Obscuring temporary user equipment identities

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74769-2117

OBSCURING TEMPORARY USER EQUIPMENT IDENTITIES

BACKGROUND

I, Field (0002( The present disclosure relates generally to communication, and more specifically to techniques for obscuring identities in wireless communication.

IL Background (0003( Wireless communication networks are widely deployed to provide various communication sendees such as voice, video, packet data, messaging, broadcast, etc, A wireless communication network, may include many Node Bs (or base stations) that may communicate with many user equipments (UEs). The UEs may be assigned various identifiers or identities (IDs) used to uniquely identify these UEs for various purposes.

In certain instances, the UE IDs may be sent over the air in the clear without any ciphering. This may make it possible for an eavesdropper or attacker to mount a linkability attack by monitoring a. communication channel for messages and determining which messages are directed to the same UE over time. The linkability attack may be able to link messages to specific UEs but may not be able to determine the true identities of the UEs. The linkability attack may be used to track the locations of the UEs and may also be the basis of other more severe security attacks. For example, the attacker may be able to determine which UE ID is assigned to a particular UE by initiating a call to that UE and observing which UE IDs are used at approximately the same time.

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74769-2117 (0004J There is therefore a need in the art for techniques to combat linkability attacks without imposing excessive computational burdens on the UEs and network entities.

SUMMARY (0005} Techniques for concealing temporaiy IDs assigned to UEs by a wireless communication network are described herein. These techniques may be used for various types of messages addressed to specific UEs and sent in the clear without ciphering via common channels. These techniques may be used to improve security, e.g., to foil linkability attacks.

(0006} tn an aspect, at a network entity (e.g., a Node B), a first ID assigned to a UE may be transformed to obtain a second ID for the UE. The first ID may be a Radio Network Temporary Identifier (RNTI) assigned to the UE in Universal Mobile Tel ecommunication System (UMTS) or some other type of ID in some other communication system. The first ID and possibly a salt value (which is a non-static value) may be transformed based on a hash function to obtain the second ID. An output message directed to the UE may be generated based on an input message, the second ID, and the salt value (if present). The input message may be a paging message, a scheduling message carrying scheduling information, a resource assignment message, etc. The output message may be sent via a common channel shared by the UE and other UEs.

(0007} In another aspect, at the UE. a message may be received via the common channel, and a salt value (if sent) may be obtained from die received message. The first ID and the salt value (if sent) may be transformed to obtain the second ID, which may be used to determine whether the received message is intended for the UE.

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2a

According to another aspect of the present invention, there is provided a network entity for sending signalling messages to user equipments, comprising: means for transforming a first identifier, ID, assigned to a user equipment, UE, to obtain a second ID for the UE, wherein the means for transforming the first

ID comprises means for hashing the first ID and a salt value to obtain the second ID; means for generating an output message directed to the UE based on an input message, the second ID, and the salt value; and means for sending the output message via a common channel shared by the UE and other UEs.

According to yet another aspect of the present invention, there is provided a method comprising: transforming, at a network entity, a first identifier,

ID, assigned to a user equipment, UE, to obtain a second ID for the UE, wherein transforming the first ID comprises means for hashing the first ID and a salt value to obtain the second ID; generating, said network entity, an output message directed to the UE based on an input message, the second ID, and the salt value; and sending, from said network entity, the output message via a common channel shared by the UE and other UEs.

According to still another aspect of the present invention, there is provided a computer-readable medium having stored thereon statements and instructions that when executed by a processor carry out the method summarized above.

According to yet another aspect of the present invention, there is provided a user equipment for receiving signalling messages, comprising: means for receiving a message via a common channel shared by a plurality of user equipments, UEs; means for transforming a first identifier, ID, assigned to a UE to obtain a second

ID for the UE, wherein the means for transforming the first ID comprises means for obtaining a salt value from the received message, and means for hashing the first ID and the salt value to obtain the second ID; and means for determining whether the received message is intended for the UE based on the second ID.

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2b

According to a further aspect of the present invention, there is provided a method comprising: receiving, at a user equipment, a message via a common channel shared by a plurality of user equipments, UEs; transforming, at a user equipment, a first identifier, ID, assigned to a UE to obtain a second ID for the UE, wherein transforming the first ID comprises means for obtaining a salt value from the received message, and means for hashing the first ID and the salt value to obtain the second ID; and determining, at a user equipment, whether the received message is intended for the UE based on the second ID.

According to another aspect of the present invention, there is provided a computer-readable medium having stored thereon statements and instructions that when executed by a processor carry out the method summarized above.

[0008] Various aspects and features of the disclosure are described in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 shows a UMTS network.

[0010] FIG. 2 shows transmissions for High-Speed Downlink Packet Access (HSDPA).

[0011] FIGS. 3A and 3B show two designs for transforming an RNTI.

[0012] FIG. 4A shows a processor that sends a transformed RNTI in the clear.

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<td> (0013]</td><td> FIG</td>

<td> (00141</td><td> FIG.</td>

<td> (0015]</td><td> FIG</td>

<td> (0016]</td><td> FIG</td>

<td> (0017]</td><td> FIG</td>

<td> (0018]</td><td> FIG</td>

4B shows a processor that embeds a transformed RNTl in a message.

shows a process for sending signaling messages to a UE.

shows an apparatus for sending signaling messages to a. UE.

shows a process for receiving signaling messages at a UE.

S shows an apparatus for receiving signaling messages at a. UE.

shows a block diagram of a UE, a Node B, and an RNC.

DETAILED DESCRIPTION (0019] The techniques described herein may be used for various wireless communication networks such as Code Division Multiple Access (CDMA) networks. Time Division Multiple Access (TDM.A) networks, frequency Division Multiple Access (FDMA) networks. Orthogonal FDMA (OFDMA) networks, Single-Carrier FDMA (SC-FDMA) networks, etc. The terms “networks” and “systems” are often used interchangeably. A CDMA network may implement a radio technology such as Universal Terrestrial Radio Access (UTRA), Evolved UTRA (E-UTRA), cdma2ÛÛO, etc. UTRA and E-UTRA are part of UMTS. UTRA includes Wideband-CDMA (WCDM A) and Low Chip Rate (LCR). cdma2000 covers IS-2Û00, ÎS-9S and IS-856 standards. A TDMA network may implement a radio technology such as Global System for .Mobile Communications (GSM). An OFDMA network may implement a radio technology such as Long Term Evolution (LTE), IEEE 802.20, Flash-OFDM.®, etc, UTRA, E-UTRA, UMTS, GSM and LTE are described in documents from an organization named “3rd Generation Partnership Project” (3GPP). cdma2000 is described in documents from an organization named “3rd Generation Partnership Project 2” (3GP.P2). These various radio technologies and standards are known in the art, For clarity, certai n aspects of the techniques are described below for UMTS, and 3 GPP terminology is used in much of the description below.

(0020j FIG, t shows a UMTS network. 1.00 that includes a Universal Terrestrial

Radio Access Network (UTRAN) and a core network 140. The UTRAN includes multiple Node Bs 110 and a Radio Network Controller (RNC) '130. A Node B is generally a fixed station that communicates with the UEs and may also be referred to as an enhanced Node B, a base station, an access point, etc. Each Node Β Π0 provides communication coverage for a particular geographic area and supports communication for the UEs located within the coverage area. The term “cell” can refer to a Node B

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PCT/US2007/061939 and/or its coverage area depending on the context in which the term is used. RNC 1 30 couples to NodeBs 110 and provides coordination and control for these Node Bs. RNC 130 also originates and terminates messages for certain protocols and applications.

Core network 140 may include various network entities that support various functions such as packet routing, user registration, mobility management, etc.

[0021] UEs 120 may be dispersed throughout the UMTS network, and each UE may be stationary or mobile. A UE may also be referred to as a mobile station, a terminal, an access terminal, a subscriber unit, a station, etc. A UE may be a cellular phone, a personal digital assistant (PDA), a wireless device, a handheld device, a wireless modem, a laptop computer, etc. A UE may communicate with one or more Node Bs on the downlink and/or uplink at any given moment The downlink (or forward link) refers to the communication link from the Node Bs to the UEs, and the uplink (or reverse link) refers to the communication link from the UEs to the Node Bs.

[0022] In UMTS, data and signaling for the UEs are processed as logical channels at a Radio Link Control (RLC) layer. The logical channels include a Dedicated Traffic Channel (DTCH), a Downlink. Shared Channel (DSCH), a Dedicated Control Channel (DCCH), a Common Control Channel (CCCH), etc·. The logical channels are mapped to transport channels at a Medium Access Control (MAC) layer. The transport, channels carry data for various services such as voice, video, packet data, etc. The transport, channels are mapped to physical channels at a physical layer. The physical channels are channelized with different channelization codes and are orthogonal to one another in the code domain.

10023 J A UE in UMTS may be assigned a variety of IDs used to identify the UE for various purposes. These UE IDs may have different context or scope (e.g., cell, paging area, etc.) and/or different life spans (e.g., temporary'· or permanent). For example, the UE may be assigned various RNTIs that may be used as temporary IDs. Table 1 lists some RNTIs that may be assigned to the UE and provides a short description of where each RNTI. may be used. The C-RNTÏ and U-RNTI. may be assigned to the UE by a serving RNC and may be scoped to an RRC connection within a particular cell. The CRNTI may be used for messages sent on the DTCH and DSCH. The U-RNTI may be used for paging messages sent on a Paging Channel (FCH) and for messages sent on the DCCH. The DSCH-RNTT, H-RNTI and E-RNT.I may he scoped to a particular cell and used for signaling messages sent on the DSCH, a High Speed Downlink Shared Channel

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PCT/US2007/061939 (HS-DSCH), and an E-DCH Absolute Grant Channel (E-AGCH), respectively. These various RNTTs may be collectively referred to as “X-RNTIs’’ and may he used as temporary IPs in local context to address the UE for signaling messages sent by Radio Resource Control (RRC) and MAC protocols. The Χ-RNTïs may be assigned by different network entities within the UTRAN (or simply, the UTRAN). Each X-RNTI may be used for signaling messages exchanged between the assigning network entity and the recipient UE,

Table 1

<td> Symbol</td><td> Name</td><td> Length</td><td> Use</td>

<td> C-RNTI</td><td> Cell RNTI</td><td> 16 bits</td><td> Used for messages sent on DT'CH and DSCH</td>

<td> U-RNTI</td><td> UTRAN-RNTI</td><td> 32 bits</td><td> Used for paging messages sent on PCH and for messages sent on DCCH</td>

<td> DSCH-RNTI</td><td> DSCH Radio Network Identifier</td><td> 16 bits</td><td> Used for signaling messages sent on DSCH</td>

<td> H-RNT1.</td><td> HS-DSCH Radio Network Identifier</td><td> 16 bi ts</td><td> Used for signaling messages sent on HS-DSCH</td>

<td> E-RNTX</td><td> E-DCH Radio Network Identifier</td><td> 16 bits</td><td> Used for signaling messages sent on E-AGCH</td>

(0024] The X-RN'ITs may be assigned to a UE at various times by the UTRAN.

The assignments may take place via unciphered signaling due to the absence of a preexisting security relationship between the UTRAN and UE at the time of assignment. However, in the assignment of an X-RNTI, the UTRAN typically addresses the UE by a Temporary Mobile Subscriber Identity (TMSI) or a Packet TMSI (P~TMSI), which is assigned to the UE in ciphered signaling at a Non Access Stratum (NAS) layer. Thus, an attacker may observe what X-RNTI was assigned by a downlink message but, in the absence of additional knowledge of the TMSI or P-TM.SI, would not be able to determine which UE was receiving the assignment.

(0025] Once an X-RNTI is assigned to a UE, the X-RNTI may be sent in the clear without ciphering in downlink and/or uplink signaling. Tor example, messages for specific UEs may be sent on the CCCH and addressed to the recipient UEs by their URNTIs. These messages may be sent on signaling radio bearer 0 (SRBG) and would be enciphered since SRB0 may carry messages for UEs that do not already have a security

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PCT/US2007/061939 relationship with the UTRAN. For messages sent unciphered on a common channel, an attacker may be abl e to determine that a message was directed toward a particular X.RNTI or UE. While the attacker may not know the identity of this UE outside of the radio context, the available information may make it possible to aggregate information about messages directed to the same UE in a so-called “linkability attack”. The attacker may monitor unciphered scheduling information sent on. a control channel and may be able to determine data transmissions addressed the same UE. The attacker may then potentially track individual UEs’ mobility between cells during a data session. In any case, sending messages in the clear on a common channel may result in a security vulnerability that may lead to more serious security threats.

10026} Techniques for reducing security vulnerability due to transmission of messages in the clear on a common channel are described herein. These techniques may be used for various signaling messages sent at various layers. The techniques may also be used for the downlink and uplink. For clarity, the techniques are described below for transmission of scheduling information and paging messages on the downlink, in UMTS.

[0027] 3GPP Release 5 and later supports HSDPA, which is a set of channels and procedures that enable high-speed packet data transmission on the downlink. For HSDPA., a Node B sends data on the HS-DSCH, which is a downiinktransport channel that is shared by all UEs in both time and code. The HS-DSCH may carry' data for one or more UEs in each transmission time interval (TIT). For HSDPA, a .10 millisecond (ms) frame is partitioned into five 2-ms subframes, each subframe covers three time slots, and each time slot has a duration of0.667 ms. For HSDPA, a TTÏ is equal to one subframe and is the smallest unit of time in which a UE may be scheduled and served. The sharing of the HS-DSCH is dynamic and may change from TTÏ to TTÏ. Data for the HS-DSCH is sent, on a High Speed Physical Downlink Shared Channel (HSPDSCH), and signaling for the HS-PDSCH is sent on a Shared Control Channel for HSDSCH (HS-SCCH).

[0028} For HSDPA, a Node B may use up to fifteen 16-chip channelization codes with spreading factor of 16 for the HS-PDSCH. The Node B may also use any number of 128-chip channelization codes with spreading factor of 128 for the HS-SCCH. The number of 16-chip channelization codes for the HS-PDSCH and the number of 128-chip channelization codes for the HS-SCCH are configurable. The channelization codes for

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PCT/US2007/061939 the HS-PDSCH and HS-SCCH are orthogonal variable spreading factor (OVSF) codes that may he generated in a structured manner. The spreading .factor (SP) is the length of a channelization code. A symbol is spread with a channelization code of length SF to generate SF chips for the symbol.

(0029] In the following description, HSDPA is considered as having (a) up to fifteen HS-PDSCHs, with each HS-PDSCH corresponding to a different Id-chip channelization code, and (b) any number of HS-SCCHs, with each HS-SCCH corresponding io a different 128-chip channelization code. A. UE may be assigned up to four HS-SCCHs at call setup and may monitor the assigned HS-SCCHs during the call. The UB may he assigned up to fifteen HS-PDSCHs in a given TTI. The HS-PDSCHs may be dy namically assigned and conveyed to the UE via signaling sent on one of the HS-SCCHs assigned to the UE.

(0030] FIG. 2 shows example transmissions on the HS-SCCHs and HS-PDSCHs for HSDPA. A Node B may serve one or more UEs in each TTI. The Node B sends a signaling message for each scheduled UE on the HS-SCCHs and sends data to the UE on the HS-PDSCHs two slots later. The signaling messages sent on the HS-SCCHs are addressed to specific UEs based on the H-RNTIs assigned to these UEs. Each UE that, might receive data on the HS-PDSCHs processes its assigned HS-SCCHs in each TTI to determine whether a signaling message has been sent for that UE. Each UE may match the signaling messages received on the HS-SCCHs with its H-RNTI to determine whether any signaling message is intended for that UE. Each UE that is scheduled in a given TTI may process the HS-PDSCHs to recover the data sent to that UE.

[0031] In file example shown in FIG, 2, a UE of interest (UE #1) monitors four HSSCCHs #1 through #4 assigned to dre UE. UE #1 Is not scheduled in TT.I », and no signaling messages are sent to UE #1 on any HS-SCCHs. UE #1 is scheduled in TTI « * I, and a signaling message is sent to the UE on HS-SCCH #1. The signaling message may convey various parameters for the transmission sent in this TTI. UE #1 is not scheduled in TTI n+2, is scheduled t.n TTI ft+ 3 and receives a signaling message on HS-SCCH #2, and is not. scheduled in TTI. n+4.

(0032] Other RNTIs may be used for other signaling messages sent to specific UEs on common channels. For example, assignment messages seat on the E-ÀGCFI are addressed to specific UEs based on the E-RNTIs assigned to these UEs. Paging messages are addressed to specific- UEs based on the U-RNTIs assigned to these UEs.

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[00331 In general, an X-RNTI may be sent in a signaling message transmitted on the downlink and may be used by each UE to match against its own X-RNTI to determine whether the signaling message is Intended for that. UE, i.e,, to ascertain “is this message for me?” All of the information in the X-RNTI may not he necessary for this matching process since the possible space of X-RNTI values may not be full. The N-RNTl may be 16 bits (or 32 bits) long and may provide .identities for many more UEs than a Node B may be able to address at any one time. For example, the Node B may have assigned only 1024 identities in the range of 0 to 1023. In this case, only .10 least significant bits (LSBs) of the X-RNTI may be used to uniquely identify a given UE. The Node B may then send random values for the higher bits and allow each UE to recognize messages directed to that UE by looking at only the lower bits, which contain the “real” portion of the identity. Sending random values for the higher bits may result in many different XRNTI values being sent for any given UE, which may mitigate linkability attack. However, this “truncation” scheme Is essentially transparent. An attacker who Is aware of the scheme can trivially penetrate it and examine the lower bits to determine which messages are addressed to the same UEs.

[0034] In an aspect, an X-RNTI may be transformed based on a function, and a transformed RNTI (instead of the original X-RNTI) may be sent in a signaling message. A. UE that already knows the X-RNTI may be able to determine whether the signaling message is intended for the UE based on the transformed RNTI. However, an attacker without prior knowledge of the X-RNTI may be unable to determine the original XRNTI based on the transformed RNTI sent in the message. The transformation may be performed in various manners,

[0035J FIG. 3A shows a design for transforming an X-RNTI. A unit 310 receives the X-RNTI, transforms the X-RNTI based, on a transform fonction //, and provides a transformed RNTI that is denoted as //(X-RNTI). Tire transform function may be a irreversible fonction that makes it difficult to determine the original X-RNTI from the transformed RNTL For example, the transform function may be a cryptographic/secure hash function that maps a. message (e,g„ tine X-RNTI) to a digest (e.g., the transformed RNTI) and has cryptographic properties so that (i) the fonction between the message and its digest is irreversible and (ii) the likelihood of two messages mapping to the same digest is very small. The output of the hash fonction may be referred to as a digest, a signature, a hashed value, etc.

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10036} The transformed RNTI may be sent in a signaling message and may allow for matching of the message by the UEs. Each UE may apply the same transform function to its X-RNTI to obtain a transformed RNTI. Each UE may then match the transformed RNTI in a received message with the locally generated transformed RNTI to determine whether the message is intended for that UE.

10037} The transformed RNTI may prevent an attacker from inferring the original X-RNTI. However, if the same transformed RNTI is included in each signaling message sent to a given UE, then the attacker may perform a correlation attack. To prevent this, the transformed RNTI may be changed with each message.

(0038} FIG, 3R shows a design for transforming an X-RNTI to obtain different transformed RNTls. A unit 320 receives the X-RNTI and a salt value o, transforms the X-RNTI and the salt value based on a transform function /./«, and provides a transformed RNTI that is denoted as //«(X-RNTI). The transform function may be a irreversible function, a cryptographic hash function, etc. A salt value is a non-static value that may be selected in any manner. Different salt values may be used for different signaling messages so that a single X-RNTI may give rise to different transformed RNTls for different messages.

(0039} A transformed .RNTI and a salt value σ may be sent in a signaling message and may allow for matching of the message by the UEs. The salt value σ may be sent in the clear along with the transformed RNTI. The salt value σ and/or the transformed RNTI may also be embedded in the signaling message, in any case, each UE may match its X-RNTI against the transformed RNTI in the signaling message. Each UE may apply the transform function //« to its X-RNTI and tlie salt value σ extracted from the message and may then compare the locally generated transformed RNTI to the received transformed RNTI in the signaling message.

(0040} FIG. 4A shows a block diagram of a design of a message processor 410 that sends a transformed RNTI in the clear in a signaling message. Message processor 410 receives an input message and an X-RNTI for a recipient. UE and generates an output message directed to the UE.

(004.1.} Within message processor 41.0, a unit 420 receives an X-RNTI and possibly a salt value σ, applies a transform function on the X-RNTI and possibly the salt value σ, and provides a transformed RNTI. A. multiplexer (Mux) 422 multiplexes the transformed RNTI, the salt value σ (if present), and an input message. An encoder 424

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PCT/US2007/061939 encodes the output of multiplexer 422 and provides an output message. Message processor 410 may be used for paging messages sent on die PCH. In this case, the UE ID or X-RNTI in TIG. 4A. may correspond to the U-RNTI.

]0042] FIG. 4B shows a block diagram of a design of a message processor 450 that embeds a transformed RNTI in a signaling message. Message processor 450 receives an input message and an X-RNTI for a recipient UE and generates an output message directed to the UE. The input message may comprise various pieces of information. 111043] Within message processor 450, a unit 460 receives an X-RNTI and possibly a salt value σ, applies a transform function on the X-RNTI and possibly the salt value o, and provides a transformed RNTI. A multiplexer 462 receives and multiplexes signaling information X_a and X], and the salt value σ (if present) and provides multiplexed information Xj. An encoder 464 encodes the multiplexed information Xi and provides coded information. A unit 466 masks the coded information based on the transformed RNTI and provides masked information Sj. A multiplexer 472 receives and multiplexes signaling information X_c through X'r and provides multiplexed information X₂. A unit. 474 generates a cyclic redundancy check (CRC) based on information Xj. and X2, then masks the CRC with the transformed RNTI to obtain a UEspecific CRC, and appends the UE-specific CRC to information Xj. An encoder 476 encodes the output of unit 474 and provides coded information R2. A multiplexer 478 receives and multiplexes the masked information Si and the coded information R₃ and provides the multiplexed information S₃ and R₃ as an output message.

(0044] Message processor 450 may be used for signaling messages sent on the HSSCCH's. In this case, X» may comprise channelization code set information, X_b may comprise modulation scheme information, X_c may comprise transport block size information, 3G may comprise HARQ process information, may comprise redundancy and constellation version information, X_t· may comprise new data indicator information, and the X-RNTI may correspond to the Η-RNTI. Information Si may be sent in the first slot of a TTI, and information R₃ may be sent in the last two slots of the TTI. In this case, the salt value 0 may be multiplexed with information X_a and Xi&gt; sent in the first slot of the TTI, as shown in TIG. 413. This may allow for early detection of the signaling message by the UEs, without having to wait for the entire message to be received.

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PCT/US2007/061939 (0045( FIG, 4A shows a design in which a transformed RNTI is sent in the clear in a signaling message. The transformed RNTI may also be sent in the clear in other manners. FIG, 4B shows a design in which a transformed RNTI is embedded in a signaling message. The embedding of the transformed RNTI may also be achieved in other manners. For example, a signaling message sent on the E-AGCH may include a UE-speciEc CRC that may be generated based on a transformed E-RNTÏ. In general, a transformed RNTI may be sent in various manners (e.g., in the clear or embedded) in a signaling message such that a recipient UE can identity the message as being directed to that UE.

(0046( As shown in FIG. 2, a UE may receive multiple (e.g., up to four) signaling messages in each TTI and may check each received message to determine whether the message is intended for the FIE. The transform function should be computationally simple so that the UE can apply the transform functionfor each received message without adversely impacting performance. Ideally, the message matching with the transformed RNTI should require only few additional instructions beyond what is normally done to match the X-RNTI.

(0047( For the design shown in FIG. SB, a UE may store a look-up table of transformed RNTIs obtained by hashing its X-RNTI with all possible salt values. The transformed RNTIs .may thus be pre-computed once and stored for later use, instead of being computed whenever signaling messages are received. For each received message, the UE may extract the salt value from the received message, retrieve the transformed RNTI for that salt value from the look-up table, and check the received message with the retrieved transformed RNTI.

(0048( The UTRAN .may assign a new X-RNTI to a UE via ciphered signaling at the start of a call and possibly during the call. Transformed versions of the new XRNTI may be sent over the air freely since only tire UE has tire unhashed version. An attacker may not have sufficient information to perform matching of signaling messages sent with the transformed RNTIs. The attacker may monitor all signaling messages in a cell over a period of time and correlate these signaling messages by maintaining a database of all possible X-RNTIs and checking each recei ved message against, all of the X-RNTIs. This type of determined eavesdropping may be combated by periodically assigning new X-RNTIs to the UEs.

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PCT/US2007/061939 (0049] Various transform functions may be used to generate transformed RNTIs. In general, a transform function should have the following qualities;

* Easy and quick computation (or amenable to a look-up table at the UE);

* Transformed RNTI and salt, value should be small;

* Difficult or impossible to reverse; and * Easy for the UTRAN to guard against collisions.

(0050] A tradeoff may be made among the qualities listed above for a given application. Different transform functions with different characteristics may be used for different applications. For example, signaling at Layer 2 may favor high bit-efficiency and quick decoding and may be able to accept a lower level of security as a. result. Signaling at Layer 3 may favor stronger security at the expense of greater overhead. (005.1.] The importance of a highly irreversible function may depend on the level of determination assumed on the part of an attacker. A transform function may simply mask out a few bits in varying positions from the Χ-RNTï and may use the salt value to select the masked bi is. This transform function may be susceptible to a brute-force attack in which the attacker collects signaling messages, tries all possible values for the deleted bits, and watches to see which ones of the resulting values are repeated. The attacker may presume that the repeated values are the real X-RNTLs of various UEs and may store these values fox' testing against future signaling messages. However, this may represent a significantly more laborious attack than the casual eavesdropping normally associated with linkability attacks.

(0052] Even if a transform function is somewhat weak cryptographically, the UTRAN may assign new X~RNTIs via. ciphered signaling. In this case, an attacker may not automatically have a pool of known X-RNTÏs to test received messages against In light of the ability to assign new X-RNTls, the cryptographic strength of the transform function may be considered less important than simple computation and over-the-air bit conservation.

(0053] A transform function may be defined based on various designs. For example, a. transform function may mcorporate design principles used In cryptographic/ secure hash functions such as SHA-1 (Secure Hash Algorithm), SHA-2 (which includes SHA-224, SHA-256, SHA-384 andSHA-512), MD-4 (Message Digest), MD-5, or other secure hash algorithms known in foe art. In one design, a single transform function is used and known ct priori by both foe UTRAN and foe UEs. In another design, a set of

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PCT/US2007/061939 transform functions is supported, and one transform function may be selected from the set, e.g., at die start of a call, and conveyed to a UE.

(0054( The length of the salt value σ may be selected based on a tradeoff between overhead and security. A longer salt value may result in more transformed RNTIs for a given X-RNTI, which may improve security at the expense of larger overhead and possibly greater collision probability. The converse may be true for a shorter salt, value. (0055( In one design, a transformed RNTI has the same or approximately the same length as the original X-RNTI. For this design, the salt value σ may be sent using additional bits. In another design, die transformed RNTI and the salt, value σ have the same or approximately the same length as the original X-RNTI in order to maintain the same or approximately the same overhead. For this design, some bits may be “reclaimed” by making the transformed RNTI shorter than the original X-RNTI. For example, the X-RNTI may be 16 bits, the transformed RNTI may be 10 bits, and the salt value may be 6 bits. The X-RNTI, transformed RNTI, and salt value may also have other lengths. The transform function may? be designed to achieve the desired cryptographic properties with the shorter transformed RNTI.

(0056( A collision occurs when two X-RNTIs for two UEs are transformed to the same transformed RNTI. e.g., Zi/x) - /Ζ_σ(χ), where x and y are the two X-RNTIs. The two UEs may have no way to resolve the collision between their transformed RNTIs, The transformed RNTI may be used to send scheduling information to one of the two UEs, e.g., as shown in FIG. 2. The recipient UE may properly detect the scheduling information as being for that UE and may decode the data sent to the UE. The nonrecipient UE may falsely detect tire scheduling information, decode the data destined for the recipient UE, and obtain nonsense result after decrypting (assuming that the data sent on the HS-DSCH was encrypted). In this case, collisions may or may not adversely impact performance, depending on the behavior of the application.

(0057( In general, impact, due to collisions of X-RNTIs may be dependent on the type of signaling being sent with these X-RNTIs. The UTRAN may attempt to prevent collisions in order to avoid possible adverse effects, (0058( In one design for avoiding collisions, the UTRAN selects X-RNTIs and salt values known to have no collisions. The UTRAN may maintain a set of all X-RNTIs assigned or assignable to the UEs. For each possible salt value σ. the UTRAN may generate a set of transformed RNTIs based on the set of X-RNTIs and that salt value.

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The UTRAN may scan the transformed set for duplicates and may reject fois salt value if duplicates are detected. In general, a salt value that causes a collision for certain XRNTIs may still be used for other X-KNTIs. However, in order to simplify implementation, the UTRAN may maintain a list of salt values that result in no duplicates over the entire set of X-RNTIs. The salt values in this list may be selected for use. Collisions may also be avoided in other manners.

(0059( FIG. 5 shows a process 500 performed by a network entity in a wireless communication network to send signaling messages to the UEs. The network entity may be a Node B, an RNC, etc., depending on the signaling messages being sent.

(9969( A first ID assigned to a UE may be transformed to obtain a second ID for the UE (block 512). The first ID may be an RNTÎ. assigned to the UE in UMTS or some other type of ID in some other communication system. The first ID may be transformed based on an irreversible fonction, a hash fonction, or some other function to obtain the second ID. An output message directed to foe UE may be generated based on an input message and foe second ID (block 514). The input message may be a paging message, a scheduling message carrying scheduling information, a. resource assignment message, etc. The output message may be sent via a common channel shared by the UE and other UEs (block 516).

(9061 ( In one design, the first ID and a salt value are bashed to obtain the second ID. The salt value may be sent in the clear in the output message. The salt vaiue may be changed each time the first ID is transformed and may be selected to avoid collisions among ail first IDs assigned to the UEs. The first ID may have a length that may be equal to the combined length of foe second ID and foe salt value.

(9062( In one design, the output message may include the input message and the second ID in the clear, e.g., as shown in FIG. 4A. In another design, foe second ID may be embedded in the output message, e.g., as shown in FIG. 4B. For example, all or a portion of the input, message may be masked with foe second ID to generate the output message. Alternatively, a UE-specific CRU may be generated based on the input message and the second ID, and the output, message may be generated based on the input message and the UE-specific CRC.

(9963( FIG. 6 shows an apparatus 600 for sending signaling messages to the UEs. Apparatus 600 includes means for transforming a fust ID assigned to a UE to obtain a second ID for the UE (module 612), means for generating an output message directed to

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PCT/US2007/061939 the UE based on an input message and tlie second ID (module 614), and means for sending the output message via a common channel shared by the UE and other UEs (module 616). Modules 612 to 616 may comprise processors, electronics devices, hardware devices, electronics components, logical circuits, memories, etc., or any combination thereof.

(0064} FIG. 7 shows a process 700 performed by a UE to receive signaling messages from a wireless communication network. A message may be .received via a common channel shared by a plurality of UEs (block 712). A first ID assigned to the UE may be transformed to obtain a second ID for the UE (block 714). The transformation maybe achieved with a look-up table, hardware, software, firmware, etc. In one design, a salt value may be obtained from the recei ved message, and the first ID and. the salt value may be hashed to obtain the second ID. Whether the received message is intended for the UE may be determined based on the second ID (block 716).. In one design of block 716, a third ID may be obtained from the received message and compared to the second ID to determine whether the received message is intended for the UE. In another design of block 716, a CRC may be generated based on the received message and the second ID, a UE-specific CRC may be obtained from the received message, and the generated CRC may be compared to the UE-specific CRC to determine whether the received message is intended for the UE. The received message may be a paging message, a scheduling message, a resource assignment message, etc. if the received message is a scheduling message, then scheduling information may be obtained from the received message and used to process a data transmission sent to the UE.

(0065} FIG. 8 show's an apparatus 800 for recei ving signaling messages. Apparatus

800 includes means for receiving a message via a common channel shared by a plurality·' of UEs (module 812)., means for transforming a first ID assigned to a UE to obtain a second ID for the LIE (module 814), and means for determining whether the received message is intended for theUE based on the second ID (module816). Modules8I2 to 816 may comprise processors, electronics devices, hardware devices, electronics components, logical circuits, memories, etc·., or any combination thereof.

(0066} FIG. 9 shows a block diagram of a design of UE 120, Node B 110, and RNC 130 in FIG. I. On the uplink, data and signaling to be sent by UE 120 are processed (e.g., formatted, encoded, and interleaved) by a.n encoder 922 and further processed

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PCT/US2007/061939 (e.g,, modulated, channelized, and scrambled) by a modulator (MOD) 924 to generate output chips. A transmitter (TMTR) 932 then conditions (e.g., converts to analog, filters, amplifies, and frequency upconverts) the output chips and generates an uplink signal, which is transmitted via an antenna 934. On the downlink, antenna 934 receives a downlink signal transmitted by NodeB 110. A receiver (RCVR) 936 conditions (e.g,, filters, amplifies, frequency dow.nconve.rts, and digitizes) the .received signal from antenna 934 and provides samples. A demodulator (DEMCD) 926 processes (e.g,, descrambles, channelizes, and demodulates) the samples and provides symbol estimates. A decoder 928 further processes (e.g., deintedeaves and decodes) the symbol estimates and provides decoded data. Encoder 922, modulator 924, demodulator 926, and decoder 928 may be implemented by a modern processor 926. These units mas' perform processing in accordance with the radio technology (e.g., UMTS) implemented by the wireless communication network.

[0067] A controller/processor 940 directs the operation at UE 120. Controller/ processor 940 may perform process 700 in FIG. 7 and/or other processes for the techniques described herein. A memory·' 942 stores program codes and data for U.E 120 and may also store temporary IDs assigned to UE 3 20, or UB IDs.

[00681 HG. 9 also shows a design of Node B 110 and RNC 130. Node B 110 includes a controller/processor .950 that performs various functions for communication with the UEs, a memory 952 that stores program codes and data for Node B 110, and a transceiver 9S4 that supports radio communication with the UEs. ControUer/processor 950 may perform process 500 in FIG. 5 and/or other processes for the techniques described herein. Memory 952 may store temporary IDs assigned to the UEs by Node B 110, or NB UE IDs. RNC· 1.30 includes a controller/processor 960 that performs various functions to support communication for the UEs and a memory 962 that stores program codes and data for RNC 130. Controller/processor 960 may perform process 500 in FIG. 5 and/or other processes for the techniques described herein. Memory' 962 may store tern porary IDs assigned to the UEs served by RNC 130, or RNC UE IDs. [0069j The techniques described herein may be used for signaling messages sent on the downlink and well as the uplink. The techniques may also be used for messages sent via a control plane as well as a user plane. A control plane is a mechanism for carrying signaling for higher-layer applications and is typically .implemented with network-specific- protocols, interfaces, and signaling messages. A user plane is a

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PCT/US2007/061939 mechanism for carrying signaling for higher-layer applications and is typically implemented with open protocols such as User Datagram Protocol (UDF), Transmission Control Protocol (TCP), and Internet Protocol (IP). Messages may be carried as part of signaling in a control plane and as part of data (from a network perspective) in a user plane.

[0070J The techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, firmware, software, or a combination thereof For a hardware implementation, the processing units used to perform the techniques at a given entity (e.g., a UE, a Node B, an KNC, etc.) may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signai processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, electronic devices, other electronic units designed to perform the functions described herein, a computer, or a combination thereof.

[0071J For a firmware and/or software implementation, the techniques may be implemented with modules (e.g., procedures, functions, etc.) that perform the functions described herein. The firmware and/or software codes may be stored in a memory (e.g., memory 942.952 or 962 in FIG. 9) and executed by a processor (e.g,, processor 940, 950 or 960), The memory may be implemented within the processor or external to the processor.

[0072] The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled In the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[00731 WHAT IS CLAIMED IS: