WO1990013192A1

WO1990013192A1 - Method for the protection of a telecommunication link

Info

Publication number: WO1990013192A1
Application number: PCT/FI1990/000110
Authority: WO
Inventors: Jyrki Penttonen; Jarmo Toivanen
Original assignee: Tecnomen Oy
Priority date: 1989-04-20
Filing date: 1990-04-17
Publication date: 1990-11-01
Also published as: FI891881A0; FI82340B; FI82340C; FI891881A

Abstract

The invention relates to a protection method for a telecommunication link, e.g. the one utilizing a telephone network. In the method, protection of information is based on a multilevel key control hierarchy effected in devices having a mutual telecommunication link therebetween, whereby the safe-keeping of a key (Km) on top level in said key control hierarchy, i.e. the key whereby the protection of other keys (Kb1-Kbn, KS) is arranged, lies on the responsibility of operator of the system.

Description

Method for the protection of a telecommunication link

The present invention relates to a protection method of a telecommunication link, e.g. the one using a telephone network, for making sure

- of the identity of a person using the link

- that the transmitted data is intact,i.e. that the information contents of the data has not been changed during transmission

- that the transmitted data retains its secrecy.

At first, reference is made to a conventional telecom¬ munication link and the threats it is subjected to in terms of data protection. A typical data transmission situation is shown in fig. 1. A user (K) is through his personal computer (PC) in communication with a computer (TK) by way of a public selective telephone network (PSTN) . This set-up is vulnerable in terms of data protection. The data transmission link is subject¬ ed to the following hazards:

- the information transmitted during the link-up can be eavesdropped, whereby a piece of information intend¬ ed to be confidential may become available to unau¬ thorized persons

- active efforts can be made to change the transmitted data and this is a remarkable hazard e.g. in terms of bank applications

- it is impossible for a computer (TK) to find out the identity of a user which facilitates break-in methods based on masking i.e. operating on a phony name.

All the above threat factors can be eliminated by using proper enciphering and authentication methods. However, the application of such methods has heretofore required the introduction of hardware specialized in these pur- poses. The requirements set on this type of hardware are quite high-standard as a result of multi-level security demands of which the requirements associated with providing physical protection have been found the most difficult to satisfy. In a method making up the object of this invention, it is possible to effect all procedures associated with the protection of security in the softwares of the user's personal computer (PC) and a computer (TK) . Thus, there is not required any hardware specialized in data protection and this means a considerable cost saving.

A method making up the object of the invention is il¬ lustrated the form of a block diagram in fig. 2. The protection of transmitted data is based on a multi¬ level key control hierarchy, wherein the top level comprises a master key Km which is used to provide a safe-keeping for other keys. Other keys include basic keys Kb1-Kbn as well as a session key Ks. The function of a session key is to conceal the information trans¬ mitted by a telecommunication link. On the other hand, the basic keys are used to conceal the distribution and exchange of a session key, which is likewise effected by using a telecommunication link.

In terms of security, the most essential question in a system like the one described is the safe-keeping of a master key, i.e. the top-level key in the key control hierarchy. In order to make the system safe in terms of data protection, the safe-keeping of a master key must be arranged in a manner that it is not available to unauthorized people. Traditionally, this has been done by placing said key in a physically protected lo¬ cation. Such location can be e.g. an electronic micro- circuit with its contents clamped so as not to be read- able. Physical protection can be effected by arranging the safe-keeping of a master key in a locked or other¬ wise difficult-to-access place.

In a method making up the object of the invention, the safe-keeping of a master key is left to the responsibil¬ ity of of a user of the system. Thus, the master is in effect a symbol, like a password without which the operation of the system is not possible. This type of arrangement offers the advantage that, in the hardware, no piece of information requires physical protection, whereby all procedures associated with protection can be effected by the software and there is not necessarily required any hardware specialized in such procedures.

In the memory of a user's personal computer (PC) there are loaded basic keys for each piece of equipment and the transmission of session keys can be effected by using these basic keys. The computer forms a session key in a random fashion separately for each call setup. If anyone happened to break in the hardware at the mo¬ ment of data transmission, when a session key is in deciphered form in the apparatus, and thus to find out a session key, the hacker could only utilize the in¬ formation until the end of a present session and this would not create a security hazard in the long run. When setting up the basic keys in the memory of the operator's personal computer (PC) , the system operator conceals these keys with either a self-selected or as¬ signed master key. Said function is effected control- lably in a manner that a unit providing the basic keys can be sure that the basic keys are immediately con¬ cealed and cannot become available to unauthorized personnel. The operator can be provided with a possi¬ bility of changing his or her master key. The following example illustrates in more detail the system operating principle. In the example, reference is made to the drawings, in which:

fig. 1 shows in a block diagram a data transmission sys¬ tem whose protection can be effected by means of a method of the invention, and

fig. 2 shows the general principles of a method of the invention on a block diagram level.

In the system shown in fig. 1, the operator's link-up with a computer proceeds in detail as follows:

1. The operator commences a telecommunication program in view of setting up a link between his or her personal computer (PC) and a computer (TK) .

2. The telecommunication program sets up a physical link between computer (TK) and personal computer (PC) . At the same time, computer (TK) receives in¬ formation about the public identifier of personal computer (PC) , e.g. on the basis of a serial number supplied thereby. In this connection, computer (TK) creates a session key to be used in a link-up set up e.g. by means of a random number generator and transmits such key to personal computer (PC) , such key being, however, concealed by a basic key of personal computer (PC) . This particular secret basic key corresponding to a public identifier is assigned to a particular piece of hardware, i.e. the same basic key is not contained in any other piece of equipment except in said personal computer (PC) and in the computer. This feature makes sure that no other piece of equipment except that personal com- puter, to which this particular message is address¬ ed, is capable of deciphering the transmitted ses¬ sion key. By using this information, computer (TK) will be able to verify the idntification of person¬ al computer (PC) .

3. The telecommunication program requests the operator for a master key or a key whereby the safe-keeping of other keys is protected in the operator's per¬ sonal computer.

4. The telecommunication program unloads the scanner of a key file by using a master key provided by the operator and checks the correctness of an unloaded file. This provides access to a basic key Kb used in the scanning of the session key, whereby the scanning of the session key can be unloaded. This is followed by bringing the master key and the de¬ ciphered basic key from the memory of an operator's personal computer, so they are no longer availbale to unauthorized people in the computer memory. In addition, these procedures are performed with no interruptions allowed or otherwise making sure that the present operation cannot be intercepted. Thus, a possible "Trojan horse" or "computer virus" will not be able to steal an operator-issued master key or a computer-assigned basic key, either. It should be appreciated that only a computer for which the message is intended will be able to carry out said function since the basic key key in question is only held by the computer in question.

5. Data transmission between an operator's personal computer and a computer can begin. The concealing of data transmission is effected by means of a ses- sion key formed in step 4. Since basic key Kb is only held by a piece of equipment in question, a computer (TK) is capable of verifying the identity of a device at the opposite end. It should be noted that the identity of the operator of a device is also verified since, according to the presumption, the loading of basic keys (Kb1-Kbn) is effected in a manner that the operator of a device has scanned them with his or her own secret key (Km) , so these particular basic keys cannot be known to anyone else except computer (TK) and operator of the de¬ vice.

Claims

1. A protection method for a telecommunication link, e.g. the one using a telephone network, in which meth¬ od the protection of information is based on a multi¬ level key control hierarchy, said method being carried out in devices (PC, TK) having a mutual telecommunica¬ tion link, c h a r a c t e r i z e d in that the safe¬ keeping of a master key (Km) placed on the top level in said key control hierarchy, i.e. a key whereby the protection of other keys (Kb1-Kbn, KS) is arranged, lies on the responsibility of a system operator.

2. A protection method as set forth in claim 1, c h a r a c t e r i z e d in that the scanning of data transmission is effected by two-level scanning, which feature makes sure that the keys (Km, Kb1-Kbn) , reserved for the distribution of keys (KS) used for data trans¬ mission, are in deciphered form in the memory of device

(PC) for a short period only and, during this period, these particular keys are protected against thefts at¬ tempted by electronic means or otherwise.

3. A protection method as set forth in claim 1 or 2, c h a r a c t e r i z e d in that the loading of basic keys (Kb1-Kbn) is effected as concealed by master key

(Km) for positively verifying the identity of operator of the device.

4. A protection method as set forth in any of claims

1 - 3, c h a r a c t e r i z e d in that identification of the device is positively verified by means of basic keys (Kbi-Kbn) .

5. A protection method as set forth in any of claims 1 - 4, c h a r a c t e r i z e d in that the master key (Km) can be replaced by the operator.