The purpose of the Nonce information element is to transfer a 32-bit nonce value to support deriving a new mapped EPS security context. The Nonce is a type 3 information element with a length of 5 octets.
In order for the UE to derive a mapped EPS security context for an inter-system change from A/Gb mode or Iu mode to S1 mode in EMM-CONNECTED mode, the MME shall generate a KSISGSN, create a nonceMME and generate the K'ASME using the created nonceMME. The MME shall include the selected NAS algorithms, nonceMME and generated KSISGSN (associated with the K'ASME) in the NAS security transparent container for handover to E-UTRAN. The MME shall derive the EPS NAS keys from K'ASME.
When the UE receives the command to perform handover to E-UTRAN, the UE shall derive K'ASME, using the nonceMME received in the NAS security transparent container. Furthermore, the UE shall associate the derived K'ASME with the received KSISGSN and derive the EPS NAS keys from K'ASME.
The MME shall include both the nonceMME and the nonceUE when creating a mapped EPS security context during inter-system change from A/Gb mode to S1 mode or Iu mode to S1 mode in EMM-IDLE mode.
Please refer 3GPP specification 24.301 for more information.
