Arrangement and method for operating the arrangement containing a substation and at least one terminal device connected to it

A method operates an arrangement having a substation and a terminal device connected to the substation. The terminal device is equipped with a terminal device certificate and a private key. The certificate enables a signed data transmission, indicates the substation as the certificate issuer, has a signature of the substation and contains a reference to a public key of the terminal device. Following a connection of a control station to the substation, the control station recertifies the public key of the substation by creating a signed certificate for the substation containing the public key of the substation, for the authentication of data which are or are intended to be transmitted from the terminal device to another terminal device. The terminal device certificate indicating the substation as the certificate issuer is transferred to the other terminal device and the certificate verification is carried out based on recertified public key.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. § 119, of European application EP 16181176.5, filed Jul. 26, 2016; the prior application is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a method for operating an arrangement containing a substation and at least one terminal device connected to the substation. According to the method, the at least one terminal device is equipped with a terminal device certificate and a corresponding private key. The certificate enables a signed data transmission, indicates the substation as the certificate issuer, has a signature of the substation and contains a reference to a public key of the terminal device.

Arrangements of this type are known, for example, in the field of automation technology, particularly in the field of electrical energy distribution. In the last-mentioned case, the substation may, for example, form an energy distribution substation and the terminal device may form an electrical measurement, monitoring and/or protection device.

Communication is normally implemented in today's automation systems by open protocols such as, for example, Internet protocol (IP), transport control protocol (TCP), user datagram protocol (UDP), Hypertext transfer protocol (http) or constraint application protocol (CoAP). If security protocols are used in this context, these are usually based on asymmetric keys (digital certificate with a public key, private keys) in order to authenticate either the server or both participants. Digital certificates, e.g. according to the X.509 standard, can be used to authenticate persons, devices and software processes. The validity of a digital certificate of a communication partner can be checked. To do this, along with the check of the information in the certificate, the certificate path, as it is referred to in the technical language, inter alia, is evaluated in order to check whether the certificate originates from a trusted source. The certificate is acknowledged as valid if the certificate path ends with a root certificate which is stored as trusted (referred to in the technical language as the RootCA (Root Certificate Authority)). A set of trusted root certificates (referred to in the technical language as “trusted root CA”) is normally stored in advance for the certificate validation. Every certificate in which the certificate path ends with a trusted root certificate can be accepted as trusted.

If the device configuration is changed, for example because the arrangement is extended, a change of RootCAs or a change in the hierarchy of the RootCAs may be required (resulting in a different certificate path).

SUMMARY OF THE INVENTION

The object of the invention is to indicate a method for operating an arrangement of the type described above which enables a connection of an additional control station at low cost.

It is thus provided according to the invention that, following the connection of a control station to the substation, the control station recertifies the public key of the substation by creating a signed certificate for the substation containing the public key of the substation. For the authentication of data which are or are intended to be transmitted from the at least one terminal device to another terminal device, to the substation and/or to the control station, the terminal device certificate indicating the substation as the certificate issuer is transferred to the other terminal device, to the substation and/or to the control station and the certificate verification of the terminal device certificate on the receiver side, i.e. on the terminal device side, the substation side and/or the control station side and/or the authentication of the data signed with the corresponding private key is/are carried out on the basis of the recertified public key of the substation.

A fundamental advantage of the method according to the invention is that no new terminal device certificates which take account of the circumstance of the connection of the control station need to be issued for the terminal devices since the public key of the substation can in fact be retained or can continue to be used due to the certificate issued by the control station, i.e. due to the recertification. The further use of the old terminal device certificates is possible even though, in the case of the old terminal device certificates, due to their previous history, the otherwise necessary root reference to the control station or a valid root certificate referring to the control station is missing; the continued use is possible according to the invention because the control station, which operates as the new root certificate authority due to the connection, confirms through the recertification of the substation(s) to all communication participants that the certificates issued earlier by the substation (or substations) should continue to be classified as trusted, even though these certificates are no longer complete in terms of the root certificate. Through the recertification, it is therefore advantageously also possible for terminal devices which are connected to different substations to communicate with one another using encryption and/or digital signatures on the basis of their old terminal device certificates, even though the terminal device certificates that are used originate from a time before the connection of the control station and therefore have no root certificate referring to the new control station.

The fact of the performed recertification is preferably made accessible to all communication participants through storage at a central storage location.

Before the connection of the control station to the substation, two or more terminal devices, referred to below as terminal devices of a first configuration type, can be connected to the substation, the terminal device certificate in the case of the terminal devices in each case indicating the substation as the certificate issuer and the certificate chain ends with the substation as the certificate issuer.

After the connection of the substation to the control station, certificates are typically issued for the substation by the control station as an additional instance. In the certificate verification and/or the signature verification in the case of data which are or have been transmitted from the terminal devices of the first configuration type, the verification of the certificate chain preferably continues to be carried out even after the connection of the control station, in each case only up to the substation as the certificate issuer of the terminal device certificates.

If a further terminal device is connected to the substation after the connection of the control station to the substation, it is advantageous if the further terminal device is equipped—before, during or after the connection—with a terminal device certificate which enables a signed data transmission, indicates the substation as the certificate issuer, has a signature of the substation and contains a reference to a public key of the terminal device. In other words, the further terminal device can be equipped with a terminal device certificate which corresponds to the terminal device certificates of the already previously connected terminal devices of the first configuration type.

The public key of the substation is in each case preferably already stored in the further device(s) which is/are connected to the substation after the control station has already been connected; alternatively, it is preferably stored there during or after the connection.

After the connection of the control station to the substation and a subsequent connection of at least one further terminal device to the substation, during the certificate verification and/or the signature verification in the case of data which are transmitted from the further terminal device to the control station, to the substation, to another further terminal device and/or to a terminal device of the first configuration type, the verification of the certificate chain is preferably carried out only up to the substation as the certificate issuer of the terminal device certificate.

In a further design of the method, it is provided that, after the control station has been connected to the substation, at least one terminal device, referred to below as a terminal device of a second configuration type, is connected to the substation, wherein the substation is or becomes indicated as the certificate issuer in the certification chain. The terminal device certificate furthermore preferably contains a substation certificate or reference is made to such a certificate containing the control station as the certificate issuer; it is thereby achieved that both an intermediate certificate containing the public key of the substation and a root certificate containing the public key of the control station is contained or referenced in the certificate chain.

If a terminal device of the second configuration type transmits a message provided with its terminal device certificate and/or signed with a private key corresponding to the terminal device certificate to another terminal device connected to the substation, in particular a terminal device of the first configuration type, it is advantageous if the terminal device receiving the message carries out the certificate verification on the basis of the intermediate certificate containing the public key of the substation and ignores the root certificate containing the public key of the control station during the authenticity check.

If a terminal device of the first configuration type transmits a message provided with its terminal device certificate and/or signed therewith to another terminal device connected to the substation, it is advantageous if the terminal device receiving the message carries out the certificate verification on the basis of the public key which is contained in the terminal device certificate chain indicating the substation as the certificate issuer or which it has stored locally.

If a terminal device of the first configuration type transmits a message provided with its terminal device certificate and/or signed with a private key corresponding to the terminal device certificate to a terminal device of the second configuration type, it is advantageous if the terminal device of the second configuration type carries out the certificate verification taking account of the public key which is contained in the terminal device certificate of the terminal device of the first configuration type, the terminal device certificate indicating the substation as the certificate issuer, and also on the basis of the public key of the substation which is already stored or referenced in the terminal device of the second configuration type.

If the validity of the terminal device certificates expires due to a predefined expiry date after the connection of the control station, it is advantageous if, when terminal device certificates are reissued, the interim connection of the control station is taken into account and terminal device certificates are created in which the respective substation is indicated as the certificate issuer in the certification chain. A substation certificate is contained in the terminal device certificate or reference is made to such a certificate containing the control station as the certificate issuer, and both an intermediate certificate containing the public key of the respective substation and a root certificate containing the public key of the control station are contained or referenced in the certificate chain.

An electrical energy distribution arrangement is preferably operated as the arrangement. In such a case, the substation preferably forms an energy distribution substation, the terminal device forms an electrical measurement, monitoring and protection device and the control station forms an energy distribution control station.

The invention furthermore relates to an arrangement with a substation and at least one terminal device connected to the substation. The at least one terminal device is equipped with a terminal device certificate which enables a signed data transmission, indicates the substation as the certificate issuer, has a signature of the substation and contains the public key of the terminal device.

It is provided according to the invention that a control station is connected to the substation. The control station having recertified the public key of the substation by signing the key as a certificate issued by it, and the at least one terminal device is designed in such a way that it signs data which are intended to be transmitted to another terminal device, to the substation and/or to the control station with its private key corresponding to the terminal device certificate and indicating the substation as the certificate issuer.

With regard to the advantages of the arrangement according to the invention, reference is made to the advantages of the method according to the invention explained above which apply accordingly to the arrangement.

The control station is preferably designed in such a way that it carries out the verification of the terminal device certificate on the control station side and/or the authentication of the data signed with the corresponding private key on the basis of the recertified public key of the substation.

Although the invention is illustrated and described herein as embodied in a method for operating an arrangement comprising a substation and at least one terminal device connected to it, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

DETAILED DESCRIPTION OF THE INVENTION

For the sake of clarity, the same reference numbers are always used in the figures for identical or comparable components.

Referring now to the figures of the drawings in detail and first, particularly toFIG. 1thereof, there is shown an arrangement10which contains a substation20and two terminal devices30and31connected to the substation20. The terminal devices30and31are equipped in each case with a terminal device certificate EZ30and EZ31respectively and a corresponding private key, the certificate enabling a signed data transmission, indicates the substation20as the certificate issuer, has a signature of the substation20and contains a public key of the respective terminal device. The terminal devices30and31can communicate with one another and with their higher-order substation20in encrypted and/or in signed form by means of their terminal device certificates EZ30and EZ31and the corresponding private keys.

The arrangement10additionally has a further substation50to which the terminals60,61and62are connected. The terminal devices60to62are equipped in each case with a terminal device certificate EZ60, EZ61and EZ62respectively and a corresponding private key. The certificate enabling a signed data transmission, indicates the substation50as the certificate issuer, has a signature of the substation50and contains a public key of the respective terminal device. The terminal devices60to62can communicate with one another and with their higher-order substation20in encrypted and/or in signed form by means of their terminal device certificates EZ60, EZ61and EZ61and the corresponding private keys.

FIG. 2shows the arrangement10according toFIG. 1after a control station100has been connected to the two substations20and50. The procedure for enabling an encrypted and/or signed communication of the terminal devices30and31and also60,61and62with the control station10, their respective own and the respective other substation20and50and even with the terminal devices of the respective other substation is explained below with reference to an example embodiment.

Following the connection of the control station100to the substations20and50, the control station100will in each case recertify the public key of the substations20and50by issuing a signed certificate RZ20and RZ50, also referred to below as the recertified certificate RZ20and RZ50, containing the public key of the substation, for the respective substation. The certificates RZ20and RZ50are preferably known and made accessible at a central location to all terminal devices and substations connected to the control station.

For the authentication of data which are transmitted or are intended to be transmitted, for example, from one terminal device30,31,60-62to another terminal device, to the substation20and50and/or to the control station100, the recertification then enables continued use in each case of the old terminal device certificate EZ30, EZ31, EZ60, EZ61and EZ62, since a certificate verification of the terminal device certificate EZ30, EZ31, EZ60, EZ61and EZ62on the receiver side, i.e. on the terminal device side, substation side and/or control station side, and/or the authentication of the data signed with the corresponding private key can be performed on the basis of the public key of the higher-order substation of the respective transmitter, i.e. by means of the respective recertified certificate RZ20and RZ50.

In other words, it is not necessary to provide all terminal devices30,31,60-62immediately with new certificates adapted to the control station100only because of the connection of the control station100to the substations20and50; this can be deferred until the respective validity period of the certificates that are still present or issued by the control station connection expires.

FIG. 3shows the arrangement10according toFIG. 2after a further terminal device32has been connected to the substation20following the connection of the control station100. In order to enable a communication in encrypted and/or signed form, the terminal device32is equipped with a terminal device certificate EZ32and a corresponding private key, the certificate enabling a signed data transmission, indicating the substation20as the certificate issuer, having a signature of the substation20and containing the public key of the respective terminal device32. The terminal device certificate EZ32thus corresponds in terms of structure to the terminal device certificates EZ30and EZ31.

Due to the recertification of the substations20and50by the control station100, it is possible for the new terminal device32to communicate with all other terminal devices30,31,60to62, the substations20and50and the control station100by means of its purely substation-related terminal device certificate EZ32.

FIG. 4shows the arrangement10according toFIG. 3after another further terminal device33has been connected to the substation20following the connection of the control station100and the terminal device32. In order to enable a communication in encrypted and/or signed form, the terminal device33is equipped with a terminal device certificate EZ33and a corresponding private key.

The substation20is indicated in the terminal device certificate EZ33of the terminal device33as the certificate issuer in the certification chain. Unlike the case of the terminal device32, the terminal device certificate EZ33contains (or refers to) a substation certificate which contains the control station100as the certificate issuer. In other words, a certificate chain in which both an intermediate certificate containing the public key of the substation20and a root certificate containing the public key of the control station100are contained or referenced is defined in the terminal device certificate EZ33. The terminal device33is therefore a terminal device which has been designated above as a terminal device of the second configuration type.

If the terminal device33transmits a message provided with its terminal device certificate EZ33and/or signed with a private key corresponding to the terminal device certificate to another terminal device30to32or60to62, the terminal device receiving the message preferably carries out the certificate verification purely on the basis of the intermediate certificate containing the public key of the substation20, i.e. the recertified certificate RZ20, and ignores the root certificate containing the public key of the control station100during the authenticity check. A procedure of this type saves processing time and processing power without incurring any loss of security.

The same applies accordingly if one of the terminal devices30to32or60to62transmits a message provided with its terminal device certificate and/or signed therewith to another terminal device30to32or60to62; the terminal device receiving the message preferably carries out the certificate verification purely on the basis of the public key which is contained in the terminal device certificate chain indicating the respective substation20and50as the certificate issuer.

Although the invention has been illustrated and described in greater detail by means of preferred example embodiments, the invention is not limited by the disclosed examples and other variations may be derived herefrom by the person skilled in the art without departing from the protective scope of the invention.