Sending Entity

Description

The Sending Entity is a fundamental concept in 3GPP security architecture, particularly within the context of secured packet transmission. It is defined as the entity from which a Secured Packet is sent and where the security mechanisms are invoked. The Sending Entity is responsible for generating the Secured Packets to be sent, applying the necessary cryptographic protections such as integrity protection, confidentiality, and replay protection. In the context of the present document (3GPP TS 23.048), the Sending Entity is specifically identified as either the USAT Interpreter Security Functionality or the Security Node. The USAT Interpreter Security Functionality resides within the (U)SIM Toolkit architecture, enabling secure communication between the UICC and external entities. The Security Node, on the other hand, is a network entity that handles security operations for secured packet exchange. The Sending Entity operates by taking an unprotected packet, applying security mechanisms according to a defined security policy, and producing a Secured Packet that can be transmitted over various bearers such as SMS, USSD, or HTTP. The security mechanisms invoked include the calculation of a Message Authentication Code (MAC) for integrity, encryption for confidentiality, and the management of counters for replay protection. The Sending Entity must maintain synchronization with the Receiving Entity regarding security parameters, such as the MAC algorithm, encryption algorithm, and counter values. This synchronization is typically achieved through the exchange of security parameters during initialization or through the use of a shared security context. The Sending Entity also plays a role in the key management process, ensuring that the appropriate keys are used for securing the packets. In the USAT environment, the Sending Entity may be the UICC itself when it sends secured packets to the network, or it may be a network entity sending secured packets to the UICC. The concept of the Sending Entity is crucial for ensuring end-to-end security in applications such as remote management of (U)SIM cards, over-the-air updates, and secure provisioning. Without a well-defined Sending Entity, the security of these transactions would be compromised, as there would be no clear point of responsibility for applying security mechanisms. The Sending Entity works in conjunction with the Receiving Entity, which verifies the security of the received packets. Together, they form a secure communication pair. The Sending Entity's role is defined in specifications such as 3GPP TS 22.048, TS 23.048, and TS 31.114, which outline the requirements and procedures for secured packet transmission. The evolution of the Sending Entity definition from Rel-5 to Rel-8 reflects a refinement in the understanding of its role, moving from a broader description that included examples like SMS-SC, UICC, USSD entry point, or dedicated (U)SIM Toolkit Server, to a more precise definition that focuses on the USAT Interpreter Security Functionality or the Security Node. This change aligns with the maturation of the USAT security architecture and the need for clarity in specification.

Purpose & Motivation

The Sending Entity concept exists to clearly define the point of origin and security invocation for secured packets in 3GPP networks. It solves the problem of ambiguity in security responsibility by designating a specific entity that generates and secures packets, ensuring that security mechanisms are consistently applied. Before this concept was formalized, there was potential for confusion about which entity should apply security, leading to vulnerabilities or interoperability issues. The Sending Entity provides a standardized role that can be implemented across different network elements, facilitating secure communication between UICCs and network servers. It was motivated by the need for secure remote management of (U)SIM cards and the growing demand for over-the-air services that require strong security. The definition has evolved to reflect the specific entities involved in USAT security, such as the USAT Interpreter Security Functionality and the Security Node, which are central to the secure packet exchange in later releases.