Title:		Integrity protection and ciphering at GSM to UMTS handover 
Source:		Ericsson 
Document for: 	Discussion

1.	Introduction
In this document some principles concerning security functions (ciphering and intergrity protection) at inter-system handover from GSM to UMTS are proposed for discussion. 
2.	Discussion
2.1 UMTS Integrity protection and ciphering
The 3GPP TS 33.102 and 3GPP TS 33.105 contain descriptions on access data link integrity and access data link confidentiality. This includes descriptions on input parameters for the UMTS integrity algorithm respective the UMTS ciphering algorithm.   
2.2 Authentication, key agreement and key conversion functions
In 3GPP TS 33.102 the conversion of GSM Kc into UMTS CK and IK and vice versa is defined. The necessary conversion is dependent on the type of used authentication procedure and is performed in the MSC/VLR and the MS. 
2.3 Abbreviations and concepts
CK	UMTS Cipher Key
IK	UMTS Integrity Key
UEA	UMTS Encryption Algorithm
UIA	UMTS Integrity protection Algorithm 
HFN	Hyper Frame Number
“UE capability and security information” contains information on the UIAs and UEAs that are supported by the MS. It also contains the HFN to be used as an input parameter for the UMTS integrity algorithm respective for the UMTS ciphering algorithm. 
2.4 References
GSM 04.18  “Radio Resource Control Protocol”
GSM 08.08  “MSC-BSS interface Layer 3 Specification”
3GPP TS 25.331  “RRC protocol specification”
3GPP TS 25.413  “UTRAN Iu Interface RANAP Signalling”
3GPP TS 33.102 “Security Architecture”
3GPP TS 33.105  “Cryptographic Algorithm Requirements”

2.5 GSM to UMTS handover
The L3 signalling performed at GSM to UMTS handover is illustrated in figure below. Each step is explained in the following list. This description has focus on the handling of ciphering and integrity protection at handover from GSM to UMTS, and therefore excludes a lot of other aspects.  

1.	Before the BSC can decide to initiate handover to UMTS, it has to get the “UE capability and security information” from the MS. This can be done by the RR Classmark interrogation procedure. The existing RR Classmark Enquiry message is extended with an IE to indicate the requested UE capability and security information. An UMTS capable MS will then include, in new IEs of the RR Classmark change message, the requested information. This new information is stored in the BSC.
2.	When the BSC decides to initiate handover to UMTS, it sends the BSSMAP message Handover Required to MSC/VLR. This message includes (among other information) a new IE “Source system to target RNC transparent container”, which includes the “UE capability and security information” received from the MS. 
3.	At reception of the BSSMAP Handover Required message, the MSC/VLR will send the RANAP Relocation Request message to the target RNC. This message includes the “Source system to target RNC transparent container” received from BSC. It also includes the CK and the IK. 
4.	At reception of the RANAP message Relocation Request message, the RNC will select an UEA based on its own capabilities and on the UE capability and security information received. After all necessary resources are allocated, target RNC sends the RANAP Relocation Request Acknowledge message including the IE “Target RNC to source RNC transparent container” to the MSC/VLR. The transparent container contains the RRC Handover to UTRAN command message to be sent transparently via MSC/VLR and BSC to the MS. This message will include information on the selected UEA.
5.	At reception of the RANAP message Relocation Request Acknowledge message, the MSC/VLR continues the procedure by sending the BSSMAP Handover command message to the BSC. The content of the IE “Target RNC to source RNC transparent container” received from the RNC is then included in the IE “Layer 3 Information” of the BSSMAP message.  The BSC will then include the received RRC message in the RR Inter System Handover command message and send this to the MS. 
6.	The target RNC shall send a RANAP Relocation Detect message to the MSC/VLR when the handover execution trigger is received, i.e. when target RNC detects the MS on the lower layers. The MS-RNC communication is started in ciphered mode according to the information in the RRC message sent to MS (see above). When the MS consider the handover as completed it sends the RRC Handover to UTRAN Complete message to the RNC.  
7.	At reception of the RRC Handover to UTRAN Complete message, the RNC shall initiate the start of integrity protection. The RNC will then generate a random parameter FRESH and select an UIA. The UIA selection is based on the RNC capabilities and on the “UE capability and security information”.  The FRESH is an input parameter to the UMTS integrity algorithm. The initial HFN value used for the integrity protection is the HFN value received in the “UE capability and security information”. 
8.	The SRNC generates the RRC message Security control command. This message includes the UIA and FRESH to be used. The RNC will then start the integrity protection of signalling messages.
9.	At reception of the Security control command message, the MS will start integrity protection by use of the received UIA and FRESH information and the stored HFN.  The MS will then send the RRC Security control command response message to the RNC.
10.	At reception of the Security control command response message indicating that integrity protection is started, the RNC will send the RANAP Relocation Complete response message to the MSC/VLR.
11.	Upon receiving the RANAP Relocation Complete message, the MSC/VLR initiates the BSSMAP Release procedure.  

3.	Conclusion
The above shows a possible solution on how ciphering and integrity protection is handled at inter system GSM to UMTS handover. The data confidentiality (ciphering) applies to both GSM and UMTS and the MS-network communication should be possible to continue in ciphered mode also at handover between GSM and UMTS. The integrity protection functionality is not supported in GSM and therefore the integrity protection is started after that the handover to UMTS is performed.  This will then also cause less impact on the GSM Um.