Key IDentifier

Description

Key IDentifier (KID) is a security-related identifier introduced in 3GPP Release 19 that provides a unique reference to cryptographic keys within the 3GPP security architecture. The KID is defined in specifications 26.804 and 33.127, which address security aspects of media streaming and lawful interception respectively. The KID serves as a standardized mechanism for identifying and managing cryptographic keys used in various security procedures, enabling network functions and user equipment to efficiently select, retrieve, and apply the correct keys for authentication, encryption, and integrity protection.

In the 3GPP security architecture, multiple keys are generated and used at different levels of the protocol stack and for different purposes. These include the long-term subscriber key (K), the authentication server function key (KAUSF), the serving network key (KSEAF), the access and mobility management function key (KAMF), and various derived keys for specific purposes such as NAS signaling, RRC signaling, and user plane protection. The KID provides a systematic way to reference these keys, particularly in contexts where keys need to be identified across different network functions, during key derivation chains, or when multiple keys are active simultaneously during key refresh procedures.

The KID is particularly relevant in the context of media streaming security as addressed in specification 26.804. In this context, the KID is used to identify the specific key required to decrypt encrypted media content, enabling efficient key management for streaming services that may use multiple encryption keys for different content segments or quality levels. The KID allows the receiving entity to quickly determine which key to apply without trial-and-error decryption attempts, improving both security and performance. This is analogous to the key identifier concept used in DRM (Digital Rights Management) systems and the ISO Base Media File Format (ISOBMFF), where key identifiers are embedded in media containers to signal the required decryption key.

In the lawful interception context addressed by specification 33.127, the KID plays a role in identifying the cryptographic keys used to protect intercepted communications. Lawful interception requires that authorized agencies can access the plaintext of communications, which necessitates knowledge of the encryption keys in use. The KID provides a standardized reference that allows the lawful interception system to identify and request the appropriate keys from the network's key management infrastructure, ensuring that interception can be performed efficiently and in compliance with legal requirements while maintaining the security of the overall system.

The implementation of KID in 3GPP networks involves coordination between multiple security functions. The Authentication Server Function (AUSF) and the Unified Data Management (UDM) are responsible for key generation and management at the network side. The UE maintains its own key hierarchy derived from the long-term credentials stored in the USIM. The KID mechanism ensures that both network and UE can unambiguously reference the same key for a given security context. This is essential for scenarios involving key refresh, handover between different network functions, and the delivery of services that require end-to-end encryption with key management handled by the network infrastructure.

Purpose & Motivation

The Key Identifier (KID) exists to solve the fundamental problem of key management in complex security architectures where multiple cryptographic keys are active simultaneously and need to be unambiguously referenced across different network functions, protocols, and service contexts. Without a standardized identifier, key selection and retrieval would rely on implicit context or proprietary mechanisms, leading to interoperability issues and potential security vulnerabilities.

Before the introduction of KID in Release 19, 3GPP security procedures primarily relied on implicit key identification through protocol context. For example, the NAS security context was identified by the ngKSI (Key Set Identifier), but this mechanism was limited to specific protocol layers and did not provide a general-purpose key reference applicable across different security domains. As 5G networks evolved to support more diverse services including media streaming, edge computing, and network slicing, the need for a more flexible and general-purpose key identification mechanism became apparent.

The KID addresses several specific limitations of previous approaches. First, it provides a unified key reference mechanism that works across different protocol layers and security domains, from NAS and RRC signaling to application-layer media encryption. Second, it enables efficient key management in scenarios where multiple keys may be valid simultaneously, such as during key refresh procedures or when different services use different encryption keys. Third, it supports lawful interception requirements by providing a standardized way to identify and request specific keys. Fourth, it facilitates interoperability between different vendors' equipment by ensuring that key references are unambiguous and standardized. The introduction of KID in Release 19 reflects the growing complexity of security requirements in 5G-Advanced and future networks, where flexible, scalable, and interoperable key management is essential.