Method and system of communication using extended sequence number

BACKGROUND

1. Field the of the Invention

0001The present invention relates to a method and system for wireless communication using an extended sequence number.

2. Description of Related Art

0002Security methods and processes relating to wireless communications have evolved in recent years. In particular, 2G CDMA security evolved into 3G CDMA security, and many of the same characteristics of 3G CDMA security are now incorporated into IMS systems as briefly described below.

0003As is well known in the art, 2G CDMA security involves cellular authentication and voice encryption (CAVE). Generally, in a 2G CDMA security protocol, a home location register (HLR) or authentication center (AC) of a network sends a challenge including a random number and a secondary key (SSD). The challenge is based on a 64-bit root key commonly referred to as the A-key, which is stored in the HLR or AC. In response to the challenge, the mobile equipment of a subscriber provides a response (AUTHR). The mobile equipment also stores the A-key. Accordingly, the mobile equipment using a CAVE on the random number and secondary key extracted from the challenge, and the A-key, prepares the AUTHR. The AUTHR, which is transmitted back to the HLR, allows the HLR to authenticate the mobile equipment. Conventional 2G CDMA security protocols generally do not provide mutual authentication. Because 2G CDMA security protocols are well-known in the art, further details are not described herein for the sake of brevity.

0004Conventional 3G CDMA security protocols are based on an authentication key agreement (AKA) and provide mutual authentication meaning (i) the mobile equipment authenticates the network and (ii) the network authenticates the mobile equipment before communications are performed. The well-known AKA security protocols used in 3G CDMA are based on quintuplets. Quintuplets include a random number RAND, expected response XRES, cipher key CK, integrity key IK and network authentication token AUTN. A conventional network authentication token AUTN is based on a sequence number SQN, an anonymity key AK, authentication management field AMF and a message authentication code MAC. It is noted that in conventional 3G CDMA security protocols, the sequence number does not include a hardware identifier of the mobile equipment.

0005<figref idref="f0001">FIG. 1</figref> is a diagram illustrating a method of creating the conventional network authentication token AUTN and a conventional message authentication vector AV, which may be performed by an AC of the network.

0006As shown in the diagram of <figref idref="f0001">FIG. 1</figref>, the message authentication code MAC is generated using function f1 to process a secret key K, the authentication management field AMF, a sequence number SQN, and the random number RAND. <figref idref="f0001">FIG. 1</figref> also illustrates that the remaining components of the conventional authentication vector AV is created using functions f2-f5 to process the secret key K and the random number RAND to generate an expected response XRES, cipher key CK, integrity key IK, and anonymity key AK, respectively. One skilled in the art will appreciate the functions f1-f5 could be a variety of functions well known in the art and thus, the specifics of the functions are omitted herein for the sake of brevity.

0007Once the conventional authentication vector AV is generated by the AC of the network, the authentication vector AV is transmitted to a serving system of the network providing service to the mobile equipment of the subscriber. The serving system extracts the network authentication token AUTN and the random number RAND from the authentication vector AV and provides the network authentication token AUTN and the random number RAND to the mobile equipment.

0008As mentioned above with respect to <figref idref="f0001">FIG. 1</figref>, the AUTN includes the sequence number SQN, authentication management field AMF and the message authentication code MAC. The mobile equipment extracts the sequence number SQN and the message authentication code MAC from the network authentication token AUTN and authenticates the network based on the sequence number SQN and message authentication code MAC.

0009In particular, the mobile equipment generates its own message authentication code MAC based on a sequence number SQN stored in the mobile equipment, a secret key K stored in the mobile equipment, the AMF, and the random number RAND. Then, the message authentication code MAC generated at the mobile equipment is compared with the MAC extracted from the network authentication token AUTN received from the serving system. Still further, the mobile equipment may determine if the sequence number SQN extracted from the network authentication token is an acceptable value. For example, the mobile equipment may determine if the sequence number extracted from the network authentication token is within an acceptable range to verify the sequence number SQN. If the mobile equipment successfully authenticates the network, the mobile equipment prepares a response RES and transmits the response RES back to the serving system of the network. The serving system of the network then compares the expected response XRES with the response RES to authenticate the mobile equipment, thereby completing a mutual authentication according to the conventional AKA security protocol.

0010If the mobile equipment during the authentication process determines the message authentication code MAC, which was extracted from the network authentication token AUTN, does not match the MAC generated in the mobile equipment, the mobile equipment transmits a failure message to the serving system of the network. Further, if the mobile equipment during the authentication process determines the MAC value, which was extracted from the network authentication token AUTN matches the MAC value generated by the mobile equipment, but that the sequence number SQN is outside of the permissible range, the mobile equipment transmits a resynchronization message to the network. As previously mentioned, the AKA security protocol used in 3G CDMA is well known in the art and thus, further information is not provided herein for the sake of brevity.

0011Conventional IMS security protocols have essentially incorporated the quintuplet based AKA security protocol described above with respect to 3G CDMA. However, in the IMS security mechanism, an HTTP AKA digest is located in an intermediary network component between the AC and the mobile equipment. For example, the HTTP AKA digest may be included in the S-CSCF of an IMS network. The HTTP AKA digest reconfigures the conventional authentication vector AV to be in the proper format for processing by various other components of the IMS network. Further details on the specifics of the HTTP AKA digest and AKA security protocol conventionally used in an IMS network can be found in the 3GPP TS 33.203 VT.4.0 standard published in December of 2006. As such, further details of the conventional IMS security protocols are omitted herein for the sake of brevity.

0012While security protocols have evolved by transitioning from 2G CDMA security protocols to 3G CDMA security protocols, which are also implemented in conventional IMS security protocols, some of the hardware equipment used for wireless communications has not been updated and/or is not capable of processing the more highly evolved protocols. For example, some companies which may have invested significant amounts of time, research and money in hardware used to process 2G CDMA security protocols have chosen not to update the hardware for various cost associated reasons. For example, some wireless devices such as mobile phones, PDAs, etc. are only capable of extracting the random number RAND and sequence number SQN from a challenge, as discussed above with respect to the 2G CDMA security protocols, and providing a response AUTHR consistent with the 2G CDMA security protocol. Therefore, some conventional 2G CDMA hardware devices are not currently capable of providing a mutually authenticated communication channel with an IMS network.

0013International patent application <patcit id="pcit0001" dnum="WO02052784A"><text>WO02/052784</text></patcit> is further prior-art.

SUMMARY

0014Example embodiments provide methods and apparatuses related to establishing communications between mobile equipment and a network using an extended sequence number. According to example embodiments, the extended sequence number includes at least a portion of the hardware identifier of the mobile equipment.

0015An example embodiment provides a method performed by mobile equipment to communicate with a network. The method includes receiving a network authentication token having a first message authentication code and a first extended sequence number that includes a first hardware identifier and first sequence number; and authenticating the network based on the first message authentication code and the first sequence number. The method performed by the mobile equipment may further include extracting the first message authentication code and the first extended sequence number from the network authentication token; calculating a second message authentication code based on the random number, the first extended sequence number, and a key stored in the mobile equipment; and separating the first extended sequence number to obtain the first hardware identifier and a first sequence number.

0016According to an example embodiment, the authenticating step compares the first message authentication code with the second message authentication code, the first hardware identifier with a second hardware identifier stored in the mobile equipment, and the first sequence number and a second sequence number stored in the mobile equipment; and authenticates the network if the first message authentication code matches the second message authentication code, the first hardware identifier matches the second hardware identifier, and the first sequence number is greater than the second sequence number.

0017According to an example embodiment, the first hardware identifier refers to a mobile equipment associated with a subscriber of the network and the second hardware identifier identifies the mobile equipment that received the network authentication token and random number.

0018According to an example embodiment, the method performed by the mobile equipment further includes generating a resynchronization pair if at least one of the first message authentication code does not match the second message authentication code, the first hardware identifier does not match the second hardware identifier, and the first sequence number is less than the second sequence number; and transmitting the resynchronization pair to the network.

0019According to an example embodiment, the method performed by the mobile equipment further includes reallocating bits of a first protocol resynchronization pair having a preset number of bits assigned to each of a first protocol resynchronization message and a first protocol sequence number; transmitting a second resynchronization pair having the same number of bits as the first protocol resynchronization pair. The bits reallocated in the reallocating step are used as bits of the second extended sequence number that has a greater number of bits than the first protocol sequence number.

0020Another example embodiment provides a method performed by a network to communicate with mobile equipment. The method includes transmitting a random number and an authentication token having a first extended sequence number, which includes a hardware identifier of the mobile equipment associated with a subscriber; and receiving a response from the transmitting step, the response being at least one of a cryptographic transformation of the random number and a resynchronization pair including a second extended sequence number and a resynchronization message.

0021According to an example embodiment, the method performed by the network further includes generating a first authentication vector including the network authentication token. The first authentication vector is a concatenation of the random number, an expected response, a cipher key, an integrity key, and the authentication token.

0022According to an example embodiment, the method performed by the network further includes comparing the response from the transmitting step to the expected response; and authenticating the mobile equipment if the response from the transmitting step matches the expected response.

0023According to an example embodiment, the method performed by the network further includes comparing the response from the transmitting step to the expected response; and generating a second authentication vector including a second network authentication token having the second extended sequence number if the response from the transmitting step does not match the expected response; and transmitting the second authentication token to the mobile equipment.

0024According to an example embodiment, the method performed by the network further includes detecting an indicator included in the response from the transmitting step; authenticating the mobile if the indicator indicates the response is a cryptographic transformation of the random number and the cryptographic transformation of the random number matches the expected response; and generating a second authentication vector including a second network authentication token having the second extended sequence number if the indicator indicates the response is the resynchronization pair; and transmitting the second authentication token to the mobile equipment.

0025According to an example embodiment, the method performed by the network further includes reallocating bits of a first protocol network authentication token, which has a preset number of bits assigned to each of a first protocol sequence number and a message authentication code; and generating an authentication vector including a second protocol network authentication token. The second protocol network authentication token has the same number of bits as the first protocol network authentication token, and the reallocated bits are used as bits of the first extended sequence number that has a greater number of bits than the first protocol sequence number.

0026Still another example embodiment provides a method of establishing a mutually authenticated communication channel between mobile equipment and a network. The method includes (a) generating an expected response, a random number, and a network authentication token including a first message authentication code and a first extended sequence number having a first hardware identifier being associated with the mobile equipment by the network; (b) transmitting the random number and the network authentication token from the network to the mobile equipment; (c) receiving the random number and the network authentication token at the mobile equipment; (d) authenticating the network based on the network authentication token; (e) transmitting a cryptographic transformation of the random number from the mobile equipment to the network; (f) authenticating the mobile equipment if the cryptographic transformation of the random number matches the expected response; and (g) establishing a mutually authenticated channel between the mobile station and the network.

0027According to an example embodiment, the network authenticating step (d) extracts the first message authentication code, and the first extended sequence number from the authentication token; calculates a second message authentication code based on the random number, the first extended sequence number, and a key stored in the mobile equipment; separates the first extended sequence number to obtain the first hardware identifier and a first sequence number; compares the first message authentication code with the second message authentication code, the first hardware identifier with a second hardware identifier stored in the mobile equipment, and the first sequence number with a second sequence number stored in the mobile equipment; and authenticates the network if the first message authentication code matches the second message authentication code, the first hardware identifier matches the second hardware identifier, and the first sequence number is greater than the second sequence number.

0028According to an example embodiment, the method of establishing the mutually authenticated channel further includes resynchronizing the mobile equipment and the network if at least one of the first message authentication code does not match the second message authentication code, the first hardware identifier does not match the second hardware identifier, and the first sequence number is less than the second sequence number.

0029According to an example embodiment, the resynchronizing step includes concatenating the second hardware identifier and the second sequence number to create a second extended sequence number; calculating a resynchronization message based on the random number, the second extended sequence number, and a key stored in the mobile equipment; grouping the second extended sequence number with the resynchronization message to form the resynchronization pair; transmitting the resynchronization pair; generating a second network authentication token using the second extended sequence number, and repeating the steps (b) - (f) referred to above while substituting the second network authentication token for the network authentication token.

0030Another example embodiment provides a method performed by mobile equipment to communicate with a network. The method includes receiving a network authentication token having a first message authentication code and a first extended sequence number that includes a hash of a first hardware identifier and first sequence number; and authenticating the network based on the first message authentication code, the hash of the first hardware identifier, and the first sequence number.

0031Another example embodiment provides a method performed by a network to communicate with mobile equipment. The method includes transmitting a random number and an authentication token having a first extended sequence number, which includes a hash of a first hardware identifier of the mobile equipment associated with a subscriber; and receiving a response from the transmitting step. The response is at least one of a cryptographic transformation of the random number and a resynchronization pair including a second extended sequence number and a resynchronization message.

BRIEF DESCRIPTION OF THE DRAWINGS

0032The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus are not limiting of the present invention and wherein: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> illustrates a method of providing a conventional authentication vector and a conventional network authentication token, which may be used in various conventional security protocols;</li><li><figref idref="f0002">FIG. 2</figref> illustrates a communication system according to an example embodiment;</li><li><figref idref="f0002">FIG. 3</figref> illustrates a mobile equipment according to an example embodiment;</li><li><figref idref="f0003">FIG. 4</figref> illustrates a method of generating a network authentication token and authentication vector according to an example embodiment;</li><li><figref idref="f0004">FIG. 5</figref> illustrates an example embodiment of a signal flow diagram;</li><li><figref idref="f0005">FIGS. 6A</figref> and <figref idref="f0006">6B</figref> are a flow chart illustrating an example embodiment of a method performed by a mobile equipment to authenticate a network; and</li><li><figref idref="f0007">FIG. 7</figref> is a flow chart illustrating an example embodiment of a method performed by a network to authenticate a mobile equipment.</li></ul>

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

0033<figref idref="f0002">FIG. 2</figref> illustrates a communication system 10 including at least one mobile station 100 and a network 20. One skilled in the art will appreciate that the network 20 should not be limited to the abbreviated portion of an IP multi-media sub-system (IMS), which is illustrated in the embodiment of <figref idref="f0002">FIG. 2</figref>. In <figref idref="f0002">FIG. 2</figref>, the IMS network 20 includes an IMS home system 300, an IMS visited system 400 and intermediary IMS components 200. While the intermediary IMS components 200 are merely shown as a block in the network 20, one skilled in the art will appreciate that the intermediary IMS components 200 may include, for example, a P-CSCF, and I-CSCF, an HSS, and an S-CSCF arranged between the mobile equipment 100 and the IMS home system 300 and IMS visited system 400. The IMS home system 300 and the IMS visited system 400 may communicate with each other directly or via the intermediary IMS components 200 to provide service to the mobile equipment 100. The location of the mobile equipment, type of service requested by the mobile equipment, etc., may determine whether the IMS home system 300 or the IMS visited system 400 provides the requested service to the mobile equipment 100.

0034According to the example embodiment as described with respect to <figref idref="f0002">FIG. 2</figref>, the IMS home system 300 includes an authentication center 310. The simplified version of the authentication center 310 illustrated in <figref idref="f0002">FIG. 2</figref> includes a memory 312, a processor 314, and a transceiver 316. Obviously one skilled in the art will appreciate that the authentication center 310 is more complex than the simplified version illustrated in <figref idref="f0002">FIG. 2</figref> and may include one or more computer systems.

0035<figref idref="f0002">FIG. 3</figref> illustrates an example embodiment of mobile equipment 100. As shown in <figref idref="f0002">FIG. 3</figref>, the mobile equipment 100 includes a removable user identity module RUIM, a memory 120, a processor 130 and a transceiver 140. The removable user identity module RUIM included in the mobile equipment 100 is a conventional removable user identity module RUIM. For example, the removable user identity module RUIM may be a module that was developed to function according to the 2G CDMA security protocols. As such, the removable user identity module RUIM may store a MIN/IMSI/TMSI as is well known in the art and will not be discussed further herein for the sake of brevity. The memory 120 and the processor 130 of the mobile equipment 100 may be used to perform example embodiments of methods described below with respect to the signal flow diagram of <figref idref="f0004">FIG. 5</figref> and the flow chart of <figref idref="f0005 f0006">FIG. 6</figref>.

0036Before describing example embodiments of methods for authentication according to the present invention, introduction of an extended sequence number used in the methods is explained with respect to <figref idref="f0003">FIG. 4</figref>.

0037According to example embodiments, the mobile equipment 100 and the authentication center 310 provide additional functionality to address deficiencies of the conventional removable unit identity module RUIM 110 included in the mobile equipment 100. The example embodiment of the mobile equipment 100 and the example embodiment of the authentication center 310 do this using an extended sequence number ESQN. An extended sequence number ESQN according to an example embodiment is a globally non-repeating sequence number for all mobile equipment. According to one example embodiment, the extended sequence number ESQN includes a hardware identifier of a subscriber's mobile equipment 100 and an example embodiment of a sequence number SQN'. In particular, the extended sequence number ESQN is the hardware identifier of the mobile equipment 100 concatenated with the sequence number SQN'.

0038Because the ESQN includes the hardware identifier of a subscriber's mobile equipment 100, and each mobile equipment 100 has a different hardware identifier, the ESQN is different for each mobile equipment 100. Further, because the ESQN includes a sequence number SQN', the ESQN may be incremented for each system access similar to how a sequence number SQN is incremented in a conventional AKA security protocol. Stated differently, the ESQN does not repeat within a mobile equipment 100 and is different for each different mobile equipment 100 that the removable user identity module RUIM is inserted into. According to one example, an ESQN includes 104 bits with 56 bits being allocated to the hardware identifier and 48 bits being allocated to an example embodiment of a sequence number SQN'.

0039According to an example embodiment, the sequence number SQN' included in an extended sequence number ESQN may be based on a time determined by the mobile equipment 100, or on a counter value, for example. A time based sequence number SQN' is determined based on a clock value that may be 0.1 seconds, for example, so that no two batch request may arrive simultaneously. An example of a sequence number SQN' based on time includes 47 bits in which 5 of the 47 bits are used for array management. This example sequence number SQN' would support approximately 65 years of operations. An example of a sequence number SQN' based on a counter includes 34 bits assuming 1 AKA/sec is the worst case rate, a lifetime of a mobile equipment 100 is around 15 years and a mechanism for allowing interleaving of requests from different visited IMS systems 400 is used requires 5 of the 34 bits. As indicated by these two examples, the number of bits for the sequence number SQN' may vary depending on the characteristics of the IMS network 20 and/or the mobile equipment 100.

0040<figref idref="f0003">FIG. 4</figref> is a diagram illustrating how the extended sequence number ESQN may be used in an authentication center 310 to generate an example embodiment of the authentication vector AV'. The memory 312 of the authentication center 310 may store various values such as a secret key K and various functions represented by f6-f10 used by the processor 314 of the authentication center 310 to generate the authentication vector AV'. One skilled in the art will appreciate the functions f6-f10 could be a variety of functions well known in the art and thus, the specifics of the functions are omitted herein for the sake of brevity.

0041As illustrated in <figref idref="f0003">FIG. 4</figref>, the processor 312 generates a message authentication code MAC' value by performing the process f6 with the secret key K, authentication message field AMF, extended sequence number ESQN, and random number RAND. Further, the processor 314 generates a expected response XRES based on the secret key K and the random number RAND using the process f7; generates the cipher key CK based on the secret key K and the random number RAND with the process f8; generates the integrity key IK based on the secret key K and the random number RAND using process f9; and generates the anonymity key AK' based on the secret key K and random number RAND using process f10. The processor 314 may then generate an example embodiment of an authentication token AUTN' using equation (1) shown below. <maths id="math0001" num="(1)"><math display="block"><mrow><mi mathvariant="normal">AUTNʹ</mi><mo>:</mo><mo>=</mo><mi mathvariant="normal">ESQN</mi><mo>⊕</mo><mi mathvariant="normal">AKʹ</mi><mrow><mo>‖</mo><mi mathvariant="normal">AMF</mi><mo>‖</mo></mrow><mi mathvariant="normal">MACʹ</mi></mrow></math><img file="EP2195997B1_D0001.tif" /></maths>

0042As such an authentication vector AV' according to an example embodiment is based on the extended sequence number ESQN, anonymity key AK', authentication message field AMF, and message authentication code MAC'.

0043Still further, the processor 314 calculates an example embodiment of an authentication vector AV' based on equation (2) shown below. <maths id="math0002" num="(2)"><math display="block"><mrow><mi>AVʹ</mi><mo>:</mo><mo>=</mo><mi>RAND</mi><mrow><mo>‖</mo><mi>XRES</mi></mrow><mo>‖</mo><mi>CK</mi><mo>‖</mo><mi>IK</mi><mo>‖</mo><mi>AUTNʹ</mi></mrow></math><img file="EP2195997B1_D0002.tif" /></maths>

0044As illustrated by equation 2, an example embodiment of an authentication vector AV' may be a concatenation of the random number RAND, expected response XRES, cipher key CK, integrity key IK, and network authentication token AUTN'.

0045<figref idref="f0004">FIG. 5</figref> is a signal flow diagram illustrating communications between the mobile equipment 100, intermediary IMS components 200a and the IMS home system 300 according to an example embodiment. It is noted that the intermediary IMS components 200a differ slightly from the intermediary IMS components 200 shown in <figref idref="f0002">FIG. 2</figref> in that intermediary IMS components 200a is considered to include the IMS visited system 400, which is considered the serving system in the example embodiment described below with respect to <figref idref="f0004">FIG. 5</figref>. In addition to the IMS visited system 400, the intermediary IMS components 200a may further include a P-CSCF, an I-CSCF, a HSS, an S-CSCF, for example.

0046<figref idref="f0004">FIG. 5</figref> illustrates the mobile equipment 100 sending a service request to the IMS home system 300 requesting service (1). In response to the service request, the IMS home system 300 generates an example embodiment of an authentication vector AV', as previously described with respect to <figref idref="f0003">FIG. 4</figref>.

0047Once the IMS home system 300 generates the authentication vector AV', the IMS home system 300 provides the authentication vector AV' to the intermediary IMS components 200a (2). The intermediary IMS components 200a then process the authentication vector AV' to extract the random number RAND, the expected response XRES, the cipher key CK, the integrity key IK, and the network authentication token AUTN' from the authentication vector AV'. The intermediary IMS components 200a determine the extended sequence number ESQN and a first message authentication code MAC' from the network authentication token AUTN' and store the expected response XRES, cipher key CK and integrity key IK, which are used to process a later response received from the mobile equipment 100.

0048The intermediary IMS components 200a provide the network authentication token AUTN' and the random number RAND to the mobile equipment 100 (3). The mobile equipment 100 receives and processes the network authentication vector AUTN' and the random number RAND to authenticate the IMS network 20.

0049The flow chart shown in <figref idref="f0005 f0006">FIGS. 6A-6B</figref> illustrates an example embodiment of a method performed by the mobile equipment 100 to authenticate the IMS network 20. In step S100 of <figref idref="f0005 f0006">FIG. 6</figref>, the transceiver 140 of the mobile equipment 100 receives the network authentication token AUTN' and the random number RAND_N from the intermediary IMS components 200a. The transceiver 140 may provide the network authentication token AUTN' and the random number RAND_N to the processor 130 and/or store the network authentication token AUTN' and the random number RAND_N in the memory 120, which can be accessed by the processor 130.

0050In step S105, the mobile equipment 100 extracts the first message authentication code MAC'_N, the first extended sequence number ESQN_N, and the authentication message field AMF from the network authentication token AUTN'. In particular, the processor 130 extracts the first message authentication code MAC'_N, the first extended sequence number ESQN_N, and the authentication message field AMF from the network authentication token AUTN' and stores the first message authentication code MAC'_N, the first extended sequence number ESQN_N and the authentication message field AMF in the memory 120 of the mobile equipment 100.

0051In step S110 of <figref idref="f0005 f0006">FIG. 6</figref>, the mobile equipment 100 calculates a second message authentication code MAC'_ME. The second message authentication code MAC'_ME is calculated using a secret key K stored in the removable unit identity module RUIM, the random number RAND_N received from the network 20, and the first extended sequence number ESQN_N and authentication message field AMF extracted from the network authentication token AUTN' in step S105. For example, the processor 130 combines the secret key k, the first extended sequence number ESQN_N, the random number RAND_N and the authentication message field AMF using function f6, which was previously mentioned in the description of <figref idref="f0003">FIG. 4</figref>, to generate the second message authentication code MAC'_ME.

0052In step S 115, the mobile equipment 100 determines if the first message authentication code MAC'_N matches the second message authentication code MAC'_ME. The processor 130 of the mobile equipment 100 may make this determination. Based on the determination of the processor 130 of the mobile equipment 100, the processor 130 may perform step S120 or step S155. In particular, if the processor 130 determines the first message authentication code MAC'_N matches the second message authentication code MAC'_ME, the processor 130 performs step S120, whereas if the processor 130 determines the first message authentication code MAC'_N does not match the second message authentication code MAC'_ME, the processor performs step S 155. Because step S 155 is described in greater detail below with respect to <figref idref="f0006">FIG. 6b</figref>, this description of an example embodiment will proceed under the assumption that the first message authentication code MAC'_N matches the second message authentication code MAC'_ME. It noted that according to another example embodiment, a failure signal is transmitted to the network 20 if the mobile equipment 100 determines the first message authentication code MAC'_N does not match the second message authentication code MAC'_ME. Further, because all the variable used to calculate the first message authentication code MAC'_N and the second message authentication code MAC'_ME are the same except the respective secret keys K_N and a K_ME and the function, there is an increased likelihood that one or more components of the network 20 and/or mobile equipment 100 has suffered a malfunction and thus, be unable to resynchronize.

0053In step S120, the mobile equipment 100 processes the first extended sequence number ESQN_N extracted from the network authentication token AUTN'. For example, the processor 130 separates the first extended sequence number ESQN_N into the first sequence number SQN'_N and a first hardware identifier ID_N. The first hardware identifier ID_N is the hardware identifier the network 20 associates with a subscriber of the IMS service. For example, when a subscriber registers for service, the subscriber may provide the authentication center 310 of the IMS home system 300 with the hardware identifier of the subscriber's mobile equipment and the authentication center may store this information in a subscriber profile stored in the memory 314, for example.

0054In step S125, the mobile equipment 100 compares the first hardware identifier ID_N with the second hardware identifier ID_ME. The second hardware identifier ID_ME is the hardware identifier of the mobile equipment 100 in which the removable unit identity module RUIM used by the used subscriber is inserted. The processor 130 may obtain the second hardware identifier ID_ME from the memory 120 and compare the obtained second hardware identifier ID_ME with the first hardware identifier ID_N.

0055In step S130, the mobile equipment 100 compares the first sequence number SQN'_N obtained from the first extended sequence number ESQN_N with the second sequence number SQN'_ME. The processor 130 may obtain the second sequence number SQN'_ME from the memory 140 and compare the obtained second sequence number SQN'_ME with the first sequence number SQN'_N.

0056In step S135, the mobile equipment 100 determines if the first hardware identifier ID_N matches the second hardware identifier ID_ME. The processor 130 may determine if both the first hardware identifier ID_N matches the second hardware identifier ID_ME by obtaining values stored in the memory 140. For example, a 1 may be stored in the memory 120 if step S135 indicates that the first hardware identifier ID_N matches the second hardware identifier ID_ME, and a 0 may be stored in the memory 140 if the first hardware identifier ID_N does not match second hardware identifier ID_ME. If the processor 130 determines the first hardware identifier ID_N matches the second hardware identifier ID_ME, the processor 130 performs step S140, whereas if the processor 130 determines the first hardware identifier ID_N does not match the hardware identifier ID_ME, the processor performs step S155. This description of an example embodiment will proceed under the assumption that the first hardware identifier ID_N matches the second hardware identifier ID_ME.

0057In step S140 of <figref idref="f0006">FIG. 6B</figref>, the mobile equipment 100 determines if the first sequence number SQN'_N obtained from the first extended sequence number ESQN_N is greater that a second sequence number SQN'_ME. The second sequence number SQN'_ME is stored in the memory 120 of the mobile equipment 100 and may be based on time or a counter value as previously discussed. The processor 130 determines the first sequence number SQN'_N is a valid sequence number if the first sequence number SQN'_N is greater than the second sequence number sequence number SQN'_ME stored in the memory 120. Further, the processor 130 determines the first sequence number SQN'_N is an invalid sequence number if the first sequence number SQN'_N is less than the second sequence number SQN'_ME stored in the memory 120. If the first sequence number SQN'_N is determined to be a valid sequence number, the first sequence number SQN'_N may be stored in the memory 120 by the processor 130 and used as the second sequence number SQN'_ME in processes the next time a network authentication token AUTN' and random number RAND_N are received from the intermediary IMS components 200a.

0058As shown in <figref idref="f0006">FIG. 6B</figref>, the mobile equipment 100 performs step S 145 if the mobile equipment 100 determines the first sequence number SQN'_N is a valid sequence number. In step S 145, the mobile equipment 100 generates a response message RES. For example, the processor 130 generates a response message RES by combining the random number RAND_N received from the intermediate IMS components 200a with the secret key K_ME stored in the removable unit identity module RUIM using function f7. Function f7 was previously mentioned with respect to <figref idref="f0003">FIG. 4</figref>.

0059In step S150, the mobile equipment 100 transmits the response message RES to the IMS network 20. For example, the transceiver 140 transmits the response message RES to the intermediary IMS components 200a of the IMS network 20.

0060As shown in <figref idref="f0005">FIGS. 6A</figref> and <figref idref="f0006">6B</figref>, the mobile equipment 100 performs step S 155 if the mobile equipment 100 determines at least one of (i) the first message authentication code MAC'_N does not match the second message authentication code MAC'_ME, (ii) the first hardware identifier ID_N does not match the second hardware ID_ME, and (iii) the first sequence number SQN'_N is not greater than the second sequence number SQN'_ME.

0061For example, condition (ii) is satisfied when the removable unit identity module RUIM is removed from a first mobile equipment and placed in a second mobile equipment that is different from the first mobile equipment. Because the hardware identities of the first and second mobile equipment are different, the network 20 would be using the hardware identifier ID_N of the first mobile equipment, which may have been the mobile equipment used by a subscriber when the subscriber first registered for an IMS service, and the hardware identifier ID_ME being used by the second mobile equipment is the hardware identifier of the second mobile equipment, which includes the removable unit identity module RUIM.

0062Still referring to <figref idref="f0006">FIG. 6B</figref>, the mobile equipment 100 generates a resynchronization pair (MACS, ESQN_ME), which includes a resynchronization message MACS and the second extended sequence number ESQN_ME in step S 155. The resynchronization message MACS is calculated in a manner similar to the second message authentication code MAC'_ME. However, resynchronization message MACS includes the second extended sequence number ESQN_ME instead of the first extended sequence number ESQN_N obtained from the network authentication token AUTN'. To generate the resynchronization message MACS, the processor 130 of the mobile equipment 100 combines the second extended sequence number ESQN_ME with the random number RAND_N and the authentication management field AMF using a function f6*, which is different from function f6 used to calculate the first message authentication code MAC'_N and the second message authentication code MAC'_ME.

0063In step S160 of <figref idref="f0006">FIG. 6B</figref>, the mobile equipment 100 transmits the generated resynchronization pair (MACS, ESQN_ME) to the IMS network 20. For example, the transceiver 140 of the mobile equipment 100 transmits the resynchronization pair (MACS, ESQN_ME) including the resynchronization message MACS and the second extended sequence number ESQN_ME to the intermediary IMS components 200a of the IMS network 20.

0064Referring back to <figref idref="f0004">FIG. 5</figref>, a response is transmitted from the mobile equipment 100 to the intermediary IMS components 200a of the IMS network 20 (4). According to an example embodiment, the response is either the response message RES generated in step S 145 of <figref idref="f0006">FIG. 6B</figref> or the resynchronization pair (MACS, ESQN_ME) generated in step S 155 of <figref idref="f0006">FIG. 6B</figref>.

0065<figref idref="f0007">FIG. 7</figref> is a flow chart illustrating an example embodiment of a method performed by the IMS network 20. In step S200 of <figref idref="f0007">FIG. 7</figref>, the IMS network 20 receives the response transmitted by the mobile equipment 100. For example, the intermediary IMS components 200a receive the response transmitted by the transceiver 140 of the mobile equipment 100.

0066In step S210 of <figref idref="f0007">FIG. 7</figref>, the IMS network 20 compares the received response with the expected response XRES previously obtained from the authentication vector AV'. For example, the intermediary IMS components 200a compare the received response RES to the expected response XRES previously extracted from the authentication vector AV' provided by the IMS home system 300. It is noted that while step S210 illustrates an actual comparison of the received response with the expected response XRES, an alternative embodiment detects an indicator included in the received response and determines if the received response is a response message RES or a resynchronization pair (MACS, ESQN_ME) from the indicator.

0067Assuming the IMS network 20 performs step S220 illustrated in <figref idref="f0007">FIG. 7</figref>, the received response matches the expected response XRES. The received response may match expected response XRES if the received response is a response message RES. In step S220, the intermediary IMS components 200a establish a mutually authenticated communication channel between mobile equipment 100 and the network 20 over which various services provided by an IMS serving system may be provided. The IMS serving system may be the IMS home system 300 or the IMS visited system 400. However, as previously mentioned, the serving system in this example embodiment is considered to be the IMS visited system 400 which is included in the intermediary IMS components 200a. The establishment of the mutually authenticated communication channel is represented in <figref idref="f0004">FIG. 5</figref> by (5a). Secure communication is provided over the mutually authenticated communication channel at least in part because both the mobile equipment 100 and the network 20 possess the cipher key CK and the integrity key IK.

0068Alternatively, the IMS network 20 performs step S230 when the received response does not match the expected response XRES. For example, if the received response is the resynchronization pair (MACS, ESQN_ME), the intermediary IMS components 200a of the IMS network 20 will determine the received response does not match the expected response XRES.

0069In step S230, the IMS network 20 calculates another authentication vector AV" based on the second extended sequence number ESQN_ME included in the resynchronization pair (MACS, ESQN_ME). For example, referring back to <figref idref="f0004">FIG. 5</figref>, the intermediary IMS components 200a transmit the resynchronization pair MACS, ESQN_ME) along with the authentication management field AMF and the random number RAND_N to the IMS home system 300 (5b). The IMS home system 300 extracts the second extended sequence number ESQN_ME from the resynchronization pair (MACS, ESQN_ME) and uses the random number RAND_N, the authentication message field AMF, and the secret key K_N to generate the authentication vector AV" as was previously described with respect to <figref idref="f0003">FIG. 4</figref>. The previously described steps illustrated in the signal diagram of <figref idref="f0004">FIG. 5</figref> and the flow charts of <figref idref="f0005">FIGS. 6A</figref>, <figref idref="f0006">6B</figref> and <figref idref="f0007">7</figref> are then repeated as necessary.

0070As described above, example embodiments use an extended sequence number ESQN to establish a mutual authentication channel between a home IMS system 300 and/or a visited IMS system 400. Further, an extended sequence number ESQN may be a hardware identifier concatenated with a sequence number SQN. Accordingly, if hardware identifier is 56 bits, the extended sequence number ESQN is 56 bits longer than a conventional sequence number.

0071Accordingly, the additional example embodiments described below are directed towards compensating for the increased length of the ESQN.

0072Referring back to <figref idref="f0004">FIG. 5</figref>, signal (3) illustrates the intermediary IMS components 200a transmitting the authentication token AUTN' and the random number RAND to the mobile equipment 100. In the following example embodiment, an assumption is made that the intermediary IMS components 200a have a limited number of bits, which may be transmitted to the mobile equipment 100. The limited number of bits corresponds to a number of bits required to transmit a random number RAND and authentication token AUTN used in conventional methods such as the IMS security protocols described in the background section of this disclosure. For example, assume the limited number of bits is 200 bits and 80 of the bits are allocated for the random number RAND with the 120 remaining bits being allocated for the authentication token AUTN. In this example, the 120 bits of the authentication token are allocated as follows: 48 bits allocated to a conventional sequence number SQN (or the sequence number SQN masked with the anonymity key AK), 16 bits allocated to AMF and 56 bits allocated to the message authentication code MAC.

0073Based on the above assumptions, in order for the intermediary IMS components 200a to perform according to the example embodiments described with respect to <figref idref="f0004">FIGS. 5</figref>, <figref idref="f0005">6A</figref>, <figref idref="f0006">6B</figref> and <figref idref="f0007">7</figref>, the mobile equipment 100 and the authentication center 310 reallocate the limited number of bits so that the bits of the hardware identifier can be included in the transmission. Further, assume the hardware identifier is 56 bits and thus, 56 bits of the 200 bits, i.e., the limited number of bits, must be reallocated.

0074In one example of reallocating bits, the authentication center 310 only includes a 34 bit sequence number SQN'_N in an example embodiment of the authentication vector AV' instead of the 48 bits originally allocated to the sequence number SQN, thereby reallocating 14 bits for the hardware identifier ID_N. In addition, the authentication center 310 may only include a 38 bit random number RAND in the example embodiment of the authentication vector AV' instead of the 80 bits originally allocated to the sequence number SQN, thereby reallocating 42 bits for the hardware identifier ID_N. As such, 56 bits are reallocated by the authentication center 310 for the hardware identifier ID_N by reducing the number of bits of the sequence number SQN by 14 and reducing the number of bits of the random number by 42 bits.

0075In another example of reallocating bits, the mobile equipment 100 may reallocate bits assigned to a conventional resynchronization message used in conventional IMS security protocols to accommodate the bits of the hardware identifier ID_ME included in the second extended sequence number ESQN_ME included in a resynchronization pair (MACS, ESQN_ME) of example embodiments of the present invention.

0076In still another example embodiment, the first extended sequence number ESQN_N included in the network authentication vector AV' includes a hash of the first hardware identifier ID_N, assuming that the intermediary IMS components 200a have a limited number of bits that may be transmitted to the mobile equipment 100. Referring back to step S120 of <figref idref="f0005">FIG. 6A</figref>, the mobile equipment 100 processes the first extended sequence number ESQN_N extracted from the network authentication token AUTN'. If the first extended sequence number ESQN_N includes a hash of the first hardware identifier ID_N, instead of the first hardware identifier ID_N, the processor 130 separates the first extended sequence number ESQN_N into the first sequence number SQN'_N and the hash of the first hardware identifier ID_N. The processor then obtains the second hardware identifier ID_ME from the memory 120 of the mobile equipment 100, processes the second hardware identifier ID_ME using the same hash function used by the network 20 to hash the first hardware identifier ID_N, and compares the hash of the second hardware identifier ID_ME generated by the mobile equipment 100 with the hash of the first hardware identifier ID_N provided by the network 20.

0077The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the present invention.