Cryptography – Key management – Having particular key generator
Patent
1997-02-18
1999-11-16
Swann, Tod R.
Cryptography
Key management
Having particular key generator
380 29, 380 46, 380 49, 380 43, 455410, 455411, H04K 100, H04L 900, H04Q 720
Patent
active
059871393
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to a method of encrypting information between a stationary network and a mobile station in a mobile radio system of the time division multiple access type (TDMA system).
More specifically, the invention relates to methods of encrypting the transmitted information in a more secure fashion in conjunction with an authorization check on the mobile by the network and when a multiple of time slots are used for the same user (mobile station).
DESCRIPTION OF THE BACKGROUND ART
The GSM-network, common in Europe, is a mobile radio network that uses time division multiple access (TDMA). As with other mobile radio networks, the GSM network employs authorization checks and encryption of transmitted messages. With regard to the GSM network, this is specified in "GSM specification 03.20", May 1994, issued by ETSI (European Telecommunication Standard Institute) and hereinafter referred to as ETSI/GSM 03.20. The various algorithms used in authorization checks and encryption are described in this reference.
As described in ETSI/GSM 03.20, an algorithm A3 is used to effect actual authorization checks between network and subscriber apparatus. As further described, an algorithm A5 is used for encryption of the payload information to be transmitted, and an algorithm A8 is used to form, from the subscriber authorization key Ki, an encryption key Kc.
As a rule, only one time slot per frame for a given connection is used in TDMA-type time division mobile radio systems; see ETSI/GSM 05.02.
The use of two or more time slots, not necessarily consecutive time slots, in a transmission frame has been proposed, see ETSI/STC SMG3, T doc SMG3 WPA 95A dated Aug. 29, 1995 (Nokia Telecommunications), see particularly point 5 "HSCSD Architecture". This provides the advantage of enabling larger quantities of information to be transmitted per unit of time (applicable particularly to data transmissions), but has the drawback of increasing bandwidth.
SUMMARY OF THE INVENTION
The inclusion in a GSM system of two or more time slots instead of one time slot for one and the same radio transmission in accordance with the aforegoing creates certain problems when encryption and authorization checks are to be employed.
The most obvious procedure would be to process each of the time slots separately and to process the information in accordance with earlier known principles. However, such procedures would require drastic modification to the existing signalling protocols and to equipment on both the network side and the mobile station side.
It would be desirable to avoid such modifications to existing standards and equipment to the greatest possible extent. The use of the same pseudo-random sequence for all time slots within one and the same frame and for a given frame number is proposed in the aforementioned ETSI document, ETSI/T doc SMG3, "First HSCSD stage 2 draft". The drawback with this method is that it is necessary to compromise between encryption safety and procedure simplicity. When two separate bursts belonging to one and the same user are transmitted in this manner while using the same encryption sequence (pseudo-random sequence), the influence of the encryption can be eliminated relatively simply, by carrying out simple EXOR operations.
The object of the present invention is therefore to provide methods for reliable encryption in a TDMA-type mobile radio system in which two or more time slots are used for one and the same transmission without needing to make substantial changes to the signalling protocol and/or system equipment.
In this regard, an inventive method is characterized by the features set forth in the following claim 1. Another inventive method is characterized by the features set forth in the accompanying claim 3. Further inventive methods are characterized by the features set forth in accompanying claims 4 and 5.
BRIEF DESCRIPTION OF THE DRAWINGS
The aforesaid inventive methods will now be described in more detail with reference to the accompanying drawings.
FIG. 1 illustrat
REFERENCES:
patent: 5148485 (1992-09-01), Dent
patent: 5442705 (1995-08-01), Miyano
Sayadian Hrayr A
Swann Tod R.
Telefonaktiebolaget LM Ericsson
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