Authentication vector

Description

An authentication vector (AV) is a data structure used in 3GPP networks to facilitate the authentication of a subscriber and the establishment of security keys. Introduced in Release 8, the term encompasses both the traditional quintet used in 3G and the SD-AV (Single-Domain Authentication Vector) used in IMS and other contexts. The authentication vector is generated by the home environment (e.g., HSS or AuC) and provided to the serving network (e.g., MME, SGSN, or AMF) to perform authentication and key agreement with the UE.

The quintet, as defined in TS 33.102, consists of five elements: RAND (random challenge), XRES (expected response), CK (cipher key), IK (integrity key), and AUTN (authentication token). These elements are used in the UMTS AKA and EPS AKA procedures. The RAND and AUTN are sent to the UE, which computes RES and verifies AUTN. If successful, the UE and network use CK and IK to derive session keys. The XRES is used by the serving network to verify the UE's response.

In LTE and 5G, the authentication vector may be transformed into a new format. For example, in EPS, the quintet is converted into a quartet (RAND, AUTN, XRES, KASME) where KASME is a key derived from CK and IK. In 5G, the authentication vector may include additional parameters such as the home network public key and the SUCI. The SD-AV is a variant used in IMS authentication, which may have a different structure.

The authentication vector is a critical element in the security architecture because it enables the serving network to authenticate the UE without direct access to the subscriber's secret key. The home environment generates the AV on demand and provides it to the serving network, which can then perform authentication locally. This approach supports roaming and reduces signaling load. The AV also ensures that the serving network cannot impersonate the home network, as the AUTN includes a MAC that only the home network can generate.

The definition of authentication vector was revised in Release 20 to explicitly list the components: 'A vector consisting of CK, IK, RAND, AUTN, and XRES.' This change provides a more precise and self-contained definition, reflecting the maturity of the specification. The authentication vector remains a fundamental concept in 3GPP security, underpinning the trust model of mobile networks.

Purpose & Motivation

The authentication vector exists to enable efficient and secure authentication in mobile networks, particularly in roaming scenarios. Before its introduction, authentication was performed using a challenge-response mechanism where the serving network had to contact the home network for every authentication attempt, which was inefficient and increased latency. The AV allows the home network to pre-compute authentication data and provide it to the serving network, which can then authenticate the UE independently.

The AV also supports mutual authentication by including the AUTN, which allows the UE to verify the network's identity. This prevents false base station attacks and enhances overall security. The evolution of the AV from a quintet to more complex structures reflects the growing security requirements and the need to support new access technologies and services. The revision in Release 20 clarifies the components, ensuring consistent implementation across vendors.