Authentication method for mobile communications

Abstract: In the preliminary authentication stage, the mobile station (35) is authenticated by sending from the home network (30) to the roamed network (31), a plurality of pairs of first random numbers (RNDa₁,...,RNDa n ) and calculation results (SRESa₁,...,SRESa n ) of the cipher function (f), which calculation is performed at the home network using the secret key (ki) and the first random numbers (RNDa₁,...,RNDa n ), by sending, from the roamed network to the mobile station, third random numbers formed by coupling second random numbers (RNDb₁,...,RNDb m ) produced at the roamed network with the first random numbers (RNDa₁,...,RNDa n ), by sending, from the mobile station to the roamed network, calculation results (SRESa₁,...,SRESa n , SRESb1,...,SRESb m )) of the cipher function (f), which calculation is performed at the mobile station using the secret key (ki) and the sent third random numbers, and by confirming, at the roamed network, coincidence of the calculation results (SRESa₁,...,SRESa n ) sent from the mobile station with the calculation results (SRESa₁,...,SRESa n ) sent from the home network. In the main authentication, the mobile station is authenticated by using a pair of the second random number (RNDb₁,...,RNDb m ) and of the calculation result (SRESb₁,...,SRESb m ) with respect to the second random number (RNDb₁,...,RNDb m ), sent from the mobile station.

Description: The present invention relates to an authentication method for mobile communications. Particularly, the invention relates to an authentication method which can be used in mobile communication networks with different algorithms, for identifying that a mobile subscriber accessing or roaming in a network different from his home network is a right subscriber in the home network. DESCRIPTION OF THE RELATED ART In the mobile communications, since the mobile subscribers are connected to a mobile communication network via radio interfaces, the mobile communication network has difficulty to confirm that the connected mobile subscriber is certainly a desired one. Therefore, it is required for the mobile communication network to authenticate the connected mobile subscriber As in such a radio environment, a communication can be easily listened by any one, enough protection will be necessary for the authentication so that only the right subscribers are correctly authenticated. Thus, in the recent digital mobile communications, a challenge-response (CR) authentication method based upon a secret key cryptography has been widely used. Referring to Fig. 1 which shows the principle of the CR authentication, this CR authentication method will be described. According to the CR authentication method, a mobile communication network and a mobile station belonging to the network have the same secret key cipher function f. The function f has two variables, a secret key ki and a random number RND. As the secret key ki may be handled as a parameter, this cipher function will be represented by f ki (RND) and its result will be represented by SRES. The mobile network has secret keys {ki} of all the right mobile stations (subscribers) belonging to this network (S101). The right mobile stations have also the respective secret keys {ki} which are different from each other (S102). These secret keys {ki} are physically protected from illegal reading out. At starting the CR authentication, although not shown, the mobile station sends its identification number to the mobile network. The mobile network then seeks for a secret key ki corresponding to this connected mobile station by retrieving its data base, and then, after having found the secret hey ki, it generates at least one random number RND (S103). The generated RND (challenge) is sent to the mobile station. The mobile station calculates f ki (RND) using the received RND and its secret key ki (S104), and then sends back the result of the calculation SRES (response) to the mobile network. Since the mobile network also has the same secret key ki and the cipher function f, the same calculation of f ki (RND) can be performed using the sent RND. The result of this latter calculation is compared with the result SRES from the mobile station (S105). If the result of f ki (RND) is equal to SRES, the authentication succeeds. Otherwise, it fails. As is described, according to the CR authentication method, the secret key ki of the mobile station does not appear at the radio interface. Only RND and SRES are transmitted between the mobile station and the network via this radio interface. Thus, the secret key ki will be securely protected from listening. Furthermore, since RND can be randomly selected by the network and the correct SRES changes depending upon the selected RND, any illegal mobile station is quite difficult to be authenticated as a right mobile station even if it sends SRES previously obtained by listening to the network. Therefore, the CR authentication method is an extremely excellent method for ensuring the security of a mobile communication system. Authentication methods describing this specification will utilize such a CR authentication method with modification. It is called "roaming" if a mobile station is accessing a visited network which is different from its home network. This visited network accessed by the mobile station is called "roamed network". Before communication with the accessing mobile station, the roamed network has to authenticate that this roaming mobile station is a right mobile subscriber registered in his home network. However, since the roamed network neither has a secret key of the roaming mobile station nor always uses the same cipher function as that home network, a particular process will be required for authenticating the roaming mobile station as being one of the mobile stations in its home network as follows. (1) In case the roamed network uses the same cipher function f as the home network, there are two methods of (a) sending a secret key ki of the accessing mobile station from the home network to the roamed network, and (b) producing at least one pair of challenge-responses {RND, SRES} which will be necessary for authentication at the home network using a secret key ki of the accessing mobile station, and sending the produced pair {RND, SRES} to the roamed network. Fig. 2a shows the method of (a) wherein the home network sends the secret key ki of the roaming mobile station to the roamed network. In this method, the mobile station and the home network have the same cipher function f and the same secret key ki (S201, S203), and the roamed network has the same cipher function (S202). If the mobile station requests roaming to the visited network with its identification number, this roamed network sends this identification number to the home network of the mobile station. The home network then seeks for a secret key ki corresponding to the roaming mobile station by retrieving its data base (S204). The found secret key ki is sent to the roamed network (S205). Thereafter, the usual CR authentication processes already described are performed between the mobile station and the roamed network using this sent secret key ki.This method (a) is simple. However, since the secret key ki which is important for security is transmitted to the roamed network, a high level protection cannot be expected.Fig. 2b shows the method of (b) wherein the home network produces a set of CR pairs which will be necessary for authentication, by using the secret key ki of the roaming mobile station, and sends them to the roamed network. In this case, only the mobile station and the home network have the same cipher function f and the same secret key ki (S211, S212). If the mobile station requests roaming to the visited network with its identification number, this roamed network sends this identification number to its home network. The home network then seeks for a secret key ki corresponding to the roaming mobile station by retrieving its data base (S213), and, having found one, generates at least one set of random numbers {RND 1 ,...,RND n } (S214). The home network then calculates f ki (RND j ) using the generated RND j (j=1,...,n) and the found secret key ki (S215), and sends back a set of CR pairs, namely the generated random numbers {RND 1 ,...,RND n } and the results of the calculation {SRES 1 ,...,SRES n }, to the roamed network (S216). Thereafter, the usual CR authentication processes already described are performed between the mobile station and the roamed network.According to this method of (b), since a set of CR pairs must be transmitted to the roamed network, the amount of transmission will be increased. However, the secret key ki which is important for security does not appear on the link between the networks causing the high level protection to be expected.Generally, the roamed network requests and receives a plurality of CR pairs from the home network at the first roaming of the mobile station, then at every authentication of that mobile station, one pair of the received CR pairs stored in the roamed network will be used without requesting a new CR pair from the home network. After all the stored CR pairs corresponding to that mobile station are used for authentication, the roamed network will request and receive a new set of CR pairs from the home network. (2) In case the roamed network does not have the same cipher function f as the home network, the aforementioned method (a) cannot be applied, and thus the method (b) has to be used. The European standard digital mobile communication system, namely GSM (Global System for Mobile communication) adopts this method (b). The method (b) wherein at least one CR pair is transmitted from the home network to the roamed network does not require that the networks have the same cipher function f. However, according to this method, the roamed network cannot use original random numbers for authentication. Also, according to this method, respective bit lengths of the random number and of the calculation result for authentication in both the roamed network and the home network are required to be equal with each other. In general, networks with different cipher functions will use, for authentication, the random number and calculation result of different bit length. Thus, in a certain case wherein the transmission of variable bit-lengths of the random number and of the calculation result is not supported by an authentication protocol, enough information may not be transmitted through radio interfaces even if the above-mentioned method (b) is used. In such a case, a good CR authentication cannot be expected. There are four cases of transmissions of the random number and of calculation result with different bit lengths, as follows. (A) The bit length of the random number in the roamed network is longer than that in the home network. In this case, since the roamed network can completely transmit via the radio interface the random number from the home network to the roaming mobile station, no problem will occur. (B) The bit length of the random number in the roamed network is shorter than that in the home network. In this case, the roamed network cannot transmit all the bits of the random number from the home network to the roaming mobile station. Thus, the mobile station will receive an incomplete random number resulting in an incorrect calculation of the cipher function. Therefore, a correct authentication at roaming cannot be expected in this case. (C) The bit length of the calculation result in the roamed network is longer than that in the home network. In this case, since the roamed network can completely transmit via radio interface the calculation result from the home network to the roaming mobile station, no problem will occur. (D) The bit length of the calculation result in the roamed network is shorter than that in the home network. In this case, the roamed network cannot transmit all the bits of the calculation result from the home network to the roaming mobile station. However, since the roamed network can obtain at least part of the calculation result from the mobile station in addition to the calculation result from the home network, although protection will be lowered a little, the authentication can be substantially performed. As aforementioned, according to the conventional authentication method, it is difficult to perform roaming in the case of (B). Since roaming will be requested between the networks in reciprocal, the above-mentioned trouble of (B) will certainly occur at the roaming of the networks using different bit-length random numbers. WO 93 17 529 A discloses a mobile communication authentication method as stated in the preamble of claim 1 of the present invention. In this prior method between the mobile station (a cordless telephone (CT)) and a mobile communication system comprising the roamed network and the home network (the Global System for Mobile Communications (GSM)) authentication data specific to the mobile station (CT specific authentication data) are transferred in the mobile communication system (the GSM network) by standard (GSM) messages and information elements without changing the format of the information elements or the message format. The authentication data (CT data) are shorter than said information elements (GSM data). Thus, the unoccupied space of the information elements is filled with constant zeros in conjunction with authentication (CT) data. These zeros may reduce the security against undesired deciphering of the transmitted data. EP-A-0 584 725 discloses another authentication method similar to the one stated in the preamble of claim 1 of the present invention. In this method a secret key is transmitted between networks without ciphering. Thus, the secret key becomes known to these networks and the subscriber. It is therefore the object of the present invention to provide a mobile communication authentication method, whereby the transmission amount of information can be reduced while maintaining the security of the communications and roaming can be performed between networks having different bit lengths of random numbers used for the mutual authentication. According to the present invention, a method for authenticating a mobile station which accesses for roaming a network different from a home network of the mobile station is provided as stated in claim 1. In the case that the bit length of the random numbers used in the roamed network is shorter than that used in the home network, the roamed network sends a random number having the shorter bit length to the mobile station. The mobile station will extend the received random number by using an extension function provided therein to obtain a random number having the same bit length as that used in the home network. The mobile station then calculates the same cipher function using the same secret key as those provided in the home network in accordance with the extended random number, and sends the calculation result to the roamed network in order to compare it with the calculation result from the home network. Therefore, according to the present invention, a mobile communication authentication method whereby roaming can be performed between networks having different bit lengths of random numbers used for the authentication from each other can be provided. The extension function may be a block cipher system of CBC mode. Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow chart showing the operation of a conventional CR authentication method already described; Figs. 2a and 2b are flow charts showing the operation of an another conventional CR authentication method already described; Fig. 3 schematically shows the constitution of a mobile communication system as a preferred embodiment according to the present invention; Fig. 4 schematically shows the constitution of a mobile station shown in Fig. 3; Fig. 5 which is illustrated in a separated form of Figs. 5A, 5B and 5C is a flow chart showing the operation of a batch preliminary authentication stage according to the embodiment of Fig. 3; Fig. 6 is a flow chart showing the operation of a main authentication stage according to the embodiment of Fig. 3; Fig. 7 which is illustrated in a separated form of Figs. 7A, 7B and 7C is a flow chart showing the operation of an another embodiment according to the present invention; Figs. 8a, 8b and 8c show examples of extension functions used in the embodiment of Fig. 7; Fig. 9 which is illustrated in a separated form of Figs. 9A, 9B and 9C is a flow chart showing the operation of a further embodiment according to the present invention; Fig. 10 which is illustrated in a separated form of Figs. 10A, 10B, 10C and 10D is a flow chart showing the operation of a batch preliminary authentication stage of a still further embodiment according to the present invention; Fig. 11 which is illustrated in a separated form of Figs. 11A and 11B is a flow chart showing the operation of a main authentication stage according to the embodiment of Fig. 10; and Fig. 12 which is illustrated in a separated form of Figs. 12A, 12B and 12C is a flow chart showing the operation of an another embodiment according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 3 which schematically shows a constitution of a mobile communication system as a preferred embodiment according to the present invention, reference numeral 30 denotes a home network and 31 shows a roamed network. In the home network 30, there are switching stations 32, a base station 33 connected to the switching station 32 by a wire line and a data base 34 connected to the switching station 32. A mobile station 35 if staying within this network 30 as illustrated by a dotted line can be connected via a radio link to the base station 33. In the roamed network 31, there are switching stations 36, a base station 37 connected to the switching station 36 by a wire line and a data base 38 connected to the switching station 36. The mobile station 35 roaming to this network 31 is now connected via a radio link to the base station 37. Fig. 4 schematically shows a constitution example of the mobile station 35 shown in Fig. 3. An antenna 40 is connected in series to an RF unit 41, a base-band processing unit 42 and a voice encoding and decoding unit 43. The voice encoding and decoding unit 43 is also connected to a speaker 44 and a microphone 45. A control unit 46 for controlling the operation of the RF unit 41, base-band processing unit 42 and voice encoding and decoding unit 43 is connected to these units. To the control unit 46 an I/O interface 47 for receiving a smart card 48 is also connected. Fig. 5 shows the operation of a batch preliminary authentication stage according to this embodiment. In this stage, it is supposed that the communication link between the mobile station 35 and the base station 37 of the roamed network 31 is a secured communication line protected from eavesdropping, such as a special wire communication line or a normal radio communication line using a ciphered message. The mobile station 35 and the home network 30 have the same cipher function f and the same secret key ki assigned to this mobile station (S501, S502). If the mobile station 35 requests a preliminary authentication by the visited network 31 with its identification number, the roamed network 31 informs its home network 30 thereof with its identification number. The home network 30 then seeks for a secret key ki corresponding to the roaming mobile station 35 by retrieving its data base 34 (S503), and, having found one, generates a set of random numbers {RNDa 1 ,...,RNDa n } (S504). The home network 30 then calculates f ki (RNDa j ) using the generated RNDa j (j=1,...,n) and the found secret key ki (S505), and sends a set of CR pairs, namely the generated random numbers {RNDa 1 ,...,RNDa n } of 62 bits and the results of the calculation {SRESa 1 ,...,SRESa n } of 32 bits, to the roamed network 31. The roamed network 31 which has been received a set of the CR pairs generates a set of random numbers {RNDb 1 ,...,RNDb m } (S506). The roamed network then couples these calculated random numbers {RNDb 1 ,...,RNDb m } with the received random numbers {RNDa 1 ,...,RNDa n }, and sends in parallel or in series (parallel in the example of Fig. 5) the coupled random numbers to the mobile station 35 by rearranging the order of the sending numbers in random. The above-mentioned n is an integer greater than one, and m is an integer much greater than n (m >> n). In a non-limiting example, m and n may be selected as m = 100 and n = 10. The mobile station 35 calculates SRESa j = f ki (RNDa j ) (j=1,...,n) and SRESb j = f ki (RNDb j ) (j=1,...,m) using the received (n + m) random numbers, the cipher function f and its secret key ki (S507), and then sends back to the roamed network 31 the results of the calculation {SRESa 1 ,...,SRESa n , SRESb 1 ,...,SRESb m } which are arranged in random order. The roamed network 31 resumes the order of the received calculation results in accordance with the sending order thereof, and thus obtains correctly ordered {SRESa 1 ,...,SRESa n } and {SRESb 1 ,... , SRESb m }. Then, the roamed network 31 compares the result {SRESa 1 ,...,SRESa n } calculated by and received from the mobile station 35 with the result {SRESa 1 ,...,SRESa n } calculated by and received from the home network 30 whether they coincide with each other (S508). If all of them coincide with each other, the mobile station 35 is deemed as a right mobile subscriber, and thus the random numbers {RNDb 1 ,...,RNDb m } and the calculation results {SRESb 1 ,...,SRESb m } from the mobile station 35 are stored in the roamed network 31 so as to be used in the main authentication as CR pairs (S509). If one or more results do not coincide with each other, it is determined that the batch preliminary authentication fails, and then the results {SRESb 1 ,... , SRESb m } are abandoned. Since the sending order of the random number from the roamed network 31 has been rearranged, the mobile station 35 cannot know which of {RNDa 1 ,...,RNDa n , RNDb 1 ,... , RNDb m } corresponds to the currently received random number. Therefore, if all the calculation results with respect to {RNDa 1 ,...,RNDa n } coincide as aforementioned, not only the mobile station 35 is deemed as a right one but also a set of the CR pairs {RNDb 1 ,...,RNDb m } and {SRESb 1 ,...,SRESb m } are considered as legal CR pairs. After succeeding the above-mentioned batch preliminary authentication of the mobile station 35, the roamed network 31 can start the main authentication shown in Fig. 6 at any time when this mobile station 35 requests roaming via the radio interface. When the mobile station 35 requests roaming with its identification number, since the results of the preliminary authentication (CR pairs) {RNDb 1 ,...,RNDb m } and {SRESb 1 ,...,SRESb m } have already been stored in the roamed network 31 (S601), the roamed network 31 sends one of the random numbers RNDb j of 64 bits to the mobile station 35. The mobile station 35 then calculates SRESb j = f ki (RNDb j ) using the received RNDb j , its cipher function f and its secret key ki (S602), and sends back the calculated result SRESb j of 32 bits to the roamed network 31. The result of this calculation SRESb j is compared with the corresponding result SRESb j stored in the roamed network at the preliminary authentication stage (S603). If they coincide with each other, the main authentication succeeds. Otherwise, it fails. It should be noted that at every main authentication of this mobile station 35, one pair of the CR pairs stored in the roamed network 31 at the batch preliminary authentication will be used. At a roaming by this mobile station 35 after all the stored CR pairs of this station 35 have been used, new preliminary authentication will be required. In the main authentication stage, the random numbers {RNDb 1 ,...,RNDb m } once sent to the mobile station are used again. Thus, if these numbers were eavesdropped during the preliminary authentication stage, the eavesdropper may disguise as a right subscriber at the main authentication. Therefore, the communication line between the mobile station and the roamed network during the preliminary authentication should be a secured communication line protected from eavesdropping. In order to simplify, it has been described that the mobile station itself has the cipher function and the secret key. However, in many mobile communication systems, smart cards which store these cipher functions and assigned secret keys may be provided. In such cases, during the preliminary authentication, only the smart cards may be connected to the roamed network by inserting them into dedicated wire line terminals in the roamed network instead of connecting them via the radio interfaces so that the communication line between the mobile station and the roamed network is secured from eavesdropping. As will be apparent from the above-description, according to this embodiment, the roamed network can obtain a desired number of CR pairs from the random numbers generated by itself and the calculation results calculated by the mobile station in accordance with the above random numbers without receiving a large number of the CR pairs from the home network. Since the number n of the random numbers sent from the home network and used for the preliminary authentication is much lower than the number m of the random numbers generated in the roamed network (m >> n), the amount of information transmitted between the networks can be extremely reduced. Furthermore, this method will be effective in case the roamed network is impossible for calculating the cipher function f. Fig. 7 shows the operation of another embodiment according to the present invention. The constitution of a mobile communication system and the constitution of a mobile station are the same as those shown in Figs. 3 and 4. This embodiment concerns a mobile communication system having an authentication protocol wherein the roamed network is allowed to produce random numbers. In this embodiment, the mobile station 35 and the home network 30 have the same cipher function f and the same secret key ki assigned to this mobile station (S701, S703).The bit length of the random numbers used in the home network 30 and in the mobile station 35 is 128 bits, and the bit length of the calculation results used therein is 32 bits (S701, S703). The roamed network 31, on the other hand, has a cipher function different from f. The bit length of the random numbers used in this roamed network 31 is 64 bits and the bit length of its calculation result is for example 32 bits (S702). Furthermore, the mobile station 35 and the roamed network 31 are provided with extension functions E( ) for extending the bit length from 64 bits to 128 5 bits. If the mobile station 35 requests roaming to the visited network 31 with its identification number, the roamed network 31 informs its home network 30 thereof with its identification number. The home network 30 then seeks for a secret key ki corresponding to the roaming mobile station 35 by retrieving its data base 34 (S704). After having found the secret key ki, the roamed network 31 generates a set of random numbers {RND 1 ,...,RND n } of 64 bits (S705), and produces extended random numbers {RND' 1 ,...,RND' n } of 128 bits by applying the extension function E( ) to the 64-bits random numbers {RND 1 ,...,RND n } (S706). The extended random numbers {RND' 1 ,...,RND' n } are sent to the home network 30. Thus, the home network 30 calculates f ki (RND' j ) using the received RND' j (j=1,...,n) and the found secret key ki (S707). The results of the calculation {SRES 1 ,...,SRES n } of 32 bits are sent back to and stored in the roamed network 31 as sets of the random numbers, their extended random numbers and their calculated results {RND j , RND' j , SRES j } (j=1,...,n). At the authentication of the mobile station, the roamed network 31 extracts an unused set of random numbers, its extended random number and its calculated result {RND j , RND' j , SRES j } from these stored sets. Then, the random number RND j in this extracted set is sent to the mobile station 35 via the radio interface. The mobile network 35 produces extended random numbers RND' j of 128 bits by applying the extension function E( ) to RND j (S708), and then calculates SRES j = f ki (RND' j ) using the extended RND' j and its secret key ki (S709). The result of the calculation SRES j of 32 bits is sent back to the roamed network 31. Then, the roamed network 31 compares the result SRES j calculated by and received from the mobile station 35 with the result SRES j stored therein whether they coincide with each other (S710). If they coincide with each other, the authentication succeeds. Otherwise, it fails. There are many extension functions. Figs. 8a, 8b and 8c show simple examples of these extension functions. The simplest extension function is shown in Fig. 8a or 8b, wherein a 64-bits input is inserted into the lower (right) or upper (left) half of an 128 bits output and the remaining half of the output is filled with "0" or a fixed numeral. However, since it will be easily revealed from the received output that an extension function is utilized to produce such a long bit output, using of these extension functions is undesirable. For an extension function, it is necessary that (1) the input value of 64 bits cannot be easily assumed from the output of 128 bits, and (2) the extended 128 bits output or the original 128 bits output cannot be easily assumed from the received output. A block cipher system of CBC mode shown in Fig. 8c may be an example of the extension function which satisfies both the above-mentioned conditions (1) and (2). In the figure, reference numeral 80 denotes an exclusive OR circuit and 81 a block cipher circuit for enciphering a 64 bits input, namely for calculating a function F( ) with respect to its input of 64 bits. If the input of this system is represented by M 1 of 64 bits, a corresponding output C 1 of the circuit 81 will be C 1 = F(M 1 ). As this output is fedback and applied to the exclusive OR circuit 80, the next output C 2 will be C 2 = F{F(M 1 ) ⊕ {M 1 }. An extended 128 bits output will be obtained by coupling these 64 bits outputs C 1 and C 2 as the upper half bits and lower half bits of the 128 bits output, respectively. According to this embodiment, although random numbers transmitted between the mobile station and the roamed network via the radio interface are 64-bits random numbers, the mobile station can perform the authentication using random numbers of 128 bits, which authentication is the same as that in the home network. Fig. 9 shows the operation of a further embodiment according to the present invention. The constitution of a mobile communication system and the constitution of a mobile station are also the same as those shown in Figs. 3 and 4. This embodiment concerns a mobile communication system having an authentication protocol wherein the home network is allowed to produce random numbers of 64 bits. In this embodiment, also, the mobile station 35 and the home network 30 have the same cipher function f and the same secret key ki assigned to this mobile station (S901, S903). The bit length of the random numbers used in the home network 30 and in the mobile station 35 is 128 bits, and the bit length of the calculation results used therein is 32 bits (S901, S903). The roamed network 31, on the other hand, has a cipher function different from f. The bit length of the random numbers used in this roamed network 31 is 64 bits and the bit length of its calculation result is for example 32 bits (S902). Furthermore, the mobile station 35 and the home network 30 are provided with extension functions E( ) for extending the bit length from 64 bits to 128 bits. If the mobile station 35 requests roaming to the visited network 31 with its identification number, this roamed network 31 sends this with its identification number to its home network 30. The home network 30 then seeks for a secret key ki corresponding to the roaming mobile station 35 by retrieving its data base 34 (S904).After having found the secret key ki, the home network 30 generates a set of random numbers {RND 1 ,...,RND n } of 64 bits (S905), and produces extended random numbers {RND' 1 ,...,RND' n } of 128 bits by applying the extension function E( ) to the 64-bits random numbers {RND 1 ,...,RHD n } (S906). Next, the home network 30 calculates f ki (RND' j ) using the calculated RND' j (j=1,...,n) and the found secret key ki (S907). The generated random numbers {RND 1 ,...,RND n } of 64 bits and the results of the calculation {SRES 1 ,...,SRES n } of 32 bits are sent to and stored in the roamed network 31 as CR pairs of {RND j , SRES j } (j=1,...,n). At the authentication of the mobile station, the roamed network 31 extracts an unused CR pair of the random number and its calculated result {RND j , SRES j } from the stored these CR pairs. Then, the random number RND j of this extracted pair is sent to the mobile station 35 via the radio interface. The mobile network 35 produces extended random numbers RND' j of 128 bits by applying the extension function E( ) to RND j (S908), and then calculates SRES j = f ki (RND' j ) using the extended RND' j and its secret key ki (S909). The result of the calculation SRES j of 32 bits is sent back to the roamed network 31. Then, the roamed network 31 compares the result SRES j calculated by and received from the mobile station 35 with the result SRES j stored therein whether they coincide with each other (S910). If they coincide with each other, the authentication succeeds. Otherwise, it fails. This embodiment differs from the embodiment of Fig. 7 in that the home network, not the roamed network, has an extension function used for extending the random numbers and that the random number of 64 bits are sent from the home network to the roamed network. Figs. 10 and 11 show the operation of a still further embodiment according to the present invention. The constitution of a mobile communication system and the constitution of a mobile station are also the same as those shown in Figs. 3 and 4. This embodiment concerns a mobile communication system having an authentication protocol wherein the roamed network 31 is not allowed to produce random numbers. The authentication process in this embodiment consists of two stages of a batch preliminary authentication stage, and a main authentication stage. In this embodiment, furthermore, a smart card 48 having a safety cipher function and a secret key, and performing the preliminary authentication operation and most of the main authentication operation is provided. Thus, the mobile station 35 will take partial charge of radio communication operation other than that performed by the smart card. It is supposed that the smart card 48 according to the embodiment has its own identification number. In another embodiment, either only the mobile station may have its identification number or both the mobile station and the smart card may have their respective identification numbers. Fig. 10 shows the operation of the batch preliminary authentication stage according to this embodiment. In this stage, it is supposed that the communication link between the smart card 48 and the roamed network 31 is a secured communication line protected from eavesdropping, such as a special wire communication line or a normal radio communication line using a ciphered message. The smart card 48 and the home network 30 have the same cipher function f and the same secret key ki assigned to this smart card (S1001, S1003).The bit length of the random numbers used in the home network 30 and in the smart card 48 is 128 bits, and the bit length of the calculation results used therein is 32 bits (S1001, S1003, S1101 of Fig. 11). The roamed network 31, on the other hand, has a cipher function different from f. The bit length of the random numbers used in this roamed network 31 is 64 bits and the bit length of its calculation result is for example 32 bits (S1002, S1102 of Fig. 11). Furthermore, the mobile station 35 and the roamed network 31 are provided with extension functions E( ) for extending the bit length from 64 bits to 128 bits. If the smart card 48 requests a preliminary authentication by the roamed network 31 with its identification number, this roamed network 31 sends this with its identification number to its home network 30. The home network 30 then seeks for a secret key ki corresponding to this smart card by retrieving its data base 34 (S1004), and, after having found it, generates a set of random numbers {RNDa 1 ,...,RNDa n } (S1005) of 128 bits. The home network 30 then calculates f ki (RNDa j ) using the generated RNDa j (j=1,...,n) and the found secret key ki (S1006), and sends back a set of CR pairs, namely the generated random numbers {RNDa 1 ,...,RNDa n } of 128 bits and the result of the calculation {SRESa 1 ,...,SRESa n } of 32 bits, to the roamed network 31. The roamed network 31 which has been received a set of CR pairs generates a set of another random numbers {RNDb 1 ,...,RNDb m } of 64 bits (S1007), and produces extended random numbers {RNDb' 1 ,...,RNDb' m } of 128 bits by applying the extension function E( ) to the 64-bits random numbers {RNDb 1 ,...,RNDb m } (S1008). The roamed network then couples these extended random numbers {RNDb' 1 ,...,RNDb' m } with the received random numbers {RNDa 1 ,...,RNDa n }, and sends in parallel or in series (parallel in the example of Fig. 10) these coupled random numbers of 128 bits to the smart card 48 by rearranging the order of the sent numbers in random. The above-mentioned n is an integer greater than one and m is an integer much greater than n (m >> n). In a non-limiting example, m and n may be selected as m = 100 and n = 10. The smart card 48 calculates SRESa j = f ki (RNDa j ) (j=1,...,n) and SRESb j = f ki (RNDb' j ) (j=1,...,m) using the received (n+m) random numbers, the cipher function f and its secret key ki (S1009), and then sends back to the roamed network 31 the results of the calculation {SRESa 1 ,...,SRESa n , SRESb 1 ,...,SRESb m } which are arranged in random order. The roamed network 31 resumes the order of the received calculation results in accordance with the sending order thereof, and thus obtains correctly ordered {SRESa 1 ,...,SRESa n } and {SRESb 1 ,...,SRESb m }. Then, the roamed network 31 compares the result {SRESa 1 ,...,SRESa n } calculated by and received from the smart card 48 with the result {SRESa 1 ,...,SRESa n } calculated by and received from the home network 30 whether they coincide with each other (S1010). If all of them coincide with each other, the smart card 48 is deemed as a right subscriber, and thus the random numbers {RNDb 1 ,...,RNDb m } and the calculation results {SRESb 1 ,...,SRESb m } from the smart card 48 are stored in the roamed network 31 so as to be used in the main authentication as CR pairs (S1011). If one or more results do not coincide with each other, it is determined that the batch preliminary authentication fails, and then the results {SRESb 1 ,...,SRESb m } are abandoned. Since the sending order of the random number from the roamed network 31 has been rearranged, the smart card 48 cannot know which of {RNDa 1 ,...,RNDa n , RNDb 1 ,...,RNDb m } corresponds to the currently received random number. Therefore, if all the calculation results with respect to {RNDa 1 ,...,RNDa n } coincide as aforementioned, not only the smart card 48 is considered as a right one but also a set of the CR pairs {RNDb 1 ,...,RNDb m } and {SRESb 1 ,...,SRESb m } is considered as legal CR pairs. After succeeding the above-mentioned batch preliminary authentication of the smart card 48, the roamed network 31 can start the main authentication shown in Fig. 11 at any time when this smart card 48 requests roaming via the mobile station 35 and the radio interface. When the smart card 48 requests roaming with its identification number via the mobile station 35, since the results of the preliminary authentication (CR pairs) {RNDb 1 ,...,RNDb m } and {SRESb 1 ,...,SRESb m } have already stored in the roamed network 31 (S1103), the network 31 sends one of the random numbers RNDb j of 64 bits to the mobile station 35. The mobile station 35 produces extended random number RNDb' j of 128 bits by applying the extension function E( ) to the received 64-bits random number RNDb j (S1104), and then sends this extended random number RNDb' j to the smart card 48. The smart card calculates SRESb j = f ki (RNDb' j ) using the received RNDb' j , its cipher function f and its secret key ki (S1105), and sends back the calculated result SRESb j of 32 bits to the roamed network 31 via the mobile station 35. The result of this calculation SRESb j is compared with the corresponding result SRESb j stored in the roamed network at the preliminary authentication stage (S1106). If they coincide with each other, the main authentication succeeds. Otherwise, it fails. As will be apparent from the above-description, according to the present invention, roaming can be performed between networks which have different cipher functions for authentication and use random numbers of different bit lengths. In near future, many smart cards which can share a part of authentication operation will be used for roaming to their visited networks. In such a case, if each of the mobile stations belonging to the visited networks has an appropriate function for extending the length of the random numbers, the smart cards can easily perform the roaming to any of their visited networks using random numbers of a shorter bit length than that in the smart cards. Fig. 12 shows the operation of an another embodiment according to the present invention. The constitution of a mobile communication system and the constitution of a mobile station are also the same as those shown in Figs. 3 and 4. This embodiment concerns a mobile communication system having an authentication protocol wherein the roamed network 31 separately manages the mobile station 35 and the smart card 48, and wherein the radio interface between the roamed network 31 and the mobile station 35 is allowed to transmit only random numbers of for example 64 bits. The authentication process in this embodiment consists of two stages of an authentication for the mobile station (mobile station authentication), and an authentication for the smart card (subscriber authentication). In this embodiment, furthermore, the mobile station belongs to this roamed network and thus it will be authenticated in accordance with the authentication process of this roamed network. Namely, the mobile station i (35) and the roamed network 31 (which is a home network for this mobile station) have the same cipher function g and the same secret key ki assigned to this mobile station (S1202, S1203). When a random number of 64 bits is received from the roamed network 31, the mobile station conducts a cipher calculation and then sends the result thereof TRES' to the roamed network. On the other hand, the smart card will be authenticated in accordance with the authentication process of the home network 30. Namely, the smart card m (48) and the home network 30 have the same cipher function f and the same secret key km assigned to this smart card (S1201, S1204). When a random number of 128 bits is received from the home network 30, the smart card conducts a cipher calculation and then sends the result thereof SRES' to the roamed network. When the smart card 48 requests roaming to the roamed network 31 with its identification number via the mobile station 35, this roamed network 31 sends this with its identification number to its home network 30. The home network 30 then seeks for a secret key km corresponding to this smart card by retrieving its data base 34 (S1205), and, having found it, generates a random number RND (S1206) of 128 bits. The home network 30 then calculates SRES = f km (RND) using the generated RND and the found secret key km (S1207), and sends back a CR pair, namely the generated random number RND of 128 bits and the result of the calculation SRES of 32 bits, to the roamed network 31. When the mobile station requests accessing to the roamed network 31 with its identification number, this roamed network seeks for a secret key ki corresponding to the mobile station by retrieving its data base 38 (S1208). After receiving the CR pair (RND, SRES) from the home network 30, the roamed network divides this received random number RND of 128 bits into two random numbers R1 and R2 of 64 bits (S1209). The divided random number R1 of 64 bits is sent to the mobile station 35 as a random number for mobile station authentication. The mobile station stores the received R1 and calculates TRES' = g ki (R1) by using the received random number R1, the cipher function g and its secret key ki (S1210). The calculated result TRES' of 32 bits is sent back to the roamed network 31. The roamed network 31 calculates g ki (R1) using the divided random number R1, the cipher function g and the found secret key ki, and confirms that its result g ki (R1) is equal to the result TRES' calculated by and received from the mobile station 35 (S1211). Then, the roamed network 31 sends the other divided random number R2 of 64 bits to the mobile station 35. The mobile station 35 couples this random number R2 with the stored random number R1 to reproduce the random number RND of 128 bits, and sends it to the smart card as a random number for subscriber authentication. The smart card calculates SRES' = f km (RND) by using the received random number RND, the cipher function f and its secret key km (S1212). The calculated result SRES' of 32 bits is sent back to the roamed network 31. The roamed network 31 confirms that the stored result SRES is equal to the result SRES' calculated by and received from the smart card (S1213). According to this embodiment, as above-described, a plurality of authentications are effected in the roamed network by using the divided random numbers which is divided to have a bit length appropriate to be used in this roamed network. The mobile network can use the random number having a longer bit length as that in the home network by coupling a plurality of the divided random numbers. In stead of generating the random numbers twice, a longer bit random number is divided into a plurality of random numbers with a shorter bit length according to this embodiment. Although the authentication is performed twice in this embodiment, three or more times of authentication may be possible with similar advantages. Many widely different embodiments of the present invention may be constructed without departing from the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.

Claims: A mobile communication authentication method for authenticating a mobile station (35) which accesses for roaming a network (31) different from a home network (30) of said mobile station, said mobile station and said home network (30) having the same secret key (ki) and using the same cipher function (f), said method including the steps of generating at said home network (30) random numbers, calculating at said home network (30) the cipher function (f) by using said secret key (ki) and the random numbers, sending, from said home network (30) to said roamed network (31), the calculation results and the random numbers, sending, from said roamed network (31) to said mobile station (35), one of the random numbers, calculating at said mobile station (35) the cipher function (f) by using said secret key (ki) and the random number, sending, from said mobile station (35) to said roamed network (31), the calculation result, and confirming, at said roamed network (31) the coincidence of the calculation result sent from said mobile station (35) with the calculation result sent from said home network (30), characterized in that in said roamed network (31) a bit length of random numbers is used which is shorter than that used in said home network (30), the step of generating random numbers at said home network (30) comprises generating the random numbers (RND 1 , ..., RND n ) with the shorter bit length used in said roamed network (31), and said method comprises the steps of: extending at said home network (30) the generated random numbers (RND 1 ',..., RND n ') to random numbers (RND 1 ,..., RND n ) with the bit length used in said home network (30) by means of an extension function; said step of calculating the cipher function (f) at said home network (30) comprises calculating the cipher function (f) by using said secret key (ki) and the extended random numbers (RND 1 ',..., RND n '); sending from said home network (30) to said roamed network (31), the calculation results (SRES 1 =f k1 (RND 1 '),... , SRES n =f k1 (RND n ')) and the random numbers before extension (RND 1 ,..., RND n ); sending, from said roamed network (31) to said mobile station (35), one of the random numbers before extension (RND j , j=1,...,n); extending at said mobile station (35) the random number (RND j ) to the random number (RND j ') with the bit length used in said home network (30) by means of said extension function; calculating at said mobile station (35) the cipher function (f) by using said secret key (ki) and the extended random number (RND j '); sending, from said mobile station (35) to said roamed network (31), the calculation result (SRES j =f ki (RND j ')); and confirming, at said roamed network (31), the coincidence of the calculation result (SRES j =f ki (RND j ')) sent from said mobile station (35) with the calculation result (SRES j =f ki (RND j ')) sent from said home network (30). The method as claimed in claim 1, wherein said extension function is an extension function of a block cipher system. The method as claimed in claim 2, wherein said block cipher system has CBC mode.

Inventors: MIZUNO TOSHIO, OHASHI MASAYOSHI, SAKAI SEIICHIRO, SUZUKI TOSHINORI, TAKEUCHI YOSHIO, YAMAGUCHI AKIRA, MIZUNO, TOSHIO, OHASHI, MASAYOSHI, SAKAI, SEIICHIRO, SUZUKI, TOSHINORI, TAKEUCHI, YOSHIO, YAMAGUCHI, AKIRA
Applicants: KOKUSAI DENSHIN DENWA CO LTD, KOKUSAI DENSHIN DENWA CO., LTD
Application year: 1995
Main classification: H04Q7
IPC codes: H04L9, H04W12