WO2017064008A1 - Method for generating a secret in a network comprising at least two users connected to a transmission medium - Google Patents

Abstract

The invention relates to a method for generating a secret in a network comprising at least one user (100) connected to a transmission medium (1), wherein the user (100) initiates, during a network communication used for generating the secret, a first user value sequence for transmission to the transmission medium (1) using bit banging.

Description

 Description title

 A method of generating a secret in a network having at least two subscribers connected to a transmission medium

The present invention relates to a method for generating a secret in a network having at least two subscribers connected to a transmission medium and to a subscriber and a computer program for carrying it out.

State of the art

In the subsequently published DE 10 2015 207 220 A1, the Applicant has proposed a method for generating a secret or key in a network, which uses a superimposition of signals of two subscribers on a common transmission medium. In this case, the network has at least a first and a second subscriber and a transmission channel between at least the first and the second subscriber. The first and second subscribers may each provide at least a first value and a second value to the transmission channel. The first subscriber or the second subscriber initiate a first subscriber value sequence or a second subscriber value sequence for transmission to the transmission channel which is largely synchronous with one another. On the basis of information about the first subscriber value sequence or the second subscriber value sequence and on the basis of an overlay value sequence resulting from a superposition of the first subscriber value sequence with the second subscriber value sequence on the transmission channel, the first subscriber or the second subscriber generate a shared secret or a common cryptographic key. Such a method is particularly well-suited for communication systems which provide for transmission of dominant and recessive bits and respectively dominant and recessive signals, whereby a dominant signal or bit of a participant of the network intersperses against recessive signals or bits. An example of this is CAN (Controller Area Network), in which the access to this bus takes place by means of a bitwise bus arbitration, which uses dominant and recessive bits for this transmission method. Further examples are TTCAN, CAN FD, LIN and l ² C. These transmission methods have been established for a long time and can be tested and standardized

Network interface devices, such as So-called network controller, easy to implement. For direct physical bus coupling is usually a transceiver module (also known as bus driver or medium attachment unit (MAU)) responsible. For a typical network connection of a computing unit (for example microcontroller), a network interface

Block, which can also be an integrated part of the arithmetic unit, used to generate the logic signals and a data transceiver connected to this transceiver module for generating the physical signals. The problem, however, is that it is not readily possible for a subscriber to generate any signal value sequences on the transmission medium by means of the network interface module, since conventional network interface modules are set up to implement protocol-compliant signal value sequences, e.g. with headers, checksums, etc., and e.g. also make the bus arbitration.

Disclosure of the invention

According to the invention, a method for generating a secret in a network with at least two subscribers connected to a transmission medium as well as a subscriber and a computer program for carrying it out with the features of the independent patent claims are proposed. Advantageous embodiments are the subject of the dependent claims and the following description. In order to advantageously make it possible for a subscriber to generate arbitrary signal value sequences on the transmission medium for secret generation, it is proposed to use bit-banging the signal value sequence (s) in the course of a secret generation and not by means of a network interface. Building blocks. If necessary, received signals are then also evaluated by means of bit-banging.

Bit banging is a technique that uses hardware and I / O (I / O) input / output (I / O) connectors to create a hardware

This interface is typically emulated with a specific peripheral device (in this case the network interface device, such as a CAN controller). On a PC both the serial and the parallel interface can be used. Microcontrollers use the I / O ports, e.g. firmly defined I / O or GPIO (General Purpose Input / Output), i. Optionally configurable as input or output ports or pins.

In other words, in the course of mystery generation serving network communication, the logical signals to be sent are not from

Network interface device, but from I / O ports to the

Transceiver module issued to generate the physical signals, and the received signals not to the network interface module, but also forwarded to the I / O ports.

With the invention, it is thus possible to establish a shared secret between two different subscribers of a network, which can be used in particular for generating a symmetric cryptographic key. However, such a shared secret can in principle also be used for purposes other than cryptographic keys in the strict sense, e.g. as a one-time pad.

Particularly advantageously, the method can be used in a network in which there is a dominant value (physically: a dominant signal) intervenes when only one subscriber applies it to the transmission medium, and a recessive value (physically: a recessive signal) that only results on the transmission medium when both or all participants transmit a recessive value. Because of the clearly defined overlay rules, the participants in such a network can derive from the following

Superimposed value sequences are particularly easy to derive information about generation of a secret.

Alternatively, the transmission of a recessive value of at least one of the subscribers can also be replaced by the fact that at this point the value sequence or, as one of the at least two possible values, nothing is transmitted.

Preferably, the signal value sequence to be transmitted in the course of the network communication serving for secret generation is generated by the computing unit (for example microcontroller) of the subscriber and transmitted to one or more users

Output configured I / O ports, especially general-purpose I / O ports, output as a logical signal value sequence to the transceiver block to generate a physical signal value sequence. A physical signal value sequence received as part of the secret generation network communication is also received by the transceiver module and output as logical signal value sequence to one or more input / output I / O ports, in particular general purpose I / O ports. The received signal value sequence is, in particular, a superposition value sequence resulting from a superposition of the first subscriber value sequence and a second subscriber value sequence on the transmission channel. The respective number of pins results from the network protocol used. For example, one input and one output are sufficient for CAN, which is particularly easy to implement. Conveniently, after completion of the secret generation and for normal or not secret generation serving network communication again the network interface module is used to generate the signal to be sent and to evaluate the received logical signals. Like him- This may be formed as a separate module or be part of the participant's computing unit.

The switchover between the use of the network interface module and the use of I / O connections for generating and evaluating the logic signals is carried out in particular by the computing unit and / or the network transceiver. It may be in accordance with a scheduler or external triggering, e.g. by receiving a corresponding switching message, done.

The invention is particularly well implemented in a CAN, TTCAN or CAN FD bus system. Here, a recessive signal level is displaced by a dominant signal level. The superimposition of values or signals of the subscribers thus follows defined rules which the subscribers can use to derive information from the superimposed value or signal and the value or signal transmitted by them. Other communication systems such as LIN and l ² C are well suited for use of these methods.

Alternatively, however, the method may also be implemented, for example, in a network with amplitude shift keying, e.g. On-Off-Keying, are used. Here, too, the overlay is fixed by allowing the subscribers to be "transmission" and "no transmission" signals and the beat signal corresponding to the "transmission" signal when one or both of the subscribers transmits and corresponds to the "no transmission" signal, if both participants do not transfer.

Preferably, a method for generating a secret based on a superimposition of dominant and recessive signals, for example, according to DE 10 2015 207 220 A1 is used, wherein the network at least a first and a second subscriber and a transmission channel between at least the first and the second subscriber having. The first and second subscribers may each provide at least a first value and a second value to the transmission channel. The first participant or the second participant initiate a first subscriber value sequence or a second subscriber value sequence for each other largely synchronous transmission to the transmission channel. On the basis of information about the first subscriber value sequence or the second subscriber value sequence and on the basis of an overlay value sequence resulting from a superposition of the first subscriber value sequence with the second subscriber value sequence on the transmission channel, the first subscriber or the second subscriber generate a shared secret.

In order to obtain an overlay value sequence, the transmission of values must have overlapping periods (ie be largely synchronous in the context of this application), so that a superimposition of the individual signals of a signal sequence takes place on the transmission medium, in particular such that the signal corresponds to the n th logical value or bit of the first subscriber with the signal according to the n-th logical value or bit of the second subscriber at least partially superimposed. In each case, this overlay should be sufficiently long for the participants to be able to record the overlay or to determine the corresponding overlay value.

The superimposition value can be determined by arbitration mechanisms or by physical signal superposition. By arbitration mechanism is meant, for example, the case that a subscriber has applied a recessive level, but detects a dominant level on the bus and thus omits the further transmission.

From the resulting value sequence of the overlay (ie overlay value sequence) and the own value sequence (ie subscriber value sequence), the participants can then generate a key that is secret to an outside attacker. The reason for this is that the outside attacker, who can listen to the effective overall signals applied to the shared transmission medium, sees only the superimposition of the value sequences, but does not have the information about the individual value sequences of the participants. Thus, the participants have more information that they can use against the attacker to generate a secret key. A further development of the method for secret generation between the subscribers based on a superimposition of dominant and recessive signals provides that the first subscriber value sequence and the second subscriber value sequence each have a first partial value sequence and a second partial value sequence, the second partial value sequence being derived from the first partial value sequence Inverting results, ie by first values to second values are exchanged and second values are exchanged to first values. The first partial value sequence and the second partial value sequence can be transmitted one after the other. Alternatively, a preferred method is proposed in which the values of the first and the second partial value sequences are combined into a subscriber value sequence in a specially sorted manner, wherein at least one value of the second partial value sequence is already transmitted before all values of the first partial value sequence have been transmitted. This makes it possible, already during transmission of the subscriber value sequence and receiving the overlay value sequence with the evaluation and Geheimbzw. Key generation to start. The solution will continue to be independent of buffer or cache memory sizes, as not complete partial value sequences must be stored before the evaluation and secret generation can be started.

An inventive participant, e.g. a control device, a sensor or an actuator, in particular a motor vehicle, an industrial plant, a home automation network, etc., is, in particular programmatically, configured to perform a method according to the invention.

The implementation of the method in the form of a computer program is also advantageous, since this causes particularly low costs, in particular if an executing arithmetic unit is still used for further tasks and therefore exists anyway. Suitable data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories, such as hard disks, flash memories, EEPROMs, DVDs and the like. It is also possible to download a program via computer networks (Internet, intranet, etc.). Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

The invention is illustrated schematically with reference to an embodiment in the drawing and will be described below with reference to the drawing.

Brief description of the drawings

Figure 1 shows a subscriber connected to a CAN bus in a configuration for normal network communication.

FIG. 2 shows the subscriber from FIG. 1 in a configuration for generating a secret or key.

Embodiment (s) of the invention

With reference to Figures 1 and 2, a preferred embodiment of a method according to the invention will be described below. In the figures, a subscriber 100 connected to a network formed here as a CAN bus 1, for example a control device, a sensor or an actuator, in particular in a motor vehicle, is shown in the figures.

The illustrated CAN bus is a two-wire bus with two CAN-H (high) and CAN-L (low) lines. A CAN bus with only one line is also possible.

The subscriber 100 is physically connected to the CAN bus 1 via a transceiver or medium attachment unit (MAU) 50. The subscriber 100 further has a central processing unit, for example a microprocessor (μΡ) 10, an I / O module (I / O) 20, in particular a certain number of E designed as general purpose I / O ports (GPIO) Provides / A terminals, as well as a trained here as a CAN controller (CAN) 30 network interface module. The elements 10, 20, 30 and 40 can also Part of a microcontroller, which is indicated in Figure 1 by a dashed line.

Now, according to a preferred embodiment of the invention optionally the logic signals from the CAN controller 30 or the I / O module 20 to the

To output transceiver 50, here, for example, a multiplexer module (MUX) 40 is provided, which provides the necessary electrical connections. A multiplexer device 40 is usually part of a microcontroller to keep the number of physical pins low. For this, the logic behind the physical pin within the microcontroller

ist der Transceiver 50 angeschlossen, der dann per interner Konfiguration wahlweise mittels E/A-Baustein oder mittels CAN- Controller angesteuert wird. 'Multiplexed'. Thus, it is possible, for example, by configuring corresponding registers to selectively connect the CAN controller 30 or GPIO to a specific pin.

the transceiver 50 is connected, which is then controlled via internal configuration either via I / O module or via CAN controller.

1 shows a configuration in which logic signals are output from the I / O module 20 and transmitted via the multiplexer module 40 to the transceiver 50, which performs the physical network communication. Likewise, the signals received by the transceiver 50 are transmitted to the I / O device 20 via the multiplexer device 40.

FIG. 2 illustrates the alternative situation in which the logic signals are output by the CAN controller 30 and transmitted to the transceiver 50 via the multiplexer module 40. Likewise, the signals received by the transceiver 50 are transmitted to the CAN controller 30 via the multiplexer device 40.

In other words, the situation for the normal network communication is shown in FIG. the network communication is transmitted via the CAN

Controller 30 and the transceiver 50 handled. On the other hand, the situation for generating the secret is shown in FIG. 1, whereby the arithmetic unit 100 is able to generate arbitrary signal value sequences, which in particular do not have to represent packets corresponding to a CAN standard, for secret generation. In this case, the network communication is performed by the I / O module 20 (which receives the logical signals to be output from the microprocessor 10) and the transceiver 50.

A changeover between the two configurations can be caused in particular by the microprocessor 10, which is indicated by the corresponding arrow in the figure. Alternatively or additionally, it may be provided that a changeover is triggered by receiving a special switching message, which is indicated in the figure by the arrow emanating from the transceiver 50.

The invention allows the output of arbitrary subscriber value sequences on network media, although this does not allow the conventional network interface devices.

For a conventional transmission, the central processing unit writes the payload (in particular the identifier, the determination of whether this frame is a data or remote transmission request frame, the specification of how many data bytes are to be sent and the data bytes to be sent) in the transmission Data buffer of the CAN controller, which then prepares it for transmission on the bus and transmits the complete frame to the transceiver block, which is responsible for the direct bus connection. This means that the CAN controller relieves the central processing unit of all data transfer work, since it independently handles the compilation of the message, the calculation of the CRC sum, the access to the bus (bus arbitration), the transmission of the frame and the error check.

If now a secret generation is to take place (for example, triggered by receipt of a corresponding switching message), the connection is interrupted by the CAN controller to the transceiver module during runtime and this is controlled directly by means of I / O. After completion of the secret generation, it is then possible to switch back to the CAN controller.

Claims

claims 1 . Method for generating a secret in a network with at least two subscribers connected to a transmission medium (1), wherein a first subscriber value sequence for transmission to the transmission medium (1) from a first subscriber (100) of the at least two subscribers in the course of a network communication serving for secret generation using bit-banging. The method of claim 1, wherein bit banging is performed by the first subscriber (100) using I / O ports. The method of claim 2, wherein the I / O ports are general purpose I / O ports. The method of any one of the preceding claims, wherein the non-secret network communication is initiated by the first agent (100) using a network interface device (30). 5. The method according to any one of the preceding claims, wherein between serving the secret generation serving network communication and the non-secret generation network communication in accordance with a computing unit of the first subscriber (100) and / or in accordance with a received from the first subscriber (100) switching message , 6. The method according to any one of the preceding claims, wherein from the first participant (100) in the course of mystery generation serving network communication from a superposition of the first subscriber value sequence and a second subscriber value sequence on the transmission medium (1) resulting in overlay value sequence is received using the bit-banging. Method according to claim 6, wherein the second subscriber value sequence is initiated by a second subscriber of the at least two subscribers in the course of the secret communication to transmit to the transmission medium (1), the first and the second subscriber (100) each having at least a first value and giving a second value to the transmission medium (1), wherein the first subscriber the secret based on information about the first subscriber value sequence and based on the overlay value sequence and the second subscriber the secret based on information about the second subscriber value sequence and on the basis of Generate Overlay Value Order. A method according to claim 7, wherein a state corresponding to the first value is set on the transmission medium (1) when both the first and second subscribers cause a transmission of the first value via the transmission medium (1), and a state corresponding to the second Set value when the first or the second party or both cause a transmission of the second value via the transmission medium (1). Method according to claim 7 or 8, wherein the first subscriber value sequence and the second subscriber value sequence each have a first partial value sequence and a second partial value sequence, wherein the second partial value sequence emerges from the first partial value sequence by exchanging first values for second values and exchanging second values for first values become. The method of claim 9, wherein the transmission of at least one value of the second partial value sequence on the transmission medium is already initiated before the transmission of all values of the first partial value sequence has been initiated on the transmission medium. 1 1. Subscriber (100) adapted to perform a method according to any one of the preceding claims. 12. subscriber (100) according to claim 1 1, comprising a central processing unit (10), an I / O module (20), a network interface module (30) and a transceiver module (50). The subscriber (100) of claim 812 including a multiplexer device (40) that selectively communicates with the transceiver device (50) to communicate with the I / O device (20) or the network interface device (30). A computer program that causes a computing unit to perform a method according to any one of claims 1 to 10 when executed on the computing unit. 15. A machine-readable storage medium having a computer program stored thereon according to claim 14.