3GPP 24.501 v20.0.0 — the document's own text

5.5.4.3 UE-initiated authentication and key agreement procedure initiation

Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).

Upon receiving a ProSe direct link establishment request from the 5G ProSe remote UE or the 5G ProSe end UE including the SUCI or the CP-PRUK ID of the 5G ProSe remote UE or the 5G ProSe end UE, for establishing a secure PC5 unicast link as specified in 3GPP TS 24.554 [19E] when the security for 5G ProSe communication via 5G ProSe UE-to-network relay or 5G ProSe UE-to-UE relay is performed over control plane as specified in 3GPP TS 33.503 [56], the UE shall:

allocate a PRTI value as specified in clause 5.5.4.2;
create a RELAY KEY REQUEST message;
set the PRTI IE of the RELAY KEY REQUEST message to the allocated PRTI value;
set the relay key request parameters IE of the RELAY KEY REQUEST message with SUCI or the CP-PRUK ID, relay service code, and nonce_1 received from the of the 5G ProSe remote UE or the 5G ProSe end UE;
send the RELAY KEY REQUEST message; and
start the timer T3527 upon sending the RELAY KEY REQUEST message.