SideLink Positioning Key

Description

SideLink Positioning Key (SLPK) is a cryptographic key defined in 3GPP Release 19 specifications 24.514 and 33.533. It is part of the security architecture for sidelink positioning, which allows user equipment (UE) to determine their position using direct sidelink communication, without relying on network coverage. SLPK is used to secure the exchange of positioning reference signals (PRS) and related measurement reports between UEs over the PC5 interface.

In sidelink positioning, UEs exchange positioning signals and measurements to compute relative or absolute positions. This is particularly useful for public safety, vehicle-to-everything (V2X), and industrial IoT applications where network coverage may be unreliable or unavailable. However, the open nature of the sidelink interface makes it vulnerable to spoofing, eavesdropping, and manipulation. SLPK addresses these threats by providing authentication and encryption for sidelink positioning messages.

SLPK is derived from a root key, typically the PC5 unicast key or a dedicated positioning key hierarchy. The key is generated during the sidelink positioning session setup, which may involve a discovery procedure and a security association establishment. The key can be refreshed periodically or on-demand to maintain forward secrecy. SLPK is used with integrity protection algorithms (e.g., AES-CMAC) and encryption algorithms (e.g., AES-CTR) to protect the positioning messages.

The key management for SLPK involves both the UE and the network (if available). In out-of-coverage scenarios, the key may be pre-provisioned or derived from a pre-shared secret. In in-coverage scenarios, the network can assist in key generation and distribution. The 3GPP security architecture ensures that SLPK is unique per session and per pair of UEs, preventing replay attacks and unauthorized access.

SLPK is a critical enabler for secure sidelink positioning, which is expected to support advanced use cases such as cooperative positioning, relative positioning for platooning, and emergency call positioning. Its standardization in Rel-19 ensures interoperability and builds trust in sidelink-based positioning services.

Purpose & Motivation

SLPK was created to secure sidelink positioning, which is a new feature in 5G-Advanced that enables UEs to determine their location without network coverage. Prior to Rel-19, sidelink communication lacked a dedicated security mechanism for positioning messages, leaving them vulnerable to attacks. SLPK provides a standardized key hierarchy and procedures to authenticate and encrypt sidelink positioning exchanges, ensuring that position information cannot be spoofed or tampered with. This is essential for safety-critical applications like V2X and public safety, where accurate and trustworthy positioning is paramount.