING), 
     wherein HASH is a hashing function, such as SHA-2, CS_2 is the second common secret and HD_SK_STRING is a static string. 
     Thus, the hearing device authentication data may be generated by encrypting a static string with the secondary hearing device session key based on the second common secret. 
     The user application  12  receives the authentication response  120  and verifies the authentication response by verifying the hearing device authentication data HD_AD by comparing HD_AD with authentication data calculated in the user application based on the second common secret. If verifying the authentication response is successful, the user application calculates the authentication key AUTH_KEY (if not calculated earlier) and stores the authentication key and the authentication key identifier AUTH_KEY_ID from the authentication response and connects by connection signalling  122  to the hearing device using the authentication key AUTH_KEY and the authentication key identifier AUTH_KEY_ID. 
       FIG. 3  shows an exemplary signaling diagram  100 A between a user application  12  and a hearing device  8  illustrating exemplary methods of creating a trusted bond between a hearing device and a user application according to a secondary authentication type. 
     The user application  12  obtains first authentication material including a hearing device identifier HD_ID and a first hearing device challenge value HD_CHALLENGE_1 by transmitting a read message  102  to the hearing device. The hearing device generates the first hearing device challenge value, stores the first hearing device challenge value in the memory unit of the hearing device, and transmits the first authentication material  104  to the user application in a read response message  106  in response to receiving the read message from the user application. 
     Further, the user application and the hearing device exchange public keys for the Diffie-Hellmann algorithm by key exchange signalling  107 . 
     The user application  12  obtains a passcode from a user of the user application by receiving the passcode via a user interface of a user accessory device running the user application and creates first authentication request  108  including first authentication data based on the passcode. The first authentication data AD_1 are given by: 
       AD_1=AES_COUNTER(APP_SK,APP_S_STRING), 
     wherein AES_COUNTER is encryption with AES in counter mode and APP_S_STRING is a static string. 
     APP_SK is an application session key and given as: 
       APP_SK=HASH(CS_1,APP_SK_STRING), 
     wherein HASH is a hashing function, such as SHA-2, and APP_SK_STRING is a static string. 
     CS_1 is a first common secret and is given as: 
       CS_1=HASH(HASH(HASH(HD_KEY,HD_CHALLENGE_1),PASSCODE_INFO),DH_SECRET) 
     wherein HASH is a hashing function, such as SHA-2, HD_KEY is a hearing device key based on the hearing device identifier, HD_CHALLENGE_1 is the first hearing device challenge value, PASSCODE_INFO is based on the passcode entered by the user, and DH_SECRET is the common secret of the Diffie-Hellman algorithm derived from the public keys of the hearing device and the user application. The passcode info PASSCODE_INFO is given as: 
       PASSCODE_INFO=HASH(HD_ID,PASSCODE_STR), 
     wherein HASH is a hashing function, such as SHA-2, HD_ID is the hearing device identifier, and PASSCODE_STR is a string of characters with the passcode entered by the user. 
     HD_KEY is a hearing device key based on the hearing device identifier and given as: 
       HD_KEY=HASH(HD_ID,APP_KEY), 
     wherein HASH is a hashing function, such as SHA-2, HD_ID is the hearing device identifier, and APP_KEY is keying material stored in the user application. Again, the same or different hashing functions may be used in different steps for determining the secondary application session key and/or first common secret. 
     The hearing device  8  receives the first authentication request  108  and verifies the first authentication data AD_1 by comparing AD_1 with authentication data calculated in the hearing device based on passcode info stored in the memory unit of the hearing device, e.g. during fitting or during hearing device update. If the hearing device  8  successfully verifies the first authentication data, and optionally the hearing device accepts the secondary authentication type indicated by the first authentication type identifier, the hearing device calculates an authentication key, stores the authentication key with an associated authentication key identifier in the memory unit, determines hearing device authentication data, and transmits an authentication response  120  comprising hearing device authentication data HD_AD and the authentication key identifier AUTH_KEY_ID to the user application  12 . 
     The hearing device authentication data HD_AD is given as: 
       HD_AD=AES_COUNTER(HD_SK,HD_S_STRING), 
     wherein AES_COUNTER is encryption with AES in counter mode, HD_SK is the hearing device session key based on the first common secret CS_1, and HD_S_STRING is a static string. Thus, the hearing device authentication data may be generated by encrypting a static string with the hearing device session key based on the first common secret. 
     The user application  12  receives the authentication response  120  and verifies the authentication response by verifying the hearing device authentication data HD_AD by comparing HD_AD with authentication data calculated in the user application based on the first common secret. If verifying the authentication response is successful, the user application calculates the authentication key AUTH_KEY (if not calculated earlier) and stores the authentication key and the authentication key identifier AUTH_KEY_ID from the authentication response and connects by connection signalling  122  to the hearing device using the authentication key AUTH_KEY and the authentication key identifier AUTH_KEY_ID. 
       FIG. 4  illustrates an exemplary method performed in a user application of creating a trusted bond between a hearing device and the user application. The method  200  comprises obtaining  202  first authentication material, e.g. with obtain module  202   a , the first authentication material comprising a hearing device identifier and a first hearing device challenge value. The method  200  optionally comprises obtaining  203  security data of the hearing device. e.g. with obtain module  202   a , the security data comprising a type identifier indicative of the authentication to be applied in the user application, and a keying material identifier enabling the user application to verify if the user application supports communication to the hearing device. The method  200  proceeds to determining  203 A first authentication data and transmitting  204  a first authentication request comprising a first authentication type identifier and the first authentication data, e.g. with transmit module  204   a , to the hearing device. Determining  203 A first authentication data optionally comprises determining  203 B a first common secret based on the first authentication material and optionally determining  203 C the first authentication data based on the first common secret, e.g. as described in relation to  FIG. 2 . Determining  203 A first authentication data optionally comprises determining  203 D, e.g. as part of  203 C, an application session key, e.g. based on the first common secret, and optionally calculating the first authentication data based on the application session key. The first authentication request comprises the first authentication type identifier based on the security data obtained in  203 . 
     The method  200  comprises receiving  206 , e.g. with receive module  206   a , an authentication response comprising an authentication key identifier and storing  208  an authentication key and the authentication key identifier, e.g. with storing module  208   a , wherein the authentication key is based on the first authentication material. Further, the method  200  comprises connecting  210  the user application to the hearing device, e.g. with connecting module  210   a , using the authentication key and the authentication key identifier. The method  200  comprises calculating  211  the authentication key based on the first authentication material, e.g. as part of  203 A or  208 . The method may comprise obtaining  226  a public key of the hearing device. In one or more exemplary methods, obtaining a public key of the hearing device may be performed before or after obtaining  202  first authentication material. Determining  203 A first authentication data may be based on public keys of the hearing device and the user application, e.g. using a Diffie-Helmann scheme as described earlier. 
       FIG. 5  illustrates an exemplary method performed in a user application of creating a trusted bond between a hearing device and the user application. The method  200 A comprises acts already described in connection with  FIG. 4  using the same reference numerals. The method  200 A optionally comprises verifying  212  the security data and/or the first authentication material, if not proceeding directly to  203 A from  202 . If the security data and/or the first authentication material are verified, the method  200 A proceeds to  203 A shown and described in more detail in  FIG. 4  If verification  212  fails, the method  200 A terminates  214 , e.g. including providing  214 A an error message in the user application. 
     The method  200 A optionally comprises verifying  216  the authentication response, if not proceeding directly from  206  to  208 . The acts of storing  208  authentication key and authentication key identifier and/or connecting  210  the user application to the hearing device using the authentication key and the authentication key identifier is performed if verifying the authentication response is successful. Verifying the authentication response is based on the first common secret and/the second common secret depending on authentication type. 
       FIG. 6  illustrates an exemplary method performed in a user application of creating a trusted bond between a hearing device and the user application. The security data obtained in  203  are indicative of a primary authentication type and the user application operates accordingly. Step  203 A is the same as described in relation to method  200  of  FIG. 4 . The method  200 B comprises receiving  218  an authentication accept message, e.g. authentication accept message  110 . The authentication accept message is indicative of the hearing device having accepted the first authentication data/first authentication type identifier of the first authentication request, e.g. first authentication request  108 . The method  200 B proceeds to obtaining  220  second authentication material comprising a second hearing device challenge value and/or the hearing device identifier, and determining  222  second authentication data based on the second authentication material. Determining  222  second authentication data optionally comprises determining  222 A a second common secret based on the second authentication material and optionally determining  222 B the second authentication data based on the second common secret, e.g. as described in relation to  FIG. 2 . Determining  222  second authentication data optionally comprises determining  222 C, e.g. as part of  222 B, a secondary application session key, e.g. based on the second common secret, and optionally calculating the second authentication data based on the secondary application session key. The method  200 B comprises transmitting  224  a second authentication request comprising a first authentication type identifier and the second authentication data to the hearing device, the first authentication type identifier being indicative of a second authentication request for the primary authentication type. In the method  200 B, verifying  216  the authentication response is based on the second common secret. 
       FIG. 7  illustrates an exemplary method performed in a user application of creating a trusted bond between a hearing device and the user application. The security data obtained in  203  are indicative of a secondary authentication type and the user application operates accordingly. The method  200 C comprises obtaining  224  a passcode from a user of the user application by receiving the passcode via a user interface of a user accessory device running the user application. Determining  203 A is based on the passcode entered by the user, and determining  211  the authentication key (see  FIG. 4 ) is based on the passcode. The method  200 C comprises obtaining  226  a public key of the hearing device. 
     In step  203 A of method  200 C, the first authentication data AD_1 are given as: 
       AD_1=AES_COUNTER(APP_SK,APP_S_STRING), 
     wherein AES_COUNTER is encryption with AES in counter mode, APP_SK is the application session key and APP_S_STRING is a static string, and wherein the application session key APP_SK is given as: 
       APP_SK=HASH(CS_1,APP_SK_STRING), 
     wherein HASH is a hashing function, CS_1 is the first common secret and APP_SK_STRING is a static string. 
     The first common secret CS_1 is in exemplary method  200 C given as: 
       CS_1=HASH(HASH(HASH(HD_KEY,HD_CHALLENGE_1),PASSCODE_INFO),DH_SECRET) 
     wherein HASH is a hashing function, HD_KEY is a hearing device key, e.g. based on the hearing device identifier, HD_CHALLENGE_1 is the first hearing device challenge value, PASSCODE_INFO is based on the passcode entered by the user, and DH_SECRET is the common secret of the Diffie-Hellman algorithm derived from the public keys of the hearing device and the user application. The passcode info PASSCODE_INFO is given as: 
       PASSCODE_INFO=HASH(HD_ID,PASSCODE_STR), 
     wherein HASH is a hashing function, HD_ID is the hearing device identifier, and PASSCODE_STR is a string of characters with the passcode entered by the user. 
     The hearing device key HD_KEY is given as: 
       HD_KEY=HASH(HD_ID,APP_KEY), 
     wherein HASH is a hashing function, HD_ID is the hearing device identifier, and APP_KEY is keying material stored in the user application. The hearing device key and the PASSCODE_STR/PASSCODE_INFO is also found or stored in the hearing device, thus enabling the hearing device to calculate the first common secret and verify the first authentication data. 
       FIG. 8  illustrates an exemplary method performed in a hearing device of creating a trusted bond between the hearing device and a user application. The method  300  comprises transmitting  302 , e.g. with transmit module  302   a , security data of the hearing device, wherein the security data are indicative of an authentication type applied in the hearing device; transmitting  304 , e.g. with transmit module  304   a , first authentication material comprising a hearing device and first hearing device challenge value; receiving  306 , e.g. with receive module  306   a , a first authentication request comprising a first authentication type identifier and first authentication data; verifying  308 , e.g. with verification module  308   a , the first authentication data and/or first authentication type identifier; and determining and storing  310 , e.g. with determine and store module  310   a , an authentication key if verifying the first authentication data is successful  312 . The method  300  optionally comprises transmitting  314  a public key of the hearing device to the user application. If the hearing device performs a primary authentication, the authentication key in step  310  is stored in a temporary memory and the method  300  proceeds to transmitting  315  an authentication accept message and starting  316  a timer. If the timer times out in  318 , i.e. the timer reaches a timer threshold, the method proceeds to step  320  of deleting or overwriting the authentication key in the temporary memory and aborting the creation of a trusted bond between the hearing device and the user application. Otherwise, the method proceeds to determine  322  if a read message has been received. If read message has been received, the method  300  proceeds to transmitting  324  second authentication material comprising a second hearing device challenge value and receiving  326  a second authentication request comprising a second authentication type identifier and second authentication data. The method  300  comprises verifying  328  the second authentication data based on the authentication key stored in the temporary memory; and storing  329  the authentication key in authentication key storage with associated authentication key identifier and generating and transmitting  330  an authentication response comprising an authentication key identifier indicative of the authentication key in the hearing device, if verifying  328  the second authentication data is successful. Generating and transmitting an authentication response comprises determining hearing device authentication data, wherein the authentication response comprises the hearing device authentication data. 
     If the security data are indicative of a secondary authentication type, the method  300  proceeds to retrieving  332  a passcode from the memory unit of the hearing device and verifying  334  the first authentication data based on the passcode. If the first authentication data are not verified in  334 , the method  300  aborts the creation of a trusted bond between the hearing device and the user application. Further, the method  300  comprises transmitting  336  an authentication response comprising an authentication key identifier indicative of the authentication key in the hearing device, if verifying the first authentication data is successful. 
       FIG. 9  shows an exemplary hearing device. The hearing device  8  comprises a processing unit  350  including a transmit module  302   a , a transmit module  304   a , a receive module  306   a , a verification module  308   a , and a determine and store module  310   a . Further, the hearing device comprises a memory unit  352  and an interface  354 , 
     wherein the hearing device  8  is configured to transmit first authentication material, e.g. with transmit module  304   a ; receive a first authentication request comprising a first authentication type identifier and first authentication data, e.g. with receive module  306   a ; verify the first authentication data, e.g. with verification module  308   a ; and determine and store an authentication key, e.g. with determine and store module  310   a , if verifying the first authentication data is successful. 
     The hearing device  8  may be arranged to execute at least parts of methods as disclosed herein. The processing unit  350  may further comprise a number of optional functional modules, such as any of a transmit module  302   a  configured to perform step  302 , a transmit module  304   a  configured to perform step  304 , a receive module  306   a  configured to perform step  306 , verification module  308   a  configured to perform step  308 , and determine and store module  310   a  configured to perform step  310 . In general terms, each functional module may be implemented in hardware and/or in software. 
     The use of the terms “first”, “second”, “third” and “fourth”, etc. does not imply any order, but are included to identify individual elements. Moreover, the use of the terms first, second, etc. does not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Note that the words first and second are used here and elsewhere for labelling purposes only and are not intended to denote any specific spatial or temporal ordering. Furthermore, the labelling of a first element does not imply the presence of a second element and vice versa. 
     Although features have been shown and described, it will be understood that they are not intended to limit the claimed invention, and it will be made obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the claimed invention. The specification and drawings are, accordingly to be regarded in an illustrative rather than restrictive sense. The claimed invention is intended to cover all alternatives, modifications, and equivalents. 
     LIST OF REFERENCES 
     
         
           2  hearing system 
           4  server device 
           6  hearing device system 
           8  hearing device 
           10  user accessory device 
           12  user application 
           20  first communication link 
           22  second communication link 
           24  antenna 
           26  radio transceiver 
           28  first microphone 
           30  second microphone 
           32  processing unit 
           34  receiver 
           36  processing unit 
           38  memory unit 
           40  interface 
           100 ,  100 A signalling diagram 
           102  read message 
           104  first authentication data 
           106  read response message 
           107  key exchange signalling 
           108  first authentication request 
           110  authentication accept message 
           112  read message 
           114  second authentication material 
           116  read response message 
           118  second authentication request 
           120  authentication response 
           122  connection signalling 
           200 ,  200 A,  200 B method of creating a trusted bond between hearing device and user application 
           202  obtaining first authentication material 
           202   a  obtain module 
           203  obtain security data 
           203 A determining first authentication data 
           203 B determining a first common secret 
           203 C determining the first authentication data based on the first common secret 
           203 D determining an application session key and calculating the first authentication data based on the application session key 
           204  transmitting first authentication request 
           204   a  transmit module 
           206  receiving an authentication response 
           206   a  receive module 
           208  storing an authentication key and the authentication key identifier 
           208   a  storing module 
           210  connecting the user application to the hearing device 
           211  calculating the authentication key 
           212  verifying security data and/or first authentication material 
           214  terminating 
           214 A providing error message 
           216  verifying the authentication response 
           218  receiving an authentication accept message 
           220  obtaining second authentication material 
           222  determining second authentication data 
           222 A determining a second common secret based on the second authentication material 
           2228  determining the second authentication data based on the second common secret 
           222 C determining a secondary application session key and calculating the second authentication data based on the secondary application session key 
           224  obtain passcode 
           226  obtain public key of the hearing device 
           300  Method performed in a hearing device of creating a trusted bond between hearing device and user application 
           302  transmitting security data 
           302   a  transmit module 
           304  transmitting first authentication material 
           304   a  transmit module 
           306  receiving first authentication request 
           306   a  receive module 
           308  verifying the first authentication data and/or first authentication type identifier 
           308   a  verification module 
           310  determining and storing authentication key 
           310   a  determine and store module 
           312  verification ok? 
           314  transmitting public key of the hearing device 
           316  starting timer 
           318  timeout? 
           320  deleting or overwriting the authentication key in the temporary memory 
           322  read message received? 
           324  transmitting second authentication material 
           326  receiving a second authentication request 
           328  verifying the second authentication data 
           329  storing the authentication key in authentication key storage 
           330  generating and transmitting an authentication response 
           332  retrieving a passcode from the memory unit of the hearing device 
           334  verifying the first authentication data based on the passcode 
           336  transmitting an authentication response 
           350  processing unit 
           352  memory unit 
           354  interface