EP1240569A2

EP1240569A2 - Method and device for verifying a file

Info

Publication number: EP1240569A2
Application number: EP00987435A
Authority: EP
Inventors: Kersten W. Heins
Original assignee: SCM Microsystems GmbH
Current assignee: Identiv GmbH
Priority date: 1999-12-21
Filing date: 2000-12-21
Publication date: 2002-09-18
Also published as: WO2001046785A2; US20030140229A1; DE19961838A1; WO2001046785A3

Abstract

The invention relates to a method and a device for verifying the authenticity and integrity of a file which has been received, or is to be transmitted from a computer (14) and which is furnished with a digital signature. For the verification process, said method accesses signals which are available at an interface (18) of the computer that is linked to an output device (16) for outputting the file furnished with the digital signature. A device (20) for carrying out the method comprises a circuit and a programme which are used to perform the verification in the device (20), in a manner which is logically separate from the central calculation unit of the computer (14). The device (20) is coupled to an interface (18) of the computer (14) that is linked to an output device (16), in such a way that it detects the signals used for the verification, in order to output the file furnished with the digital signature.

Description

Method and device for checking a file

The invention relates to a method for checking the authenticity and integrity of a file received or to be sent from a computer and provided with a digital signature. The invention further relates to an apparatus for performing the method.

Sending and receiving files electronically has become increasingly important with the advancing development of the Internet. Especially when exchanging important data (sensitive data), such as takes place when trading over the Internet (e-commerce), there is a need to ensure secure data transmission. This results from the fact that the information sent over the Inlernet from one computer to a remote other computer passes through a number of intermediate computers and separate networks before they reach their destination. So there is

There is a risk that the transmission of data by means of files before they are received can be undesirably disrupted by transmission errors as well as by third parties.

In particular, the recipient of a transmitted file has an interest in knowing that the authenticity and integrity of the received file has been checked. Authenticity in this context means the guarantee that the file actually comes from the person (or company, etc.) who claims to be the sender of the file. The integrity of a file is given if its content during the

Transmission was not changed - intentionally or accidentally. In certain applications, there are additional requirements on the part of the recipient that the confidentiality of the transmitted data is guaranteed and / or that the sender does not object to the sending of the data.

Securing the data transmission taking into account the aspects listed above is carried out in a known manner using established techniques and standards that are internationally accepted and are known as public key cryplography. An essential aspect of this method is the provision of a file to be sent with a digital signature, which is checked on the recipient's computer after receipt of the "signed" file. In this context, a signed file is understood to mean a file together with its associated digital signature.

When checking, however, there is a risk that certain viruses or other malicious programs (e.g. special Java, ActiveX applications, etc.) on the recipient's computer can interfere with the checking processes or influence them so that the recipient does not notice that the data displayed on the screen of his computer do not match the data sent. On the other hand, it is also possible. that the received data is checked correctly and correctly leads to a positive result, but that manipulated data are output on the screen without a warning to the recipient.

The reverse problem can occur on the sender's side of the file. If when signing a file to be sent, one for the

If there is an undetectable malfunction caused by a virus or the like, the sender is not able to identify the error on the basis of the signed file displayed on the screen, in particular if there is an error in the digital signature.

These problems could be solved with a completely independent signature architecture, i.e. with a special system that is shielded from the environment only for checking files. However, since such a system has its own processor and its own peripheral devices such as a keyboard. Screen, etc., it is too expensive for the intended purpose.

A security device for a computer is known from US Pat. No. 5,406,624, with which security-related data is kept away from the computer which may be infected with viruses or the like. The

The device also serves to perform processes such as the generation of keys and the writing of the keys on Smarl cards independently of the computer. For this purpose, the computer is isolated from its peripheral devices,

"a by connecting them to the computer not directly but via the intermediate security device. In order to carry out the relevant procedures, the device takes control of the peripheral devices and independently carries out the necessary operations, such as reading or writing a Smarl-Caid.

However, the security device is not suitable for checking the authenticity and integrity of a file that has been output on an output device of the computer, received online or to be sent. Another disadvantage of this device is that special commands or a separate swilch box are required to activate it. In addition, the security device is very complex and therefore expensive, since it is designed to carry out complex processes such as reading and writing a smart card. In addition, a complete, separate screen control must be present in the safety device.

It is therefore an object of the invention to provide a possibility for checking a received or ready-to-send signed file, which provides as secure as possible information regarding the authenticity and integrity of the file output on an output device of a computer.

This object is achieved by a method of the type mentioned at the beginning, in which signals are accessed for checking that are present at an interface of the computer to an output device for the output of the file provided with the digital signature. This enables the data to be checked as it is output on the computer's output device and perceived by the user. The invention is based on the knowledge that the signals which are output to an output device of the computer cannot be attacked by viruses or the like, since the output device is a passive unit which no longer processes the data. The viewer of the signed file can thus be informed whether the file output on the output device and the digital signature match. If the result is positive, this ensures that the data used for the check

(File and digital signature) were not subsequently manipulated on the recipient's computer or in the network. Because is provided. To carry out the method according to the invention in a device logically separated from the central processing unit (CPU) of the computer, the checking of the file cannot be disturbed by viruses or the like, which may have an effect on the data processing taking place in the computer.

The reconstruction of the file output on the output device and its digital signature from the signals that are present at the interface enables a relatively uncomplicated checking of the output of the signed file using known methods.

The method according to the invention preferably includes the decryption of the digital signature of the reconstructed signed file, the decryption generating a first digest number. This first digest number can then be compared in a simple manner with a second digest number, which is determined from the reconstructed file. The result of this comparison provides reliable information about the authenticity and integrity of the output file, provided that the key used actually belongs to the sender. However, this assignment between the public key and the sender is usually ensured by an independent certification body. In addition, if the result of the comparison is positive, if the file is a received file, the recipient can be sure that the file was actually sent by the sender. Thus, e.g. B. The sender does not make an offer contained in the file irrelevant by denying that he has ever sent this file.

According to a further development of the method, provision is made to also check the creation time of the file provided with the digital signature. So z. B. In the case of received files, reliable information about the validity of a temporary offer contained in the signed file can be given at the time of receipt.

The method according to the invention is particularly suitable for files which have been received online from a network or are sent online via a network, since such files are subject to an increased risk of faulty transmission or manipulation. Finally, it proves advantageous to carry out at least part of the method using a chip card. If the computer is equipped, for example, with a Smarl-Cad terminal, a corresponding Smarl-Card can be used to support decryption processes required in connection with the method according to the invention as well as checking of keys.

The invention also provides a device for performing the method, which comprises a circuit and a program with which in the device and logically separated from the central processing unit of the

Computer the check is carried out, the device being coupled to an interface of the computer to an output device so that it detects the signals used for the check to output the file provided with the digital signature. With the device according to the invention, the non-attackable signals provided for the output of the signed file can be scanned and evaluated in a simple manner. Also the checking of the file cannot be disturbed due to the separation of the device from the data processing of the computer.

The device is preferably coupled to the interface of the computer to a screen. For example, the recipient of a file receives the secure information as to whether the received file in the form as it is displayed on the screen originally came from the specified sender and was transmitted without interference. However, the device can also be coupled to the interface of the computer to a printer.

For a cost-effective production of the device, it is advantageous that the device comprises an ASIC (application-specific integrated circuit) which houses the circuit necessary for the check. The

ASIC can also have a microprocessor that works under program control.

Flexibility with regard to the selection of the computer on which the device is to be used is achieved in that the

Isl device suitable for retrofitting the computer, ie it is designed as a so-called add-on system. The device can be easily set up on the desired computer and if required be uninstalled again to equip another computer with the device.

The device can be arranged internally on the base board (motherboard) or on a lick of the computer. But it can also be implemented in an external device that is connected to the computer. For example, it is possible to place the device in a chip card terminal. z. B. to integrate a smart card reader / writer. The device preferably has a chip card associated with the chip card terminal, which is linked to the rest of the device in such a way that it at least partially carries out a decryption process or provides data for a decryption process. It is thus possible to have at least part of the method according to the invention carried out with the aid of or directly from a microprocessor of the smart card. However, other functions relating to the method according to the invention can also be carried out with the terminal.

In order to inform the user simply and easily about the result of the file check, the device comprises a TRUE / FALSE display.

A preferred embodiment of the device according to the invention comprises a real-time clock, by means of which the everything of a signed file can be determined. This can e.g. it may be necessary to check whether an offer contained in the file is still valid.

If the device is to be set up at different locations, the device can be coupled to the interface of the computer wirelessly. This means that the choice of locations is not affected by the length of a cable or its undesirable visibility.

Further features and advantages of the invention result from the following exemplary description with reference to the drawing. In this show:

1 is a schematic flow diagram for processing a file to be sent; and Fig. 2 is a schematic flow diagram for checking a received file with the device according to the invention, which works according to the method according to the invention.

In the following, the method according to the invention and the device according to the invention provided therefor are described using the example of checking a received file. However, it is also possible to use the method and the device on the ropes of the recipient to check a ready-to-send file that is present at the interface to the network.

FIG. 1 shows the processes which, according to the concept of public key cryplography, usually run on a computer 10 of the sender before a file is sent. A so-called digest number is calculated from the file created by the sender, which is to be sent to a recipient, using a predetermined mathematical algorithm. A digest number has a certain length and is specific to the respective file, i.e. the smallest change in the file would result in a different value. On the other hand, however, the original file can never be obtained from the Digesl number. The digest number of the file is encrypted using a private key of the sender, which is only known to the sender. The result of this encryption is called the digital signature of the file. The digital signature is attached to the file to be sent. The (signed) file provided with the digital signature can now either be sent to the recipient immediately via a network 12 or, if the data is confidential, be encrypted beforehand.

The optional encryption of the signed file is usually carried out using a randomly generated one-time key (one timc key). The unique key itself is in turn encrypted with a public key and then attached to the signed, encrypted file. Both of them are finally sent to the recipient as a "saved file".

FIG. 2 shows the processes that are carried out on the ropes of the receiver to check the received file. The file received by a computer 14 is saved or only a signed file recognized. In the first case, the saved file is first decrypted on the computer 14 by means of a private key of the recipient, as a result of which a signed but still encrypted file and a unique key are obtained. The signed, encrypted file can now be decrypted with the unique key. The resulting signed file is then further processed as an unencrypted received file that is provided with a signature.

In order to make the signed file visible to the recipient, it is output on an output device 16 which is connected to the computer 14 via an interface 18. The output device 16 is usually a screen, but it can also be a printer or the like, for example. be provided. The signals emitted by the computer 14 to the output device 16 for displaying the signed file are logically separated from the central processing unit of the computer 14, i.e. these signals cannot be influenced by programs that run on the computer 14. These signals are therefore not vulnerable to viruses or the like.

In addition to the output device 16, there is also one at the interface 18

Device 20 connected, which can access the signals intended for the output device 16. Usually, an interface 18 to a screen is an analog interface. In modern screens that convert the data to be displayed into analog signals themselves, a digital interface is accordingly provided. For the sake of simplicity, the data present at the interface in this case are also referred to as "signals". Both the connection of the output device 16 and the device 20 to the interface 18 of the computer 14 can take place wirelessly. e.g. by means of coordinated infrared interfaces on the participants

Devices.

The device 20 has an electronic circuit, which can be accommodated in an ASIC, and a suitable program for checking the signed file. Since the device 20 is logically separated from the central computing unit of the computer 14, no viruses or the like, which are located, for example, in the main memory of the computer 14 and which undesirably influence the data processing, can interfere with the verification of the signed file.

The verification of the signed file in the device 20 is described in the following for the case of a screen as an output device 16: The signals present at the interface 18 are from the

Device 20 scanned and evaluated. In this way, the image output on the screen can be reconstructed and the file "displayed" together with the associated digital signature is located. The digital signature is decrypted using a public key that has been made publicly available by the sender and is matched to the private key that was used to encrypt the digest number generated by the sender from the original file. The public key is certified by an independent certification body. The result of this decryption is a first digest number. A second digest number is calculated from the file itself. The same mathematical algorithm is used for this, which generated the original digest number on the sender's computer 10. The information about the mathematical algorithm necessary for this process has been sent with the digital signature. The two digest numbers are finally compared and the result is over one

TRUE / FALSE output device 22 of device 20 is output. The result can be indicated, for example, by a green light-emitting diode if the digest numbers match (TRUE) and by a red light-emitting diode if the digest numbers do not match (FALSE).

If the two digest numbers match, the file was not changed after it was signed by the sender. In addition, the recipient has certainty about the identity of the sender, since certification of the public key ensures that the sender belongs to the sender. Since the sender alone has access to the private key that was used to sign the file, the sender cannot deny that he sent the file. If the two digest numbers do not match, it must be assumed that the file was either not correctly transmitted or manipulated, or that the signature was generated with a private key that does not match the public key used to decrypt the digital signature. A preferred embodiment of the device 20 additionally comprises a real-time clock 24 for the secure determination of the everything in the file, for example the time difference between the reception and the replacement line point of the file. For this purpose, the file is sent next to the digital signature with an indication of the creation or Abscndezeilpunkl or the

Validity period, which can be referred to as a line stamp. In the device 20, a comparison of this line specification with the current time can now be used to determine whether, for. B. a temporary offer contained in the file is still valid. This check is then taken into account when the result of the Dalei check is displayed.

The device 20 is designed as an add-on system, i.e. a computer can be retrofitted with the device 20. The device 20 can be arranged both internally in the computer 14 on the base board or on a plug-in card. According to a further embodiment, the device 20 is integrated in a Smarl card terminal. With the help of the smart card terminal and the corresponding smart card, the certification of the public key needed to decrypt the digital signature can be checked at the same time.

Furthermore, with the help of a suitable smart card, the decryption of the digital signature or, if necessary, the saved file can be supported. The smart card contains, for example, a key necessary for the respective decryption and / or a decryption program. Some or all of the decryption can be performed or controlled by a smart card microprocessor.

Claims

- fl patent claims

1. A method for checking the authenticity and integrity of a file received from a computer (10; 14) or to be sent, which is provided with a digital signature, characterized in that

Check aul signals are accessed, which are available at an interface (18) of the computer (10; 14) to an output device (16) for the output of the file provided with the digital signature.

2. The method according to claim 1, characterized in that the method is carried out in a logically separate device (20) from the central processing unit of the computer (10; 14).

3. The method according to claim 1 or 2, characterized in that the method comprises the reconstruction of the output on the output device (16), provided with the digital signature file from the signals that are present at the interface.

4. The method according to claim 3, characterized in that the method comprises the decryption of the digital signature of the reconstructed signed file, wherein a first digest number is generated by the decryption.

5. The method according to claim 4, characterized in that the method comprises determining a second digest number from the reconstructed file and comparing the first digest number with the second digest number.

6. The method according to any one of the preceding claims, characterized in that the method comprises checking the creation line point of the file provided with the digital signature.

7. The method according to any one of the preceding claims, characterized in that the file provided with the digital signature was received online from a network or is sent online via a network.

8. The method according to any one of the preceding claims, characterized in that at least part of the method is carried out by means of a chip card.

9. Device for performing the method according to one of the preceding claims, characterized in that the device (20) comprises a circuit and a program with which in the device (20) and logically separated from the central processing unit of the computer (10; 14th ) the check is carried out, and the device (20) is coupled to an interface (18) of the computer (10; 14) to an output device (16) so that it uses the signals required for the check to output the digital signature provided file.

10. The device according to claim 9, characterized in that the device (20) to the interface (18) of the computer (10: 14) to one

Screen coupled isl.

11. The device according to claim 9, characterized in that the device (20) to the interface (18) of the computer (10: 14) is coupled to a printer.

12. Device according to one of claims 9 to 11, characterized in that the device (20) comprises an ASIC.

13. Device according to one of claims 9 to 12, characterized in that the device (20) is suitable for retrofitting the computer (10; 14).

14. Device according to one of claims 9 to 13, characterized in that the device (20) on the base board of the computer (10;

14) is arranged.

15. The device according to one of claims 9 to 13, characterized in that the device (20) is arranged on a plug-in card of the computer (10; 14).

16. The device according to any one of claims 9 to 13, characterized records that the device (20) is integrated in a chip card terminal.

17. The apparatus according to claim 16, characterized in that the device (20) has a chip card associated with the smart card terminal, which is linked to the rest of the device so that it performs a decryption process at least partially or provides data for a decryption process.

18. Device according to one of claims 9 to 17, characterized in that the device (20) comprises a TRUE / FALSE display device.

19. Device according to one of claims 9 to 18, characterized in that the device (20) comprises a real-time clock (22).

20. Device according to one of claims 9 to 19, characterized in that the coupling of the device (20) to the interface (18) of the computer (10; 14) takes place wirelessly.