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Handover from UTRAN/GERAN to E-UTRAN
Ref: [33.401:9.2.2, 10.3.2], [23.401:5.5.2.2], [36.300:19.2.2.5.6], [36.331], [24.301]
Below diagram depicts handover from E-UTRAN to UTRAN. The same procedure is applicable for handover to GERAN; GPRS security context is mapped after mapping EPS context to UMTS one.
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During HO procedure [23.401:5.5.2.2], MME generates K'ASME based on CK, IK, and (random 32 bit) NONCEMME.
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Network while sending HO Command to UE also includes NAS Security context parameters (Ciphering algorithm, Integrity algorithm, eKSI) and NONCEMME in nas-securityParamToEUTRA [24.301:9.9.2.7] in RRC Reconfiguration Request.
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RRC Handover From UTRAN Command
(RRC Reconfiguration Request)
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UE generates mapped EPS NAS security context (Fig 1.5.3).
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UMTS
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mapped EPS NAS
eKSI=KSISGSN/y
KSGSN
UL NAS count=0
DL NAS count=0
UE security capability
KNASint/Enc algo id
KNASenc/Int algo id
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Following handover, UE is required to do Traking Area Update (based on criteria listed in [23.401:5.3.3.0] "Triggers for TAU"). During TAU, MME may trigger NAS Security Mode procedure if change in security algorithms is needed.
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This concludes brief discussion on LTE NAS security during inter RAT scenarios. More details can be found in references listed on next page.
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