United States Patent [w]

Transcription

1 United States Patent [w] Karppanen et al. US A [ii] Patent Number: 5,987,137 [45] Date f Patent: Nv. 16,1999 [54] METHD FR THE ENCRYPTIN F DATA TRANSFER [75] Inventrs: Art Karppanen, Helsinki; Hannu Kari, Veikkla; Jari Hamalainen, Tampere; Jari Jupperi, Helsinki, all f Finland [73] Assignee: Nkia Mbile Phnes, Ltd., Esp, Finland [21] Appl. N.: 08868,914 [22] Filed: Jun. 4, 1997 [30] Freign Applicatin Pririty Data Jun. 6, 1996 [FI] Finland [51] Int. CI. 6 H04L 928; H04L 906; H04L 900 [52] U.S. CI 38028; 3809; 38029; 38037; 38043; 38048; [58] Field f Search 3809, 28, 29, 38037, 43, 49, 23, 25, 48, 21 [56] References Cited U.S. PATENT DCUMENTS 5,099, Gupta et al ,161, Lampsn et al ,235, Gupta et al ,257, Chen et al.. 5,319, Finkelstein et al ,455, Brwn et al ,640, Hamalainen et al.. FREIGN PATENT DCUMENTS A Eurpean Pat. ff A Eurpean Pat. ff.. W WIP. W WIP. Primary Examiner Bernarr E. Gregry Attrney, Agent, r Firm Perman & Green, LLP [57] ABSTRACT The bject f the inventin is a methd fr the encryptin f infrmatin transferred between data transfer devices (MS, SGSN) in a data cmmunicatin system wherein ne r mre data frames are created frm ne r mre data packets frmed frm the infrmatin by the applicatin. The data frames cmprise at least a header field and a data field. In the methd, at least sme part f the data packets is ciphered by using a ciphering key (Kc). T the data frames, synchrnizatin data (UNT) is attached, the value f which is changed at least at the transmissin f each data frame. 21 Claims, 8 Drawing Sheets Netwrk side MS side UNT (TLLI) I Key Kc (TLLI) > A5 UNTb BLCNT UNT (TLLI) Key Kc (TLLI) H A5 UNTb BLCNT BLCK1 clear BLCK1 clear plain text in bit wise binary additin channel encrypted text bit wise binary additin plain text ut

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10 5,987,137 METHD FR THE ENCRYPTIN F DATA TRANSFER BACKGRUND F THE INVENTIN The present inventin relates t a methd fr the encryptin f infrmatin being transferred between data cmmunicatin devices in a data cmmunicatin system wherein ne r mre data frames are frmed frm ne r mre data packets frmed frm the infmatin by an applicatin, and these data frames cmprise at least a header field and a data field. The inventin relates additinally t a data cmmunicatin system which cmprises the means fr the encryptin f infrmatin being transferred between data transfer devices, the means fr frming ne r mre data packets frm the infrmatin and the means fr frming data frames frm the data packets. Data transfer between separate data transfer devices can be achieved in such a way that thse data transfer devices between which data at that time is t be transferred, are linked tgether fr the time needed fr the data transfer. In such a case, the link is maintained until the user stps the data transfer. In such cases, mst part f the linkage time is spent in entering cmmands prvided by the user and nly a small part f the time is actual data transfer. This limits, fr example, the maximum number f simultaneus users. Anther pssibility is t explit a s-called packet switched data transmissin. In this case, data is transferred between data transfer devices in a packet mde, in which case the time between the packets is freely available and can be used by ther data transfer devices. In this case, the number f simultaneus users can be increased, especially in wireless data transfer netwrks, such as cellular netwrks, since in this case the mbile statins which are in the same cellular area can use the same transfer channel. ne such a cellular system is the GSM system (Grup Special Mbile) fr which a packet mde data transfer service GPRS (General Packet Radi Service) has been develped. FIG. 1 shws a blck diagram f principal blcks in the peratin f the GPRS system. Apacket switching cntrller SGSN (Serving GPRS Supprt Nde) cntrls the peratin f packet switching service n the cellular netwrk side. The packet switching cntrller SGSN cntrls the sign-n and sign-ff f the mbile statin MS, the updating f the lcatin f the mbile statin MS and the ruting f data packets t their crrect destinatins. The mbile statin MS is cnnected t the base statin subsystem BSS thrugh a radi interface Um (FIG. 1). The base statin subsystem is cnnected t the packet switching cntrller SGSN thrugh the BSS-SGSN interface Gb. In the base statin subsystem BSS, the base statin BTS and the base statin cntrller BSC have been cnnected t each ther by a BTS-BSC interface Abis. The lcatin f the packet switching cntrller SGSN in the mbile statin netwrk can vary, fr example, accrding t which technical implementatin is being used. Althugh in FIG. 1, the packet switching cntrller SGSN has been marked utside the base statin subsystem BSS, the packet switching cntrller SGSN can be placed, fr example, as a part f the base statin BTS cnnected t the base statin subsystem BSS r as a part f the base statin cntrller BSC. The GPRS system has been described, fr example, in draft prpsals GSM 01.60, GSM 02.60, GSM and GSM which have been dated prir t the applicatin date f the present inventin. The peratin f bth the mbile statin MS and the packet switching cntrller SGSN can be divided int varius layers, each prviding a different functin, as has been shwn in FIG. 2. The Internatinal Standardisatin rganisatin, IS, has frmulated an SI mdel (pen Systems Intercnnectin) fr gruping data transfer int different functinal layers. In this mdel, there are seven layers which are nt necessarily needed in all data cmmunicatin systems. Transferable infrmatin, such as cntrl signalling and data transmitted by the user, between a mbile statin MS and a packet switching cntrller SGSN is exchanged preferably in a data frame mde. The data frame f each layer cnsists f a header field and a data field. FIG. 2 shws als the structure f data frames being used in the GPRS system in different layers. The infrmatin cntained in the data field can be, fr example, data entered by the user f the mbile statin r signalling data. The data field may cntain cnfidential infrmatin which has t be secured as reliably as pssible befre transmitting it t the radi path. In such a case, the encryptin has t be executed in such a way that in all simultaneus cnnectins between the packet switching cntrller SGSN and mbile statins MS cnnected t it, a separate encryptin key is used. Cnversely, it is nt preferable t cipher the address data f the data frame by the same encryptin key used in the ciphering f the data field, since mbile statins MS use a shared radi path resurce, i.e. infrmatin in many different cnnectins is transferred in the same channel, fr example, at different time intervals. In this case, each mbile statin shuld receive all messages transmitted in the channel cncerned and decrypt at least the encryptin f the address data t identify t which mbile statin the message is intended. Als the packet switching cntrller SGSN des nt knw which encryptin key shuld be used. In the fllwing, the peratinal functins f the layers f the GPRS system have been presented. The lwest layer is called an MAC layer (Media Access Cntrl) which cntrls the use f the radi path in the cmmunicatin between the mbile statin MS and the base statin subsystem BSS, such as allcating channels fr transmitting and receiving packets. Data transmissin between the base statin subsystem and the packet cntrller SGSN in the lwest level is executed at the L2 layer (link layer) in which link layer prtcl is used, such as LAPD prtcl accrding t standard Q.921, frame relay prtcl r the equivalent. The L2 layer may additinally cntain als quality r ruting data accrding t GPRS specificatins. Layer L2 has prperties f the physical layer and the link layer f the SI mdel. The physical transmissin line between the base statin subsystem BSS and the packet cntrller SGSN depends, fr example, n where the packet cntrller SGSN has been lcated in the system. Abve the MAC layer, there is an RLC layer (Radi Link Cntrl) and its functin is t divide the data frames frmed by the LLC layer int fixed sized packets t be transmitted t the radi path and their transmissin and retransmissin when necessary. The length f the packets in the GRPS system is the length f ne GSM time slt (apprximately ms). LLC layer (Lgical Link Cntrl) prvides a reliable transmissin link between the mbile statin MS and the packet cntrller SGSN. The LLC layer, fr example, adds t the transmitted message errr checking data by means f which it is intended t crrect thse uncrrectly received messages and when necessary, the message can be retransmitted.

11 5,987,137 SNDC layer (Sub-Netwrk Dependent Cnvergence) cmprises functins like prtcl cnversins f transmitted infrmatin, cmpressin, segmentatin and segmentatin f messages cming frm the upper layer. Additinally, ciphering and deciphering are accmplished at the SNDC 5 layer. The structure f the SNDC frame has been presented als in FIG. 2. The SNDC frame cmprises an SNDC header field (SNDC header) and an SNDC data field (SNDC data). The SNDC header field cnsists f prtcl data (Netwrk Layer Service access pint Identity, NLSI) and f SNDC 10 cntrl data, such as determinatins f cmpressin, segmentatin and ciphering. The SNDC layer functins as a prtcl adapter between prtcls used at the upper level and the prtcl f the LLC layer (link layer). The transmitted infrmatin cmes preferably as data 15 packets t the SNDC layer frm sme applicatin, such as messages accrding t the GPRS system r packets f the Internet prtcl (IP). The applicatin can be, fr example, a data applicatin f a mbile statin, a telecpy applicatin, a cmputer prgram which has a data transmissin link t a 20 mbile statin, etc. The MAC layer, RLC layer, LLC layer and the L2 layer cntain prperties which are described at layer 2 in the SI mdel. The abve mentined layers and the layers described in the SI mdel are nt, hwever, distinctly cherent. The SNDC frame is transferred t the LLC layer where an LLC header field is added t the frame. The LLC header field cnsists f a Temprary Lgical Link Identity (TLLI) and an LLC cntrl part. The packet cntrller GPRS establishes a TLLI identity fr each data transmissin link between a mbile statin MS and a packet cntrller GPRS. This data is used in data transmissin fr defining which data transmissin link each message belngs t. Simultaneusly, the same TLLI identity can nly be used in ne data transmissin link. After the terminatin f the link, the TLLI identity used in the link can be allcated t a new link t be subsequently frmed. The LLC cntrl part defines the frame number and the cmmand type (inf, acknwledge, retransmissin request etc.) fr ensuring an errr free data transfer. Ciphering in the GSM system is executed at the physical layer as a bit per bit ciphering, i.e. bit stream transmitted t the radi path is frmed by summing t the transmitted data ciphering bits which are frmed by using algrithm A5 45 knwn per se, by using a ciphering key Kc. Algrithm A5 ciphers transmitted data and signalling infrmatin at the physical layer n the channels dedicated t data transfer (Traffic Channel, TCH r Dedicated Cntrl Channel, DCCH). 50 Synchrnizatin f transmitted messages is ensured in such a way that algrithm 5 is driven by means f a special synchrnizatin data (UNT). The synchrnizatin data UNT is frmed n the basis f a TDMA frame number. Then the cntents f each 114-bit blck frmed by algrithm 55 A5 depend nly n the frame numbering and the ciphering key Kc. The setting f the ciphering key Kc is mst preferably executed at the stage when the cmmunicatin traffic f the dedicated channel has nt yet been encrypted and the mbile 60 statin netwrk being used has identified the mbile statin MS. In the identificatin in the GSM system, an Internatinal Mbile Subscriber Identity, IMSI, is used which identifies the mbile statin and which has been stred in the mbile statin, r a Temprary Mbile Subscriber Identity, 65 TMSI, is used which has been frmed n the basis f the subscriber identity. In a mbile statin, als a subscriber 40 identificatin key, Ki, has been stred. The subscriber identificatin key Ki is als knwn by the mbile statin netwrk. T ensure that the ciphering key Kc is knwn nly by the mbile statin MS and the mbile statin netwrk, the transmissin f the ciphering key frm the base statin subsystem BSS t the mbile statin MS is indirect. Then, in the base statin subsystem BSS, a Randm Access Number, RAND, is frmed which is transmitted t the mbile statin MS. The ciphering key Kc is frmed frm the randm access number RAND and frm the subscriber identificatin key Ki by using algrithm A8, as has been shwn in FIG. 3. The calculatin and string f the ciphering key Kc are executed bth in the mbile statin MS and in the mbile statin netwrk. Data transfer between the mbile statin MS and the base statin subsystem BSS is nnciphered at the start f the cnnectin. The transitin t the ciphered mde prceeds preferably in such a way that the base statin subsystem BSS transmits t the mbile statin a certain cmmand (unciphered) which in this cntext is called the "start cipher". After the mbile statin MS has received the cmmand "start cipher", it starts the enciphering f the transmitted messages and deciphering f the received messages. Crrespndingly, the base statin subsystem BSS starts the enciphering f messages transmitted t the mbile statin after the base statin subsystem has received the ciphered message transmitted by the mbile statin and deciphered the ciphering crrectly. In the abve described ciphering, the synchrnizatin was based, fr example, n the TDMA frame numbering f the physical layer. It is nt pssible t use it in all applicatins, particularly when infrmatin belnging t different cnnectins is transmitted n the same channel, such as in packet switched data transmissin methds. In the Eurpean patent applicatin EP , a methd has been presented fr the encryptin f data transfer wherein, fr example, encryptin data which cmprises an encryptin key is attached t the transmitted data frames. A U.S. Pat. N. 5,319,712 cmprises a methd and equipment fr the encryptin f data transfer s that a sequence number is attached t the data frames f the link layer and the data frame is ciphered. A disadvantage f these ciphering methds accrding t the prir art is, fr example, that the receiver des nt knw withut deciphering, t whm the received data frame is intended, in which case the unnecessary receptin f data frames and deciphering causes a deteriratin in the efficiency f the system. SUMMARY F THE INVENTIN The aim f the present inventin is t prvide a methd and equipment fr the encryptin f data transfer in a data transfer system wherein the transferred data is in a data frame mde and which data transfer system has been divided int functinal layers in which case the data frame structure can be different in the different layers. The methd accrding t the inventin is characterized in that at least sme part f the data packets is ciphered by a ciphering key and that synchrnizatin data is attached t the data frames and its value is changed at least at the transmissin f each data frame. The system accrding t the inventin is characterized in that the means fr ciphering the infrmatin cmprise at least: means fr ciphering data packets with a ciphering key, means fr attaching synchrnizatin data t the data frames,

12 means fr changing the value f the synchrnizatin data at the transmissin f each data frame, and means fr interpreting the synchrnizatin data in the data transfer device f the receiver. Cnsiderable advantages are achieved by the inventin, 5,987,137 cmpared t the ciphering methds accrding t the prir art. In the methd accrding t the inventin, the header field f the data frame f the physical layer can be transmitted in a nn-ciphered mde, r methds which are presently knwn can be used in the ciphering. In the methd accrding 1 t the preferable embdiment f the inventin, the ciphering key is changed fr each transmissin blck f the physical layer, in which case deciphering withut knwledge abut the ciphering key is virtually impssible. By using the methd accrding t the inventin, it is pssible additinally 1 5 t implement a partial enciphering, in which case nly a part f the transmitted data frames is ciphered. In this case, fr example, advertisements can be delivered nn-ciphered and ther infrmatin ciphered nly t thse wh have the right t receive ciphered data frames and t decipher them. BRIEF DESCRIPTIN F THE DRAWING The inventin is described in mre detail in the fllwing by referring t the attached drawings in which FIG. 1 shws the lgical structure f the GPRS system as a schematic blck diagram, FIG. 2 shws the layer structure f the GPRS system and the data frame structure f the layers, FIG. 3 shws definitin f the ciphering key accrding t 30 the prir art in mbile statins and in a mbile statin netwrk as a schematic blck diagram, FIG. 4a shws ciphering accrding t a preferable embdiment f the inventin, FIG. 4b shws ciphering accrding t anther preferable 35 embdiment f the inventin, FIGS. 5a-5d shw the data frame structure f the link layer accrding t an embdiment, FIG. 6a shws the data frame structure f the adapting layer accrding t an embdiment with Pint-t-Pint cnnectin, and FIG. 6b shws the data frame structure f the adapting layer accrding t an embdiment with multipint cnnectin. 45 DETAILED DESCRIPTIN F THE INVENTIN In the fllwing, the inventin has been visualized by means f a packet switching service GPRS implemented in 50 the GSM system, but the inventin has nt, hwever, been limited nly t this system. In the inventin, ne has aimed at the implementatin wherein as much as pssible f the existing ciphering technique is explited, such as the ciphering f the GSM 55 system which is adjusted s that it can be applied in the transmissin f data frames, fr example, in the GPRS system. ne advantage f the inventin is that it can be applied in many peratinal mdes, such as the Pint-t- Pint, PTP, cnnectin, multipint cnnectin (Pint-t- 60 Multipint-Multicast, PTM-M; Pint-t-Multipint-Grup, PTM-G) etc. The ciphering methds are classified mainly n the basis f the TLLI identity. A distinct TLLI identity is allcated fr each cnnectin type between the mbile statin MS and the packet switching cntrller SGSN. The 65 fllwing different types are available fr use in the GPRS system accrding t present standards: 5 20 Pint-t-Pint (PTP) uses unique TLLI identity in the cmmunicatin between the mbile statin MS and the packet switching cntrller SGSN. Pint-t-Multipint-Multicast (PTM-M) uses TLLI allcated fr the cmmunicatin between the mbile statin MS and the multicast service prvider. Pint-t-Multipint-Grup (PTM-G) uses TLLI allcated fr mutual cmmunicatin via multicast service prvider f mbile statins MS within the mbile statin grup. Pint-t-Pint cnnectin typically uses the acknwledged mde at the link layer level, i.e. the receiver f the transmissin transmits data as an acknwledgement f a crrect receptin. At Pint-t-Multipint cnnectins, data frames are usually transmitted by using peratin mde in which acknwledgements are nt transmitted. As already has been stated earlier in this descriptin, in systems where data f different cnnectins is transmitted in the same channel, it is nt preferable t cipher the header field f data frames by a unique ciphering key fr each cnnectin. In this case, the data, frames are ciphered at least partly at sme ther layer than the physical layer. In the GPRS system, the ciphering is executed at the LLC layer. The transmitted data is ciphered in such a way that t each bit f the data frame, a crrespnding bit f the ciphering bit string is summed. The ciphering bit string has been frmed preferably by a ciphering algrithm by using an individual and unique ciphering key Kc. The ciphering algrithm is, fr example, the A5 algrithm knwn frm the GSM system. In additin t the crrect address, ne has t ensure that the data frames can be sequenced in the receiver. This can be implemented in a manner knwn per se, s that synchrnizatin data UNT is entered int the ciphering algrithm, in which case the receiver is able, after deciphering, t find ut the sequence f the data frames. Fr example, in TDMA systems (Time Divisin Multiple Access), like GSM, the TDMA frame number can be used fr numbering the data frames f the physical layer. Hwever, the packet switching cntrller SGSN f the GPRS system des nt knw the TDMA frame number, s in this inventin a methd has been develped fr synchrnizing data frames, and in this methd the sequence number f data frames (data frame number) is used as a synchrnizatin data. Thus the cntents f each transmitted blck are determined by, fr example, the frame numbering and the ciphering key Kc. The amunt f data t be ciphered varies in different cnnectins, but this is nt significant in the applicatin f the inventin since the ciphering can be executed by dividing the transmitted data preferably int sub-blcks f standard length. Then the first bit f each sub-blck is ciphered by the first bit f the ciphering algrithm, the secnd bit f the sub-blck by the secnd bit f the ciphering algrithm etc. In the GPRS system, the length f a sub-blck can be, fr example, 114 bits, such as in the present GSM system. The length f the sub-blck can be, preferably, als divisible by the length f a byte. In many applicatins, the length f the byte is eight, in which case a suitable length fr a sub-blck culd be 64 bits. In the GSM system, a mbile statin MS can use nly ne ciphering key Kc at a time. In the GPRS system, ne ciphering key per mbile statin MS is nt necessarily sufhcient in every situatin, since the mbile statin can simultaneusly have many different types f active cnnectins (PTP, PTM) with each cnnectin having mst preferably a separate ciphering key Kc which has been preferably frmed by different means. The ciphered data frame

13 5,987,137 cntains thus the ciphering key Kc being used, the synchrnizatin data UNT and pssibly als the values UNTb f a blck cunter BLCNT attached t the TLLI. FIG. 4a shws a preferable ciphering methd accrding t the inventin as a schematic blck diagram in a situatin 5 where a nn-encrypted sub-blck (plain text in) is transferred encrypted (encrypted text) frm the netwrk t the mbile statin. In this embdiment, als the value UNTb f the blck cunter is used in the determinatin f the ciphering blck BLCK 1. The blck cunter can be set t its initial value by means f a setting line "clear", preferably at the start f the data frame f each adapting layer. Bth at the netwrk side and in the mbile statin MS, the value f the synchrnizatin data UNT is calculated fr each transmitted blck, with the value f the synchrnizatin data UNT and the ciphering key Kc entered int the ciphering 15 algrithm A5. At the transmissin side, the utput bit string (BLCK1) is summed t the sub-blck (plain text in). The encrypted sub-blck is transferred in the channel t the mbile statin MS. The mbile statin MS deciphers it crrespndingly by summing the utput bit string 20 (BLCK1) f the ciphering algrithm A5 t the received encrypted sub-blck and, as a result f the summing, a nn-encrypted sub-blck (plain text ut) crrespnding t the transmitted sub-blck is btained. FIG. 4b shws anther preferable ciphering methd accrding t the inventin as a 25 schematic blck diagram. This embdiment differs frm the embdiment f FIG. 4a mainly in that the blck cunter BLCNT is nt used. A typical length f a frame sequence number is frm six t eight bits. Frm the ciphering security pint f view, this value as UNT variable alne is nt sufhcient, and therefre als ther variables can be used in the determinatin f the UNT value f the synchrnizatin data in additin t the frame sequence number, fr example, the base statin identificatin. The base statin identificatin is knwn by bth the netwrk and the mbile statin, since the mbile 35 statin, which is being used, ntifies the packet switching cntrller SGSN abut the changing f the base statin. The changing f the base statin alters thus the UNT value f the synchrnizatin data in this embdiment. In the Pint-t-Pint cnnectin mde, the fllwing 40 variables are available in the determinatin f the UNT value f the synchrnizatin data: a) The frame number f the Lgical Link Cntrl layer (LLC frame number, LLC #) which is cnveyed t the adapting layer (SNDC). 45 b) The data frame number f the adapting layer (SNDC data blck number, SDU#) which can be attached t the transmitted data frame r initialized at the start f the cnnectin when it is maintained at bth ends f the cnnectin. c) Identity f a ruting area (Ruting area #) which is knwn at bth ends f the cnnectin s that the identity need nt be attached t the transmitted data frame d) Identity f the area f a packet switching cntrller (SGSN #) which is knwn at bth ends f the cnnectin s that the identity need nt be attached t the transmitted data frame. e) Identity f a base statin (Cell #) which is knwn at bth ends f the cnnectin s that the identity need nt be attached t the transmitted data frame. In Pint-t-Multipint cnnectin mde, the fllwing variables are available in the determinatin f the UNT value f the synchrnizatin data: a) The data frame number f the adapting layer (SNDC data blck number, SDU #) which is transmitted within the SNDC data frame. b) Identity f a ruting area (Ruting area #) which is knwn at bth ends f the cnnectin s that the identity need nt be attached t the transmitted data frame. c) Identity f the area f a packet switching cntrller (SGSN #) which is knwn at bth ends f the cnnectin s that the identity need nt be attached t the transmitted data frame. d) Identity f a base statin (Cell #) which is knwn at bth ends f the cnnectin s that the identity need nt be attached t the transmitted data frame. Additinally, in bth cnnectin mdes, the value f the blck cunter BLCNT can be used, which makes cracking f an encrypted data field even mre difecult fr an intruder, since the same ciphering bit string is nt used in the encryptin f sequential data fields. therwise, the recalculatin is executed nly nce fr each transmissin f a data frame f the adapting layer. The length f the data frame f the adapting layer can be thusands f bits, s that it may be pssible t find ut the encryptin key if the encryptin algrithm is nt calculated sufeciently ften. The abve presented variables defining the synchrnizatin data UNT can either be used alne r in cmbinatin. Sme f the variables thus have t be delivered t the receiver within data frames and sme f them can be managed lcally. The use f lcally managed variables increases the level f the security and t sme extent it reduces the amunt f transferred data. The fllwing tables give an example f the cntents f the synchrnizatin data UNT. Table 1.1 shws sme synchrnizatin data accrding t the mst preferable embdiment f the inventin and in it, a blck cunter BLCNT has been used, and table 1.2 shws anther preferable embdiment f the inventin and in it, the identity f the base statin has been used instead f the value f the blck cunter UNTb. TABLE 1.1 Bit mde FTP SDU # (lcal r delivered) PTM SDU # (delivered) LLC # (delivered) UNTb UNTb

14 5,987, TABLE1.2 Bit mde FTP SDU # (lcal r delivered) PTM SDU # (delivered) LLC # (delivered) Cell #, Ruting area # r SGSN # (lcal) Cell #, Ruting area # r SGSN # (lcal) In the fllwing, the setting f the ciphering key Kc is described. The setting f the ciphering key Kc is initiated by the netwrk as ften as the netwrk peratr finds it necessary. Additinally, a unique ciphering key has t be generated fr each TLLI cnnectin. A table f the ciphering key Kc-TLLI identity pairs is mst preferably maintained bth in the packet switching cntrller GPRS and in the mbile statin MS. The setting f the ciphering key is different fr different cnnectin types. In a Pint-t-Pint cnnectin, the ciphering key Kc is 20 transmitted indirectly by using a randm access number RAND. The ciphering key Kc is frmed in the GPRS system preferably frm the randm access number RAND and frm the subscriber identificatin key Ki f the mbile statin by using the algrithm A8, just as in the GSM system. The 25 identificatin key f a mbile statin has been stred n the SIM card (Subscriber Identity Mdule) f the mbile statin and in the Authenticatin Centre AuC f the netwrk. In a multipint cnnectin, all mbile statins which are cnnected t the same service use the same ciphering key 30 Kc. The ciphering key Kc is activated when the cnnectin t the service is created. The ciphering key Kc can be entered t the mbile statin MS by using different methds. A multipint service prvider can enter the ciphering key, fr example, in a ciphered mde, in which case the mbile 35 statin MS has t be lgged t the packet switching cntrller GPRS thrugh a Pint-t-Pint cnnectin prir t gaining access t the multipint cnnectin. During the lgn stage f the Pint-t-Pint cnnectin, a ciphering key Kc has been defined fr the cnnectin and it is used in the 40 encryptin f the ciphering key f the multipint cnnectin when it is transmitted t the mbile statin MS. The ciphering key f the multipint cnnectin can als be entered, fr example, by using the keypad f the mbile statin MS, such as, fr example, a PIN cde, r a kind f 45 SIM card can be used where, amng ther parameters, the ciphering key Kc has been stred. The ciphering key Kc need nt be regenerated when the mbile statin MS changes its lcatin t the area f anther packet switching cntrller GPRS because the ciphering key 50 can be delivered frm the previus packet switching cntrller t the new ne. The transitin frm clear text mde t ciphered mde prceeds preferably in such a way that the packet switching cntrller GPRS transmits in clear text a special "start 55 cipher" cmmand. In the mbile statin MS, the enciphering f the transmissin and the deciphering f the receiving start after the "start cipher" cmmand has been crrectly received by the mbile statin. n the packet switching cntrller GPRS side, the enciphering starts crrespndingly after the 60 packet switching cntrller has received the message transmitted by the mbile statin MS and deciphered it. The abve described peratin crrespnds, in its main parts, t the start f the enciphering f the GSM system. In sme packet switching applicatins, ciphering can be 65 applied als in such a way that nly messages ging in ne directin are ciphered, i.e. messages frm the mbile statin MS t the packet switching cntrller GPRS r frm the packet switching cntrller GPRS t the mbile statin MS. Applicatins like this include, fr example, delivering f advertisements which are usually transmitted nn-ciphered. Additinally, ciphering accrding t the inventin can be applied als in such a way that nly sme part f the transmitted data frames f the adapting layer SNDC is ciphered. In this case, ne encryptin bit is mst preferably added t the data frame f the adapting layer and it will indicate whether the data frame cncerned is ciphered r nn-ciphered. Fr example, when the encryptin bit has the value zer, the data frame is nn-ciphered and when the encryptin bit has the value ne, the data frame is ciphered. This can be used, fr example, in situatins where the access rights t a service require registratin r the equivalent, in which case the registered users can decipher the ciphered data frames. Fr ther users, the service prvider can deliver infrmatin cncerning services and advertisements in nnciphered data frames. FIG. 5fl shws an example f a data frame structure f a link layer accrding t a preferable embdiment. The header field f the data frame (frame header) cmprises a TLLI identity f three bytes and a cntrl part (Cntrl) f tw bytes. A byte cmprises, as is knwn per se, eight binary infrmatin (bits). The infrmatin field f the data frame cmprises the transmitted infrmatin. The length f the infrmatin field may vary. The data frame als cntains a check field (Check sequence) f tw bytes which includes, fr example, errr crrectin infrmatin. FIG. 5b shws the structure f the cntrl part f the data frame f FIG. 5a when the data frame is an infrmatin delivery and system supervisry data frame (Infrmatin+ supervisry) wherein: CR indicates whether it is a questin f a cmmand r a respnse (CmmandRespnse), SI and S2 describe the type f the supervisry cmmand, N(S) is the number f the sending sequence (Send sequence number), PF indicates whether it is a questin f a cnfirmatin request message (P) r a cnfirmatin message (F) (PllFinal), and N(R) is the number f the receptin sequence (Receive sequence number). FIG. 5c shws the structure f the cntrl part f the data frame f FIG. 5a when the data frame is a system supervisry data frame (Supervisry). The significance f bits has been described abve. FIG. 5d shws the structure f the cntrl part f the data frame f FIG. 5a when the data frame is an unnumbered data frame (Unnumbered) wherein: M 1-5 are unnumbered cmmands and respnses, GD indicates whether it is a questin f a cntrl r a data frame (CntrlData), and x-bits are nt significant.

15 5,987, FIG. 6a shws an example f a data frame structure with identity f a ruting area, a Pint-t-Pint cnnectin f an adapting layer accrding identity f an area f a packet switching cntrller, and t a preferable embdiment. The first byte cntains cntrl. data in which: ldentlt f a cell y,,.,.., ,.. r r 7. A methd accrding t claim 6, wherein a data frame M indicates whether it is a questin f the last segment f >.,,.. '., number ls frmed and the infrmatin frmed by the applicatin, malntalned lcauy in data transfer r,.,,,,, i devices linked t a data transfer cnnectin in which case the b indicates whether the ciphering is in use,..,. ^ ^. v,. i. sequence number is set t its initial value at a start f the data Pn indicates the pririty classificatin,.,,.,,,,,. T.,,...,., transfer cnnectin and is updated in a previusly defined NLM is prtcl data which can be, tr example, ln lu,., manner during the cnnectin. TCPIP ' 8. A methd accrding t claim 7, wherein the data CLNP ' transfer cnnectin is a data transfer cnnectin f a packet X 25 ' ' switching system. ' ' 9. A methd accrding t claim 7, wherein the data FIG. 6b shws an example f a data frame structure with 1 -'. c,,.,,,,.,,.. p,.,,. transfer cnnectin is a Pint-t-Pint cnnectin, a multipint cnnectin f an adapting layer accrding t a.,,,.,,.,, t j.i u J-» 'n. -a t u-» u u 1". A methd accrding t claim 7, wherein the data & preferable embdiment. Ihe significance f bits has been ' described abve transfer cnnectin is a multipint cnnectin. Althugh the inventin has been described abve in a data n A methd accrding t claim 10, wherein there are a transfer system where mbile statins MS, base statin 20 plurality f data transfer cnnectins including said data subsystems BSS and packet switching cntrllers SGSN f transfer cnnectin, and infrmatin is transferred between a GPRS system are used, the inventin can be applied als a file data transfer device f a data service prvider and data in ther data transfer systems, such as TDMA and MA transfer devices f data service users, wherein a ciphering data transfer systems, mst preferably in packet switching key (allcated separately t each f said data cnnectins is data transfer systems. set t resp e cti ve nes f the data transfer devices by trans- The inventin has nt been limited nly t the abve r.,,,,,,.,,.,...,.,,.,.,., terring the ciphering key in a ciphered mde in the data presented embdiments, but it can be mdified within the..,.,,,-, r c tu tt u J i cmmunicatin system, by using a keypad f a data transfer J J b Jir scpe f the attached claims.. What is claimed is: device. 1. A methd fr the encryptin f infrmatin transferred A methd accrding t claim 11, wherein between data transfer devices in a data cmmunicatin nly data transmitted frm the data transfer device f a system wherein plural data frames are created frm ne r data service prvider t data transfer devices f data mre data packets frmed frm the infrmatin by a cm- service users is ciphered at least partly, municatin applicatin, and individual nes f these data,,,,.,.., r,,,,, r.,,, j.r.ij«nly data transmitted frm data transfer devices f data frames cmprise at least a header field and a data field, JD J wherein the methd cmprises steps f: servlce users t the data transfer devlce f data servlce ciphering at least ne part f the data packets by using a P rvider is ci P hered at least P^' r ciphering key; data transmitted in bth directins is ciphered at least attaching synchrnizatin data t the data frames, wherein partly. the synchrnizatin data includes a sequence number f 13. A methd accrding t claim 12, wherein at a start f each f said data frames; and ciphering, data, cncerning a directin in which data transfer changing the value f the ciphering key at least at the is ciphered, is transmitted t data transfer devices. transmissin f each f said data frames. 14. A methd accrding t claim 10 wherein there are a 2. A methd accrding t claim 1 wherein a data transfer 45 plurality f data transfer cnnectins including said data cnnectin is frmed between tw r mre data transfer transfer cnnectin, and infrmatin is transferred between devices cnnected t the data cmmunicatin system, a file data transfer device f a data service prvider and data wherein a separate ciphering key is allcated t each transfer devices f data ^ - ^ ^ ^ wherein a ci herin cnnectin, in which case in a cmmn data transfer,,,,,,, r,,.,, ',. key allcated separately t each f said data cnnectins is channel, data frames f at least tw separate cnnectins are i{] <.. r..,.. r,., r,,..,,,.,,, j-, 4, set t respective nes f the data transfer devices by transtransferable in ciphered mde independent f each ther..,.,.,..,,,,,,. tu j J- * i i u f-j terring the ciphering key in a ciphered mde in the data 3. A methd accrding t claim 1 wherein a grup f said..,. data frames is divided int sub-blcks, wherein synchrni- cmmunicatin system, by using a smart card, zatin data cmprises a blck cunter which is allcated 15. A methd accrding t claim 6, wherein a data frame separately t each cnnectin and t which an initial value 55 number f the lmk la y er ls maintained in ne data transfer device f a is set at a start f a cnnectin and a value f which is transfer cnnectin and is delivered t ther changed at a transmissin f each f said sub-blcks. da t a transfer devices in a data frame f the link layer. 4. A methd accrding t claim 1 wherein data frames are 16. A methd accrding t claim 1, wherein nly sme frmed at an adapting layer. part f the data frames f an adapting layer is ciphered, in 5. A methd accrding t claim 4, wherein data frames f 60 which case data f a ciphering f each f said data frames the adapting layer are transferred t a tile link layer wherein is transmitted in the header field f the respective data frame, data frames f the tile link layer are frmed frm data frames 17. Adata cmmunicatin system which cmprises means f the adapting layer fr transmissin t a transmissin path. fr encryptin f infrmatin transferred between data trans- 6. A methd accrding t claim 5, wherein synchrniza- f er devices, means fr frming ne r mre data packets f tin data cmprises at least ne f the fllwing: 65 the infrmatin and means fr frming data frames f the data frame number f the tile link layer, data packets, the means fr encryptin f infrmatin crndata frame number f the adapting layer, prise at least:

16 13 means fr ciphering data packets by a ciphering key, 5,987,137 means fr changing a value f the ciphering key fr each f a plurality f transmissin blcks; means fr attaching synchrnizatin data t data frames, 5 means fr changing the value f the synchrnizatin data (UNT) at the transmissin f each data frame, wherein the synchrnizatin data includes sequence numbers f respective nes f the data frames, and r means fr interpreting synchrnizatin data in the data transfer device f the receiver A data cmmunicatin system accrding t claim 17, characterized in that the data transfer devices cmprise at least ne mbile statin. 19. A data cmmunicatin system accrding t claim 18, characterized in that the mbile statin is a GSM mbile statin. 20. A data cmmunicatin system accrding t claim 17, characterized in that the data transfer devices cmprise at least ne base statin. 21. A data cmmunicatin system accrding t claim 20, characterized in that the base statin is a GSM base statin.