AUTHENTICATION METHODS FOR CELLULAR COMMUNICATIONS SYSTEMS

Abstract: In one way to improve the security of bilateral authentication procedures while keeping transmission overhead to a minimum, a first party wishing to initiate communications with a second party generates a random number having a first number of digits. The first party then enciphers the random number using a secret ciphering key to obtain an enciphered version of the random number that has a second number of digits greater or equal to the first number. A third number of selected digits of the enciphered random number are then transmitted to the second party, where the third number is less than the first number. The second party receives the digits transmitted from the first party and expands the third number of digits in a pre-agreed manner to obtain the second number of digits. The second party then deciphers the second number digits using the second party's secret key to obtain the first number of digits, which is not equal to the original random number chosen by the first party. The so-obtained first number of digits is then used by the second party as a ciphering key to encipher the second party's secret key thereby obtaining an enciphered key containing the second number of digits. A fourth number of digits less than the second number are then selected by the second party and transmitted to the first party.

Description: AUTHENTICATION METHODS FOR CELLULAR COMMUNICATIONS SYSTEMS BACKGROUND This invention relates to telecommunication and more particularly to multiple access telecommunication and even more particularly to cellular radio telephone communication. This invention enables a first party electronically to verify the identity of a second party and vice versa before the parties engage in enciphered communications using a ciphering key provided as a byproduct of the identity- verification process. The problem of ensuring the security of a communication session, whether the session involves audio, video, or another kind of data, is not a new one. U.S. Patent No. 4,799,061 to Abraham et al. describes a system for authenticating components in a communication system that is illustrated by FIG. 1. A first party wishing to initiate communication with a second party chooses a random number which is applied at point A with the first party's secret key as inputs of an enciphering algorithm 1. The first party's algorithm 1 enciphers the random number according to the secret key, and the resulting enciphered random number is transmitted by the first party to the second party. The second party applies the received enciphered random number with its own secret key as inputs of a deciphering algorithm 2. The deciphering algorithm 2 reproduces at point B the random number applied by the first party at point A only if the second party's secret key is the same as the first party's secret key and their algorithms are the same. The second party applies the reproduced random number with its secret key as inputs of the second party's enciphering algorithm 1. The second party's algorithm 1 enciphers the second party's secret key according to the reproduced random number, and the resulting enciphered second party's secret key is transmitted by the second party to the first party. The first party deciphers the received enciphered second party's secret key using its random number as the deciphering key in its deciphering algorithm 2. A comparator 3 determines whether the first party's secret key and the deciphered second party's secret key are the same; if they are, the first party has authenticated the second party, which in this context is to say that both parties are using the same algorithms and keys. Communication then proceeds using the random number, which both parties now possess, as a ciphering and deciphering key. The system described in the Abraham patent has a number of disadvantages. In order for the second party to be able to reproduce at point B the same random number inserted by the first party at point A, the transmission from A to B must be "information lossless". One of the consequences of information-lossless transmission is that the number of digits of the enciphered random number transmitted from the first party to the second party cannot be less than the number of digits of the random number. If fewer digits were transmitted, there would inevitably be less information transmitted than necessary to reproduce the random number. Indeed, the characteristics of cipher algorithms are such that not even a single digit of the random number would be guaranteed to be reproduced error free at point B unless sufficient information is contained in the enciphered random number. Furthermore, the Abraham patent uses the random number for enciphering the second party's key for transmission. If this transmission is intercepted and the random number contains a lower number of digits (i.e., is shorter than) the key, it will be easier for an eavesdropper to crack the ciphering and thereby read the secret key than it would be to crack the secret key itself. Thus, the security of the system is no greater than that determined by the length of the random number. To avoid compromising the degree of security afforded by the secret key, the length of the random number must be greater than or equal to the length of the key, and thus the enciphered random number transmitted from the first party to the second party must also be longer than the key. Choosing a long key for adequate security, however, requires significant transmission time, which is a scarce or expensive resource in some communication systems, for example, cellular telephone systems. Nevertheless, the need for communication security is acute in flexible systems like cellular telephony. In the United States, losses due to cellular telephone fraud run into the hundreds of millions of dollars, forcing manufacturers, service providers, the Federal Communications Commission (FCC) and industry trade groups to investigate a number of techniques for combating such fraud. One technique involves authenticating both the radio base station and the mobile station, i.e., both ends of a communication link, in order to avoid connection to fraudulent entities. A simplified layout of a cellular communications system is depicted in FIG. 2. Mobile telephones M1-M10 communicate with the fixed part of a public switched network by transmitting radio signals to, and receiving radio signals from, cellular base stations B1-B10. The cellular base stations B1-B10 are, in turn, connected to the public switched network via a Mobile Switching Center (MSC). Each base station B1- B10 transmits signals within a corresponding area, or "cell" C1-C10. As depicted in FIG. 2, an idealized arrangement of base stations are organized so that the cells substantially cover an area in which mobile telephone communication ordinarily occurs (e.g., a metropolitan area), with a minimum amount of overlap. When a user activates a mobile telephone within a cell, the mobile telephone transmits a signal indicating the mobile telephone's presence to the cell's base station. The mobile telephone transmits the signal, which may include its electronic serial number (ESN), in a designated set-up channel that is continuously monitored by each base station. When the base station receives the mobile telephone's signal, it registers the mobile telephone's presence within the cell. This process can be repeated periodically so that the mobile telephone is appropriately registered in the event that it moves into another cell. When a mobile telephone number is dialed, a telephone company central office recognizes the number as a mobile telephone and forwards the call to the MSC. The MSC sends a paging message to certain base stations based on the dialed mobile telephone number and current registration information. One or more of the base stations transmits a page on its set-up channel. The dialed mobile telephone recognizes its identification on the set-up channel, and responds to the base station page. The mobile telephone also follows an instruction to tune to an assigned voice channel and then initiates ringing. When a mobile user terminates a communication, a signaling tone is transmitted to the base station, and both sides release the voice channel. In the above described operation, mobile telephones are not connected permanently to a fixed network but instead communicate through a so-called "air interface" with a base station. This, of course, provides the flexibility of cellular communication systems, since a user can readily transport a mobile telephone without the restriction of being physically linked to a communication system. This same feature, however, also creates difficulties with respect to securing information transmitted over cellular telephone systems. For example, in ordinary wired telephone systems, a central office exchange can identify a particular subscriber to be billed for use of a telephone set by the communication line to which it is physically attached. Thus, fraudulent use of a subscriber's account typically requires that a physical connection be made to the subscriber's line. This presents a risk of discovery to a would-be fraudulent user. Cellular telecommunication systems, on the other hand, pose no such connection problem for the would-be fraudulent user since these systems communicate over an air interface. Absent protection schemes, fraudulent users can use another subscriber's account by accessing the other subscriber's ESN as it is transmitted to the network at various times for establishing and maintaining communications. In establishing a standard cellular connection, two identification codes are typically transmitted by a mobile telephone to the system. These are the Mobile Identification Number (MIN) and the ESN. The MIN identifies a subscriber, while the ESN identifies the actual hardware being used by the subscriber. Accordingly, it is expected that the MIN corresponding to a particular ESN can, due to subscribers purchasing new equipment, change over time. The MIN is a 34-bit binary number derived from a 10-digit directory telephone number, while the ESN is a 32-bit binary number that uniquely identifies a mobile telephone. The ESN is typically set by the mobile telephone manufacturer. A conventional authentication method utilized in setting up communications in, for example, the Advanced Mobile Phone System (AMPS), is illustrated by the flowchart depicted in FIG. 3. According to this method, a base station receives both an ESN and a MIN from the mobile telephone at block 200. These identification codes are designated ESN m and MIN m to indicate that they are received from the mobile telephone. Next, at block 202 the base station retrieves an ESN sys which corresponds to MIN m from a system memory. ESN sys is then compared with ESN m at block 204. If the two serial numbers are the same, the flow proceeds to block 206 and system access is permitted. Otherwise, system access is denied at block 208. One drawback to this system is that it is relatively simple for a fraudulent user to assemble valid MIN/ESN combinations by eavesdropping on the air interface or from other sources. Since accesses according to this conventional system are presumed valid if the MIN and ESN received from the mobile telephone correspond with those stored in system memory, all of the necessary information for fraudulent access can be obtained by electronic eavesdropping. U.S. Patents No. 5,091 ,942 to Dent; and No. 5,282,250; No. 5,390,245; and No. 5,559,886, all to Dent et al., are directed to the problem of communication security in systems like cellular telephone systems, and all of these patents are expressly incorporated here by reference. These patents solve the above-mentioned problems with prior cellular telephone methods and the method of the Abraham patent by methods such as that illustrated by FIG. 4. The first party, who generally would be the party initiating communication, chooses a random number rand and transmits rand to the second party. In addition, the first party uses an authentication algorithm 10 to encipher rand according to the first party's secret key, generating an enciphered random number having two parts, respl and resp2. The first party also transmits resp2 to the second party. The second party enciphers the received rand with its local version of the authentication algorithm 10 according to the second party's secret key, generating an enciphered random number having two parts, respl' and resp2'. The second party uses a comparator 11 to determine whether resp2' matches resp2; a match indicates to the second party that the first party is probably authentic and merits a reply. The second party's reply to the first party includes respl'. The first party uses a comparator 12 to determine whether the received respl ' matches respl ; a match indicates to the first party that the second party is probably authentic. Such authentication of both ends of a communication can be called bilateral authentication. In contrast to the prior methods, the lengths of the quantities rand, resp2, and respl ' that are exchanged by the parties are not critical to the security of the system depicted in FIG. 4. In fact, they may have fewer digits than the secret keys. The numbers of digits need only be sufficient that a second party not in possession of the same secret key as the first party has a negligible probability by chance of getting respl ' equal to the value respl expected by the first party and that the first party has a negligible probability by chance of getting a value resp2 equal to the value of resp2' that will be computed by the second party. For example if respl and resp2 both have sixteen binary digits (bits), the chances are only 1/65,536 of getting such a value right by accident, which is sufficiently low to render a guessing game unprofitable for many useful communication systems. Moreover, using a combined length of respl and resp2 that is much shorter than the length of the secret key means that insufficient information is transmitted to permit a unique determination of the key, even if sufficient computing power is available to try all possible keys. Even so, the method of FIG. 4 leaves one at liberty to choose long keys to make such computation impractical, without incurring the transmission time overhead of exchanging long authentication quantities between the parties. On the other hand, the method of FIG. 4 might be weakened if an eavesdropper can collect several sets of rand, resp2 pairs from a genuine first party that could then be used to address a genuine second party or to attempt to crack the secret key. The patents incorporated by reference above disclose ways of hindering the latter weakness, including the use of rolling keys. U.S. Patent Application No. 08/706,574 by Osborn for a "System for Preventing Electronic Memory Tampering" addresses the former weakness, and is expressly incorporated here by reference. It is, however, still important to improve the security of bilateral authentication procedures while keeping transmission overhead to a minimum. These objectives or advantages are obtained when practicing the authentication methods described in this application. SUMMARY A first party wishing to initiate communications with a second party generates a random number comprising a first number of digits. The first party then enciphers the random number using a secret ciphering key to obtain an enciphered version of the random number comprising a second number of digits greater or equal to the first number. A third number of selected digits of the enciphered random number are then transmitted to the second party, where the third number is less than the first number. The second party receives the digits transmitted from the first party and expands the third number of digits in a pre-agreed manner to obtain the second number of digits. The expansion process can be as simple as padding out the third number of digits with constant digits such as zeros. The second party then deciphers the so-obtained second number digits using the second party's secret key to obtain the first number of digits, which is not equal to the original random number chosen by the first party. The so-obtained first number of digits is then used by the second party as a ciphering key to encipher the second party's secret key thereby obtaining an enciphered key containing the second number of digits. A fourth number of digits less than the second number are then selected by the second party and transmitted to the first party. The first party also expands the third number of digits it transmitted to the first party in the same pre-agreed manner to obtain the second number of digits and then deciphers this so-obtained second number of digits using its secret key to obtain a first number of digits once more. The first party then selects from these a fourth number of digits less than the second number in the same manner as the second party, and compares the selected fourth number of selected digits with the fourth number of digits received from the second party. If a complete match is detected, the first party has verified that the second party possesses the same secret key as itself. The roles may then be reversed to allow the second party to verify the identity of the first party, either by repeating the above procedure starting with a new random number chosen by the second party, or else continuing to derive a number of digits to be returned from the first party to the second party based on the digits received from the second party and the first party's secret key. In a more general implementation of the above, the steps of selecting a smaller of digits from a larger number of digits may be replaced by a step of generating a smaller number of digits from a larger number of digits using a pre-agreed compression algorithm, which can also depend on the secret key available to the party performing the compression. In addition, if savings in computational effort are thereby achieved, whenever a smaller number of digits only are needed, the larger number of digits do not all need to be generated. BRIEF DESCRIPTION OF THE DRAWINGS The features, objects, and advantages of Applicant's invention will be understood by reading this description in conjunction with the accompanying drawings, in which: FIG. 1 illustrates a prior system for authenticating components in a communication system; FIG. 2 illustrates a simplified layout of a cellular communications system; FIG. 3 illustrates a conventional authentication method; FIG. 4 illustrates a prior bilateral authentication method; FIG. 5 illustrates an authentication method in accordance with the invention; FIG. 6 illustrates an authentication method in accordance with the invention; and FIG. 7 illustrates an authentication method in accordance with the invention. DETAILED DESCRIPTION The invention is applicable in a communication system having user terminals, such as cellular telephones, that seek to receive service from a communication network, such as a group of cellular base stations. A first implementation of the invention may be described with the aid of FIG. 5, which is based on FIG. 4. Suppose in a first case that the network wishes to initiate communication with a terminal. Then, the first party of FIGS. 4 and 5 is the network, which sends the pair (rand, resp2) to the terminal, which is the second party. In FIG. 5, the network uses for the "first party" secret key the secret key it assigned to the genuine second party (the user terminal), rather than its own secret key as in FIG. 4. The second party replies with respl' only if the received resp2 matches its locally computed resp2', as shown in FIGS. 4 and 5. In this way, a false network that cannot compute resp2 cannot cause a genuine terminal to reply with a value respl', and this the method depicted in FIG. 5 avoids a problem with prior authentication methods. In principle, an eavesdropper can attack the method of FIG. 5 by performing the complicated sequence of: 1. placing a call via the network to a genuine terminal, thereby causing the network to transmit the (rand, resp2) pair to the genuine terminal; 2. intercepting the (rand, resp2) pair transmitted by the network; using the pair to address the same genuine terminal, thereby causing the terminal to respond with respl'; (of course, the eavesdropper can also merely wait for the genuine terminal to respond to the network with respl ';) and intercepting respl' transmitted by the terminal; and 3. later, initiating a call to the network, receiving an authentication challenge (rand, resp2) pair and replying with the respl' obtained in step 2 in order to gain fraudulent access. This kind of fraud, which may be called "fraud with the help of a genuine terminal", can be prevented for telephone systems by including dialed digits as inputs to the authentication algorithm 10, as described in the '942, '250, '245, and '886 patents incorporated by reference above. Thus, when the network initiates a call to a telephone, the value of resp2 is generated by the algorithm 10 in FIG. 5 based on the random number rand, the secret key assigned to the called party, and all or part of the called party's telephone number. On the other hand when a telephone initiates a call to the network (the first party in FIG. 5 is the caller), the value respl ' that the network (the second party in FIG. 5) requires for authentication is generated by both the network and the caller based on the random number rand, the caller's secret key, and all or part of the telephone number of the called party. Thus an eavesdropper at step 3 would only possess a respl' that is valid for calling himself and could not place calls to a third party. Although the bilateral authentication method depicted in FIG. 5 has a number of advantages over prior methods, practical problems in some communication systems, such as distributed mobile telephone switching systems, may hinder the use of dialed digits in this way, as described in Applicant's '942 patent cited above. Accordingly, further bilateral authentication methods are described below. Another method of bilateral authentication in accordance with Applicant's invention is illustrated by FIG. 6. In a first implementation, a first party, e.g., one wishing to initiate communications, generates a random number, which may be long, i.e., the random number can comprise any number of digits. The random number is applied at point A to an enciphering algorithm 100, which generates an enciphered random number in accordance with the first party's secret key that is also applied to the algorithm 100. In contrast to the Abraham patent and FIG. 1 , however, the enciphered random number need not comprise as many digits as the unenciphered random number, e.g., the enciphering algorithm 100 can be an "information-lossy" algorithm. It will be understood that if the algorithm 100 is not information-lossy, the enciphered random number can also be shortened by selecting predetermined digits or by otherwise truncating or compressing it, as indicated by a select/compress operation 101. The combination of the enciphering algorithm 100 and the select/compress operation 101 forms an information-lossy algorithm in that an output of the algorithm can never be inverted to reveal the inputs to the algorithm, i.e., the random number and the secret key. The shortened enciphered random number is then transmitted from the first party to the second party, who deciphers the received value using a deciphering algorithm 200. If the algorithm 200, in correspondence with a pseudo-inverse of the algorithm 100, requires a number of input digits equal to the number of output digits generated by the algorithm 100, then the received shortened enciphered random number is expanded as necessary by a pad/expand operation 201. The shortened enciphered random number is decompressed or padded out with predetermined digits such as zeros to obtain the desired number of input digits for the second party's algorithm 200. The second party's algorithm 200 deciphers the enciphered random number, expanded if necessary, thereby producing a modified random number at point B. The modified random number may have the same number of digits as the first party's random number, although none of those digits are guaranteed to be the same as the digits of the first party's random number. The modified random number is applied as an enciphering key to the second party's enciphering algorithm 110 for enciphering the second party's secret key, which is applied as data to the algorithm 110. In contrast to the Abraham patent and FIG. 1 , the algorithm 110 can be an information-lossy algorithm like algorithm 100, and may even be the same as the algorithm 100, although the latter is not necessary. The number of digits of the enciphered second party's secret key that is generated by the algorithm 110 may therefore be fewer than the number of digits of either the modified random number or the second party's secret key. If desired, the number of digits can be further reduced by a select/compress operation 102 that selects predetermined output digits or otherwise compresses the enciphered second party's secret key. As noted above, the combination of the algorithm 110, even if it is information-lossless, and the select/compress operation 102 forms an information-lossy algorithm, which is to say that it is impossible to invert the output of the algorithm to obtain both inputs to the algorithm 110. The second party transmits the enciphered second key, compressed if desired, to the first party. In contrast to the Abraham patent and FIG. 1 , the first party in FIG. 6 is not able to decipher the received enciphered second party's key using the random number from point A as a deciphering key, and so the first party would not be able to compare the second party's key to its own key. Instead in accordance with Applicant's invention, an expected value is generated by the first party using the shortened random number it transmitted to the second party and copies 300 of the pad/expand operation 201 , the deciphering algorithm 200, the enciphering algorithm 110, and the select/compress operation 102 and using the first party's own secret key. A comparator 120 determines whether the value generated by the copies 300 is the same as the value received from the second party. If the comparator indicates complete agreement, the probability that the second party has the same secret key as the first party is high, thereby authenticating the second party to the first party. It will be understood that the each party provides information to the other in a manner that is error-free, thereby permitting the comparator's indication of agreement/disagreement to be also an authentication indication. For example, the parties may exchange information by messages sent through the air interface of a cellular communication system. Such messages are usually protected by error- correction coding and automatic repeat request (ARQ) techniques that ensure substantially error-free communication. It is not necessary, of course, to use a cellular air interface; other ways of providing the necessary information to each party can be employed. If desired, the roles of the second party and the first party may now be reversed and a different random number can be used to authenticate the first party to the second party, thereby achieving bilateral authentication. The parties can then continue enciphered communication using their secret keys or derivations thereof for enciphering. Any derivations used must be derived by a mutually agreed process such as combining the secret key with one or more of a time/date stamp, a call number, a called party's telephone number, a caller's telephone number, a modified random number, and indeed all or any mixture of digits the two parties have in common. Such digits can be obtained in a manner such as those described in U.S. Patents No. 5,604,806 and No. 5,745,578 to Hassan et al. and U.S. Patent Applications No. 08/555,968 and No. 09/015,774, both by Hassan et al., which relate to using characteristics of a radio channel, such as the short-term reciprocity and rapid decorrelation of phase, to establish key sequences for enciphered communication. In perhaps the simplest case, the modified random number may be used as an enciphering key for the call or communication session. If in FIG. 6 the details within the boxes 300 are omitted, then FIG. 6 may be readily compared to the authentication method practiced in communication systems operating under the Global System for Mobile (GSM) communications standard. One apparent difference is Applicant's method of generating the rand value, comprising blocks 100, 101 in FIG. 6. The '942, '250, '245, and '886 patents disclose that a preferred bilateral authentication method would have the effect of preventing the recipient of an attempt to initiate communication from making any reply or taking any inappropriate action if the attempt was received from a non-authentic source. Such attempts by a non-authentic party to engage an authentic party in communication usually have some fraud as an objective, but may alternatively be intended merely to cause a nuisance. For example, even formulating a "no" reply to a fraudulent signal can consume battery power in a handheld telephone, thereby reducing the battery life between charges. It would thus be preferable for information to be contained in the very first transmission from a communication-initiating first party to a second party that would allow the second party to authenticate the first party before replying. Such a method in accordance with Applicant's invention is depicted by FIG. 7. In FIG. 7, the first party performs the operations of blocks 100, 101 , 201 , 200, and 110 as described in connection with the identically numbered blocks in FIG. 6. One difference between FIGS. 6 and 7 is that block 103 in FIG. 7 indicates a modified select/compress operation that yields two selected or compressed output values based on the enciphered first party's secret key. To assist in comparison with the techniques described in the '250 and '245 patents, these two values can be called "respl" and "resp2" and the shortened enciphered random number generated by the select/compress operation 101 can be called "rand". (If the operation 101 is not present, the enciphered random number can be called "rand".) Now the first party transmits both rand and resp2 to the second party. The second party performs the operations of blocks 201 , 200, and 110 as described in connection with the identically numbered blocks in FIG. 6. In FIG. 7, however, the second party has a counterpart of the first party's block 103. In this way, the second party's secret key is used to obtain corresponding values of respl and resp2, which will be equal to the first party's corresponding values only if the first party and the second party have the same secret key. A comparator 130 determines whether the second party's value resp2 matches the resp2 value received from the first party, and if they match, there is a high probability that the first party is authentic. Only if the second party authenticates the first party will the second party transmit to the first party a reply that includes the second party's respl value. The first party then compares its own respl value to the respl value received from the second party using a comparator 120. If the values match, the first party has confirmed that the second party is probably authentic, completing the bilateral authentication process. If desired, further bilateral authentication may take place by interchanging the roles of the first and second parties and, using a different random number, again performing the bilateral authentication illustrated by FIG. 7. This provides additional security over a bilateral authentication procedure performed only once as it permits the party desirous of authenticating the other to choose the random number, thereby eliminating the risk that previously intercepted (rand, respl , resp2) values could be repeated by an eavesdropper. To reduce transmission overhead, the (rand, resp2) pair transmitted from the second party to the first party (in the second iteration of the bilateral authentication process) may be combined with the second party's transmission of the respl value (in the first iteration). If in FIG. 7 the details within box 300 are omitted, then FIG. 7 may be readily compared to the methods described in the '942, '250, '245, and '886 patents such as that depicted in FIG. 4. One apparent difference is Applicant's method of generating the rand value, comprising blocks 100, 101 in FIG. 9. The security provided by the method depicted in FIG. 7 can be enhanced by allowing both the first rand value transmitted from the first party to the second party and the second rand value transmitted from the second party to the first party to affect the second (respl , resp2) pair of values transmitted from the second party to the first party. In this way, an eavesdropper would have had previously to intercept precisely that particular combination of the two rands in order to masquerade as a genuine party using the intercepted values. In this way, the second party receives a more powerful indication of the first party's authenticity, which is logical bearing in mind that fraud attempts would usually involve the fraudster being the initiating or first party. In one aspect of Applicant's invention, the advantage of inhibiting responses to non-authentic challenges for improved security in both directions can be sacrificed if desired to obtain faster bilateral authentication. In this variation of the method depicted in FIG. 7, the first and second parties exchange respectively chosen random numbers and then each party computes a respective response (i.e., a respl' value) using its respective secret key and the random number received from the other party. The parties then exchange these responses and compare them to the expected responses (i.e., respl values) that each can compute in the same manner as the other party. Calling the first party A and the second party B, it is desirable to avoid the possibility of A's response to B being the same as B's response to A because such a condition has authentication value only if the responses are exchanged at precisely the same moment, precluding one party from merely echoing the other party's response. Accordingly, A and B should compute their responses in different manners. This may be generally recognized as an "A-to-B" algorithm that is used for one direction and a "B-to-A" algorithm that is used for the other direction. This distinguishability can be achieved in several ways. The algorithm used by a party can depend on the party's status. For example, A might be a communication network and B might be a network subscriber terminal. For another example, a party could use one algorithm when it initiates a communication session and another algorithm when it responds to a session initiated by another. Having decided on an algorithm, the algorithms may be further distinguished by varying their inputs. For example, A's response to B can be computed by reversing the order of the digits of the random number received from B. For another example, A and/or B can change one of the bits of the random number or key from a ONE to a ZERO. For yet another example, A and B can swap the odd or even bits or digits of the response values calculated, the odd values being transmitted in one direction and the even values in the other direction. Several such distinctions can be combined to modify or choose the algorithms for the respective directions such that they are distinguishable from each other. Another aspect of Applicant's invention can be called "cumulative authentication" that is advantageous in communication systems that are more tolerant of transmission overhead due to authentication. In cumulative authentication, A first releases a first digit of an authentication value to B, and B, upon verifying this first authentication digit, releases a second authentication digit to A, whereupon A, after verifying the second authentication digit, releases a third authentication digit to B, and so forth. Such cumulative authentication prevents release of a complete response in either direction upon any digit being detected to be incorrect, suggesting a non-authentic party. Applicant's invention is described above with reference to particular embodiments, and it will be readily apparent to those skilled in the art that it is possible to embody the invention in forms other than those described above. The particular embodiments described above are merely illustrative and should not be considered restrictive in any way. The scope of the invention is determined given by the following claims, and all variations and equivalents that fall within the range of the claims are intended to be embraced therein.

Claims: I Claim: 1. In a cellular wireless communication system having a network of base stations for communicating with a plurality of subscriber terminals, a method of authenticating an identity of at least one subscriber terminal to the network, comprising the steps of: generating a random number having a first number of digits; enciphering the random number using a first secret key, thereby producing an enciphered random number having a second number of digits less than the first number; providing the enciphered random number to the network and the at least one subscriber terminal; computing, in the network, an expected response value based on the enciphered random number and the first secret key; computing, in the at least one subscriber terminal, a response value based on the enciphered random number and a second secret key stored in the terminal; transmitting the response value from the at least one subscriber terminal to the network via at least one of the base stations; and comparing, in the network, the transmitted response value to the expected response value, thereby producing an authentication indication for the at least one subscriber terminal to the network. 2. In a cellular wireless communication system having a network of base stations for communicating with a plurality of subscriber terminals, a method of authenticating an identity of at least one subscriber terminal to the network, comprising the steps of: generating a random number having a first number of digits; enciphering the random number using a first secret key, thereby producing an enciphered random number having a second number of digits at least equal to the first number; selecting a third number of digits of the enciphered random number, wherein the third number is less than the first number, thereby producing a shortened random number; providing the shortened random number to the network and the at least one subscriber terminal; computing, in the network, an expected response value based on the shortened random number and the first secret key; computing, in the at least one subscriber terminal, a response value based on the shortened random number and a second secret key stored in the at least one subscriber terminal; transmitting the response value to the network via at least one of the base stations; comparing, in the network, the transmitted response value to the expected response value, thereby producing an authentication indication for the terminal to the network. 3. In a cellular wireless communication system having a network of base stations for communicating with a plurality of terminal units, a method of authenticating an identity of at least one terminal unit to the network, comprising the steps of: generating a random number having a first number of digits; enciphering the random number using a first secret key, thereby producing an enciphered random number having a second number of digits at least equal to the first number; compressing the enciphered random number, thereby producing a shortened random number having a number of digits less than the first number, wherein the compressing is information-lossy; providing the shortened random number to the network and the at least one terminal unit; computing, in the network, an expected response value based on the shortened random number and the first secret key; computing, in the at least one terminal unit, a response value based on the shortened random number and a second secret key stored in the at least one terminal unit; transmitting the response value to the network via at least one of the base stations; and comparing, in the network, the transmitted response value to the expected response value, thereby producing an authentication indication for the at least one terminal unit to the network. 4. In a cellular wireless communication system having a network of base stations for communicating with a plurality of terminal units, a method of authenticating an identity of at least one terminal unit to the network, comprising the steps of: appending, in the network, a number of predetermined digits to an authentication challenge, thereby producing an extended challenge; deciphering, in the network, the extended challenge using a first secret key, thereby producing a first deciphered challenge having at least as many digits as the first secret key; enciphering, in the network, the first secret key using the first deciphered challenge as an enciphering key, thereby producing an enciphered key; appending, in the at least one terminal unit, the predetermined digits to the authentication challenge, thereby producing the extended challenge; deciphering, in the at least one terminal unit, the extended challenge using a second secret key stored in the at least one terminal unit, thereby producing a second deciphered challenge having at least as many digits as the second secret key; enciphering, in the at least one terminal unit, the second secret key using the second deciphered challenge as an enciphering key, thereby producing an authentication response; transmitting the authentication response to the network via at least one of the base stations; and comparing, in the network, the transmitted authentication response to the enciphered key, thereby producing an authentication indication for the at least one terminal unit to the network. 5. In a cellular wireless communication system having a network of base stations for communicating with a plurality of terminal units, a method of authenticating an identity of at least one terminal unit to the network, comprising the steps of: deciphering, in the network, an authentication challenge using a first secret key having more digits than the authentication challenge, thereby producing a first deciphered challenge having at least as many digits as the first secret key; enciphering, in the network, the first secret key using the first deciphered challenge as an enciphering key, thereby producing a first enciphered key; selecting a predetermined number of digits of the first enciphered key, wherein the predetermined number is less than the number of digits of the secret key, thereby producing a shortened enciphered key; deciphering, in the at least one terminal unit, the authentication challenge using a second secret key stored in the at least one terminal unit, thereby producing a second deciphered challenge having at least as many digits as the second secret key; enciphering, in the at least one terminal unit, the second secret key using the second deciphered challenge as an enciphering key, thereby producing a second enciphered key; selecting the predetermined number of digits of the second enciphered key, thereby producing an authentication response; transmitting the authentication response to the network via at least one of the base stations; and comparing, in the network, the transmitted authentication response to the shortened enciphered key, thereby producing an authentication indication for the at least one terminal unit to the network. 6. In a cellular wireless communication system having a network of base stations for communicating with a plurality of subscriber terminals, a method of authenticating an identity of at least one subscriber terminal to the network, comprising the steps of: expanding, in the network, an authentication challenge, thereby producing a first expanded challenge, wherein the expanding is information-lossless; deciphering, in the network, the expanded challenge using a first secret key, thereby producing a first deciphered challenge having at least as many digits as the first secret key; enciphering, in the network, the first secret key using the first deciphered challenge as an enciphering key, thereby producing an enciphered key; expanding, in the at least one subscriber terminal, the authentication challenge, thereby producing a second expanded challenge, wherein the expanding in the at least one subscriber terminal is substantially the same as the expanding in the network; deciphering, in the at least one subscriber terminal, the expanded challenge using a second secret key stored in the at least one subscriber terminal, thereby producing a second deciphered challenge having at least as many digits as the second secret key; enciphering, in the at least one subscriber terminal, the second secret key using the second deciphered challenge as an enciphering key, thereby producing an authentication response; transmitting the authentication response to the network via at least one of the base stations; and comparing, in the network, the transmitted authentication response to the enciphered key, thereby producing an authentication indication for the at least one subscriber terminal to the network. 7. In a cellular wireless communication system having a network of base stations for communicating with a plurality of subscriber terminals, a method of authenticating an identity of at least one subscriber terminal to the network, comprising the steps of: deciphering, in the network, an authentication challenge using a first secret key having more digits than the authentication challenge, thereby producing a first deciphered challenge containing at least as many digits as the first secret key; enciphering, in the network, the first secret key using the first deciphered challenge as an enciphering key, thereby producing a first enciphered key; compressing, in the network, the first enciphered key, thereby producing a shortened key having a number of digits less than the number of digits of the secret key, wherein the compressing is information-lossy; deciphering, in the at least one subscriber terminal, the authentication challenge using a second secret key stored in the at least one subscriber terminal, thereby producing a second deciphered challenge having at least as many digits as the second secret key; enciphering, in the at least one subscriber terminal, the second secret key using the second deciphered challenge as an enciphering key, thereby producing a second enciphered key; compressing, in the at least one subscriber terminal, the second enciphered key, thereby producing an authentication response, wherein the compressing in the at least one subscriber terminal is substantially the same as the compressing in the network; transmitting the authentication response to the network via at least one of the base stations; and comparing, in the network, the transmitted authentication response to the shortened enciphered key, thereby producing an authentication indication for the at least one subscriber terminal to the network. 8. In a cellular wireless communications system having a network of base stations for communicating with a plurality of terminal units, a method of authenticating the network to at least one terminal unit, comprising the steps of: generating a random number having a first number of digits; enciphering the random number using a first secret key, thereby producing an enciphered random number having a second number of digits less than the first number; providing the enciphered random number to the network and the at least one terminal unit; computing, in the at least one terminal unit, an expected response based on the enciphered random number and the first secret key; computing, in the network, a response based on the enciphered random number and a second secret key stored in the network; transmitting the response to the at least one terminal unit via at least one of the base stations; and comparing, in the at least one terminal unit, the transmitted response to the expected response, thereby producing an authentication indication for the network to the at least one terminal. 9. In a cellular wireless communication system having a network of base stations for communicating with a plurality of terminal units, a method of authenticating the network to at least one terminal unit, comprising the steps of: generating a random number having a first number of digits; enciphering the random number using a first secret key, thereby producing an enciphered random number having a second number of digits at least equal to the first number; selecting a third number of digits of the enciphered random number, wherein the third number is less than the first number, thereby producing a shortened random number; providing the shortened random number to the network and the at least one terminal unit; computing, in the at least one terminal unit, an expected response based on the shortened random number and the first secret key; computing, in the network, a response based on the shortened random number and a second secret key stored in the network; transmitting the response to the at least one terminal unit via at least one of the base stations; and comparing, in the at least one terminal unit, the transmitted response to the expected response, thereby producing an authentication indication for the network to the at least one terminal unit. 10. In a cellular wireless communication system including a network of base stations for communicating with a plurality of terminal units, a method of authenticating an identity of the network to at least one terminal unit, comprising the steps of: generating a random number having a first number of digits; enciphering the random number using a first secret key, thereby producing an enciphered random number having a second number of digits at least equal to the first number of digits; compressing the enciphered random number, thereby obtaining a shortened random number having a third number of digits that is less than the first number, wherein the compressing is information-lossy; providing the shortened random number to the network and at least one of the terminal units; computing, in the at least one terminal unit, an expected response based on the shortened random number and the first secret key; computing, in the network, a response based on the shortened random number and a second secret key stored in the network; transmitting the response to the at least one terminal unit via at least one of the base stations; and comparing, in the at least one terminal unit, the transmitted response to the expected response, thereby producing an authentication indication for the network to the at least one terminal unit. 11. In a cellular wireless communication system including a network of base stations for communicating with a plurality of terminal units, a method of authenticating an identity of the network to at least one terminal unit, comprising the steps of: appending, in the at least one terminal unit, at least one predetermined digit to an authentication challenge, thereby producing an extended challenge; deciphering, in the at least one terminal unit, the extended challenge using a first secret key stored in the at least one terminal unit, thereby producing a first deciphered challenge having at least as many digits as the first secret key; enciphering, in the at least one terminal unit, the first secret key using the first deciphered challenge as an enciphering key, thereby obtaining an enciphered key; appending, in the network, the at least one predetermined digit to the authentication challenge, thereby producing the extended challenge; deciphering, in the network, the extended challenge using a second secret key stored in the network, thereby producing a second deciphered challenge having at least as many digits as the second secret key; enciphering, in the network, the second secret key using the second deciphered challenge as an enciphering key, thereby producing an authentication response; transmitting the authentication response to the at least one terminal unit via at least one of the base stations; and comparing, in the at least one terminal unit, the transmitted authentication response to the enciphered key, thereby producing an authentication indication for the network to the at least one terminal unit. 12. In a cellular wireless communication system having a network of base stations for communicating with a plurality of terminal units, a method of authenticating an identity of the network to at least one of the terminal units, comprising the steps of: deciphering, in the at least one terminal unit, an authentication challenge using a first secret key having more digits than the authentication challenge, thereby producing a first deciphered challenge having at least as many digits as the first secret key; enciphering, in the at least one terminal unit, the first secret key using the first deciphered challenge as an enciphering key, thereby obtaining a first enciphered key; selecting a predetermined number of digits of the first enciphered key, thereby producing a shortened enciphered key, wherein the predetermined number is less than the number of digits of the secret key; deciphering, in the network, the authentication challenge using a second secret key stored in the network, thereby producing a second deciphered challenge having at least as many digits as the second secret key; enciphering, in the network, the second secret key using the second deciphered challenge as an enciphering key, thereby producing a second enciphered key; selecting the predetermined number of digits of the second enciphered key, thereby producing an authentication response; transmitting the authentication response to the at least one terminal unit via at least one of the base stations; and comparing, in the at least one terminal unit, the transmitted authentication response to the shortened enciphered key, thereby producing an authentication indication for the network to the at least one terminal unit. 13. In a cellular wireless communication system having a network of base stations for communicating with a plurality of subscriber terminals, a method of authenticating an identity of the network to at least one subscriber terminal, comprising the steps of: expanding, in the at least one subscriber terminal, an authentication challenge, thereby producing a first expanded challenge, wherein the expansion is information- lossless; deciphering, in the at least one subscriber terminal, the expanded challenge using a first secret key, thereby producing a first deciphered challenge having at least as many digits as the first secret key; enciphering, in the at least one subscriber terminal, the first secret key using the first deciphered challenge as an enciphering key, thereby producing an enciphered key; expanding, in the network, the authentication challenge, thereby producing a second expanded challenge, wherein the expanding in the network is substantially the same as the expanding in the at least one subscriber terminal; deciphering, in the network, the second expanded challenge using a second secret key stored in the network, thereby producing a second deciphered challenge having at least as many digits as the second secret key; enciphering, in the network, the second secret key using the second deciphered challenge as an enciphering key, thereby producing an authentication response; transmitting the authentication response to the at least one subscriber terminal via at least one of the base stations; and comparing, in the at least one subscriber terminal, the transmitted authentication response to the enciphered key, thereby producing an authentication indication for the network to the at least one subscriber terminal. 14. In a cellular wireless communication system having a network of base stations for communicating with a plurality of subscriber terminals, a method of authenticating an identity of the network to at least one subscriber terminal, comprising the steps of: providing an authentication challenge to the network and one of the terminal units, the challenge comprising fewer digits than a secret authentication key; deciphering, in the at least one subscriber terminal, an authentication challenge using a first secret key having more digits than the authentication challenge, thereby producing a first deciphered challenge having at least as many digits as the first secret key; enciphering, in the at least one subscriber terminal, the first secret key using the first deciphered challenge as an enciphering key, thereby producing a first enciphered key; compressing, in the at least one subscriber terminal, the first enciphered key, wherein the compressing is information-lossy, thereby producing a shortened key having a number of digits that is less than the number of digits of the first secret key; deciphering, in the network, the authentication challenge using a second secret key stored in the network, thereby producing a second deciphered challenge having at least as many digits as the second secret key; enciphering the second secret key using the second deciphered challenge as an enciphering key, thereby producing a second enciphered key; compressing, in the network, the second enciphered key, thereby producing an authentication response, wherein the compressing in the network is substantially the same as the compressing in the at least one subscriber terminal; transmitting the authentication response to the at least one subscriber terminal via at least one of the base stations; and comparing, in the at least one subscriber terminal, the transmitted authentication response to the shortened enciphered key, thereby producing an authentication indication for the network to the at least one subscriber terminal. 15. In a cellular wireless communication system having a network of base stations for communicating with a plurality of terminal units, a method of bilaterally authenticating an identity of at least one terminal unit and an identity of the network, comprising the steps of: generating first and second random numbers, each having a first number of digits; enciphering the first and second random numbers using respective secret keys, thereby producing first and second enciphered random numbers, each having a second number of digits less than the first number; providing the first and second enciphered random numbers to the network and to the at least one terminal unit; computing in the network a first expected response based on one of the first and second enciphered random numbers and the one's respective secret key, the one's respective secret key being stored in the network, and computing in the at least one terminal unit a second expected response based on the other of the first and second enciphered random numbers and the other's respective secret key, the other's respective secret key being stored in the at least one terminal unit; computing in the at least one terminal unit a first response based on the one of the first and second enciphered random numbers and the secret key stored in the at least one terminal unit, and computing in the network a second response based on the other of the first and second enciphered random numbers and the secret key stored in the network; transmitting, via at least one of the base stations, the first response to the network and the second response to the at least one terminal unit; and comparing in the network the transmitted first response to the first expected response, thereby producing an authentication indication for the at least one terminal unit to the network, and comparing in the at least one terminal unit the transmitted second response to the second expected response, thereby producing an authentication indication for the network to the at least one terminal unit. 16. In a cellular wireless communication system having a network of base stations for communicating with a plurality of subscriber terminals, a method of bilaterally authenticating identities of subscriber terminals and the network, comprising the steps of: generating a first authentication challenge in the network and a second authentication challenge in at least one subscriber terminal; providing the second authentication challenge to the network and the first authentication challenge to the at least one subscriber terminal; computing in the network a first response based on the first and second authentication challenges and a first secret key stored in the network, and computing in the at least one subscriber terminal a second response based on the first and second authentication challenges in a reverse order and a second secret key stored in the at least one subscriber terminal; transmitting, via at least one of the base stations, the first response to the at least one subscriber terminal and the second response to the network; and authenticating the network in the at least one subscriber terminal based on the transmitted first response and authenticating the at least one subscriber terminal in the network based on the transmitted second response. 17. In a cellular wireless communication system having a network of base stations for communicating with a plurality of subscriber terminals, a method of bilaterally authenticating identities of subscriber terminals and the network, comprising the steps of: generating a first authentication challenge in the network and a second authentication challenge in at least one subscriber terminal; providing the second authentication challenge to the network and the first authentication challenge to the at least one subscriber terminal; computing, in the network in a first manner, a first response value based on the first and second authentication challenges and a first secret key stored in the network and computing, in the at least one subscriber terminal in a second manner, a second response based on the first and second authentication challenges and a second secret key stored in the at least one subscriber terminal; transmitting, via at least one of the base stations, the first response to the at least one subscriber terminal and the second response to the network; and authenticating the network in the at least one subscriber terminal based on the transmitted first response and authenticating the at least one subscriber terminal in the network based on the transmitted second response. 18. In a cellular wireless communication system having a network of base stations for communicating with a plurality of terminal units, a method of bilaterally authenticating identities of terminal units and the network, comprising the steps of: generating a first random challenge in the network and computing first and second authentication values based on the first random challenge and a first secret key; providing the first random challenge and the first authentication value to at least one terminal unit; computing in the at least one terminal unit first and second authentication responses based on the first random challenge and a second secret key stored in the at least one terminal unit; comparing, in the at least one terminal unit, the first authentication value to the first authentication response, thereby producing an authenticity indication for the network to the at least one terminal unit; if the authenticity indication for the network indicates that the network is not authentic, placing the at least one terminal unit in a quiescent state with respect to the network, else performing the steps of: generating a second random challenge in the at least one terminal unit and computing third and fourth authentication values based on the second random challenge and the second secret key; providing the second random challenge, the second authentication response and the third authentication value to the network; comparing in the network the second authentication value to the second authentication response, thereby producing an authenticity indication for the at least one terminal unit to the network; if the authenticity indication for the at least one terminal unit indicates that the at least one terminal unit is not authentic, placing the network to a quiescent state with respect to the at least one terminal unit, else performing the steps of: computing in the network third and fourth authentication responses based on the second random number and the first secret key; comparing in the network the third authentication value to the third authentication response, thereby producing a further authenticity indication for the at least one terminal unit to the network; and if the further authenticity indication indicates that the terminal is not authentic, placing the network in a quiescent state with respect to the at least one terminal unit, else providing the fourth authentication response to the at least one terminal unit. 19. The method of claim 18, further comprising the step of comparing, in the at least one terminal unit, the fourth authentication response to the fourth authentication value, thereby producing a further authentication indication for the network to the at least one terminal unit. 20. In a cellular wireless communication system having a network of base stations for communicating with a plurality of subscriber terminals, a method of authenticating identities of the subscriber terminals and the network, comprising the steps of: generating a first random challenge in the network and computing first and second authentication values based on the first random challenge and a first secret key; providing the first random challenge and the first authentication value to at least one subscriber terminal; computing in the at least one subscriber terminal first and second authentication responses based on the first random challenge and a second secret key; comparing in the at least one subscriber terminal the first authentication value to the first authentication response, thereby producing an authenticity indication for the network to the at least one subscriber terminal; generating a second random challenge in the at least one subscriber terminal and computing third and fourth authentication values based on the second random challenge and the second secret key; providing the second random challenge, the second authentication response, and the third authentication value to the network; comparing in the network the second authentication value to the second authentication response, thereby producing a first authenticity indication for the at least one subscriber terminal to the network; computing in the network third and fourth authentication responses based on the second random number and the first secret key; comparing in the network the third authentication value to the third authentication response, thereby producing a second authenticity indication for the at least one subscriber terminal to the network; and if the first and second authenticity indications indicate that the at least one subscriber terminal is authentic, providing the fourth authentication response from the network to the at least one subscriber terminal. 21. In a cellular wireless communication system having a network of base stations for communicating with a plurality of terminal units, a method of authenticating identities of the terminal units and the network, comprising the steps of: generating a first random challenge in the network and computing first and second authentication values based on the first random challenge and a first secret key; providing the first random challenge and the first authentication value to at least one terminal unit; computing in the at least one terminal unit first and second authentication responses based on the first random challenge and a second secret key; comparing in the at least one terminal unit the first authentication value to the first authentication response, thereby producing an authenticity indication for the network to the at least one terminal unit; generating a second random challenge in the at least one terminal unit and computing third and fourth authentication values based on the second random challenge and the second secret key; providing the second random challenge, the second authentication response and the third authentication value to the network; comparing in the network the second authentication value with the second authentication response to obtain a first authenticity indication for the terminal; computing in the network third and fourth authentication responses based on the second random number and the first secret key; comparing in the network the third authentication value to the third authentication response, thereby producing a second authenticity indication for the at least one terminal unit to the network; and providing the fourth authentication response from the network to the at least one terminal unit. 22. In a cellular wireless communication system having a network of base stations for communicating with a plurality of terminal units, a method of authenticating identities of the terminal units and the network, comprising the steps of: generating a first random challenge in at least one terminal unit and computing first and second authentication values based on the first random challenge and a first secret key; providing the first random challenge and the first authentication value to the network; computing in the network first and second authentication responses based on the first random challenge and a second secret key; comparing in the network the first authentication value to the first authentication response, thereby producing an authenticity indication for the at least one terminal unit to the network; if the authenticity indication indicates that the at least one terminal unit is not authentic, placing the network in a quiescent state with respect to the at least one terminal unit, else performing the steps of: generating a second random challenge in the network and computing third and fourth authentication values based on the second random challenge and the second secret key; providing the second random challenge, the second authentication response and the third authentication value to the at least one terminal unit; comparing in the at least one terminal unit the second authentication value to the second authentication response, thereby producing an authenticity indication for the network; if the authenticity indication for the network indicates that the network is not authentic, placing the at least one terminal unit in a quiescent state, else performing the steps of: computing in the at least one terminal unit third and fourth authentication responses based on the second random number and the first secret key; comparing in the at least one terminal unit the third authentication value to the third authentication response, thereby producing a further authenticity indication for the network to the at least one terminal unit; and if the further authenticity indication indicates that the network is not authentic, placing the at least one terminal in the quiescent state, else providing the fourth authentication response to the network. 23. The method of claim 22, further comprising the step of comparing, in the network, the fourth authentication response to the fourth authentication value, thereby producing a further authentication indication for the at least one terminal unit to the network. 24. In a cellular wireless communications system including a network of base stations for communicating with a plurality of subscriber terminals, a method of authenticating identities of subscriber terminals to the network and vice versa, comprising the steps of: generating a first random challenge in at least one of the subscriber terminals and computing in dependence on a secret key therefrom first and second authentication values; providing the first random challenge and the first authentication value to the network; generating in the network first and second authentication responses from the first random challenge and a secret key stored in the network; comparing in the network the first authentication value and the first authentication response to provide an authenticity indication for the at least one subscriber terminal to the network; generating a second random challenge in the network and computing therefrom in dependence on the secret key stored in the network third and fourth authentication values; providing the second random challenge, the second authentication response and the third authentication value to the at least one subscriber terminal; comparing in the at least one subscriber terminal the second authentication value and the second authentication response to obtain a first authenticity indication for the network; generating in the at least one subscriber terminal third and fourth authentication responses from the second random number in dependence on the secret key stored in the at least one subscriber terminal; comparing in the at least one subscriber terminal the third authentication value and the third authentication response to provide a second authenticity indication for the network to the at least one subscriber terminal; and if the first and second authenticity indications indicate that the network is authentic, providing the fourth authentication response from the at least one subscriber terminal to the network. 25. The method of claim 24, further comprising the steps of receiving the fourth authentication response at the network and comparing it with the fourth authentication value to provide a further authentication indication for the at least one subscriber terminal to the network. 26. In a cellular wireless communications system having a network of base stations for communicating with a plurality of subscriber terminals, a method of authenticating identities of the subscriber terminals and the network, comprising the steps of: generating in a subscriber terminal a. first random challenge and first and second authentication values, the first and second authentication values being based on a first secret key and the first random challenge; providing the first random challenge and the first authentication value to the network; computing in the network first and second authentication responses based on the first random challenge and a second secret key; comparing in the network the first authentication value and the first authentication response to provide an authenticity indication for the subscriber terminal to the network; generating in the network a second random challenge and third and fourth authentication values, the third and fourth authentication values being based on the second secret key and the second random challenge; providing the second random challenge, the second authentication response, and the third authentication value to the subscriber terminal; comparing in the subscriber terminal the second authentication value and the second authentication response to obtain a first authenticity indication for the network; generating in the subscriber terminal third and fourth authentication responses based on the second random number and the first secret key; comparing in the subscriber terminal the third authentication value and the third authentication response to provide a second authenticity indicatibn for the network to the subscriber terminal; and providing the fourth authentication response to the network. 27. The method of claim 26, further comprising the steps of receiving the fourth authentication response at the network and comparing it with the fourth authentication value to provide a further authentication indication for the subscriber terminal to the network.

Inventors: DENT PAUL W, DENT, PAUL, W.
Applicants: ERICSSON INC, ERICSSON, INC.
Application year: 2000
Main classification: H04Q7
IPC codes: H04L9, H04L29, H04W12