Evolved Packet System Security Context

Description

The EPS security context is a fundamental concept in 3GPP Evolved Packet System (EPS) security architecture, defined in specifications such as TS 24.301 and TS 33.401. It represents a state that is established locally at both the User Equipment (UE) and the serving network domain (e.g., MME for NAS, eNB for AS). At both ends, 'EPS security context data' is stored, which consists of the EPS NAS (Non-Access Stratum) security context and the EPS AS (Access Stratum) security context. This context is essential for protecting the integrity and confidentiality of signalling messages and user data over the air interface and within the core network. The term was introduced in Release 8, the first release of LTE, and has remained unchanged through Release 19.

The EPS NAS security context secures signalling between the UE and the Mobility Management Entity (MME), including authentication, security mode control, and mobility management messages. It includes parameters such as the Key Set Identifier (KSI), the NAS ciphering and integrity keys (K_NASenc and K_NASint), and the selected algorithms. The EPS AS security context secures the radio interface between the UE and the eNodeB, including Radio Resource Control (RRC) signalling and user plane data. It comprises the AS keys (K_RRCenc, K_RRCint, K_UPenc) and algorithms. Both contexts are derived from a root key, K_ASME, which is generated during the authentication procedure (e.g., EPS AKA). The security context also includes a negotiation of security capabilities and a unique identifier to manage key freshness.

The establishment of the EPS security context begins with the UE and network performing mutual authentication. The MME initiates an Authentication Request, and upon successful authentication, both parties derive K_ASME. The MME then sends a NAS Security Mode Command to the UE, which includes the selected NAS algorithms and a replayed UE security capability. The UE verifies the command, derives the NAS keys, and responds with a NAS Security Mode Complete. Similar procedures exist for AS security, where the eNB receives keys from the MME and initiates an AS Security Mode Command. Once the security contexts are active, all subsequent NAS and AS messages are integrity protected and ciphered according to the negotiated algorithms.

The EPS security context supports key handling for mobility and interworking. When a UE moves between MMEs or eNBs, the context is transferred to ensure continuity of security. For example, during an S1 handover, the source eNB forwards the AS security context to the target eNB. During an inter-MME handover, the MME transfers the NAS security context. The context also supports idle mode mobility with the use of a temporary identity (GUTI) and a KSI to allow the network to identify the correct security context without full authentication. This reduces signalling overhead and improves efficiency.

In summary, the EPS security context is the cornerstone of EPS security, providing a comprehensive framework for key management, algorithm negotiation, and secure communication. Its standardized definition across releases ensures interoperability and robust security for LTE networks, which continue to be widely deployed alongside 5G. The concept is also extended to 5G in the form of the 5G security context, but the EPS security context remains distinct for EPS access.

Purpose & Motivation

The EPS security context was created to provide a unified and robust security framework for the EPS, replacing the more fragmented security mechanisms of earlier generations like UMTS and GPRS. In those legacy networks, security was handled separately for different domains (e.g., ciphering in RNC, integrity in SGSN), leading to complexity and potential gaps. The EPS security context consolidates both NAS and AS security into a single state that is established after mutual authentication, ensuring that all signalling and user data are protected from the outset. This design addresses the need for stronger security in an all-IP network, where threats like eavesdropping, tampering, and replay attacks are more prevalent.

The motivation for the EPS security context also stems from the requirement to support seamless mobility and interworking with non-3GPP accesses. By maintaining a security context that can be transferred between network nodes during handovers, EPS ensures that security is not compromised when the UE moves. Furthermore, the context includes key freshness mechanisms (e.g., KSI, re-authentication) to prevent long-term key reuse, which is a security best practice. The introduction in Release 8 coincided with the launch of LTE, and its unchanged definition through Release 19 reflects its enduring effectiveness. It has been instrumental in enabling secure mobile broadband, VoLTE, and mission-critical services over LTE.