The user equipment (UE) and the Mobility Management Entity (MME) in an evolved 3GPP system generate authentication material that can be carried inside a packet switched network temporary mobile station identifier (P-TMSI) signature field of a Universal Mobile Telecommunications System (UMTS) signaling message from the UE to a UMTS/GPRS serving GPRS support node (SGSN) in a UMTS or GPRS Terrestrial Radio Access Network (UTRAN) or in a GSM/Edge Radio Access Network (GERAN), as well as from the SGSN to the MME of the evolved 3GPP system. The MME authenticates a context transfer request from the UTRAN/GERAN system based on the transferred authentication material and knowledge of how to create or to verify the authentication material. Additionally, the MME and the UE derive or verify authentication material, based on at least one user-specific key, for embedding in the P-TMSI signature field in legacy 3GPP signalling.