Question: What are the parameters used to form the input S for the SHA-256 hashing algorithm when deriving HRES* from RES* in the SEAF and HXRES* from XRES* in the AUSF?
Option 1: P0 = RAND, P1 = RES* or XRES*, and the input S is equal to the concatenation P0||P1 of the P0 and P1.
Option 2: P0 = RES* or XRES*, P1 = RAND, and the input S is equal to the concatenation P0||P1 of the P0 and P1.
Option 3: P0 = RAND, P1 = SUPI, and the input S is equal to the concatenation P0||P1 of the P0 and P1.
Option 4: P0 = RES* or XRES*, P1 = NULL, and the input S is equal to the concatenation P0||P1 of the P0 and P1.
Explanation: According to the security architecture and procedures for 5G System, when deriving HRES* from RES* in the SEAF and HXRES* from XRES* in the AUSF, the parameters used to form the input S for the SHA-256 hashing algorithm are P0 = RAND and P1 = RES* or XRES*. The input S is equal to the concatenation P0||P1 of the P0 and P1. This ensures secure derivation of H(X)RES*, which is identified with the 128 least significant bits of the output of the SHA-256 function.