DE102016012189B4 - Diffie Hellman key derivation, subscription loading, authentication - Google Patents

Abstract

Method for deriving session keys, based on the key derivation according to Diffie Hellman, for secure communication between two communication partners, namely a Secure Element (B) and a server (A) implemented or implementable in a mobile radio-capable terminal device, comprising the steps, each for Secure Element (B) and at the server (A) :( i) Generate or obtain a random number (b, a) and provide the random number (b, a) or a value derived from it using a one-way function as a private key ( B-Pr, A-Pr); (ii) Deriving your own public key (B-Pub, A-Pub) based on your own private key (B-Pr, A-Pr); (iii) Transferring the derived own public key (B-Pub, A-Pub) or a value derived therefrom by means of a one-way function to the respective other communication partner (A, B); (iv) deriving at least one common session key (K) for the communication ationpartner based on their own private key (B-Pr, A-Pr) and the transmitted public key (A-Pub, B-Pub) of the other communication partner (A, B), characterized in that in step (i) in the secure element (B), a plurality of at least two random numbers (b1, b2, ...) is generated by a pseudo-random number generator implemented in the secure element and a corresponding plurality of at least two own private keys (B-Prl, B-Pr2, ...) is provided; in step (ii) in the secure element (B) a corresponding plurality of at least two own public keys (B-Publ, B-Pub2, ...) is derived and transmitted to the server (A) each key pair of private key (B-Prl, B-Pr2, ...) and public key (B-Publ, B-Pub2, ...) of the Secure Element becomes a key index (11, 12, ...) is assigned and in step (iii) the keys of the plurality of public keys (B-Publ, B-Pub2, .. .) are transmitted to the server (A) together with the key index (11, 12, ...) assigned to them; and in step (iv) a corresponding plurality of at least two common session keys (K-1, K-2, ...) is derived.

Description

Field of the Invention

The invention relates to methods for deriving session keys, based on the key derivation according to Diffie Hellman, in a secure element. The invention further relates to a method for loading a subscription profile into a secure element and a method for authentication using the method for deriving session keys.

State of the art

In procedures for deriving session keys, the in 1 is shown, based on the key derivation according to Diffie Hellman DHE , agree on two communication partners A . B a prime number p and a large number G , In the case of the elliptical curve Diffie Hellman ECDHE, parameters of an elliptical curve are also agreed. Both communication partners each generate or procure their own random number. This random number or a value derived by means of a one-way function (one-way function) forms the own private key of an asymmetrical key pair. Starting from your own private key, your own public key is derived using the prime number and the large number. The derived public key or a value derived from it using a one-way function is transferred to the other communication partner. Both communication partners derive a common session key based on their own private key and the transmitted public key of the other communication partner. Both come to the same session key.

To use a mobile radio-compatible device, such as smartphones or cell phones, in a mobile network of a network operator, the device contains a secure element with a subscription profile or profile for short.

The profile manages the configuration of the end device and the connection of the end device in the mobile radio network. The profile is formed by a data record that enables the establishment, operation and termination of a connection of the terminal in the mobile radio network and includes, for example, a cryptographic authentication key Ki and an International Mobile Subscriber Identity IMSI.

The terminal itself has one or more terminal chips for operating functions of the terminal. Current smartphones typically have, for example, at least three terminal chips, namely a transceiver IC, which carries out the physical radio communication, a baseband processor (or synonymous modem), which carries out functions for data transmission via radio communication at the protocol level, and an application processor AP on which the operating system and application software are executed. Transceiver ICs for other radio channels can be provided as further terminal chips, in particular for short-range radio channels such as NFC (NFC: near field communication) or Bluetooth.

The secure element can be designed in different form factors, in particular plug-in, embedded, integrated and software. Secure elements of the form factor plug-in and embedded are arranged on a dedicated, dedicated chip or SoC (Silicon-on-Chip). Examples of plug-ins are SIM card (SIM = Subscriber Identity Module) or USIM card (Universal SIM) or UICC (Universal Integrated Circuit Card) and contact the end device via a card reader. Alternatively, the dedicated chip can be integrated in a housing which can be permanently soldered or permanently soldered into the terminal. A solderable / soldered secure element is provided with the addition “embedded” and is referred to as eUICC or eSIM or eUSIM, where e stands for embedded and the further designation is adopted from the correspondingly equipped plug-in. Further possible form factors of a secure element are integrated secure elements which are integrated on a terminal chip or SoC (system-on-chip) of the terminal, ie do not have their own chip. Integrated Secure Elements are provided with the addition "integrated" and e.g. referred to as integrated UICC, iUICC, iSIM or iUSIM. Further possible form factors of a secure element are pure software modules with the functionality of a secure element, which are integrated in a terminal chip.

Subscription profiles are typically loaded into a secure element after it is already out of the control of the secure element manufacturer and has passed into the possession and thus under the control of the terminal manufacturer or a mobile network operator. In the meantime, multi-subscription secure elements have also been made available, which contain several subscription profiles.

If a secure element should later enable an encrypted subscription profile to be loaded into the secure element, it is recommended that a Diffie Hellman key pair is saved in the secure element during production at the secure element manufacturer. If it should be possible to load several subscription profiles, it is advisable to save several Diffie Hellman key pairs in the Secure Element. The multiple Diffie Hellman key pairs need Trusted storage space that is scarce in the Secure Element. The same problem exists if several authentication keys are to be available for network authentication in the Secure Element.

The document US 20160065370 A1 from the prior art discloses a method for deriving session keys for secure communication between two communication partners, namely one in a user terminal and a server computer. In the process, the communication partners agree on a shared secret, from which both derive a session key. The server computer encrypts key derivation parameters with the session key and transmits the encrypted key derivation parameters to the user terminal. A cryptogram key is derived using the key derivation parameters and the shared secret. The communication partners do not need to save this permanently in order to use it.

The document US 20160241389 A1 from the prior art discloses a method for secure communication between a server and a client. In this method, the server sends a message to the client with an identifier of a private server key with which the message was encrypted. In a later message from the client back to the server, which also contains the identifier and the encrypted message, the server can use the identifier to determine the private server key with which the message was encrypted and decrypt the message.

The document US 20120144193 A1 discloses a protocol for agreeing common keys between a terminal with a secure element and a host terminal, in which the terminal is identified by the host terminal on the basis of information encrypted by the terminal and an anonymous identifier of the terminal.

Summary of the invention

The invention is based on the object of providing a method for key derivation which generates a plurality of session keys and which is suitable for using the memory present in the secure element economically.

The object is achieved by a method according to claim 1. Advantageous refinements of the invention are specified in dependent claims.

The method according to claim 1 is based in principle on the Diffie Hellman key exchange described above. In contrast to the basic form of Diffie Hellman, a plurality of at least two random numbers is generated in the secure element by means of a pseudo-random number generator implemented in the secure element, instead of just a random number, and is provided as a corresponding plurality of at least two separate private keys. Furthermore, a corresponding plurality of at least two separate public keys are derived in the secure element and transmitted to the server. In addition, a key index is assigned to each key pair consisting of the private key and the public key of the Secure Element and transmitted to the server.

The server now has information in the form of the multiple public keys of the secure element for deriving several session keys, as well as a key index for each future session key, which enables the server to reference a desired session key with respect to the secure element. The secure element also has a pseudo-random number generator, with which it can generate the own random number generated at the beginning of the Diffie Hellman method at any time. As a result, the secure element does not necessarily have to store the key pairs it has generated. Rather, after the server has received all the information required for key derivation, including the key index, the secure element can delete the generated key pairs again. It is sufficient to store the key indexes in the Secure Element, because with their help the pseudo-random number generator of the Secure Element can generate the key pairs again.

Therefore, according to claim 1, a method for key derivation is created which generates several session keys and which is suitable for using the memory present in the secure element economically.

The final derivation of the session key, a plurality of at least two common session keys being derived accordingly, can take place at any point in time and does not have to take place in a close temporal connection to derive the key pairs. As explained below, in preferred embodiments, public / private key pairs are derived in a first method section. The session keys are only derived in a downstream, separate second process section, which can be separated in time and space from the first process section, and in which the secure element may already be in the possession of another entity than in the first stage of the procedure.

According to some embodiments of the invention, the method is therefore divided into two, in that it comprises at least two method sections.

The method, which is divided in two, comprises a first method section comprising steps (i) to (iii) and not comprising step (iv). In the first step of the method, the plurality of public keys transmitted in step (iii) are stored together with the indices at the server. In the first step of the procedure, the server is equipped with the parameters (public keys and indices) that it needs to derive session keys for secure communication with the Secure Element.

The two-part method further comprises, in the second part of the method, at least one second part of the method. More precisely, the second part of the method comprises either exactly a second part of the method or a number of second parts of the method corresponding to the number of key pairs generated in the secure element. The or each second process section comprises at least one step (iv), i.e. deriving at least one common session key from the server and the secure element. The second step of the method is only started when the server requests the secure element, in which case the public key of the server is retransmitted to the secure element.

The two-part method has the additional advantage that the server can be equipped with the public keys for deriving several session keys at any time in advance, in the first part of the method. At the time when session keys are derived (i.e. in the second stage of the procedure), the transfer effort for the transfer of public keys and indexes to the server is eliminated and the session key generation can be carried out more quickly. The server has a large storage capacity. Therefore, permanent storage of previously received public keys and indexes from Secure Elements is comparatively uncritical. The secure element, on the other hand, has low storage capacities, so that as little data as possible, e.g. Key to be saved there permanently.

According to a preferred embodiment (claim 3), the two-part method further comprises the following steps. In the course of the first section of the method, or after completion of the first section of the method, the key indexes are stored in the secure element and the plurality of key pairs generated are deleted from the secure element. When the server requests the secure element to start the second method section, the server transmits at least one or exactly one key index to the secure element. Upon request or transmission of the key index, the private element of the secure element is generated again in the secure element by means of the pseudo random number generator using the respective key index, and step (iv), deriving at least one or exactly one common session key is carried out in the Secure element performed.

This preferred embodiment of the two-part method has the additional advantage that the keys of the secure element do not have to be stored permanently in the secure element, which has only comparatively little memory compared to the server. At the same time, the server has a pool of several public keys and indexes of the Secure Element in order to be able to gradually derive several session keys. In addition, they have the two process steps carried out separately from one another in terms of time and / or space. This enables one party (e.g. secure element manufacturer) to perform the first stage of the procedure itself and to delegate the second stage of the procedure to another party (e.g. party loading content, e.g. subscription profiles into the secure element), as will be explained below.

The first and the second method section are optionally carried out during different production sections in a production process for producing the secure element or the terminal device and / or under the control of different owners of the secure element or the terminal device.

Optionally, the first process step is carried out during the production of the Secure Element by a Secure Element manufacturer or / and the second process step is carried out during the provisioning of the Secure Element with one or more subscription profiles by a manufacturer of the terminal device or by a mobile network operator. The device manufacturer or mobile network operator is the owner of the Secure Element. However, ownership of the Secure Element remains with the manufacturer of the Secure Element. By only transferring the data required for the second process step to the new owner, the secure element manufacturer only transfers limited uses of the secure element.

A method according to the invention for loading a subscription profile from a server into a secure element comprises the following Steps. Deriving a common session key as described above, using the two-part method in the embodiment that the private keys generated in the secure element are deleted again from the secure element. On the server, encrypting a subscription profile to be loaded with the session key. Load the encrypted subscription profile from the server into the Secure Element. In the secure element, decrypting the subscription profile with the session key and implementing the decrypted subscription profile. The server received public keys and indexes of the secure element in advance, so that the server is able to derive session keys for the secure element. The secure element initially no longer has its private key. Only when the server sends a key index to the Secure Element does the Secure Element generate its own private key again using the pseudo-random number generator, and the session key using the newly generated private key and the public key of the server. The public key of the server can optionally be transmitted to the secure element in the first process section and stored there. Alternatively, the public key of the server is only transferred to the secure element in the second method step, together with the request from the server to the secure element, so that it does not have to be stored permanently in the secure element, but only during the derivation of the session key.

In a method for loading multiple subscription profiles, several session keys are derived using the method described above. Each subscription profile is encrypted with one of the several session keys.

A method for authentication between a server of a mobile radio network and a secure element comprises the following steps: deriving a common session key by means of the two-part method with deleting the keys generated in the first method section from the secure element; - at the server, encrypting an authentication message with the session key; - Transmission of the encrypted authentication message from the server to the Secure Element; - In the secure element, decrypting the encrypted authentication message with the session key and, in the case of successful decryption, successful authentication of the server.

The authentication method optionally also includes transferring a challenge from the secure element to the server. The challenge is included in the encrypted authentication message. The encrypted authentication message represents a response to the challenge.

According to embodiments of the invention, further parameters for deriving the key pair are agreed between the secure element and the server, in particular a prime number p and a large number g and, if necessary, elliptic curve parameters, in particular the parameters provided for a Diffie-Hellman method.

In the case of a two-part method, the public key of the server is optionally only transferred to the secure element in the second method section, together with the request from the server, so that the public key of the server only needs to be stored in the secure element for deriving session keys, but preferably not permanent.

list of figures

• 1 eine vereinfachte schematische Darstellung der Schlüsselableitung nach Diffie Hellman;
• 2 eine vereinfachte schematische Darstellung einer Schlüsselableitung, nach einer Ausführungsform der Erfindung.

The invention is explained in more detail below on the basis of exemplary embodiments and with reference to the drawings, in which:

• 1 a simplified schematic representation of the key derivation according to Diffie Hellman;
• 2 a simplified schematic representation of a key derivation, according to an embodiment of the invention.

Detailed description of exemplary embodiments

2 shows a simplified schematic representation of a key derivation, according to an embodiment of the invention. The basic process is the same as for the Diffie Hellman process 1 , Generating private and public keys A-Pr . A-Pub . B-Pr . B Pub forms a first stage of the process. Deriving the session key K forms a second stage of the process. In addition, one in the secure element B implemented pseudo-random number generator multiple random numbers b1 . b2 . b3 . b4 . b5 , ... as private key of the Secure Element B derived, as well as a key index for each derived private key 11 . 12 . 13 . 14 . 15 , .... The Secure Element forwards a corresponding number 1 . 2 . 3 . 4 . 5 , ... public key B Pub and sends it along with the indices 11 . 12 . 13 . 14 . 15 , ... to the server A , In the secure element B itself only the indices 11 . 12 . 13 . 14 . 15 , ... saved. The generated private and public keys themselves are deleted. Takes the same or a different server later A (who got the indices and public keys from the first server), the Secure Element generates the private keys again based on the indices and can therefore use the public key received from the server A-Pub the server session key K derived. The public key A-Pub of the server can optionally be connected to the secure element in the first method section B transferred and stored there. Alternatively, the public key A-Pub of the server A only in the second stage of the process to the Secure Element B transmitted, along with the request from the server A to the Secure Element B so that it is not permanently in the secure element B must be saved, but only during the derivation of the session key.

Claims (11)

Method for deriving session keys, based on the key derivation according to Diffie Hellman, for secure communication between two communication partners, namely a Secure Element (B) and a server (A) implemented or implementable in a mobile radio-capable terminal device, comprising the steps, each for Secure Element (B) and at the server (A): (i) generating or obtaining a random number (b, a) and providing the random number (b, a) or a value derived from it using a one-way function as a private key ( B-Pr, A-Pr); (ii) deriving one's own public key (B-Pub, A-Pub) starting from one's own private key (B-Pr, A-Pr); (iii) transferring the derived own public key (B-Pub, A-Pub) or a value derived therefrom by means of a one-way function to the respective other communication partner (A, B); (iv) deriving at least one common session key (K) of the communication partners based on their own private key (B-Pr, A-Pr) and on the transmitted public key (A-Pub, B-Pub) of the other communication partner (A, B) , characterized in that in step (i) in the secure element (B), by means of a pseudo-random number generator implemented in the secure element, a plurality of at least two random numbers (b1, b2, ...) and a corresponding plurality of at least two private keys (B-Prl, B-Pr2, ...) are provided; in step (ii) in the secure element (B) a corresponding plurality of at least two own public keys (B-Publ, B-Pub2, ...) is derived and transmitted to the server (A); a key index (11, 12, ...) is assigned to each key pair of private key (B-Prl, B-Pr2, ...) and public key (B-Publ, B-Pub2, ...) of the Secure Element and in step (iii) the keys of the plurality of public keys (B-Publ, B-Pub2, ...) are each transmitted to the server (A) together with the key index (11, 12, ...) assigned to them ; and in step (iv) a corresponding plurality of at least two common session keys (K-1, K-2, ...) is derived. Procedure according to Claim 1 , the method comprising at least two method sections, namely: a first method section comprising steps (i) to (iii), and not comprising step (iv), the plurality of public keys transmitted in step (iii) in the first method section ( B-Publ, B-Pub2, ...) is saved at the server (A); and at least one second method section or a number of second method sections corresponding to the number of key pairs (B-Pr1, B-Pr2, ...; B-Pub1, B-Pub2, ...) generated in the Secure Element, each comprising at least a step (iv), the second method section only being started when the server (A) requests the secure element (B). Procedure according to Claim 2 , further comprising the steps: - in the course of the first process section or after completion of the first process section in the Secure Element (B): --- storing the key indexes (11, 12, ...) in the Secure Element (B); --- Delete the plurality of generated key pairs (B-Prl, B-Pr2, ...; B-Publ, B-Pub2, ...) from the Secure Element (B); and - when the server (A) is requested to the secure element (B) to start the second process step: --- by the server (A), transferring at least one or exactly one key index (11, 12, ... ) to the Secure Element (B); --- with the Secure Element (B), using the pseudo-random number generator based on the respective key index (11, 12, ...), generating the private key again (B-Prl, B-Pr2, ...) and performing Step (iv) deriving at least one or exactly one common session key (K-1, K-2, ...). Procedure according to Claim 2 or 3 , wherein the first and the second process section are carried out during different production sections in a production process for producing the Secure Element (B) or the end device and / or under the control of different owners of the Secure Element (B) or the end device. Procedure according to Claim 4 , wherein the first process step is carried out during the production of the Secure Element (B) at a Secure Element manufacturer and / or the second process step during the provisioning of the Secure Element (B) with one or more subscription Profiles with a manufacturer of the end device or with a mobile network operator who owns the Secure Element (B), whereby the ownership of the Secure Element (B) remains with the manufacturer of the Secure Element (B). Method for loading a subscription profile from a server (A) into a secure element (B), comprising the steps: - Deriving a common session key (K-1) by means of the method according to one of the Claims 3 to 5 , - at the server (A), encrypting the subscription profile with the session key (K-1), - loading the encrypted subscription profile from the server (A) into the secure element (B), - in the secure element (B), Decrypt the subscription profile with the session key (K-1) and implement the decrypted subscription profile. Procedure for loading multiple subscription profiles, using the procedure after Claim 6 multiple session keys (K-1, K-2, ...) are derived and each subscription profile is encrypted with one of the session keys (K-1, K-2, ...). Method for authentication between a server (A) of a mobile radio network and a secure element (B), comprising the steps: - Deriving a common session key (K-1) by means of the method according to one of the Claims 3 to 5 , - at the server (A), encrypting an authentication message with the session key (K-1), - transmitting the encrypted authentication message from the server (A) to the secure element (B), - in the secure element (B), decrypting the encrypted authentication message with the session key (Kl) and in the case of successful decryption successful authentication of the server (A). Procedure according to Claim 8 , further comprising the step: transferring a challenge from the Secure Element (B) to the server (A); the challenge being included in the encrypted authentication message and the encrypted authentication message representing a response to the challenge. Procedure according to one of the Claims 1 to 9 , whereby further parameters for deriving the key pair are agreed between the secure element (B) and the server (A). Procedure according to one of the Claims 1 to 10 combined with Claim 2 , wherein the public key (A pub) of the server (A) is only transferred to the secure element (B) in the second method section, together with the request from the server (A), so that the public key (A pub) of the Servers (A) need only be stored in the Secure Element (B) to derive session keys (K).

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