Location Privacy for Cellular Systems; Analysis and Solution

1
Location Privacy for Cellular Systems Analysis
and Solution

• Geir M. Køien
• Telenor RD (Norway) and Agder University College
  (Norway)
• and
• Vladimir A. Oleshchuk
• Agder University College (Norway)

2
Background and Motivation

• The Principals
• User Entity (UE)
• The Mobile Station (MS) w/radio access
• A tamper resistant security module (smartcard
  etc)
• Serving Network (SN)
• Core Network nodes
• Gateways etc
• Network Access Servers
• Access Network
• Radio network controllers
• Access Points (AP)
• Home Server (HS)
• Home Location Register
• Authentication Center etc

3
Background and Motivation

• The 2G/3G Solutions
• 3G Access Security
• The UMTS Authentication and Key Agreement (AKA)
  protocol
• Off-line delegated protocol
• Home network distributes complete session
  credentials to serving network..
• Which executes the network initiated mutual
  entity authentication (USIM and Network)
• Authentication based on knowledge of pre-shared
  secret (only at USIM and HLR/AuC)
• Challenge-Response with signed challenge to
  allow a one-pass scheme
• Location/Identity Privacy
• Permanent identity (IMSI) only used in clear
  when necessary
• After initial identity presentation with IMSI in
  clear
• the AKA protocol is executed
• then encryption/integrity protection started
• then the Serving Network assigns a temporary
  identity (TMSI)
• Subsequent identity presentation with TMSI (in
  clear)

4
Background and Motivation

• Mobility Management
• Cellular Control Model
• Users subscribe to services at mobile operator
  (home environment/operator)
• Infrastructure in control of a central authority
  (the serving network operator)
• Operative control is at Serving Network, while
  administrative (incl. charging) control at Home
• Handovers under network control (performance/QoS
  reasons)
• 2G/3G Mobility Management
• Location Registration (incl. loc. updating)
• No existing UE-SN relationship ? IMSI transferred
  in clear on common channel
• Call to Mobile Station
• Paging (call announcement) is in cleartext on
  broadcast channel (IMSI or TMSI)
• Call from Mobile Station
• Access in cleartext over common channel until
  identity is presented (IMSI or TMSI)
• Handover (HO)

5
UMTS Authentication and Key Agreement

• 3G Security and Privacy
• Principals USIM, SN and HE
• Mutual off-line (delegated) challenge-response,
  executed between SN and USIM
• Confidentiality on all user/control plane data
  and integrity on control plane data
• Limited privacy (IMSI presented in clear, but
  protected TMSI used when possible)

6
Background and Motivation

• Privacy Issues and Location Issues
• 3GPP Privacy Requirements
• User Identity Confidentiality
• The property that the identity cannot be
  eavesdropped over the radio access link
• User Location Confidentiality
• Presence/arrival of a user cannot be detected by
  eavesdropping on the radio link
• User untraceability
• Protection against tracking of users
• Location Issues
• The Serving Network (SN) will necessarily know
  where the subscriber is
• During active calls/session through radio derived
  methods (this is a E112/E911 req.)
• During idle time through the registration (tied
  to a location area)
• The Home Server network will only know which SN
  the UE is attached to
• The UE must depend on infrastructure support to
  determine location
• Satellite (GPS), possibly with SN support
  (kick-start measurements and timing)
• Location can also be provided by SN (commercial
  service)

7
Background and Motivation

• Control and Trust Issues
• Trust Relationships
• UE HS
• UE is a subscriber with the HS. HS has security
  jurisdiction over UE. With current subscription
  models the relationships is relatively long (even
  for pre-paid).
• SN HS
• Mutual relationship based on legally binding
  roaming agreements. Both parties wants to limit
  the trust needed to maintain the relationship.
• UE SN
• No a priori relationship. Relationship created
  on-the-fly with the HS as the mediator.
• Control Issues
• Home Control
• Large no.of serving network operators
• For commercial reasons the operators sign even
  with bad operators
• Particularly problematic in the delegated
  off-line model in current cellular systems
• Remedy On-line authentication (Home
  Subscriber)
• Remedy Spatial home control may be needed for
  large pan-national
• serving networks

8
Enhanced Security and Privacy

• Requirements
• Performance is King (AKA requirements)
• The most critical performance aspect is temporal
  (real-time response)
• Processing time may be significant but Moores
  law is on our side
• Message Propagation Delays Physical laws
• Important to reduce no of round-trips to a
  minimum
• 3-Way AKA
• We have three principals that all should be
  active in the security context
• Security Context Hierarchy
• Long-term contexts is the basis (Roaming
  agreements and Subscription contacts)
• Medium-term contexts needed to establish
  credentials for 3-way context
• Short-term session contexts needed for
  over-the-air protection
• Computational and Communication Balance
• AKA computation should be possible on secure
  device (smartcard)
• Air-interface may have severe capacity
  restrictions during establishment

9
Enhanced Security and Privacy

• The Architectural Context
• Minimizing Total Setup Execution Time
• Typical 3G scenario for initial registration
• UE?SN Access Request (access channels are
  narrow minimal message)
• SN?UE Go to control channel and identify
  yourself
• UE?SN Present IMSI Request to be registered
• SN?HS Request credentials for IMSI
• HS?SN Reply( Authentication Vector )
• SN?UE Challenge( RAND,AUTN )
• UE?SN Response ( RES )
• Identity Presentation, Initial Registration and
  AKA triggered by same event
• Historic reasons that lead to sequential/serial
  procedure execution
• Combined procedures means fewer round-trips
• Location Privacy vs. Spatial Home Control
• Problematic to allow spatial home control and
  provide location privacy
• Spatial resolution important

10
Enhanced Security and Privacy

• The Initiator-Responder Scheme
• Combined Identity Presentation, AKA and Location
  Registration
• Location Registration is invariably triggered by
  the UE
• Combined procedure must therefore be trigger by
  UE
• The Context Reference Identity (CRID)
• To provide location privacy an anonymous identity
  should be used
• Context Reference Identity
• Pseduo-random value created by UE
• Valid for exactly one medium-term 3-way security
  context
• HS must be allowed to learn both CRID and
  permanent identity
• SN shall not learn permanent identity, but will
  know that HS acknowledges CRID
• An additional Temporary Alias Identity (TAID)
• Medium-term context valid for several sessions
• TAID is (pseudo-random) session identity assigned
  by SN
• SN and UE knows (TAID,CRID) association

11
Enhanced Security and Privacy

• Home Control
• Secure Multi-party Computation (SMC)
• HS defines a Validity Area (VA) were UE is
  permitted to be
• UE location (x,y) should not be revealed to HS
• Point-Inclusion scheme allows HS to receive
  privacy protected location E(x,y) and still
  determine if is (x,y) inside VA.
• Spatio-Temporal Binding of Medium-Term Security
  Context
• SN identity tied to context
• HS identity tied to context
• Context Reference Identity (CRID) tied to context
• Area identity (large area) tied to context
• Validity period tied to context

12
Privacy Preserving 3-Way AKA

• Cryptographic basis
• Secure Multi-party Computation (SMC)
• Homomorphic crypto
• Operation X on encrypted data is equivalent to
  some operation Y on cleartext data
• Identity-Based Encryption (IBE)
• Identity string used as public-key key (for
  instance bob_at_operator.net)
• Alice must know system parameters before she can
  encrypt with ID
• Private Key Generator (PKG) creates corresponding
  private key
• Bob receives private key from PKG
• No authentication in basic scheme
• Challenge-Response
• Two-way challenge-response between UE and HS
• Symmetric MAC signed response (based on
  long-term pre-shared secret)
• Diffie-Hellman (DH) Exchange
• DH used for generation of medium-term shared
  secret

13
Privacy Preserving 3-Way AKA

• Secure 2-Party Location Inclusion Protocol
  (S2PLIP)
• The S2PLIP concept
• Executed between SN and HS
• SN provides UE position (x,y), HS provides
  polygon P
• SN does not want HS to learn (x,y) and HS do not
  want to disclose P
• The S2PLIP protocol
• 0. Distribution of public-key pair from Bob (HS)
  to Alice (SN). Use same E/D.
• Location z (x,y) Polygon P ai,bi i
  1,2,..n
• HS?SN E(P)
• SN?HS Please decrypt parameter ? (for some i)
• HS?SN D(?)
• SN?HS Vector of values e
• HS Iff D(e)gt0 for all e then z is inside P
• HS-SN Interface is high capasity
• S2PLIP has few round-trips

14
Privacy Preserving 3-Way AKA

• Outline of the PP3WAKA protocol
• Always initiated by UE
• UE generates CRID
• IBE to provide confidentiality (UE?SN and UE?HS)
• Challenge-Response (UE??HS)
• DH over SN-HS interface
• SMC to protect location while allowing spatial
  home control

15
Privacy Preserving 3-Way AKA

• Outline of the PP3WAKA protocol
• 1 UE prepares PP3WAKA
• Prf(?) ? CRID
• Generate UE?HS challenge/response data incl.
  keys
• HSK HSIDSNIDLONG_TERM_PERIOD (HS IBE
  public key)
• ID HSIDSNIDHashed_Area_CodePERIOD (SN
  IBE public key)
• EID(CRID) ? A
• EHSK(UEID,CRID,Challenge) ? B
• UE?SN (A,B,PERIOD,HSID)
• 2 SN prepares to contact HS
• SN observes UE location (x,y) (we presume
  polygon E(P) present at SN)
• SN generates ?
• SN generates DH value DHA
• C PERIODHAC ? DHA
• SN?HS B,CBKEY

16
Privacy Preserving 3-Way AKA

• 3 HS responds
• Validity of PERIOD verified. HS constructs ID
  and HSK, and generates corresponding private keys
  dID and dHSK.
• Decrypt B. Associate CRID-UEID. Compute response
  to UE.
• Generate challenge to UE. Generate UE-HS shared
  key, and use it to protect data sent to UE.
• EUE-HS key(Challenge, Response,DH secret s) ? D
• In parallel S2PLIP continues (HS return decrypt
  ?))
• HS?SN D,DHB,dID,CRID, ?BKEY
• 4 SN receives HS response and continues setup
  with UE
• SN, which now has dID , decrypts A to get CRID.
• Continue iff (CRIDUE CRIDHS). Compute DH
  secret s. Generate pseudo-random key derivation
  element RNDSN.
• Derive session keys KeyGens(CRID,RNDSN) ? KSN
• Generate TAID protect and bind to CRID
  EKsn(CRID,TAID) ? E
• In parallel S2PLIP continues (SN start
  computation of e-values)
• SN?UE (D,RNDSN,E)

17
Privacy Preserving 3-Way AKA

• 5 UE responds
• Decrypts D. Then verify HS response, and compute
  own response to HS.
• UE accepts s, and generates session keys
  KeyGens(CRID,RNDSN) ? KSN
• KSN is used to decrypt E. UE then gets
  CRID,TAID.
• Generate pseudo-random key derivation element
  RNDUE.
• Derive session keys KeyGens(CRID,RNDUE) ? KUE
• EKue(TAID,RESHE) ? F
• UE believes that SN has possession of s. With
  msg-5 the UE has demonstrated possession of s to
  SN. UE and SN also believe that KSN and KUE are
  shared session keys.
• UE?SN (RNDUE,F)
• 6 SN receives UE response and forward response
  to HS
• Derive session keys KeyGens(CRID,RNDUE) ? KUE
• Decrypt F. Verify TAID. Forward RESUE.
• SN now believes that s is a shared secret for
  CRID. Only outstanding is a verification that HS
  has authenticated CRID(and thereby UE).
• S2PLIP continues (SN forwards e-values)
• SN?HS CRID,RESSN,e-valuesBKEY

18
Privacy Preserving 3-Way AKA

• 7 HS responds to SN
• HS verifies UE response. HS now considers UE to
  be authenticated and CRID a valid UE identity.
• It then completes spatial verification (of
  e-values).
• Message 7 is sent to SN to verify that HS
  acknowledges CRID.
• HS?SN CRID,successBKEY
• 8 SN receives HS acknowledge
• SN now has assurance that HS acknowledges CRID.
• UE has not yet verification that HS accepted its
  response, but UE can continue without this
  knowledge (any subsequent SN usage of the PP3WAKA
  credentials will demonstrate SN belief in the
  credentials).

19
Analysis of the PP3WAKA protocol

• Complexity
• The PP3WAKA protocol is complex (by necessity)
• Computation
• feasible on advanced smartcards
• Pre-computation possible for SN and HS
• Communication
• Over-the-air message sizes seems feasible
• Round-trip count OK due to combined Mobility Mngt
  and Security procedures
• Hard to convince one self of correctness
• Difficult to apply formal methods
• Privacy not easy to model in most formalisms
• 3-Way protocols not easy to model
• IBE not easy to model (easy to assume too much)
• SMC not easy to model (again, what is correct
  assumptions to be made)
• But formal verification is on our agenda!

20
Analysis of the PP3WAKA protocol

• Privacy
• Permanent UE identity is never revealed over the
  air or to the SN
• Context Reference Identity (CRID)
• Known to UE,SN,HS
• Relatively short-lived
• Never revealed over the air (or elsewhere)
• The Temporary Alias Identity
• Not know by HS
• Very short lifespan
• Will be revealed over the air (paging and access
  request)
• Location information
• SN knows location, but SN can only associate it
  with CRID/TAID
• HS will only be given very coarse grained
  information (SN area)
• Adversary
• Can now that there is a subscriber at location
  (x,y)

21
Analysis of the PP3WAKA protocol

• Security
• An informal argument
• DH-exchange and Challenge-Response are well know
  mechanisms
• We assume security of IBE (and SMC)
• UE-HS authentication
• By means of online challenge-response (MAC with
  pre-shared secret)
• SN assurance of CRID
• SN-HS relationship exists
• HS received CRID from UE (IBE protected and tied
  to challenge-response)
• HS assert that CRID is a valid identity (over
  protected channel)
• Belief in DH-secret
• Created online over protected channel (SN and HS
  believes in s)
• HS has jurisdiction over UE
• UE knows that CRID is fresh and so it believes
  that s is a valid shared secret

22
Privacy Preserving 3-Way AKA

• Conclusion
• PP3WAKA
• Successfully provides credible subscriber
  location/identity privacy
• Successfully provides a 3-way security context
  (UE,SN,HS)
• Successfully provides a flexible session key
  scheme (UE-SN)
• Successfully provides a measure of Spatial Home
  Control
• Intelligent combination of Mobility Management
  and Security procedures
• Efficient in terms of round-trips (total
  execution time)
• Good balance in terms of computation requirements
  between principals
• Good balance in terms of communication
  requirements over interfaces
• Outstanding issues
• Formalize security arguments
• Formalize privacy arguments
• Get some experience (pilot implementation)