S3-030147 LS to SAGE.DOC

3GPP SA3 currently works on the security for application services based on the IP multimedia core network subsystem (IMS) of UMTS. One of the alternatives under discussion (see attachments) proposes the use of a key derivation procedure satisfying particular requirements, which are outlined in this liaison statement. 3GPP SA3 kindly asks ETSI SAGE to assist in the selection of the discussed alternatives by commenting on the feasibility of such a key derivation procedure and the possibility for ETSI SAGE to take on the work, if considered feasible by ETSI SAGE and 3GPP SA3 decides to go for this alternative. Statement of the problem The problem statement contained in this introduction is somewhat simplified, but it is hoped that it contains the information necessary for ETSI SAGE to provide initial comments. If this is not the case, 3GPP SA3 or the contact person named above will be happy to provide more information. The entities involved: a User Equipment (UE) communicates via a proxy signalling server (P-CSCF) in a visited network with a signalling server (S-CSCF) in the home network. The UE also communicates with an application server (AS) to obtain particular services. There may be several application servers with which a UE communicates. The situation is depicted in the figure below. UE Application Server