Programmable Hardware Security Module and Methods on a Programmable Hardware Security Module

Description

[0001] The invention relates to a programmable hardware security module, a method for securing a private key of a cryptographic key pair, and a method for securely providing a private key of a cryptographic key pair on a programmable hardware security module.

[0002] In industrial automation environments, cryptographic security methods are necessary for data protection and the authentication of communication partners. Cryptographic security mechanisms require cryptographic key material. Specifically, cryptographic key pairs consisting of private and public keys are used. The respective private keys must be kept secret, while public keys or associated data structures such as digital certificates or security tokens are made available to the relevant components for processing. In small automation systems, a suitable security infrastructure for setting up a key infrastructure, particularly for distributing cryptographic key material, is often unavailable.Cloud-based solutions often cannot be used due to applicable regulations concerning closed networks or requirements regarding local control.

[0003] It is generally known that building and operating central security infrastructures, so-called PKI or security data centers, is important. In practice, however, this is not feasible in all application scenarios.

[0004] US 2011296172 A1 discloses a method and system for server-side key generation for non-token clients.

[0005] US 2006291664 A1 discloses a system and method for automatic management of cryptographic keys comprising a key control system, a key management agent system and a key system application program interface.

[0006] US 2009086977 A1 discloses a method comprising receiving, over a network, a request for the provision and supply of a private key, wherein the private key is intended for use in a public-private key system.

[0007] Against this background, the purpose of the present application is to provide a secure and easily implementable decentralized security infrastructure.

[0008] This problem is solved by the subject matter of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0009] The invention relates to a programmable hardware security module comprising an application device for generating a cryptographic key pair, wherein the key pair comprises a private key and a public key, a memory area containing a master transport key, a key derivation module for deriving a key pair-specific transport key from the master transport key using a key pair-specific derivation parameter, wherein the key pair-specific derivation parameter is for the key pair orThe key pair-specific transport key is characteristic of the private key or the public key of the key pair, wherein the key pair-specific derivation parameter is formed from certificate information of the digital certificate, in particular the public key, a device identifier, or a hash value of the digital certificate, wherein it is transmitted to the programmable hardware security module or the corresponding programmable hardware security module together with a key request message, and wherein the key pair-specific transport key is usable for encrypting the private key of the cryptographic key pair.

[0010] A programmable hardware security module (PCM) is, for example, an open hardware security module (HQM) that allows existing commercial or open-source security software to run in a protected environment. Software packages can be installed and executed with minimal effort. A programmable hardware security module is also protected, for example, by hardware or software security mechanisms. The execution environment can be protected with software, configuration data, or runtime data. Such an open hardware security module offers the possibility of executing security-critical data and program code, for example, in a Linux runtime environment. Furthermore, a local PKI (Public Key Infrastructure) for generating certificates can be implemented with less effort.

[0011] The hardware security module includes an application interface that provides access to an application for generating a key pair. This is a cryptographic key pair consisting of a private key and a public key. Additionally, the hardware security module contains a memory area with a master transport key. This master transport key is intended to protect the private key.

[0012] The hardware security module includes a key derivation module for deriving a key-pair-specific transport key using a key-pair-specific derivation parameter. This transport key can then be used to encrypt the private key. A specific derivation parameter is used for each key pair, ensuring that the transport key is ideally generated only once per key pair. In other words, all transport keys for different key pairs are unique. The key-pair-specific derivation parameter thus identifies the key pair, its private key, or its public key.The key pair-specific derivation parameter is provided to the programmable hardware security module, for example via an interface, or is stored in a memory area.

[0013] A simple, decentralized PKI can be advantageously built to generate device keys for a target device and, in particular, device certificates. The described programmable hardware security module can ensure that a transport key is key-pair specific to the generated private key. For example, on such a module, using scripts and open-source software such as OpenSSL, the generation of key pairs—that is, private keys and their corresponding certificates—can be easily implemented, while simultaneously ensuring the security of a private key generated in this way by securing it with the key-pair-specific transport key.

[0014] According to one embodiment, the programmable hardware security module also includes an output unit for printing the private key as an encrypted file. For example, this could be a so-called PEM file with the file format "privacy enhanced mail certificate". The output is sent specifically to the devices on which the private key of the generated cryptographic key pair is to be stored. Alternatively, a different file format can be used instead of a PEM file, such as a PKCS#12 file.

[0015] According to one embodiment, the programmable hardware security module further comprises a device for generating a digital certificate containing the public key of the cryptographic key pair. Advantageously, the digital certificate is generated directly during the generation of the cryptographic key pair, particularly as a further step. The digital certificate is preferably issued, for example, to a target device, to associate the public key contained in the digital certificate with the programmable hardware security module. In particular, the digital certificate contains identification data of the programmable hardware security module.

[0016] The invention further relates to a corresponding programmable hardware security module, comprising a receiving device for receiving a private key of a cryptographic key pair as an encrypted file and a digital certificate comprising a public key of the cryptographic key pair, a storage area with a master transport key, a key derivation module for deriving a key pair-specific transport key from the master transport key using a key pair-specific derivation parameter, wherein the key pair-specific derivation parameter for the key pair orThe key pair is characterized by the private key or the public key of the key pair, wherein the key pair-specific derivation parameter is formed from certificate information of the digital certificate, in particular the public key, a device identifier, or a hash value of the digital certificate, wherein it is transmitted to the programmable hardware security module or the corresponding programmable hardware security module together with a key request message, and wherein the key pair-specific transport key is usable for decrypting the encrypted file.

[0017] Advantageously, the two modules, referred to in the present application as the programmable hardware security module and the corresponding programmable hardware security module for differentiation, are used together in one implementation variant. The corresponding programmable hardware security module is used on the target device side or the field device side and communicates, in particular, with the target device. Both security modules share the key derivation module according to the invention; the only difference between the two modules lies in the use of the generated key-pair-specific transport key: The programmable hardware security module generates the private key of the cryptographic key pair to be protected and can advantageously be used to generate a key file encrypted with the key-pair-specific transport key.On the other hand, the corresponding programmable hardware security module uses the key-pair-specific transport key, which is also generated, for example to decrypt the received key file and ultimately to provide it to a target device or field device that is to be equipped with the key pair.

[0018] According to the guidelines for examination F IV 3.2, separate independent claims can be formulated for several items that complement or interact with each other. The present case involves such interacting, corresponding components.

[0019] For example, the encrypted key file is re-encrypted on the corresponding programmable hardware security module and provided to the target device in encrypted form. Specifically, this involves using a negotiated key between the target device and the corresponding programmable hardware security module.

[0020] According to one embodiment, the corresponding programmable hardware security module also has an interface for transferring the key-pair-specific transport key to a target device. The target device thus receives the key-pair-specific transport key for decrypting the key file. After decryption of the key file, the private key, if required, and the digital certificate, if transmitted to the target device, are then available on the field device.

[0021] According to an alternative embodiment, the corresponding programmable hardware security module also has an interface for transferring the private key to a target device. For example, this private key is secured with a symmetric key specifically negotiated between the field device and the corresponding programmable hardware security module.

[0022] The invention further relates to a method for securing a private key of a cryptographic key pair, comprising the following steps: Generating the cryptographic key pair on a programmable hardware security module, deriving a key pair-specific transport key from a master transport key using a key pair-specific derivation parameter, wherein the key pair-specific derivation parameter is formed from certificate information of the digital certificate, in particular the public key, a device identifier, or a hash value of the digital certificate, and transmitting it to the programmable hardware security module or the corresponding programmable hardware security module together with a key request message. Encrypting the private key with the key pair-specific transport key.

[0023] According to one embodiment, the procedure also includes the generation of a digital certificate comprising a public key of the cryptographic key pair.

[0024] The invention further relates to a method for securely providing a private key of a cryptographic key pair on a corresponding programmable hardware security module, comprising the following steps: Receiving the private key as an encrypted file and a digital certificate comprising a public key of the cryptographic key pair, deriving a key-pair-specific transport key from a master transport key using a key-pair-specific derivation parameter, wherein the key-pair-specific derivation parameter is formed from certificate information of the digital certificate, in particular the public key, a device identifier, or a hash value of the digital certificate, and transmitting it to the programmable hardware security module or the corresponding programmable hardware security module together with a key request message, wherein the key-pair-specific transport key is usable for decrypting the encrypted file.

[0025] Again, the methods for securing a private key and for securely providing a private key on a corresponding programmable hardware security module work together, such that both methods, which are executable on corresponding devices, are claimed as independent claims in accordance with the provisions in the directives as set out above.

[0026] According to one version, the procedure also includes: Providing the digital certificate to a target device, decrypting the encrypted file using the key pair-specific transport key, and providing the decrypted private key to the target device.

[0027] In this variant, the corresponding programmable hardware security module first decrypts the private key by having the key-pair-specific transport key available to it and applying it to the key file, and then transmits the private key of the key pair. This transmission preferably does not occur in plaintext, but rather using a key agreed upon with the target device, in particular a symmetric key negotiated via a Diffie-Hellman key exchange.

[0028] According to one version, the procedure also includes: Providing the digital certificate to a target device, providing the encrypted file to the target device, providing the key pair-specific transport key to the target device.

[0029] In this variant, the corresponding programmable hardware security module transmits the key-specific transport key to the target device, which is then capable of decrypting the key file independently.

[0030] According to one embodiment, the corresponding programmable hardware security module authenticates the target device. For example, communication takes place between the corresponding programmable hardware security module and the target device to exchange identification information or credentials of the target device. Thus, the transmission of the decrypted private key or the key-pair-specific transport key to the target device is advantageously contingent upon the target device having been previously authenticated.

[0031] According to one embodiment, the corresponding programmable hardware security module generates a target device-specific key and provides the target device with the private key or the key pair-specific transport key protected with the target device-specific key.

[0032] In one implementation, a key derivation function is used to derive the key-pair-specific transport key. This function is given at least the key-pair-specific derivation parameter and the master transport key as input. For example, a so-called Key Derivation Function is used, such as an HKDF or HMAC-Based Key Derivation Function, which employs an HMAC hash function, or a PBKDF2 or Password-Based Key Derivation Function 2, a standardized function for deriving a key from a password. These functions, mentioned as examples, are given the master transport key, which forms the basis of the key derivation. Additionally, the key-pair-specific derivation parameter is specified as an input parameter.

[0033] According to one embodiment, the key pair-specific derivation parameter is formed from certificate information of the digital certificate, in particular the public key, a device identifier, or a hash value of the digital certificate.

[0034] For example, the key derivation parameter is generated from information in the digital certificate associated with the private key. The digital certificate associated with a private key could be, for example, an X.509 certificate or a JSON token. The derivation parameter can be specifically generated using, for example, the public key of the generated key pair, a device identifier, or a hash value of the digital certificate.

[0035] For example, the key derivation parameter is generated from information in the digital certificate associated with the private key. The digital certificate associated with a private key could be, for example, an X.509 certificate or a JSON token. The derivation parameter can be generated, for example, using the public key of the generated key pair or using a device identifier that is transmitted to the programmable hardware security module (PSM) or the corresponding PSM along with a key request message, such as a serial number. Furthermore, another attribute of the generated certificate, such as the certificate signature, the certificate serial number, or an additional attribute from a certificate extension, can be used.Alternatively, the entire certificate can be used, or a hash value of the entire certificate, for example a hash value generated using the SHA256 function.

[0036] The parameters mentioned, which are used in the derivation of the derivative parameter, can each be used directly, or a hash value can be generated from this data using a hash function, which is then used as a derivative parameter in the key derivation function. For all the examples mentioned, an additional derivative parameter chosen by the programmable security module or the corresponding programmable hardware security module itself can optionally be included. This can provide additional security when generating the key-pair-specific transport key. The derivative parameter is generated by a Derivation Parameter Construction Function (DPCF).

[0037] Advantageously, another node that knows the master transport key can determine the key pair-specific transport key solely on the basis of the certificate or a partial piece of information from the certificate.

[0038] The interaction of the described protected devices and methods makes it possible to generate and input device keys securely with minimal effort.

[0039] The modules and devices can be implemented using hardware and / or software. In a hardware implementation, the respective unit can be designed as a device or as part of a device, for example, as a computer, a processor, or a microprocessor. In a software implementation, the respective device or module can be designed as a computer program product, a function, a routine, part of program code, or an executable object. A processor is understood to be an electronic circuit for computer-based data processing, for example, a CPU. This can be the CPU of a computer or the microprocessor of a microchip.

[0040] The invention is explained in more detail below with reference to exemplary embodiments and the figures. The figures show: Figure 1: A schematic representation of a programmable hardware security module according to a first embodiment of the invention; Figure 2 : A schematic representation of a corresponding programmable hardware security module according to a second embodiment of the invention; Figure 3 : A block diagram for the schematic representation of a method for securing a private key according to the first embodiment of the invention; Figure 4 : A block diagram for the schematic representation of a method for the secure provision of a private key according to the second embodiment of the invention.

[0041] Figure 1Figure 10 shows a programmable hardware security module (HSP) that can exchange data with an industrial PC. The programmable hardware security module (HSP) is, for example, an open hardware security module (OHS) or industrial security module (ISP). The programmable hardware security module includes, for example, an application unit (11) on which an application is installed that generates an asymmetric key pair. This cryptographic key pair consists of a public and a private key. For example, the application issues certificate C using a digital certificate generation unit.

[0042] The computer PC, which in a scenario is to be used to equip a target device, for example a field device, with cryptographic key material, transmits an identifier or identifier ID of the target device to the programmable hardware security module 10. This module uses this information to generate a key pair for the target device. The digital certificate C, for example, is stored on the computer PC so that it can later be transmitted to the field device. The programmable hardware security module 10 also includes a memory area 12 containing a master transport key MK. This master transport key MK is preferably stored in a protected manner. For example, tamper protection measures are provided to protect the secret master transport key MK from unauthorized reading or manipulation.The programmable hardware security module 10 also includes a key derivation module 13, which derives a key-pair-specific transport key TK from the master transport key MK using a key-pair-specific derivation parameter DP. The master transport key MK is provided internally to the key derivation module 13 from memory. In one embodiment, the key-pair-specific derivation parameter DP can be provided on the programmable hardware security module 10 or, alternatively, provided to the key derivation module 13 externally. The derived key-pair-specific transport key TK is available on the programmable hardware security module 10 and can be used, in particular advantageously, for encrypting the generated private key of the cryptographic key pair.For example, an encrypted file KF is created and made available to the computer PC at an output unit 14 of the programmable hardware security module 10.

[0043] Figure 2 shows a corresponding programmable hardware security module 20, which advantageously works in conjunction with the one associated with Figure 1The computer PC described above can be used. Furthermore, the corresponding programmable hardware security module 20 is in contact with a field device FD or target device. In particular, the corresponding programmable hardware security module 20 can transmit data to the field device FD. This occurs via an interface 24, 25. The corresponding programmable hardware security module 20 has a receiving device 21 for receiving a private key, which is received by the computer PC in encrypted or unencrypted form. In particular, the device in connection with Figure 1The computer PC described above sends an encrypted key file KF to the corresponding programmable hardware security module 20. Likewise, a certificate C, which was stored on the computer PC according to the description of the first embodiment, is advantageously transmitted from the computer PC to the receiving device 21. Furthermore, a memory area 22 containing a master transport key MK is provided on the corresponding programmable hardware security module 20.

[0044] For the described scenario, in which the programmable hardware security module 10, explained with reference to Figure 1, is combined with the one shown in Figure 1, Figure 2For the described corresponding programmable hardware security module 20 to interact, the identical master transport key MK must be stored on both modules. Then, the identical key-pair-specific transport key can advantageously be derived from the master transport key MK using a key-pair-specific derivation parameter DP via a key-derivation module 23 provided on the corresponding programmable hardware security module 20. For this to work, the key-derivation function provided must also match that on the programmable hardware security module 10, as must the key-pair-specific derivation parameter DP used.If the key-pair-specific derivation parameter DP is generated directly on the respective module from the public key or a digital certificate of the public key, then the identical key-pair-specific derivation parameter DP can be generated on both modules. Other incoming parameters and the function for generating the derivation parameter are identical for this purpose. For example, the key-pair-specific derivation parameter DP is generated on both modules by creating a hash value of the entire digital certificate C. This is advantageously possible if the information of the digital certificate C is available on both modules, either through self-generation or by receiving it from the computer PC, and an identical hash function is applied.

[0045] Using the key-pair-specific transport key TK generated on the corresponding programmable hardware security module 20, the received encrypted file KF can be decrypted, for example. The private key of the key pair intended for the target device FD is then available on the corresponding programmable hardware security module 20 and can be transferred to the field device FD, for example, via interface 25. The private key Kpriv is then available on the field device FD. The certificate C is also advantageously transmitted to the target device, so that the key material is thus stored securely on the field device FD. It is also advantageously possible to transmit the encrypted file KF and the generated key-pair-specific transport key TK to the field device FD via interface 24.The field device FD can then decrypt the key file KF, so that ultimately the private key Kpriv is also present on the field device FD in this variant.

[0046] The two modules are advantageously used in the manufacturing or commissioning of a field device when it comes to storing the key material, i.e., a certificate and the corresponding private key, on the field device FD. Advantageously, different interfaces are provided for outputting the key-pair-specific transport key TK and the encrypted file containing the private key KF, so that the two pieces of information are output via different methods. For example, a display and another output unit, such as a wired connection, are provided.

[0047] In one variant, all application keys, such as the master transport key MK and the certificate authority's signature for signing the certificate, are hard-coded within the application. Furthermore, fields of the certificate to be issued, C, can be predefined in the application, so that when using the application, only the device ID of the target device, FD, needs to be transmitted to create the digital certificate C for the identified field device, FD. Alternatively, the parameters in the application can be set via configuration messages. Once the application has been activated by an activation command, these parameters can no longer be changed.

[0048] Particularly advantageous is the decryption of the encrypted key file KF, which contains the encrypted private key, on the corresponding programmable hardware security module and re-encryption with a key specific to the target device DF. For example, this key is dynamically negotiated between the corresponding hardware security module 20 and the target device DF using a Diffie-Hellman key exchange. The re-encrypted key file KF is then transferred to the target device DF and decrypted by it. Specifically, a separate communication interface such as JTAG or SPI is used for this transfer.

[0049] Figure 3Figure 1 schematically shows the process steps of a method for securing a private key of a cryptographic key pair according to the first embodiment. The cryptographic key pair is generated on a programmable hardware security module, for example, the one based on… Figure 1 The programmable hardware security module described in the first embodiment. Furthermore, a key-pair-specific transport key is derived 102 from a master transport key using a key-pair-specific derivation parameter. Finally, a file containing the private key is encrypted 103 with the key-pair-specific transport key. Steps 101 and 102 can be executed separately and, in particular, in a variable order.

[0050] In addition, and either simultaneously or subsequently to step 101, a digital certificate is generated (101') comprising a public key of the cryptographic key pair.

[0051] Figure 4 describes a method for securely providing a private key of a cryptographic key pair on a corresponding programmable hardware security module, in particular on the one based on Figure 2The corresponding programmable hardware security module described in the second embodiment is used. The process involves receiving (201) the private key as an encrypted file and a digital certificate comprising a public key of the cryptographic key pair. Furthermore, (202) a key-pair-specific transport key is derived from a master transport key using a key-pair-specific derivation parameter. Steps 201 and 202 can be performed separately and, in particular, variably in terms of timing, either in parallel or sequentially.

[0052] Furthermore, the digital certificate is provided to a target device (step 203). In one variant, the encrypted file is decrypted (step 204) using the key pair-specific transport key, and the decrypted private key is provided to the target device (step 205). Steps 204 and 205 can be performed either sequentially or simultaneously with step 203. Alternatively, instead of step 204, the encrypted file can be provided to the target device (step 204'), and the key pair-specific transport key can be provided to the target device (step 205'). Steps 204' and 205' can again be performed either sequentially or simultaneously with step 203. Steps 204' and 205' can be performed in any order.

[0053] Although the invention has been illustrated and described in detail by the exemplary embodiments, the invention is not limited by the disclosed examples and other variations can be derived from them by the person skilled in the art without leaving the scope of protection of the invention.

[0054] The described devices and methods establish a decentralized PKI (Public Key Infrastructure) that can generate device keys and certificates and securely provide them to target devices. A key-pair-specific transport key plays a central role in protecting the generated and transmitted private key. This transport key is linked to the respective key pair intended for each target device by deriving it from a master key using a key-pair-specific derivation parameter. This enables the generation and deployment of device keys to be implemented securely and with minimal effort.

Claims

1. Programmable hardware security module (10), comprising: - an application device (11) for generating a cryptographic key pair, wherein the cryptographic key pair comprises a private key and a public key, - a memory area (12) containing a master transport key (MK), - a device (15) for generating a digital certificate comprising the public key of the cryptographic key pair, wherein the cryptographic key pair is generated for a corresponding programmable hardware security module, - a key derivation module (13) for deriving a key-pair-specific transport key (TK) from the master transport key (MK) by using a key-pair-specific derivation parameter (DP), wherein the key-pair-specific derivation parameter is characteristic of the key pair or of the private key or the public key of the key pair, wherein the key-pair-specific derivation parameter is formed from certificate information of the digital certificate, in particular the public key of the digital certificate or a hash value of the digital certificate or a device identifier, wherein the device identifier is transmitted to the programmable hardware security module or the corresponding programmable hardware security module together with a key request message, and wherein the key-pair-specific transport key (TK) is able to be used for encrypting the private key of the cryptographic key pair.

2. Programmable hardware security module (10) according to Claim 1, further comprising an output unit (14) for outputting the private key as an encrypted file (KF).

3. Corresponding programmable hardware security module 20, comprising - a receiving device (21) for receiving a private key of a cryptographic key pair as an encrypted file and a digital certificate comprising a public key of the cryptographic key pair from a programmable hardware security module (10), - a memory area (22) containing a master transport key (MK), - a key derivation module (23) for deriving a key-pair-specific transport key (TK) from the master transport key (MK) by using a key-pair-specific derivation parameter (DP), wherein the key-pair-specific derivation parameter is formed from certificate information of the digital certificate, in particular the public key of the digital certificate, a device identifier or a hash value of the digital certificate, wherein the device identifier is transmitted to the programmable hardware security module or the corresponding programmable hardware security module together with a key request message, wherein the key-pair-specific derivation parameter is characteristic of the key pair or of the private key or the public key of the key pair and wherein the key-pair-specific transport key (TK) is able to be used for decrypting the encrypted file.

4. Corresponding programmable hardware security module (20) according to Claim 3, further comprising an interface (24) for transferring the key-pair-specific transport key (TK) to a target device (30).

5. Corresponding programmable hardware security module (20) according to Claim 3, further comprising an interface (25) for transferring the private key to a target device.

6. Method for securing a private key of a cryptographic key pair, having the following steps: - generating (101) the cryptographic key pair on a programmable hardware security module, - generating a digital certificate comprising the public key of the cryptographic key pair, wherein the cryptographic key pair is generated for a corresponding programmable hardware security module, - deriving (102) a key-pair-specific transport key from a master transport key by using a key-pair-specific derivation parameter, wherein the key-pair-specific derivation parameter is formed from certificate information of the digital certificate, in particular the public key of the digital certificate, a device identifier or a hash value of the digital certificate, and wherein the device identifier is transmitted to the programmable hardware security module or the corresponding programmable hardware security module together with a key request message, - encrypting (103) the private key using the key-pair-specific transport key.

7. Method according to Claim 6, further comprising: - generating (101') a digital certificate comprising a public key of the cryptographic key pair.

8. Method for securely providing a private key of a cryptographic key pair on a corresponding programmable hardware security module (20), having the following steps: - receiving (201) the private key as an encrypted file and a digital certificate comprising a public key of the cryptographic key pair from a programmable hardware security module (10), - deriving (202) a key-pair-specific transport key from a master transport key by using a key-pair-specific derivation parameter, wherein the key-pair-specific derivation parameter is formed from certificate information of the digital certificate, in particular the public key of the digital certificate, a device identifier or a hash value of the digital certificate, wherein the device identifier is transmitted to the programmable hardware security module or the corresponding programmable hardware security module together with a key request message, and wherein the key-pair-specific transport key is able to be used for decrypting the encrypted file.

9. Method according to Claim 8, further comprising: - providing (203) the digital certificate to a target device, - decrypting (204) the encrypted file by means of the key-pair-specific transport key, - providing (205) the decrypted private key to the target device.

10. Method according to Claim 8, further comprising: - providing (203) the digital certificate to a target device, - providing (204') the encrypted file to the target device, - providing (205') the key-pair specific transport key to the target device.

11. Method according to one of Claims 8 to 10, wherein the corresponding programmable hardware security module authenticates the target device.

12. Method according to one of Claims 8 to 11, wherein the corresponding programmable hardware security module generates a target-device-specific key (206) and provides the target device with the private key or the key-pair-specific transport key in a manner protected by the target-device-specific key.

13. Method according to one of Claims 6 to 12, wherein the key-pair-specific transport key is derived by using a key derivation function for which at least the key-pair-specific derivation parameter and the master transport key are predetermined as input parameters.