Security Association

Description

A Security Association (SA) is a simplex logical connection that provides security services to the traffic carried over it. In 3GPP architectures, an SA is established between two entities (e.g., a UE and a network node, or between two network nodes) and embodies a shared state information necessary for cryptographic operations. This shared state typically includes identifiers (like a Security Parameter Index - SPI), the specific cryptographic algorithm to be used (e.g., AES-CBC, SNOW 3G), the operational mode, the secret keys, key lifetimes, and the associated security policy dictating what traffic is to be protected and how.

SAs are often unidirectional. For a bidirectional secure exchange, a pair of SAs (one for each direction) is usually established. The creation of an SA involves a security association establishment protocol, such as Internet Key Exchange (IKE) for IPsec, or a protocol within the 3GPP stack like the NAS or AS security setup procedures. For example, when a UE attaches to a 5G network, a NAS security context is established between the UE and the AMF, which constitutes an SA for protecting NAS signaling messages. This context includes the selected NAS integrity and ciphering algorithms (NIA, NEA), the derived keys (K_{AMF}), and counters.

In the context of IPsec, which is widely used in 3GPP for securing N interfaces (e.g., N2, N3) and for User Plane security, SAs are managed by a Security Association Database (SAD) and a Security Policy Database (SPD). The SPD contains rules that select which traffic requires protection and which SA (or bypass/ discard action) to apply. The SAD contains the parameters for each active SA. The lifecycle of an SA includes establishment, use for packet processing (applying encryption/integrity protection on outbound, verification/decryption on inbound), and eventual deletion upon expiry or termination of the secure session.

Purpose & Motivation

Security Associations were created to provide a structured, scalable framework for applying cryptographic protection to communications in packet-switched networks, particularly within the Internet Protocol (IP) suite. The problem they solve is the need for communicating peers to have a consistent, agreed-upon set of security parameters. Without such an association, endpoints might attempt to use mismatched algorithms, keys, or policies, resulting in communication failure or insecure connections.

In 3GPP networks, the concept was adopted to secure various interfaces and protocols. For instance, in GPRS and later architectures, IPsec SAs are used to protect the GTP tunnels between network nodes (like between SGSN and GGSN, or SGW and PGW). In 5G, SAs are fundamental to the service-based architecture's security, protecting HTTP/2 messages between network functions. Their creation was motivated by the move towards all-IP core networks, which introduced new threats compared to traditional circuit-switched signaling. SAs provide a standardized way to negotiate and manage security contexts dynamically, enabling features like algorithm agility (the ability to switch to stronger algorithms as needed) and key freshness, addressing the limitations of static, pre-configured security setups.