GB2365687A

GB2365687A - Authentication process using sequence numbers

Info

Publication number: GB2365687A
Application number: GB0019067A
Authority: GB
Inventors: Peter Thomas Howard
Original assignee: Vodafone Ltd
Current assignee: Vodafone Ltd
Priority date: 2000-08-02
Filing date: 2000-08-03
Publication date: 2002-02-20
Anticipated expiration: 2020-08-03
Also published as: GB0019067D0; GB2365687B; GB0018950D0

Abstract

A process for authenticating communication between a Third Generation (3G) telecommunication network and a user services identity module (USIM) for use therein is described. The Home Location Register (HLR)/ Authentication Centre (AuC) of the network generates sequence numbers having values which advance between a start value and an end value in a sequence and are transmitted to a USIM intending to become authenticated in the network. Each USIM compares the value of each received sequence number with the value of the highest value sequence number previously accepted, and accepts the received serial number, and confirms part of the authentication process, only if its value is higher than the value of that previously accepted sequence number. To guard against the inevitable failure of this part of the authentication process if the received sequence number has the highest possible value (e.g. beyond which a counter generating the sequence numbers will wrap around to zero), the USIM compares the value of each received sequence number with a predetermined limit value (e.g. equal to the maximum value) and disables further sequence number comparison if the value of the received sequence number exceeds the predetermined limit value. In addition, the said part of the authentication process is then assumed to be satisfied for the future.

Description

<Desc/Clms Page number 1> TELECOMMUNICATIONS SYSTEMS AND METHODS The invention relates to telecommunication systems and methods. Telecommunication systems embodying the invention, and to be described in more detail below by way of example only, are systems, such as radio telecommunication systems, in which an authentication process is carried out when a user wishes to become active in the system. According to the invention, there is provided an authentication communication between a telecommunications network and a user device for use therein, in which numbers (sequence numbers) having values which advance from a start value towards an end value in the sequence are generated and transmitted between the network and the user device, a comparison step carries out a check of the value of each transmitted sequence number before acceptance thereof to determine whether to accept it, acceptance of the transmitted sequence number confirming at least part of the authentication process, that part of the authentication process also or instead being confirmed if the value of the transmitted sequence number is determined to be at least as advanced in the sequence as a predetermined limit value.

According to the invention, there is also provided a telecommunication system, comprising a telecommunications network and a plurality of user devices each for use therein after completion of a respective authentication process, the network including number generating means for generating numbers (sequence numbers) having values

which advance from a start value towards an end value in a sequence, means in the network for transmitting the sequence numbers to the user devices, first comparison means in each user device for carrying out a check of each received sequence number fore acceptance thereof to determine whether to accept it, means responsive to acceptance of the received sequence number for confirming at least part of the authentication process in relation to that user device, and second comparison means for comparing the value of each sequence number for a particular user device with a predetermined limit value and operative to confirm the said part of the authentication process in relation to that user device if the value of that sequence number is at least as advanced in the sequence as the predetermined limit value.

According to the invention, there is further provided a telecommunication network arranged to carry out an authentication process for authenticating communication between the network and a user device, comprising number generating means in the network for generating numbers (sequence numbers) having values which advance from a start value towards an end value in a sequence, means for transmitting the sequence numbers to the user device, means operative to confirm at least part of the authentication process in response to receipt from the user device of a signal confirming that a sequence number received thereby has been accepted by the user device, and means responsive to the generation of a sequence number having a value at least as advanced in the sequence as a predetermined limit value for assuming further confirmation of the said part of the authentication process for that user device.

According to the invention, there is yet further provided a user device for a telecommunication network, including means for accepting and storing numbers (sequence numbers) having values which advance from a start value towards an end value in a sequence and which are received from the network, first comparing means for carrying out a check of each received sequence number for acceptance thereof to determine whether to accept it, acceptance of the sequence number confirming at least part of a process of authenticating communication between the network and the user device, and second comparison means for comparing the value of the received sequence number with a predetermined limit value and permanently confirming the said part of the authentication process if the value of the received serial number is at least as advanced in the sequence as the predetermined limit value.

Mobile telecommunication systems, networks and methods according to the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a block diagram of one of the networks; and Figure 2 is a flow chat.

The network to be described with reference to Figure 1 is a cellular telecommunications network, more particularly a Third Generation (3G) or UMTS network. However, the

invention is not restricted to such a network.

As shown in Figure 1, the network comprises a switching centre (MSC) 10 which controls a plurality of radio network controllers (RNC), of which one is shown at 12 in Figure 1. Each RNC 12 controls several base transceiver stations (BTS) 14. Each BTS 14 communicates with a plurality of mobile stations (MS) 16 - that is, mobile telephone handsets for example. The network includes a home location register (HLR) 20 with which is associated an authentication centre (AuC) 22. Communication between each BTS 14 and the MS 16 is normally by means of a wireless link.

Each BTS 14 has a respective service area or cell 24 having a particular size (which varies according to the radio range of the BTS, the local terrain and the local propagation characteristics). Communication can take place between the BTS and an MS within the cell. As an MS moves out of the area of the cell of one BTS and into the area of the cell of another BTS, communication is handed over from the first to the second BTS as is known in cellular telecommunication technology.

Users of the network are provided with smart cards (known as User Services Identity Modules, UNITS Subscriber Identity Modules or Universal Subscriber Identity Modules - USIMs). Each USIM carries data identifying the user and other data relating to the user such as the type of service to which the user is entitled, and also data, including algorithms, for authentication purposes. When a user wishes to become active in the

network, he inserts his USIM into a suitable terminal such as a telephone handset. When the handset has been activated and the USIM has been authenticated within the network, he can then make and receive calls (which may be voice or data calls).

When a telephone handset containing a customer's USIM is switched on, radio communication will be established between that MS and the BTS 14 covering the area or cell in which the MS is currently located (assuming, of course, that the MS is within range of a BTS). An authentication process then takes place.

The main purpose of the authentication process is to enable the serving network to corroborate the identity of the user's USIM - that is, that the USIM is genuine and authorised to use the particular network. Secondly, the authentication process establishes on behalf of the user that he is connected to a serving network that is authorised to provide him with services. Thus, for example, if the user is roaming in areas covered by networks other than his home network (that is, the network with which he is registered), this part of the authentication process establishes that the particular network with which he is in communication is one of the networks authorised by his home network to offer roaming services.

The authentication process is initially controlled by the AuC 22. It involves the issue of a challenge by the AuC to the USIM. At the same time, the AuC generates the correct response to the challenge. During the authentication process, that response is compared

with the actual response received from the USIM. If the correct response is received, then this part of the authentication process is satisfactorily completed.

In addition, the authentication process includes the generation of a "sequence number", and the transmission of this sequence number to the USIM. The sequence numbers are generated by the AuC and advance sequentially in a given direction from a start value towards an end value: in a simple case, for example, the numbers could be considered to be generated by a counter and to increase progressively. They could, for example, be linked to a global clock. Each sequence number would therefore comprise a part according to the actual date which would obviously be incremented each day and a part which would be a serial number in a series which increases from a datum value (e.g. zero) each day to a maximum value and then repeated the next day. In principle, though, sequence numbers could instead advance in the sequence by decreasing in value from a high value towards zero.

Each sequence number (contained within an Authentication Token - AUTN) is transmitted with a Message Authentication Code CMAC) by means of which the USIM can check that the AUTN has been issued by a genuine network (that is, the USIM carries out this check independently of checking the value of the sequence number). The USIM then checks the sequence number for "freshness". The aim of the freshness check is to determine if the sequence number has previously been received by the USIM. This check can be carried out using various methods.

According to one method, the USIM is satisfied that a sequence number is fresh (that is, it achieves the required quality of "freshness") if the sequence number is further advanced in the sequence than the most advanced sequence number in the sequence that it has previously received and accepted. Normally, the sequence will be a sequence of increasing numbers. Therefore, a USIM will only determine that a received sequence number is "fresh" if it is higher in value than the highest sequence number that was previously accepted as being fresh.

If the sequence is a decreasing sequence, a sequence number will be considered to be more advanced in the sequence than a previous accepted sequence number if it is lower in value than the sequence number previously accepted as being fresh.

If the USIM determines that a received sequence number has the required quality of freshness, then that part of the authentication process has been satisfactorily completed. The use of sequence numbers, and the process of checking their value by the USIM, helps to protect against a non-authorised network "replaying" a previous authentication process and thus "capturing" that user.

If the USIM determines that a received sequence number does not have freshness (that is, it determines that the received sequence number is nat further advanced in the sequence that the most advanced sequence number in the sequence which it had

previously accepted as being fresh), then it indicates a failure of authentication (a "synchronisation failure") to the AuC. The authentication process will be repeated. The AuC will issue another sequence number, further advanced in the sequence, and the USIM will repeat the check for freshness. Such re-synchronisation will obviously cause delay in the authentication process. This may be inevitable in some cases (e.g. because of temporary corruption of the radio signal). However, circumstances may arise in which repeated attempts at re-synchronisation fail and are bound to fail. In particular, if the sequence is a sequence with increasing values, and the numbers are generated by a counter and become very high within the sequence, it may become almost inevitable that the counter will reach its maximum value and then wrap around to zero. Although the counter may be designed to have a very large maximum capacity, circumstances may nevertheless arise in which it reaches its maximum value: for example, special circumstances may cause a "jump" in the value of sequence numbers generated by the AuC or a particular user may perform a particularly large number of registrations (and thus a correspondingly large number of authentications). If wrap around does occur, then of course it will be inevitable that all new sequence numbers thereafter received will be lower than the highest values previously accepted. In such circumstances, the USIM will therefore indicate a synchronisation failure. Furthermore, a failure of this type will make it practically impossible for the USIM to recover from a state of permanent re- synchronisation - where every attempted authentication causes a synchronisation failure thereby causing the USIM to be denied service permanently.

In order to overcome this problem, the invention responds to receipt of a sequence number having a predetermined limit value within the sequence by permanently disabling all subsequent checks of sequence numbers for freshness. If, therefore, the received sequence number has this limit value, the USIM immediately disables freshness checking, that is, it ignores freshness checking for the current authentication and for all subsequent authentications. That part of the authentication process dependent on freshness checking is assumed to be permanently satisfied. The predetermined limit value is a value at or before the most advanced possible number in the sequence. Thus, for example, if the sequence is a sequence of sequentially increasing numbers, the end of the sequence will be the maximum possible number (for example, if the sequence numbers are generated by a counter, then the maximum number will be the greatest number before the counter wraps around to recommence counting). The predetermined limit value can therefore be at or near this maximum value.

In this way, subsequent failures of synchronisation because of failure of the freshness check carried out by the USIM are impossible. It follows, of course, that the overall authentication process carried out by the USIM becomes less secure - because that part of the authentication process dependent on the freshness checking of sequence numbers by the USIM is no longer being implemented. However, the consequence of this is normally much less severe than the consequence (permanent denial of service) of USIMs entering into a state of "permanent re-synchronisation".

The process is illustrated in the flow charge in Figure 2.

At A, the AuC transmits a sequence number to the USIM. At B, the USIM receives a sequence number from the AuC.

At step C, the USIM checks whether the value of the sequence number is greater or less than the predetermined value (for example, as explained above, a value at or near the maximum possible sequence number before wrap around occurs). If the USIM determines that the received sequence number is at least as great as or greater than the predetermined limit value, it disables all future freshness checking (step D) and sends a corresponding signal to the AuC (step E).

If, however, the USIM determines that the received sequence number is not equal to the predetermined limit value, it compares the value of the received sequence number with the highest sequence number previously accepted as being fresh (step F). If it determines that the new sequence number is higher than the highest previously accepted sequence number, synchronisation is achieved (step G).

If, however, it determines that the newly received sequence number is less than the highest sequence number that was previously accepted as being fresh, it determines synchronisation failure (step H) and signals accordingly to the AuC (step I). The AuC then generates a further sequence number and the process repeats.

Other methods can be used by the USIM to determine freshness of a newly received sequence number. For example, the sequence numbers could be generated in batches and the numbers in a batch could be used in any order. In such a case, the USIM would regard a received sequence numbers fresh if it was more advanced in the sequence than the values of the sequence numbers of the preceding batches and had not already been received. Other methods are possible.

Systems have been proposed in which a USIM only accepts a received sequence number if its value does not exceed the highest value sequence number previously accepted as fresh by not more than a predetermined amount. This limits the maximum increase in sequence numbers that can be accepted and makes it very unlikely that the counter will reach the maximum value during the life of the USIM and then wrap around (causing the problems discussed above). However, if in such a system wrap around does occur, the problem which the invention overcomes will be present and the invention is thus advantageous compared with such a system. In the invention, also, if the current sequence number in the AuC is lost for any reason, the AuC can be recovered by incrementing a previously backed-up value by a sufficiently large amount. In the alternative system described above, this may not be possible because it places restrictions on the amount by which a previously backed-up value can be implemented.

Claims

CLAIMS 1. An authentication communication between a telecommunications network and a user device for use therein, in which numbers (sequence numbers) having values which advance from a start value towards an end value in the sequence are generated and transmitted between the network and the user device, a comparison step carries out a check of the value of each transmitted sequence number before acceptance thereof to determine whether to accept it, acceptance of the transmitted sequence number confirming at least part of the authentication process, that part of the authentication process also or instead being confirmed if the value of the transmitted sequence number is determined to be at least as advanced in the sequence as a predetermined limit value.
2. A process according to claim 1, in which the comparison step carries out the said check by assessing whether the transmitted sequence number has previously been accepted and only accepts it if it decides that it has not been previously accepted.
3. A process according to claim 2, in which the comparison step carries out the said check by comparing the value of each transmitted sequence number before acceptance thereof with the value of the sequence number previously accepted having the value most advanced in the sequence and only accepts the transmitted sequence number if its value is more advanced in the sequence than that previously accepted sequence number.

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4. A process according to any preceding claim, in which the said part of the authentication process is deemed to be confirmed for the future when the value of the generated sequence number has been determined to be at least as advanced in the sequence as the predetermined limit value.
5. A process according to any preceding claim, in which the predetermined limit value is equal to the end value.
6. A process according to any preceding claim, in which the sequences numbers are generated by the network and transmitted to the user device and the comparison step is carried out by the user device.
7. A process according to claim 6, in which the step of determining if the value of the generated sequence number is at least as advanced in the sequence as the predetermined limit value is carried out in the user device.
8. A process according to claim 6, in which the step of determining if the value of the generated sequence number is at least as advanced in the sequence as the predetermined limit value is carried out in the network.
9. A process according to any preceding claim, in which the values of the sequence numbers advance in the sequence by increasing in value therein.

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10. A process according to any preceding claim, in which the network is of the Third Generation or UMTS type.
11. A process according to any preceding claim, in which the user device is a USIM (User Services Identity Module).
12. A telecommunication system, comprising a telecommunications network and a plurality of user devices each for use therein after completion of a respective authentication process, the network including number generating means for generating numbers (sequence numbers) having values which advance from a start value towards an end value in a sequence, means in the network for transmitting the sequence numbers to the user devices, first comparison means in each user device for carrying out a check of each received sequence number for acceptance thereof to determine whether to accept it, means responsive to acceptance of the received sequence number for confirming at least part of the authentication process in relation to that user device, and second comparison means for comparing the value of each sequence number for a particular user device with a predetermined limit value and operative to confirm the said part of the authentication process in relation to that user device if the value of that sequence number is at least as advanced in the sequence as the predetermined limit value.
13. A system according to claim 12, in which the first comparison means carries out the said check by assessing whether the transmitted sequence number has previously been

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accepted and only accepts it if it decides that it has not been previously accepted.
14. A system according to claim 13, in which the first comparison means carries out the said check by comparing the value of each received sequence number before acceptance thereof with the value of the sequence number previously accepted by that user device having the value most advanced in the sequence, and means in the user device for accepting that received sequence number only if its value is more advanced in the sequence than that previously accepted sequence number.
15. A system according to any one of claims 12 to 14, in which the second comparison means is also operative to confirm the said part of the authentication process for the future in relation to the user device.
16. A system according to claim 15, in which the second comparison means comprises second comparison means in each user device for comparing the value of each received sequence number with the predetermined limit value.
17. A system according to any one of claims 12 to 16, in which the predetermined limit value is the end value.
18. A system according to any one of claims 12 to 17, in which the values of the

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sequence numbers advance in the sequence by increasing in value therein.
19. A system according to any one of claims 12 to 18, in which the network is of the Third Generation or UMTS type.
20. A system according to claim 19, in which each device is a USIM (User Services Identity Module).
21. A telecommunication network arranged to carry out an authentication process for authenticating communication between the network and a user device, comprising number generating means in the network for generating numbers (sequence numbers) having values which advance from a start value towards an end value in a sequence, means for transmitting the sequence numbers to the user device, means operative to confirm at least part of the authentication process in response to receipt from the user device of a signal confirming that a sequence number received thereby has been accepted by the user device, and means responsive to the generation of a sequence number having a value at least as advanced in the sequence as a predetermined limit value for assuming further confirmation of the said part of the authentication process for that user device.
22. A network according to claim 21, in which the predetermined limit value is the end value.

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23. A network according to claim 21 or 22, in which the values of the sequence numbers advance in the sequence by increasing in value therein.
24. A network according to any one of claims 21 to 23, which is of the Third Generation or UNITS type.
25. A network according to claim 24, in which the user device is USIM (User Services Identity Module).
26. A user device for a telecommunication network, including means for accepting and storing numbers (sequence numbers) having values which advance from a start value towards an end value in a sequence and which are received from the network, first comparing menas for carrying out a check of each received sequence number for acceptance thereof to determine whether to accept it, acceptance of the sequence number confirming at least part of a process of authenticating communication between the network and the user device, and second comparison means for comparing the value of the received sequence number with a predetermined limit value and permanently confirming the said part of the authentication process if the value of the received serial number is at least as advanced in the sequence as the predetermined limit value.
27. A user device according to claim 26, in which the first comparison means carries out the said check by assessing whether the transmitted sequence number has previously

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been accepted and only accepts it if it decides that it has not been previously accepted.
28. A device according to claim 27, in which the first comparison means carries out the said check by comparing each received sequence number before acceptance thereof with the value of the sequence number previously accepted and having a value most advanced in the sequence and is accepted only if its value is more advanced in the sequence than that previously accepted sequence number.
29. A user device according to claims 26 to 28, in which the predetermined limit value is the end value.
30. A user device according to any one of claims 26 to 29, in which the values of the sequence numbers advance in the sequence by increasing in value therein.
31. An authentication process for use in authenticating communication between a telecommunication network and a user device, substantially as described with reference to the accompanying drawings.
32. A telecommunication network, substantially as described with reference to the accompanying drawings.
33. A user device for use in a telecommunication network, the user device being

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substantially as described with reference to the accompanying drawings.