DE102015220024A1 - A method of creating a secret or key in a network - Google Patents

Abstract

A method is proposed for generating a secret in a first network subscriber, in which the first network subscriber initiates transmission of at least one first value sequence on a transmission channel at least partially synchronously with transmission of at least one second value sequence on the transmission channel by a second network participant. The first network subscriber determines the secret on the basis of the at least one first value sequence and on the basis of a superposition of the at least one first value sequence and the at least one second value sequence on the transmission channel. A key based on the secret is stored in a volatile memory of the first network participant and a communication of the first network participant is secured by means of the key.

Description

The present invention relates to a method for generating a secret in a network, in particular for generating a common, secret key in two users of the network.

Also point-to-point connections are usually counted as networks and should also be addressed here with this term. The two participants communicate via a shared transmission medium. In this case, logical bit sequences (or, more generally, value sequences) are transmitted physically by means of corresponding transmission methods as signals or signal sequences. The underlying communication system may e.g. be a CAN bus. This provides for transmission of dominant and recessive bits or correspondingly dominant and recessive signals, whereby a dominant signal or bit of a participant of the network intersperses against recessive signals or bits. A state corresponding to the recessive signal adjusts itself to the transmission medium only if all participants involved provide a recessive signal for transmission or if all participants transmitting at the same time transmit a recessive signal level.

State of the art

Secure communication between different devices is becoming more and more important in an increasingly networked world and is an essential prerequisite for the acceptance and thus the economic success of the corresponding applications in many areas of application. Depending on the application, this includes various protection goals, such as safeguarding the confidentiality of the data to be transferred, the mutual authentication of the nodes involved or the assurance of data integrity.

In order to achieve these protection goals, suitable cryptographic methods are usually used, which can generally be subdivided into two different categories: on the one hand, symmetrical methods in which the sender and receiver have the same cryptographic key, and on the other asymmetrical methods in which the sender transfers the ones to be transmitted Encrypt data with the public key (that may also be known to a potential attacker) of the recipient, but decrypt it only with the associated private key, ideally known only to the recipient.

One of the disadvantages of asymmetric methods is that they usually have a very high computational complexity. Thus, they are only conditionally for resource constrained nodes, such as e.g. Sensors, actuators, or similar, suitable, which usually have only a relatively low computing power and low memory and energy-efficient work, for example due to battery operation or the use of energy harvesting. In addition, there is often limited bandwidth available for data transmission, making the replacement of asymmetric keys with lengths of 2048 bits or even more unattractive.

For symmetric methods, however, it must be ensured that both the receiver and the transmitter have the same key. The associated key management generally represents a very demanding task. In the area of mobile telephony, for example, keys are inserted into a mobile telephone with the aid of SIM cards, and the associated network can then assign the unique identifier of a SIM card to the corresponding key. In the case of wireless LANs, on the other hand, a manual entry of the keys to be used (usually by entering a password) usually takes place when setting up a network. However, such key management quickly becomes very cumbersome and impractical if one has a very large number of nodes, for example in a sensor network or other machine-to-machine communication systems, e.g. also CAN-based vehicle networks. In addition, a change in the key to be used is often not possible or only with great effort. In current procedures, the keys are generated centrally. The assignment to individual ECUs takes place in a secure environment z. B. in the factory of the vehicle manufacturer. There, the keys are also activated.

A method for securing sensor data against manipulation and ensuring transaction authentication, for example in a motor vehicle network, using conventional encryption methods are, for example, in DE 102009002396 A1 and in the DE 102009045133 A1 disclosed.

For some time now, the term "physical layer security" has been used to investigate and develop novel approaches by means of which keys for symmetrical methods can be generated automatically on the basis of physical properties of the transmission channels between the nodes involved. One uses the reciprocity and the inherent randomness of these transmission channels. In particular, in wired or optical systems, this approach is often only partially suitable because such channels usually only a very have limited temporal variability and an attacker can draw relatively well on the channel parameters between the transmitter and the receiver, for example, with the help of modeling. Such methods for secure communication in a distributed system based on channel characteristics of the connected units are, for example, in the non-prepublished applications DE 10 2014 208975 A1 such as DE 10 2014 209042 A1 described.

The not pre-published DE 10 2015 207220 A1 discloses a method for generating a shared secret by means of public discussion between two communicants.

Disclosure of the invention

The presented methods for generating a secret or a cryptographic key do not require any manual intervention and thus enable the automated establishment of secure communication relationships or connections between two nodes. In addition, the methods have a very low complexity, in particular with regard to the required hardware design, such as e.g. the required memory resources and computing power, and they are associated with a low energy and time requirements. In addition, the methods offer very high key generation rates with a very low probability of error.

The methods assume that participants in a network communicate with each other via a communication channel. In particular, they transfer logical sequences of values (in the case of binary logic, bit sequences) with the aid of physical signals on the transmission channel. Even if possible superimpositions take place on the transmission channel through the signals, that is to say on the physical level, in the description below the logical level is primarily considered. Thus, the transferred, logical value sequences as well as their logical overlay are considered.

Subscribers of the network can thus give first signals (for example associated with logical bit "1") and second signals (associated, for example, with logical bit "0") to the communication channel and detect resulting signals on the communication channel. Now transmit two participants (largely) at the same time each one signal sequence, the participants can detect the resulting overlay on the communication channel. The effective signal resulting from the (largely) simultaneous transmission of two (independent) signals on the communication channel can then in turn be assigned to one (or more) specific logical values (or values).

The transmission should be largely synchronous in that a superimposition of the individual signals of a signal sequence on the transmission medium takes place, in particular, that the signal corresponding to the n-th logical value or bit of the first subscriber with the signal corresponding to the n-th logical Value or bit of the second participant at least partially superimposed. This overlay should be sufficiently long for the participants to be able to record the overlay or determine the corresponding overlay value.

The superimposition can be determined by arbitration mechanisms or by physical signal superposition. By arbitration mechanism is meant, for example, the case that a node wants to apply a recessive level, but detects a dominant level on the bus and thus omits the transmission. In this case, there is no physical interference between two signals, but only the dominant signal is seen on the transmission channel.

From the resulting value sequence of the overlay and its own 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.

Key distribution presents a general challenge in symmetric cryptosystems. For this reason, once distributed symmetric keys are typically used over an extended period of time or are not updated during the lifetime of a device. In order to be able to switch off such devices temporarily, however, a non-volatile memory is needed in which the corresponding (symmetric) keys can be stored. With the methods described, it is now possible to easily and automatically establish new symmetric keys between two network users. Therefore, it may be possible to dispense with storing the key in a non-volatile memory, because after each (temporary) shutdown of one or both network participants new keys can be generated as needed. The methods are thus particularly for simple devices, such as simple control devices in a vehicle or sensors or actuators in a vehicle attractive because it can be dispensed with a non-volatile memory in these devices. As described, this is due to the fact that network participants can generate new secrets or keys as required by the described methods (eg after restarting a component of a network participant, the network participant or a system comprising the network participant). Another benefit in this context is a potentially higher level of security against certain attacks that can be achieved. Since keys are never stored in non-volatile memory, invasive attacks that aim to extract the key from the non-volatile memory are ineffective.

The described methods can be implemented particularly well in a CAN, TTCAN or CAN FD bus system. Here, a recessive bus level is replaced by a dominant bus 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. The methods are also well suited for other communication systems such as LIN and I2C.

A network or a participant of a network are set up to carry out the described methods in particular by having the corresponding electronic memory and computing resources. Also stored on a storage medium of such a user or on the distributed storage resources of a network may be a computer program configured to perform all the steps of a corresponding method when executed in the subscriber or in the network.

drawings

The invention is described in more detail below with reference to the accompanying drawings and to exemplary embodiments. Show

1 schematically the structure of an exemplary, underlying communication system,

2 schematically a linear bus as an example of an underlying communication system,

3 schematically exemplary signal sequences of two participants of a network and a resulting overlay value sequence on a transmission channel between the participants, and

4 schematically the flow of an exemplary method for generating a key between two participants of a network.

Description of the embodiments

The present invention relates to a method for generating a shared secret or (secret) symmetric cryptographic key between two nodes of a communication system (participants of a network) communicating with each other via a shared medium (transmission channel of the network). The generation or negotiation of the cryptographic keys is based on a public data exchange between the two participants, although a possible listening third party as an attacker is not or only very difficult to draw conclusions about the generated key. With the invention, it is thus possible to completely automatically and securely establish corresponding symmetrical cryptographic keys between two different subscribers of a network in order then to build certain security functions, such as e.g. a data encryption, to realize. As described in detail, a common secret is first established for this, which can be used to generate the 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.

The invention is suitable for a variety of wired or wireless as well as optical networks or communication systems, especially those in which the various participants communicate with each other via a linear bus and the media access to this bus using a bitwise bus arbitration. This principle represents, for example, the basis of the widespread CAN bus. Possible fields of application of the invention accordingly include, in particular, CAN-based vehicle networks as well as CAN-based networks in automation technology.

The present invention describes an approach with which automatically symmetric cryptographic keys can be generated in one, or in particular between two nodes of a network. This generation takes place by exploiting properties of the corresponding transfer layer. Unlike the However, physical approaches to "physical layer security" are not evaluated physical parameters of the transmission channel such as transmission strength, etc. Rather, there is a public data exchange between the nodes involved, thanks to the characteristics of the communication system and / or the modulation method used a possible listening attacker no, or no sufficient conclusions on the negotiated key allows.

The following is an arrangement as abstract in 1 is shown. There are different participants 2 . 3 and 4 via a so-called shared medium ("shared medium") 10 communicate with each other. In an advantageous embodiment of the invention, this divided transmission medium corresponds to a linear bus (wired or optical) 30 as he exemplifies in 2 is shown. The network 20 in 2 consists of just this linear bus 30 as a shared transmission medium (for example as a wired transmission channel), subscribers or nodes 21 . 22 and 23 and (optional) bus terminations 31 and 32 ,

The following is for communication between the various nodes 21 . 22 and 23 assumed that it is characterized by the distinction between dominant and recessive values. In this example, the possible values are the bits "0" and "1". In this case, a dominant bit (eg, the logical bit, 0 ') quasi displace or overwrite a simultaneously transmitted recessive bit (eg, the logical bit, 1').

An example of such a transmission method is the so-called on-off-keying (amplitude-shift keying), in which exactly two transmission states are distinguished: in the first case (value 'on' or '0') becomes Signal transmitted, for example in the form of a simple carrier signal, in the other case (value, Off ', or "1") no signal is transmitted. The state 'On' is dominant while the state 'Off' is recessive.

Another example of a corresponding communication system that supports this distinction between dominant and recessive bits is a (wired or optical) system based on bitwise bus arbitration, such as that used in the CAN bus. The basic idea here is also that if, for example, two nodes want to transmit a signal at the same time and one node transmits a '1', whereas the second node transmits a '0' which 'gains' '0' (ie the dominant bit) ie the signal level which can be measured on the bus corresponds to a logical '0'. In CAN, this mechanism is used in particular for the resolution of possible collisions. In doing so, higher priority messages (i.e., previous dominant signal level messages) are prioritized by each node simultaneously monitoring the signal level on the bus bit by bit in the transmission of its CAN identifier. If the node itself transmits a recessive bit but a dominant bit is detected on the bus, the corresponding node aborts its transmission attempt in favor of the higher priority message (with the earlier dominant bit).

The distinction between dominant and recessive bits makes it possible to interpret the shared transmission medium as a kind of binary operator that combines the various input bits (= all bits transmitted simultaneously) using a logical AND function.

In 3 For example, it shows how a subscriber 1 (T1) keeps the bit sequence 0, 1, 1, 0, 1 ready for transmission between the times t0 and t5 via the transmission channel. Subscriber 2 (T2) holds the bit sequence 0, 1, 0, 1, 1 ready for transmission between times t0 and t5 over the transmission channel. With the above-described characteristics of the communication system and assuming that the bit level "0" in this example is the dominant bit, the bit string 0, 1, 0, 0, 1 will be seen on the bus (B) be. Only between the times t1 and t2 as well as between t4 and t5, both subscriber 1 (T1) and subscriber 2 (T2) provide a recessive bit "1", so that only here does the logical AND link have a bit level of "1" on the bus (B) results.

Taking advantage of these characteristics of the transmission method of the communication system, a key generation between two participants of a corresponding network can now be done by the participants detect a superposition of bit sequences of the two participants on the transmission medium and from this information together with information about the self-transmitted bit sequence a common (symmetric ), generate secret keys.

An exemplary, particularly preferred implementation will be described below with reference to FIG 4 explained.

The process for generating a symmetric key pair is in step 41 started by one of the two nodes involved in this example (subscriber 1 and subscriber 2). This can be done, for example, by sending a special message or a special message header.

Both participant 1 and participant 2 generate in step 42 first locally (ie internally and independently) a bit sequence. Preferably, this bit sequence is at least twice, in particular at least three times as long as the common key desired as a result of the method. The bit sequence is preferably generated in each case as a random or pseudo-random bit sequence, for example with the aid of a suitable random number generator or pseudorandom number generator.

Example of local bit sequences of length 20 bits:

• - Generated bit sequence of subscriber 1: S _T1 = 01001101110010110010
• - Generated bit sequence of subscriber 2: S _T2 = 10010001101101001011

In one step 43 Participants 1 and 2 transmit to each other (largely) synchronously their respective generated bit sequences over the shared transmission medium (using the transmission method with dominant and recessive bits, as previously explained). Different possibilities for synchronizing the corresponding transmissions are conceivable. Thus, for example, either subscriber 1 or subscriber 2 could first send a suitable synchronization message to the respective other node and then start the transmission of the actual bit sequences after a certain period of time following the complete transmission of this message. Equally, however, it is also conceivable that only one suitable message header is transmitted by one of the two nodes (eg a CAN header consisting of arbitration field and control field) and during the associated payload phase then both nodes simultaneously (largely) transmit their generated bit sequences synchronously , In a variant of the method, in step 42 generated bit sequences of a participant in step 43 can also be transmitted to several messages distributed, for example, if this requires the (maximum) sizes of the corresponding messages. In this variant too, the transmission of the correspondingly large number of correspondingly large messages distributed bit sequences of the other subscriber takes place again (largely) synchronously.

On the shared transmission medium, the two bit sequences then overlap, whereby due to the previously required property of the system with the distinction of dominant and recessive bits, the individual bits of subscriber 1 and subscriber 2 result in an overlay, in the example mentioned de facto AND-linked. This results in a corresponding overlay on the transmission channel, which could detect, for example, a listening third party.

Example of a overlay bit string for the above local bit strings:

• • Effective bit sequence on the transmission channel: S _eff = S _T1 AND S _T2 = 00000001100000000010

Both subscriber 1 and subscriber 2 detect during the transmission of their bit sequences of the step 43 in a parallel step 44 the effective (overlaid) bit sequences S _eff on the shared transmission medium. For the example of the CAN bus, this is usually done in conventional systems during the arbitration phase anyway.

This is likewise possible for systems with on-off keying (wireless, wired or optical). The practical feasibility here benefits in particular from the fact that in such a system the state 'On' is dominant and the state 'Off' is recessive (as already explained above). Thus, even without measurement, a node knows that the effective state is dominant on the shared medium if the node itself has sent a dominant bit. On the other hand, if a node has sent a recessive bit, it does not readily know the state on the shared transmission medium, but in this case it can determine how it looks by means of a suitable measurement. In fact, since the node does not transmit anything in this case, there are no problems with so-called self-interference, which would otherwise require complex echo cancellation, especially in the case of wireless systems.

In a next step 45 Both subscriber 1 and subscriber 2 likewise transmit (largely) their initial bit sequences S _T1 and S _T2 again synchronously, but this time inverted. The synchronization of the corresponding transmissions can again be realized exactly in the same way as described above. On the shared communication medium, the two sequences are then ANDed together again. Subscribers 1 and 2 in turn determine the effective superimposed bit sequences S _eff on the shared transmission medium.

Example of the above bit sequences:

• Inverted bit sequence of subscriber 1: S _T1 '= 10110010001101001101
• Inverted bit sequence of subscriber 2: S _T2 '= 01101110010010110100
• - Effective, superimposed bit sequence on the channel: S _eff '= S _T1 ' AND S _T2 '= 00100010000000000100

Both subscriber 1 and subscriber 2 determine during the transmission of their now inverted bit sequences then again the effective, superimposed bit sequences on the shared transmission medium. At this time, both nodes (subscriber 1 and subscriber 2), as well as a possible attacker (eg subscriber 3) who is listening to the communication on the shared transmission medium, thus know the effective, superimposed bit sequences S _eff and S _eff '. In contrast to the attacker or third participant, however, participant 1 still knows his initially generated, local bit sequence S _T1 and participant 2 his initially generated, local bit sequence S _T2 . However, subscriber 1 in turn does not know the initially generated, local bit sequence of subscriber 2 and subscriber 2 does not know the initially generated, local bit sequence of subscriber 1. The detection of the overlay bit sequence again takes place during the transmission in step 46 ,

As an alternative to this exemplary embodiment, subscriber 1 and subscriber 2 can also send their inverted, local bit sequence directly with or directly after their original, local bit sequence, ie steps 45 and 46 done with the steps 43 and 44 , The original and the inverted bit sequence can be transmitted in a message, but also in separate messages as part bit sequences.

In step 47 Subscriber 1 and subscriber 2 now each locally (ie internally) link the effective, superposed bit sequences (S _eff and S _eff '), in particular with a logical OR function.

Example of the above bit sequences:

• S _tot = S _eff OR S _eff '= 00100011100000000110

The individual bits in the bit sequence (S _ges ) resulting from the OR operation now indicate whether the corresponding bits of S _T1 and S _{T2 are} identical or different. For example, if the nth bit within S _tot is a '0', it means that the nth bit within S _{T1 is} inverse to the corresponding bit within S _T2 . Likewise, if the nth bit within S _{tot is} a '1', the corresponding bits within S _T1 and S _{T2 are} identical.

Participant 1 and Participant 2 will then step out 48 based on the bit sequence S _ges obtained from the OR combination in their original, initial bit sequences S _T1 and S _T2 all bits which are identical in both sequences. This consequently leads to correspondingly shortened bit sequences.

Example of the above bit sequences:

• - Abbreviated bit sequence of user 1: S _{T1, V} = 01011100101100
• - Abbreviated bit sequence of subscriber 2: S _{T2, V} = 10100011010011

The resulting, shortened bit sequences S _{T1, V} and S _{T2, V} are now just inverse to each other. Thus, by inversion of its shortened bit sequence, one of the two subscribers can determine exactly the shortened bit sequence that already exists in the other subscriber.

The thus shared, shortened bit sequence will now be from participant 1 and participant 2 in step 49 each locally processed in a suitable manner to generate the actually desired key of the desired length N. Again, there are a variety of ways that this treatment can be done. One possibility is to select N bits from the common truncated bit sequence, where it must be clearly defined which N bits are to be taken, eg simply by selecting the first N bits of the sequence. It is also possible to calculate a hash function via the shared, shortened bit sequence which provides a hash of length N. In general, the rendering can be done with any linear and nonlinear function that returns a N bit length bit sequence when applied to the co-present truncated bit sequence. The mechanism of key generation from the common truncated bit sequence is preferably identical in both subscribers 1 and 2 and is performed accordingly in the same way.

Following the key generation, it may still be possible to verify that the keys generated by subscribers 1 and 2 are actually identical.

For this purpose, for example, a checksum could be calculated using the generated keys and exchanged between subscribers 1 and 2. If both checksums are not identical, then obviously something has failed. In this case, the described method for key generation could be repeated.

In a preferred variant of the method for generating a key, in a number of passes, a whole series of resulting shortened bit sequences, which are each present in the case of subscribers 1 and 2, can be generated, which are then combined into a single large sequence before the actual key is derived therefrom , This may also be adaptive respectively. If, for example, the length of the common, shortened bit sequence is, for example, less than the desired key length N after a single pass through the described procedure, then one could, for example, generate further bits before the actual key derivation.

Finally, the generated symmetric key pair may be subsumed by Subscriber 1 and Subscriber 2 in conjunction with established (symmetric) cryptographic methods, e.g. Ciphers for data encryption.

A potential attacker (eg subscriber 3) can listen to the public data transmission between subscriber 1 and subscriber 2 and thus gain knowledge of the effective, superimposed bit sequences (S _eff and S _eff ') as described. The attacker then only knows which bits in the locally generated bit sequences of nodes 1 and 2 are identical and which are not. In addition, with the identical bits, the attacker can even determine whether it is a '1' or a '0'. For a complete knowledge of the resulting, shortened bit sequence (and thus the basis for the key generation), however, he lacks the information about the non-identical bits. In order to further aggravate possible attacks for the attacker, in a preferred variant the bit values identical in the original, locally generated bit sequences of the users 1 and 2 are additionally deleted. This means that participant 3 has only information that is not used for key generation. Although he knows that correspondingly shortened bit sequences emerge from the different between the local bit sequences of the participants 1 and 2 participants bits. However, he does not know which bits have been sent by subscriber 1 and subscriber 2 respectively.

In addition to the information about the superimposed overall bit sequence, subscriber 1 and subscriber 2 also have the information about the locally generated bit sequence transmitted by them in each case. The fact that the keys generated in subscribers 1 and 2 remain secret as a basis despite the public data transmission results from this information advantage over a subscriber 3 following only the public data transmission.

A key derived from the secret is now to be stored in a volatile memory, in particular in a RAM, of the network participants. This can be saved in the network participants non-volatile memory. Upon restarting one of the network participants involved, the secret and / or the key will then be lost. The network participants can initiate a new secret generation in this case. This can e.g. happen by one of the network participants informing one or more remaining network participants that he no longer has a secret or a key. In principle, a secret generation or a key exchange between the communication partners of the network can also take place with each startup of a network participant or of a network participant's parent system. In particular, such a key can then be used to secure communication over the transmission channel, e.g. be encrypted or secured via a message authentication code (Message Authentication Code).

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102009002396 A1 [0007]
• DE 102009045133 A1 [0007]
• DE 102014208975 A1 [0008]
• DE 102014209042 A1 [0008]
• DE 102015207220 A1 [0009]

Claims (7)

Method for generating a secret in a first network subscriber ( 21 ), the first network participant ( 21 ) transmission of at least one first value sequence on a transmission channel ( 30 ) substantially in synchronism with a transmission of at least one second value sequence on the transmission channel by a second network participant ( 22 ) and the secret based on the at least one first value sequence and on the basis of an overlay of the at least one first value sequence and the at least one second value sequence on the transmission channel ( 30 ), characterized in that a key generated on the basis of the secret in a volatile memory of the first network participant ( 21 ) and a communication of the first network participant ( 21 ) is secured by the key. A method according to claim 1, characterized in that during or after a restart of the volatile memory or the network participant ( 21 ) or one of the first network participants ( 21 ) system is triggered in order to identify a new secret for securing and to generate a new key. Method according to one of claims 1 or 2, characterized in that the key is stored only in a volatile memory. Method according to one of the preceding claims, characterized in that the first network participant ( 21 ) is a control device, an actuator or a sensor of a vehicle. Method according to one of the preceding claims, characterized in that the transmission channel is part of a CAN bus system and the communication is carried out according to a CAN protocol.  Computer program adapted to carry out the steps of a method according to one of claims 1 to 5.  Network subscriber adapted to perform the steps of a method according to any one of claims 1 to 5.

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