# Patent

## Title
Method of establishing authentication keys and secure wireless communication

## Abstract

A method of establishing authentication keys at both a network and mobile equipment are provided. The authentication key generated by the mobile equipment is based on both mobile keys and network keys, which are each calculated by the mobile equipment. The authentication key generated by the network is based on both mobile keys and network keys, which are each calculated by the network. The mobile keys are calculated from a challenge generated by the mobile equipment and the network keys generated by the mobile based on a challenge generated by network.

## Background
BACKGROUND
1. Field the of the Invention
The present invention relates to a method and system for secure wireless communications. In particular, example embodiments relate to a method of establishing authentication keys at both the network and mobile equipment in order to establish a mutually authenticated communication channel.
2. Description of Related Art
Security 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 new systems such as IMS systems, for example.
As is well known in the art, 2G CDMA security involves cellular authentication and voice encryption (CAVE). In particular, 2G CDMA security uses at least a root key commonly referred to as an AKey and shared secret data (SSD) keys. The SSD keys are generated via a well-known SSD update procedure. The SSD keys are semi long term keys and are treated as root keys herein. The SSD keys may be shared with a Visitor Location Register (VLR) of a network if the VLR is the Home Serving System, for example. Further, conventional 2G CDMA security protocols may involve a global challenge and response procedure and a unique challenge and response procedure.
For the global challenge procedure, the network broadcasts a random challenge RAND to the mobile equipment. A mobile equipment performing system access (e.g. registration, call origination, and call termination) in a network that requires authentication, creates and sends an authentication response AUTHR using a long term key. The pair RAND/AUTHR is forwarded to the Home Location Register/Authentication Center (HLR/AC) for verification. Also for calls of type call origination, last 6 digits are used in calculating AUTHR. For both call origination and call termination the mobile generates keys that are useful for the call (i.e SMEKEY and PLCM). The HLR/AC returns to the VLR the SMEKEY and PLCM if the RAND/AUTHR pair verifies.
A unique challenge procedure can be performed by the network towards a mobile equipment at any time on either the control or traffic channel. For example, the VLR requests a unique challenge and expected response pair, RANDU and AUTHU from the HLR/AC. The network sends the RANDU to the mobile equipment and the mobile equipment calculates a response AUTHU using a long term key and sends a response AUTHU to the network. The network verifies the RANDU/AUTHU pair.
Conventional 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.
For example, 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. 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.
If 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. The AKA security protocol briefly described above and used in 3G CDMA is well known in the art and thus, further information is not provided herein for the sake of brevity.
While security protocols have evolved by transitioning from 2G CDMA security protocols to 3G CDMA security protocols, which are also implemented in some 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. Therefore, some conventional 2G CDMA hardware devices are not currently capable of providing a mutually authenticated communication channel using the AKA security protocols of conventional 3G CDMA.
Accordingly, proposals have been made, which attempt to establish a mutually authenticated communication channel without using the quintuplet based AKA security protocol described above with respect to 3G CDMA. Stated differently, these proposals are attempting to use IS-41 authentication procedures previously used in 2G CDMA security protocols. However, all of these proposals suffer from at least the following deficiency. In particular, a compromise of a past IS-41 session key (e.g., SMEKEY and PLCM) would allow an attacker to replay a random number and successfully complete the key agreement protocol and communicate with a mobile equipment or a network. As such, these proposals are insecure when a previously used IS-41 session key is revealed.
SUMMARY
Example embodiments provide methods and apparatuses related to establishing communications between mobile equipment and a network that leverages ANSI-41 security protocols.
An example embodiment provides a method performed by a mobile equipment to communication with a network that leverages ANSI-41 security protocols. The method includes receiving a first challenge from the network; calculating a network authorization response and network keys based on the received first challenge; generating a second challenge; calculating a mobile authorization response and mobile keys based on the generated second challenge; and obtaining an authentication key based on the mobile keys and the network keys calculated by mobile equipment.
According to an example embodiment, the method may further include generating a mobile authentication code based on the authentication key; sending the second challenge, the network authorization response, the mobile authorization response, and the mobile authentication code to the network; receiving a network message authentication code from the network; generating a network message authentication code based on the first challenge and the authentication key; and establishing a communication channel between the mobile equipment and the network if the network message authentication code received from the network matches the network message authentication code generated at the mobile equipment.
According to an example embodiment, a UIM of the mobile equipment calculates the network authorization response and network keys using the first challenge and a value of a secret key stored in the UIM of the mobile equipment, and calculates the mobile authorization response and mobile keys using the second challenge and a value of a secret key stored in the UIM of the mobile equipment.
According to an example embodiment, the mobile keys and the network keys include a PLCM and SMEKEY. Further, the authentication key includes a hash of the mobile keys and the network keys. Still further, the first and second challenges are first and second random numbers, respectively.
According to an example embodiment, a UIM of the mobile equipment receives a call type indicator indicating a call origination and additional bits allocated to the first challenge to effectively increase a length of the first challenge along with the first challenge, and calculates the network authorization response and the network keys using the first challenge, the additional bits, and a value of a secret key stored in the UIM of the mobile equipment, and receives the call type indicator indicating a call origination and additional bits allocated to the second challenge to effectively increase a length of the second challenge along with the second challenge, and calculates the mobile authorization response and the mobile keys using the second challenge, the additional bits allocated to the second challenge, and a value of a secret key stored in the UIM of the mobile equipment.
According to an example embodiment, a UIM of the mobile equipment receives the first challenge along with last call digits, and calculates the network authorization response and the network keys using the first challenge, the bits of the last call digits, and a value of a secret key stored in the UIM of the mobile equipment, and receives the second challenge along with last call digits, and calculates the mobile authorization response and the mobile keys using the second challenge, the last call digits, and a value of a secret key stored in the UIM of the mobile equipment. The last call digits are a number that is not callable by the mobile equipment.
According to an example embodiment, a pair of the first challenge and network authorization response or a pair of the second challenge and mobile authorization response are used in a subsequent protocol run to generate a PLCM and SMEKEY.
Another example embodiment provides a method performed by a mobile equipment to communicate with a network that leverages ANSI-41 security protocols. The method includes generating a first challenge; calculating a mobile authorization response and mobile keys based on the generated first challenge; sending the first challenge and the mobile authorization response to the network; receiving a second challenge from the network; calculating a network authorization response and network keys based on the second challenge; and generating an authentication key based on the mobile keys and network keys calculated by the mobile equipment.
According to an example embodiment, the method further includes receiving a network message authentication code from the network; generating a network message authentication code at the mobile based on the second challenge and the authentication key; generating a mobile message authentication code if the generated network message authentication code matches the received network message authentication code; and sending the mobile message authentication code to the network in an attempt to establish a mutually authenticated communication channel with the network.
According to an example embodiment, a UIM of the mobile equipment calculates the mobile authorization response and mobile keys using the first challenge and a value of a secret key stored in the UIM of the mobile equipment, and calculates the network authorization response and network keys using the second challenge and a value of a secret key stored in the UIM of the mobile equipment.
According to an example embodiment, a UIM of the mobile equipment receives a call type indicator indicating a call origination and additional bits allocated to the first challenge to effectively increase a length of the first challenge along with the first challenge, and calculates the mobile authorization response and the mobile keys using the first challenge, the additional bits, and a value of a secret key stored in the UIM of the mobile equipment, and receives the call type indicator indicating a call origination and additional bits allocated to the second challenge to effectively increase a length of the second challenge along with the second challenge, and calculates the network authorization response and the network keys using the second challenge, the additional bits-allocated to the second challenge, and a value of a secret key stored in the UIM of the mobile equipment.
According to an example embodiment, a UIM of the mobile equipment receives the first challenge along with last call digits, and calculates the mobile authorization response and the mobile keys using the first challenge, the last call digits, and a value of a secret key stored in the UIM of the mobile equipment, and receives the second challenge along with last call digits, and calculates the network authorization response and the network keys using the second challenge, the last call digits, and a value of a secret key stored in the UIM of the mobile equipment. The last call digits are a number that is not callable by the mobile equipment, such as 00000 or 01101, for example.
According to an example embodiment, a pair of the first challenge and mobile authorization response or a pair of the second challenge and network authorization response are used in a subsequent protocol run to generate a PLCM and SMEKEY.
Another example embodiment provides a method performed by a network to communicate with a mobile equipment that leverages ANSI-41 security protocols. The method includes sending a first challenge to the mobile equipment; receiving a network authorization response, a second challenge, and a mobile authorization response from the mobile equipment; calculating network keys based on the network authorization response received from the mobile and the first challenge; calculating mobile keys based on the second challenge and the mobile authorization response; and generating an authentication key based on the networks keys and the mobile keys calculated by the network.
According to an example embodiment, the method further includes receiving a mobile message authentication code from the mobile equipment; generating a mobile message authentication code at the network based on the second challenge and the authentication key; generating an network message authentication code if the generated mobile message authentication code matches the mobile message authentication code received from the mobile equipment; and sending the network message authentication code to the mobile equipment in an attempt to establish a mutually authenticated communication channel with the mobile equipment.
According to an example embodiment, an authentication center of the network receives a call type indicator indicating a call origination and additional bits allocated to the first challenge to effectively increase a length of the first challenge along with the first challenge, and calculates the network authorization response and the network keys using the first challenge, the additional bits, and a value of a secret key stored in the authentication center of the network, and receives the call type indicator indicating a call origination and additional bits allocated to the second challenge to effectively increase a length of the second challenge along with the second challenge, and calculates the mobile authorization response and the mobile keys using the second challenge, the additional bits allocated to the second challenge, and a value of a secret key stored in the authentication center.
According to an example embodiment, an authentication center of the network receives the first challenge along with last call digits, and calculates the network authorization response and the network keys using the first challenge, the last call digits, and a value of a secret key stored in the authentication center, and receives the second challenge along with last call digits, and calculates the mobile authorization response and the mobile keys using the second challenge, the last call digits, and a value of a secret key stored in the authentication center. The last call digits are a number that is not callable by the mobile equipment.
Still another example embodiment provides a method performed by a network to communicate with a mobile equipment that leverages ANSI-41 security protocols. The method includes receiving a first challenge and a mobile authorization response; calculating mobile keys based on the first challenge and the mobile authorization response; generating a second challenge and a network authorization response; calculating network keys based on the second challenge and the network authorization response; and generating an authentication key based on the mobile keys and network keys calculated by the network.
According to an example embodiment, the method further includes generating a network message authentication code based on the second challenge and the authentication key; sending the second challenge and the network message authentication code to the mobile equipment; receiving a mobile message authentication code from the mobile equipment; generating a mobile message authentication code at the network based on the first challenge and the authentication key; and establishing a mutually authenticated communication channel with the mobile equipment if the received mobile message authentication code matches the mobile message authentication code generated at the network.
According to an example embodiment, generating the second challenge and the network authorization response includes sending a unique challenge request from a HSS/VLR to a HLR/AC of the network; receiving unique challenge and a network authorization response from the HLR/AC of the network; and concatenating the unique challenge with a mobile equipment identification number to generate the second challenge.
According to an example embodiment, an authentication center of the network receives a call type indicator indicating a call origination and additional bits allocated to the first challenge to effectively increase a length of the first challenge along with the first challenge, and calculates the mobile authorization response and the mobile keys using the first challenge, the additional bits, and a value of a secret key stored in the authentication center, and receives the call type indicator indicating a call origination and additional bits allocated to the second challenge to effectively increase a length of the second challenge along with the second challenge, and calculates the network authorization response and the network keys using the second challenge, the additional bits allocated to the second challenge, and a value of a secret key stored in the authentication center.
According to an example embodiment, an authentication center of the network receives the first challenge along with last call digits, and calculates the mobile authorization response and the mobile keys using the first challenge, the last call digits, and a value of a secret key stored in the authentication center, and receives the second challenge along with last call digits, and calculates the network authorization response and the network keys using the second challenge, the last call digits, and a value of a secret key stored in the authentication center. The last call digits are a number that is not callable by the mobile equipment.



BRIEF DESCRIPTION OF THE DRAWINGS
The 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:
FIG. 1 illustrates a communication system according to an example embodiment;
FIG. 2 illustrates an example embodiment of a mobile equipment;
FIG. 3 illustrates a signal flow diagram illustrating communication between the ME of the mobile equipment, the UIM of the mobile equipment, a home serving system/visitor location register (HSS/VLR) of the network, and the HLR/AC of the network according to an example embodiment;
FIG. 4 is a flow chart illustrating an example embodiment of a method performed by the mobile equipment to generate an authentication key;
FIG. 5 is a flow chart illustrating an example embodiment of a method performed by the network to generate an authentication key;
FIG. 6 illustrates a signal flow diagram illustrating communication between the ME, the UIM, the HSS/VLR, and the HLR/AC according to another example embodiment;
FIG. 7 is a flow chart illustrating another example embodiment of a method performed by the mobile equipment to generate an authentication key;
FIG. 8 is a flow chart illustrating another example embodiment of a method performed by the network to generate an authentication key;
FIG. 9 is a signal flow diagram illustrating yet another example embodiment of method performed by the mobile equipment including the UIM; and
FIG. 10 is a signal flow diagram illustrating yet another example embodiment of method performed by the network.



DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
FIG. 1 illustrates a communication system 10 including at least one mobile equipment 100 and a network 20. In FIG. 1, the network 20 includes a home location register (HLR) 300 and a visitor location register (VLR) 400. One skilled in the art will appreciate that the communication system 100 illustrated in FIG. 1 is simplified and would include various intermediary components used to communicate between the mobile equipment 100, the HLR 300 and the VLR 400. The location of the mobile equipment 100, type of service requested by the mobile equipment 100, etc., may determine whether the HLR 300 or the VLR 400 provides a requested service to the mobile equipment 100.
According to the example embodiment as described with respect to FIG. 1, the HLR 300 includes an authentication center (AC) 310. One skilled in the art will appreciate the HLR 300 and the AC 310 may be separate and distinct components of the communication system instead of the AC 310 being included with the HLR 300 as shown in FIG. 1. In the remainder of this application the HLR 300 and authentication center will be referred to collectively as a Home Location Register/Authentication Center (HLR/AC).
FIG. 2 illustrates an example embodiment of mobile equipment 100. As shown in FIG. 2, the mobile equipment 100 includes a user identity module (UIM), a memory 120, a processor 130 and a transceiver 140. The UIM may be a conventional user identity module. Alternatively, one skilled in the art will appreciate the UIM of mobile equipment 100 could be a conventional removable user identity module (RUIM). For example, the UIM may be a module that was developed to function according to the 2G CDMA security protocols. As such, the UIM 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, the processor 130 and the transceiver 140 may be used in conjunction with the UIM to perform example embodiments of methods described below with respect to FIGS. 3-10. For ease of explanation, the memory 120, the processor 130 and the transceiver 140 are collectively referred to as ME in example embodiments described below. Accordingly, although the signal flow diagrams of FIGS. 3 and 6 illustrate the ME and the UIM as separate entities, one skilled in the art will appreciate that the UIM is actually inserted into or a component of the mobile equipment 100, and the ME (i.e., the memory 120, the processor 130 and the transceiver 140) is a component of the mobile equipment 100.
FIG. 3 is a signal flow diagram illustrating communications between the ME of the mobile equipment 100, the UIM of the mobile equipment 100, a home serving system/visitor location register (HSS/VLR) of the network 20, and the HLR/AC of the network 20.
Referring to FIG. 3, the HSS/VLR provides a first challenge, which is generated by HSS/VLR to the ME. The first challenge is a first random number RANDN, which was generated by the HSS/VLR, to the ME. For example, the transceiver 140 of the ME receives the random number first RANDN and may store the first random number RANDN in the memory 120 and/or provide the first random number RANDN to the processor 130. The ME provides the random number RANDN to the UIM. In response to receiving the random number RANDN, the UIM calculates a network authentication response AUTHRN and network keys KEYSN based on the random number RANDN. In particular, the UIM calculates the network authorization response AUTHRN using the random number RANDN and a value of a secret key, which is stored in the UIM. The network keys KEYSN calculated by the UIM may include a PLCM and SMEKEY, for example. Calculating the network authorization response AUTHRN and network keys KEYSN from a random number RANDN is well known in the art and thus, will not be discussed further herein for the sake of brevity.
Still referring to FIG. 3, the ME then generates a second random number RANDM and provides the second random number RANDM as a challenge to the UIM. For example, the processor 130 of the ME generates the second random number by any well-known method and provides the second random number RANDM to the UIM.
The UIM then generates a mobile authentication response AUTHRM and mobile keys KEYSM based on this second random number. As previously mentioned, calculating an authentication response and keys such as the PLCM and SMEKEY from a random number are well-known in the art.
According to an example embodiment, the ME then generates an authentication key KM based on both the network keys KEYSN and mobile keys KEYSM, which are generated by the UIM. For example, the authentication key KM may be a hash of the mobile KEYSM and network KEYSN as shown by the following equation, which is also shown in FIG. 3.

KM=H1(KEYSM,KEYSN)

Following the operations illustrated by signals 1-5 of FIG. 3, the ME has an authentication key KM which includes keys associated with both the mobile equipment 100 and the network 20 including the HSS/VLR and the HLR/AC.

Still referring to FIG. 3, the ME then provides the network authentication response AUTHRN, the second random number RANDM, and the mobile authentication response AUTHRM to the HSS/VLR of the network 20. In addition to the network authentication response AUTHRN, the random RANDM and the mobile authentication response AUTHRM, the ME may send a mobile message authentication code MACMm to the HSS/VLR. If this is the case, the ME first generates the mobile message authentication code MACMm based on the authentication key KM. For example, the mobile message authentication code MACMm may be a pseudo random function calculated using the authentication key KM and the random number RANDM, which was generated by the ME. The generation of a message authentication code is well known in the art and thus, will not be discussed further in here for the sake of brevity.
In FIG. 3, once the HSS/VLR receives the network authentication response AUTHRN from the ME, the HSS/VLR provides the first random number RANDN, which was previously generated by the HSS/VLR and the network authentication response AUTHRN received from the ME to the HLR/AC. The HLR/AC calculates network keys KEYSN based on the first random number RANDN and the network authorization response AUTHRN and provides the calculated network KEYSN to the HSS/VLR.
As shown in FIG. 3, the HSS/VLR also provides the received second random number RANDM and authorization response AUTHRM received from the ME to the HLR/AC, thereby requesting keys associated with the ME be generated by the HLR/AC. Accordingly, the HLR/AC calculates the mobile keys KEYSM from the random number RANDM and mobile authorization response AUTHRM, and provides the mobile keys KEYSM back to the HSS/VLR.
The HSS/VLR then generates an example embodiment of an authentication key KN based on the mobile keys KEYSM and network keys KEYSN received from the HLR/AC. The authentication key KN is calculated in the same manner as the authentication key KM was calculated at the ME, except that the authentication key KN is calculated based on the network keys KEYSN and mobile keys KEYSM, which were provided by the HLR/AC, instead of the mobile KEYSM and network KEYSN calculated by the UIM.
According to the example embodiment shown in FIG. 3, the authentication key KM generated by the ME includes at least a portion of the mobile KEYSM calculated from a random number RANDM generated by the mobile equipment 100. As such, an attacker cannot merely use a past session key such as an SMEKEY and PLCM and replay a random number to complete the key agreement protocol. Similarly, the authentication key KN generated by the network 20 is based on at least a portion of network keys KEYSN generated based on the first random number RANDN generated by the network and thus, an attacker cannot merely use a compromised past session key and replay a random number to complete the key agreement protocol.
Still referring to FIG. 3, the HSS/VLR may verify the ME by comparing a mobile message authentication code MACMm received from the ME with a mobile message authentication code MACMn generated by the HSS/VLR. The mobile message authentication code MACMn is generated using the authentication key KN. For example, the mobile message authentication code MACMn is a pseudo random function calculated using the authentication key KN. If the mobile message authentication code MACNme received from the ME matches the mobile message authentication code MACMn generated by the HSS/VLR, the HSS/VLR determines the ME is authentic. The HSS/VLR may then calculate a network message authentication code MACNn and provide the network message authentication code MACNn to the ME as illustrated by signal 11 of FIG. 3.
In response to receiving the network message authentication code MACNn from the HSS/VLR, the ME may authenticate the HSS/VLR. For example, the ME may generate its own network message authentication code MACNm based on authentication key KM and compare this generated network message authentication code MACNm with network message authentication code MACNn received from the HSS/VLR. The ME determines the HSS/VLR is authentic if the network message authentication code MACNm generated by the mobile matches the network message authentication code MACMn received from the HSS/VLR.
FIG. 3 also illustrates that a cryptographic key CK and an integrity key IK may be generated by the HSS/VLR if the HSS/VLR determines the ME is authentic, and a cryptographic key CK and a integrity key IK may be generated by the ME if the ME determines the HSS/VLR 400 is authentic. The generation of a cryptographic key CK and an integrity key IK is well known in the art and thus, will not discussed herein for the sake of brevity. Further, the use of a cryptographic key CK and an integrity key IK in communications between a mobile equipment 100 and an HSS/VLR is well known in the art, and thus will be omitted herein for the sake of brevity.
The flow chart shown in FIG. 4 illustrates an example embodiment of a method performed by the mobile equipment 100 to generate an authentication key KM.
In step S100, the transceiver 140 of the ME receives a first random number RANDN from the network 20. In response to receiving the first random number RANDN, the transceiver 140 may store the first random number RANDN in the memory 120 and/or provide the first random number RANDN to the processor 130. As such, the UIM may obtain the first random number RANDN from the memory 120 and/or receive the first random number RANDN from the processor 130. In step S105, the UIM calculates a network authentication response AUTHRN and network keys KEYSN based on the first random number RANDN. As previously mentioned, these calculations are well known in the art.
In step S110 of FIG. 4, the ME generates a second random number RANDM. For example, the processor 130 generates a second random number RANDM and provides the random number RANDM to the UIM.
The UIM then calculates a mobile authentication response AUTHRM and mobile keys KEYSM based on the random number RANDM in step S115. As such, the ME obtains network keys KEYSN based on the random number RANDN received from the network and mobile keys KEYSM based on the random number RANDM generated by the ME according to an example embodiment.
In step S120, the ME generates an example embodiment of an authentication key KM based on the mobile keys KEYSM and the network keys KEYSN. For example, the processor 130 of the ME may generate the authentication KM, and the authentication key KM is a hash including at least a portion of the mobile keys KEYSM and the network keys KEYSN.
Steps S100-S120 described above illustrate an example embodiment of a method performed by the mobile equipment 100 to generate the authentication key KM.
Steps S125-155 of FIG. 4 described below are steps the mobile equipment 100 may perform following the generation of the authentication key KM to establish a mutually authenticated communication channel with the network 20. In step S125, the ME generates a mobile message authentication code MACMme using the authentication key KM and the random number RANDM generated by the ME. Step S125 may be performed by the processor 130 of the ME. As shown in step S130, the transceiver 140 of the ME may then send the network authorization response AUTHRN, the random number RANDN, the mobile authorization response AUTHRM, and the mobile message authentication code MACMm to the HSS/VLR of the network 20.
In step S135, the transceiver 140 of the ME receives a network message authentication code MACNn from the network 20. In step S140, the ME generates a network message authentication code MACNm based on the random number RANDN received from the network in step S100 and the authentication key KM generated in step S120. In step S145, the ME compares the network message authentication code MACNn received from the network with the network message authentication code MACNm generated by the ME.
If the two network message authentication codes do not match, the ME may abort the method the security protocol as shown in step S150. For example, the ME may send a failure notice to the network or just silently abandon the rest of the protocol. Alternatively, if the two network message authentication codes do match, a mutually authenticated communication channel is established between the ME and the network 20 in step S150.
The flow chart shown in FIG. 5 illustrates an example embodiment of a method performed by the network 20 to generate an authentication key KN.
In step S200, the network 20 generates a first random number RANDN and sends the first random number RANDN to the mobile equipment 100. For example, the HSS/VLR may generate and send the first random number RANDN to the transceiver 140 of the ME.
In step S205, the network 20 receives a network authorization response AUTHRN, a second random number RANDM, and a mobile authorization response AUTHRM from the ME. In step S205, the network 20 is also shown to receive a mobile message authentication code MACMm generated by the ME, however, it is noted that this mobile message authentication code MACMm may be received at a later time.
In response to receiving the network authorization response AUTHRN, the random number RANDM and the mobile authorization response AUTHRM from the ME, the network 20 obtains network KEYSN based on the first random number RANDN and the network authorization response AUTHRN, and obtains mobile KEYSM based on the second random number RANDM and the mobile authorization response AUTHRM. For example, as illustrated in the signal flow diagram of FIG. 3, the HSS/VLR provides the first random number RANDN and the network authorization response AUTHRN to the HLR/AC and in return, the HLR/AC calculates and provides the network keys KEYSN back to the HSS/VLR. In addition, the HSS/VLR provides the random number RANDM and the mobile authorization response AUTHRM to the HLR/AC and in return, the HLR/AC calculates the mobile keys KEYSM and provides the mobile keys KEYSM back to HSS/VLR. As previously mentioned, both the network keys KEYSN and the mobile keys KEYSM may include PLCM and SMEKEY associated with the network 20 and mobile equipment 100, respectively.
In step S215, the network 20 generates an example embodiment of an authentication key KN based on the network keys KEYSN and the mobile keys KEYSM generated by the network.
Step S200-S215 described above illustrate an example embodiment of a method performed by the mobile equipment 100 to generate the authentication key KN. Steps S220-S235 described below are steps the network 20 may perform following the generation of the authentication key KN to establish a mutually authenticated communication channel with the mobile equipment 100.
In step S220, the network 20 generates a mobile message authentication code MACMn based on the authentication key KN and the random number RANDM. In step S225, the network 20 compares the mobile message authentication code MACMn generated by the network 20 with the mobile message authentication code MACMm received from the ME in step S205, for example. The HLS/VLR of the network 20 determines the ME is authentic if the mobile message authentication code MACMn generated at the network matches the mobile message authentication code MACMm received from the ME.
If in step S225 the HLS/VLR of the network 20 determines the mobile message authentication code MACMn generated by the network 20 matches the mobile message authentication code MACMm received from the mobile equipment 100, the network 20 may generate a network message authentication code MACNn based on the random number RANDN and the authentication key KN. In step S235, the network 20 transmits the network authentication code MACNn to the mobile equipment 100 in attempt to establish a mutually authenticated communication channel between the mobile equipment 100 and the network 20. Alternatively, if the network 20 determines the mobile message authentication code MACMn generated at the network 20 does not match the mobile message authentication code MACMm received from the ME, the method illustrated in FIG. 5 is aborted by the network 20 in step S240.
FIG. 6 is a signal flow diagram illustrating communications between ME, a UIM, a HSS/VLR, and HLR/AC according to another example embodiment. In this signal flow diagram the ME initiates the communication between the mobile equipment 100 and the network 20.
Referring to FIG. 6, the ME generates a first challenge, which is a first random number RANDM, and provides the first challenge to the UIM. In response, the UIM calculates the mobile authorization response AUTHRM and mobile keys KEYSM based on the first random number RANDM and provides the mobile authorization response AUTHRM and mobile KEYSM back to the ME. The ME then provides the first random number RANDM and the mobile authorization response AUTHRM to the network 20. As shown in FIG. 6, the first random number RANDM and the mobile authorization response AUTHRM is provided to the HSS/VLR of the network 20.
The HSS/VLR provides the received first random number RANDM and the received mobile authorization response AUTHRM to the HLR/AC of the network 20. The HLR/AC calculates the mobile keys KEYSM based on the first random number RANDM and the network authentication response AUTHRM and provides the calculated mobile keys KEYSM to the HSS/VLR. Further, the HSS/VLR generates and sends a unique challenge request to the HLR/AC. In response to the unique challenge request, the HLR/AC generates a unique random number RANDU and a unique authorization response AUTHRU, which is provided to the HSS/VLR. The HSS/VLR uses the unique random number RANDU and the unique authorization response AUTHRU to calculate a second random number RANDN and a network authorization response AUTHRN, which is provided to the HLR/AC. For example, the second random number RANDN may be concatenation of a mobile identification number MIN and the unique random number RANDU, and the network authorization AUTHRN may be the unique authorization response AUTHRU as indicated in FIG. 6. The HLR/AC then calculates network keys KEYSN based on the second random number RANDN and the network authorization response AUTHRN. The HLR/AC provides the network keys KEYSN to the HSS/VLR.
The HSS/VLR then generates an example embodiment of an authentication key KN based on the mobile keys KEYSM and the network keys KEYSN, which were generated by the HLR/AC.
Still referring to FIG. 6, the HSS/VLR sends the second random number RANDN, which was generated by the HSS/VLR as a second challenge to the ME. Further, the HSS/VLR may also send a network message authentication code MACNn to the ME along with the second random number RANDN. However, one skilled in the art will appreciate that the network message authentication code MACNn may be sent in a later step. In response to receiving the second random number RANDN from the network, the ME provides the second random number RANDN to the UIM. The UIM then calculates a network authorization AUTHRN and network keys KEYSN from the second random number RANDN.
The ME then calculates an example embodiment of an authentication key KM based on the mobile keys KEYSM and the network keys KEYSN, which are each calculated by the UIM. For example, the authentication key KM may be a hash including at least a portion of the mobile keys KEYSM and the network keys KEYSN. As such, according to an example embodiment, the mobile equipment 100 generates an authentication key KM based on mobile keys KEYSM and network keys KEYSN which are generated by the UIM, and the network 20 generates an authentication key KN from mobile keys KEYSM and network keys KEYSN that are calculated in the network 20.
According to the example embodiment shown in FIG. 6, the authentication key KM generated by the mobile equipment 100 includes at least a portion of the mobile KEYSM calculated from a random number RANDM generated by the mobile equipment 100. As such, an attacker cannot merely use a past session key such as an SMEKEY and PLCM and replay a random number to complete the key agreement protocol. Similarly, the authentication key KN generated by the network 20 based on at least a portion of network keys KEYSN generated based on the first random number RANDN generated by the network and thus, an attacker cannot merely use a comprised past session key and replay a random number to complete the key agreement protocol.
Still referring to FIG. 6, if the network 20 has already provided the network message authentication code MACNn to the mobile equipment 100 as shown by signal 10 in FIG. 6, the ME may then verify the network message authentication code MACMn. In particular, the ME may generate its own network message authentication code MACNm and compare this network message authentication code MACNm with the network message authentication code MACNn received from the HSS/VLR of the network 20. If the network message authentication code MACNm generated by the ME matches the network message authentication code MACNn received from the HSS/VLR, the ME determines the HSS/VLR is authentic.
If the HSS/VLR of the network 20 is determined to be authentic, the ME generates a mobile message authentication code MACMm and provides the mobile message authentication code MACMm to the HSS/VLR. The HSS/VLR then attempts to verify the mobile equipment 100 based on the received mobile message authentication code MACMm. As such, the HSS/VLR generates its own mobile message authentication code MACMn and compares this generated mobile message authentication code MACMn with the mobile message authentication code MACMm received from the ME. If the received mobile message authentication code MACMm matches the generated mobile message authentication code MACMn, the HSS/VLR determines the ME is authentic and attempts to establish a mutually authenticated communication channel with the ME.
FIG. 6, like FIG. 3, illustrates that a cryptographic key CK and an integrity key IK may be generated by the HSS/VLR if the HSS/VLR determines the ME is authentic, and a cryptographic key CK and a integrity key IK may be generated by the ME if the ME determines the HSS/VLR is authentic. The generation of a cryptographic key CK and an integrity key IK is well known, and the use of a cryptographic key CK and an integrity key IK in communications between a ME and an HSS/VLR is well known.
The flow chart shown in FIG. 7 illustrates another example embodiment of a method performed by the mobile equipment 100 to generate authentication key KM.
In step S300, the mobile equipment 100 generates a first random number RANDM. In particular, the processor 130 of the ME generates a random number RANDM as is well known in the art. The processor 130 of the ME provides the generated random number RANDM to the UIM. Based on the random number RANDM, the UIM calculates a mobile authorization response AUTHRM and mobile keys KEYSM in step S305. For example, the mobile keys KEYSM are a PLCM and SMEKEY.
The ME then transmits the random number RANDM and the mobile authorization response AUTHRM to the network 20 in step S310. For example, the transceiver 140 of the ME provides the random number RANDM and the mobile authorization response AUTHRM to the HSS/VLR of the network 20.
In step S315, the ME receives a second random number RANDN and a network message authentication code MACN from the network 20. In step S320, the ME calculates the network authorization response AUTHRN and network keys KEYSN based on the received second random number RANDN. For example, the processor 140 of the ME provides the received second random number RANDN to the UIM, and the UIM calculates the network authorization response AUTHRN and the network keys KEYSN. The UIM provides the network authorization response AUTHRN and the network keys KEYSN to the processor 130 and/or the memory 120 of the ME.
Based on the network keys KEYSN and mobile keys KEYSM, which are generated by the UIM according to an example embodiment, the ME generates an authentication key KM. In particular, the processor 130 of the ME may generate the authentication key KM by calculating a hash including at least a portion of the network keys KEYSN and mobile keys KEYSM.
After the performance of steps S300 to S325, the mobile equipment 100 possesses an example embodiment of the authentication key KM. The mobile equipment 100 may then continue the method illustrated in FIG. 7 in an attempt to establish a mutually authentication communication channel with the network 20.
In step S330, the mobile equipment 10 generates a network message authentication code MACNm based on the random number RANDN received from network 20 using the example embodiment of the authentication key KM.
In step S335, the ME compares the network message authentication code MACNm generated by the ME with the network message authentication code MACNn received from the network 20 to determine if the two network message authentication codes match. If the two network message authentication codes do not match, the ME may abort the security protocol as shown in step S350. For example, the ME may send a failure notice to the network or just silently abandon the rest of the protocol.
Alternatively, if the network message authentication code MACNm generated by the ME of the mobile equipment 100 does match the network message authentication code MACNn received from the network 20, the method shown in FIG. 7 proceeds to step S340.
In step S340, the ME generates a mobile message authentication code MACMm. In step S345, the ME transmits the generated mobile message authentication code MACMm to the network 20 in an attempt to establish a mutually authenticated communication channel with the network 20 which was verified as discussed above in step S335.
The flow chart shown in FIG. 8 illustrates another example embodiment of a method performed by the network 20 to generate an authentication key KN.
In step S400, the network 20 receives a random number RANDM and a mobile authorization response AUTHRM from mobile equipment 100. For example, the random number RANDM and the mobile authorization response AUTHRM may be received by the HSS/VLR 400 of the network 20.
In step S405, the network 20 obtains mobile keys KEYSM based on the received random number RANDM and mobile authorization response AUTHRM. For example, the HSS/VLR of the network 20 provides the random number RANDM and the mobile authorization response AUTHRM received from the mobile equipment 100 to the HLR/AC of the network 20, and the HLR/AC calculates the mobile keys KEYSM based on the random number RANDM and the mobile authorization response AUTHRM.
In step S410, the network 20 generates a second random number RANDN and a network authorization response AUTHRN. In particular, as illustrated by signals 6-9 of FIG. 6, the HSS/VLR generates a unique challenge request and provides this unique challenge request to the HLR/AC. In response to receiving the unique challenge request, the HLR/AC generates a unique random number RANDU and unique authorization response AUTHRU and provides this information to the HSS/VLR. The HSS/VLR then generates a random number RANDN and a network authorization response AUTHRN from the unique random number RANDU and unique authorization response AUTHRU and provides the random number RANDN and the network authorization response AUTHRN to the HLR/AC.
In step S415, the network 20 obtains network keys KEYSN based on the random number RANDN and the network authorization response AUTHRN. In particular, the HLR/AC calculates the network keys KEYSN based on the random number RANDN and the AUTHRN received from the HSS/VLR and provides the calculated network keys KEYSN to the HSS/VLR.
In step S420, the network 20 generates an example embodiment of an authentication key KN based on the network keys KEYSN and the mobile keys KEYSM generated by the network 20. For example, the example embodiment of the authentication key KN is a hash including at least a portion of the mobile keys KEYSM and in network keys KEYSN.
As a result of the performance of steps S400 to S425, the network 20 possesses an example embodiment of the authentication key KN. The network 20 may then continue the method illustrated in FIG. 8 in an attempt to establish a mutually authentication communication channel with the mobile equipment 100.
In step S425, the network 20 generates a network message authentication code MACNn based on the authentication key KN and the random number RANDN. For example, the HSS/VLR may perform this operation on behalf of the network 20.
The HSS/VLR of the network 20 then sends the random number RANDN and network authentication code MACNn, which are both generated by the network 20 to the mobile equipment 100 in step S430. In response, the HSS/VLR of the network 20 may receive a mobile message authentication code MACMm from the mobile equipment 100 in step S435. In step S440, the network 20 generates its own mobile message authentication code MACMn based on the authentication key KN and the random number RANDM previously received from the mobile equipment 100 as described in step S400.
In step S445, the network 20 attempts to verify the authenticity of the mobile equipment 100 by comparing the mobile message authentication code MACMn generated at the network 20 with the mobile message authentication code MACMm received from the mobile equipment 100.
If the mobile message authentication code MACMn generated at the network 20 does not match the mobile message authentication code MACMm received from the mobile equipment 100, the network 20 may abort the security protocol as shown in step S455.
Alternatively, if the mobile message authentication code MACMn generated at the network 20 does match the mobile message authentication code MACMm received from the mobile equipment 10, the network 20 determines the mobile equipment 100 is authentic and attempts to establish a mutually authenticated communication channel with the mobile equipment 10 as shown in step S450.
According to another example embodiment, a RAND/AUTHR value created in a first protocol run by the mobile equipment is used in a future protocol run. Since the network cannot create a RAND/AUTHR pair, the network requests the ME to create a RAND/AUTHR pair during the first protocol run, and in a subsequent second protocol run, the network requests that the ME use the RAND/AUTHR created in the first protocol run to create a SMEKEY/PLCM used to generate an authentication key. According to this example embodiment, the number of communications between the network and mobile equipment may be reduced while still providing a mutually authenticated communication channel.
The example embodiments described above enable the establishment of a mutually authenticated communication channel with use of the IS-41 authentication procedures. However, it is noted that typically, 64-bit random challenges are used with 64-bit keys to provide sufficient security. However, security would be further improved if larger random challenges, i.e., larger random numbers can be used. Accordingly, the example embodiment described below may be used to increase the number of bits available for the random numbers used in the previously described example embodiments.
As is well known in the art, the call type of a communication may be defined as a call registration, a call origination, or a call termination. Conventionally, for a call origination calculation, the six last call digits are used instead of a 24-bit IMSI in the CAVE calculations performed by the UIM. Six digits can be encoded in approximately 20 bits. The example embodiments described below with respect to FIGS. 9 and 10 exploit the difference between the calculations relating to a call origination type and the call registration type or call termination type.
FIG. 9 illustrates an example embodiment of method performed by the mobile equipment 100 including the UIM. As shown in FIG. 9, the ME sends a communication including a random number, a call type indicator indicating a call origination, and six digits, which can be encoded in about 20 bits, to the UIM. Because the call type is designated as a call origination, bits used for the six digits are reallocated to extend the random number. For example, if the random number is allocated 32 bits, and the 20 bits conventionally used for the six last call digits are reallocated to the random number, according to example embodiment a random number of about 52 bits is provided. The 52 bits including the 32 bits used normally and the 20 reallocated bits.
In response to receiving the a communication including a random number, a call type indicator indicating a call origination, and six digits, the UIM calculates an authorization response AUTHR and keys using the extended random number, which may include about 52-bits based on the assumptions specified above.
FIG. 10 illustrates an example embodiment of method performed by the HSS/VLR and the HLR/AC of the network 20. As shown in FIG. 10, the HSS/VLR sends a communication AUTHREQ including a random number RAND, an authorization response AUTHR, a call type indicator indicating a call origination, six digits encoded in about 20 bits normally allocated to the last 6 call digits to the HLR/AC. Because the call type is designated as a call origination, the bits normally for the six call digits are reallocated to extend the random number. In response to receiving the communication including a random number, the call type indicator indicating a call origination, the six digits, and the authorization response, the HLR/AC calculates the keys essentially using the extended random number, which may include about 52-bits based on the assumptions specified above.
According to yet another example embodiment, a challenge is created by setting the call digits to a value a user would probably never enter and establish a call with. For example, certain values of call digits, such as six 0 digits, for example, rarely or never are used. Another example of rarely used values for the six digits is 01101 since 011 is used in many places in the world to place an international call and thus, it is unlikely that a 011 will be followed by another 011 or 01. According to this embodiment, the challenge may be a 32 bit random number RAND or a sequence number. Since a CDMA call would not likely be placed using call digits 01101, then no SMEKEY or PLCM keys would be created with any of the random challenges. Hence, a predictable sequence number with a rarely used call digit pattern may be used according to this example embodiment. By providing a newly created protocol using such a random number and call digit combination, this example embodiment ensures the new protocol uses random number and call digit combination securely.
The 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.

## Claims

1. A method performed by a mobile equipment to communicate with a network that leverages ANSI-41 security protocols, the method comprising:
receiving a first challenge from the network, the first challenge having associated last call digits, the last call digits being a number that is not callable by the mobile equipment;
calculating a network authorization response and network keys based on the received first challenge, the bits of the last call digits, and a value of a secret key stored in a user identity module (UIM) of the mobile equipment;
generating a second challenge;
calculating a mobile authorization response and mobile keys based on the generated second challenge, the last call digits, and the value of the secret key stored in the UIM of the mobile equipment; and
obtaining an authentication key based on the mobile keys and the network keys calculated by mobile equipment.

2. The method of claim 1, further comprising:
generating a mobile authentication code based on the authentication key;
sending the second challenge, the network authorization response, the mobile authorization response, and the mobile authentication code to the network;
receiving a network message authentication code from the network;
generating a network message authentication code based on the first challenge and the authentication key; and
establishing a communication channel between the mobile equipment and the network if the network message authentication code received from the network matches the network message authentication code generated at the mobile equipment.

3. The method of claim 1, wherein the UIM of the mobile equipment calculates the network authorization response and network keys using the first challenge and a value of a secret key stored in the UIM of the mobile equipment, and calculates the mobile authorization response and mobile keys using the second challenge and a value of a secret key stored in the UIM of the mobile equipment.

4. The method of claim 1, wherein the mobile keys and the network keys include a PLCM and SMEKEY, the authentication key includes a hash of the mobile keys and the network keys, and the first and second challenges are first and second random numbers.

5. The method of claim 1, wherein the UIM of the mobile equipment receives the first challenge along with a call type indicator indicating a call origination and last call digits, bits of the last call digits being allocated to the first challenge to effectively increase a length of the first challenge, and calculates the network authorization response and the network keys using the first challenge, the bits of the last call digits, and a value of a secret key stored in the UIM of the mobile equipment, and receives the second challenge along with the call type indicator indicating a call origination and last call digits, bits of the last call digits being allocated to the second challenge to effectively increase a length of the second challenge, and calculates the mobile authorization response and the mobile keys using the second challenge, the bits of the last of the last call digits, and a value of a secret key stored in the UIM of the mobile equipment.

6. The method of claim 1, wherein at least one of a pair of the first challenge and network authorization response and a pair of the second challenge and mobile authorization response are used in a subsequent protocol run to generate a PLCM and SMEKEY.

7. A method performed by a mobile equipment to communicate with a network that leverages ANSI-41 security protocols, the method comprising:
generating a first challenge;
calculating a mobile authorization response and mobile keys based on the generated first challenge, bits of a last call digits received by a user identity module (UIM) of the mobile equipment, and a value of a secret key stored in the UIM of the mobile equipment, the last call digits being a number that is not callable by the mobile equipment;
sending the first challenge and the mobile authorization response to the network;
receiving a second challenge along with the last call digits from the network;
calculating a network authorization response and network keys based on the second challenge, the last call digits, and the value of the secret key stored in the UIM of the mobile equipment; and
generating an authentication key based on the mobile keys and network keys calculated by the mobile equipment.

8. The method of claim 7, further comprising:
receiving a network message authentication code from the network;
generating a network message authentication code at the mobile based on the second challenge and the authentication key;
generating a mobile message authentication code if the generated network message authentication code matches the received network message authentication code; and
sending the mobile message authentication code to the network in an attempt to establish a mutually authenticated communication channel with the network.

9. The method of claim 7, wherein the UIM of the mobile equipment calculates the mobile authorization response and mobile keys using the first challenge and a value of a secret key stored in the UIM of the mobile equipment, and calculates the network authorization response and network keys using the second challenge and a value of a secret key stored in the UIM of the mobile equipment.

10. The method of claim 7, wherein the mobile keys and the network keys include a PLCM and SMEKEY, the authentication key includes a hash of the mobile keys and the network keys, and the first and second challenges are first and second random numbers.

11. The method of claim 7, wherein the UIM of the mobile equipment receives the first challenge along with a call type indicator indicating a call origination and call digits, bits of the call digits being allocated to the first challenge to effectively increase a length of the first challenge, and calculates the mobile authorization response and the mobile keys using the first challenge, the additional bits, and a value of a secret key stored in the UIM of the mobile equipment, and receives the call type indicator indicating a call origination and additional bits allocated to the second challenge to effectively increase a length of the second challenge along with the second challenge, and calculates the network authorization response and the network keys using the second challenge, the additional bits allocated to the second challenge, and a value of a secret key stored in the UIM of the mobile equipment.

12. The method of claim 7, wherein at least one of a pair of the first challenge and mobile authorization response and a pair of the second challenge and network authorization response are used in a subsequent protocol run to generate a PLCM and SMEKEY.

13. A method performed by a network to communicate with a mobile equipment that leverages ANSI-41 security protocols, the method comprising:
sending a first challenge and last call digits to the mobile equipment, the last call digits being a number that is not callable by the mobile equipment;
receiving a network authorization response, a second challenge, the last call digits and a mobile authorization response from the mobile equipment, the network authorization response being based on the first challenge, the last call digits, and a value of a secret key associated with the mobile equipment;
calculating network keys based on the network authorization response received from the mobile, the last call digits, a value of a secret key stored in an authentication center associated with the network and the first challenge;
calculating mobile keys based on the second challenge, the last call digits, the value of the secret key stored in the authentication center and the mobile authorization response; and
generating an authentication key based on the networks keys and the mobile keys calculated by the network.

14. The method of claim 13, further comprising:
receiving a mobile message authentication code from the mobile equipment; generating a mobile message authentication code at the network based on the second challenge and the authentication key;
generating an network message authentication code if the generated mobile message authentication code matches the mobile message authentication code received from the mobile equipment; and
sending the network message authentication code to the mobile equipment in an attempt to establish a mutually authenticated communication channel with the mobile equipment.

15. The method of claim 13, wherein the mobile keys and the network keys include a PLCM and SMEKEY, the authentication key includes a hash of the mobile keys and the network keys, and the first and second challenges are first and second random numbers.

16. The method of claim 13, wherein the authentication center of the network receives the first challenge along with a call type indicator indicating a call origination and last call digits, bits of the last call digits being allocated to the first challenge to effectively increase a length of the first challenge, and calculates the network authorization response and the network keys using the first challenge, the last call digits, and a value of a secret key stored in the authentication center of the network, and receives the second challenge along with call type indicator indicating a call origination and last call digits, bits of the last call digits being allocated to the second challenge to effectively increase a length of the second challenge, and calculates the mobile authorization response and the mobile keys using the second challenge, the last call digits, and a value of a secret key stored in the authentication center.

17. A method performed by a network to communicate with a mobile equipment that leverages ANSI-41 security protocols, the method comprising:
receiving a first challenge, last call digits and a mobile authorization response, the last call digits being a number that is not callable by the mobile equipment;
calculating mobile keys based on the first challenge, the last call digits, a value of a secret key stored in the authentication center associated with the network and the mobile authorization response;
generating a second challenge and a network authorization response;
calculating network keys based on the second challenge, the last call digits, the value of the secret key stored in the authentication center and the network authorization response; and
generating an authentication key based on the mobile keys and network keys calculated by the network.

18. The method of claim 17, further comprising:
generating a network message authentication code based on the second challenge and the authentication key;
sending the second challenge and the network message authentication code to the mobile equipment;
receiving a mobile message authentication code from the mobile equipment;
generating a mobile message authentication code at the network based on the first challenge and the authentication key; and
establishing a mutually authenticated communication channel with the mobile equipment if the received mobile message authentication code matches the mobile message authentication code generated at the network.

19. The method of claim 17, wherein generating the second challenge and the network authorization response comprises:
sending a unique challenge request from a HSS/VLR to a HLR/AC of the network;
receiving unique random number and a network authorization response from the HLR/AC of the network; and
concatenating the unique random number with a mobile equipment identification number to generate the second challenge.

20. The method of claim 16, wherein the mobile keys and the network keys include a PLCM and SMEKEY, the authentication key includes a hash of the mobile keys and the network keys, and the first challenge and the second challenge are a first random number and a second random number.

21. The method of claim 17, wherein the authentication center of the network receives a call type indicator indicating a call origination and additional bits allocated to the first challenge to effectively increase a length of the first challenge along with the first challenge, and calculates the mobile authorization response and the mobile keys using the first challenge, the additional bits, and a value of a secret key stored in the authentication center, and receives the call type indicator indicating a call origination and additional bits allocated to the second challenge to effectively increase a length of the second challenge along with the second challenge, and calculates the network authorization response and the network keys using the second challenge, the additional bits allocated to the second challenge, and a value of a secret key stored in the authentication center.