TSG-SA Working Group 1 (Services) meeting #4	TSG S1#4 (99)432
Quebec City, Quebec, Canada 5th-9th, July 1999                   Agenda Item:6.6.1

TSG-SA WG3 (Security) meeting #4	TSGS-WG3#4(99)190
London, 16-18 June 1999	

To:	3GPP TSGs S2, T3

Copy:	3GPP TSGs N2

Source:	3GPP TSG S3

Title:	Interoperation between UMTS and GSM
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SA3 have started to consider solutions for secure interoperation between GSM and UMTS. The following scenarios are being considered:

1.	Registration of a UMTS user in a GSM BSS, including authentication and key agreement
2.	Intersystem handover of a UMTS user from UTRAN to GSM BSS
3.	Intersystem handover of a UMTS user from GSM BSS to UTRAN
4.	Registration of a GSM user in a UTRAN, including authentication and key agreement
5.	Intersystem handover of a GSM user from GSM BSS to UTRAN
6.	Intersystem handover of a GSM user from UTRAN to GSM BSS

We currently assume that the UMTS users with dual mode terminals only have an association with one (UMTS) HLR/AuC. This HLR/AuC can send appropriate authentication vectors to both UMTS and GSM networks. Solutions being considered involve the distribution and use of combined GSM/UMTS authentication vectors or the use of conversion functions to derive GSM security parameters from UMTS security parameters and vice versa.

For example using conversion functions, authentication and key agreement involving UMTS users registering on a GSM BSS could be achieved using a GSM challenge, response and cipher key derived from the corresponding parameters in a UMTS authentication vector produced by the user’s UMTS HLR/AuC. In this case the user’s USIM would have to implement the same conversion functions to derive the appropriate GSM parameters.  Similarly, authentication and key agreement involving GSM users registering on a UTRAN could be achieved using a UMTS challenge, response, cipher key and integrity key derived from the corresponding parameters in a GSM authentication vector produced by the user’s GSM HLR/AuC. In this case the user’s dual mode mobile would have to implement the same conversion functions to derive the appropriate UMTS parameters (the functions are implemented on the dual mode mobile to avoid modifications to GSM SIMs).

A current assumption is that it is not feasible to perform a reauthentication at intersystem handover. Instead, it is assumed that the appropriate keys must be available in the access network at handover so that GSM ciphering, or UMTS ciphering and integrity protection can be enabled. Various options are being considered for passing these keys between and within GSM BSS and UTRAN, some of which involve the use of conversion functions. However, many of the details cannot be worked out until UMTS inter-system handover procedures have been defined. 

Preliminary evaluation of the current proposals has focused on security issues. This has involved considering the level of security provided for various schemes in different interoperation scenarios. The efficiency of the schemes and their impact on the system architecture has also been studied. An important goal has been to take maximum advantage of the compatibility between the GSM and UMTS authentication and key agreement schemes to minimise the amount of extra functionality required to support interoperation. It should be noted that the conversion functions under study are unkeyed and are computationally quite simple.

To complete this work, extensive liaison with S2 and T3 is envisaged. It is hoped that this work item can be progressed at the proposed joint meeting between S2, S3 and T3 on 24th August in Bonn.