LTE S1 NAS Deciphering

Dear experts,

I am researching in the LTE S1 NAS deciphering. I presume the ciphering and deciphering of traffic for LTE should not differs much as to GERAN and UTRAN.

LTE’s predecessors like GERAN and UTRAN requires monitoring of two interfaces to decipher the traffic.

GERAN – Gb and Gr/Gn
UMTS – Iub and IuCS/IuPS

For LTE, apart from S1 interface, which other interface should i consider? Must the PDCP message and sort of stuff be considered?

Thank you very much.