Serial Number

Description

The Serial Number is a two-octet field used in 3GPP specifications for secured packet transmission, particularly in the context of (U)SIM Toolkit and remote management. It is defined in 3GPP TS 23.048 and TS 31.115. The Serial Number identifies a particular message and is linked to the Message Identifier. It is altered every time the message is changed, ensuring that each version of a message has a unique identifier. This mechanism is crucial for replay protection and for ensuring that the receiving entity can detect duplicate or outdated messages. The Serial Number works in conjunction with the Message Identifier, which identifies the type or category of the message. Together, they provide a unique reference for a specific message instance. When a message is sent, the Sending Entity includes the Serial Number in the secured packet. The Receiving Entity checks the Serial Number against previously received values to ensure that the message is fresh and not a replay. If the Serial Number is not as expected, the message may be rejected. The Serial Number is also used in the calculation of the Message Authentication Code (MAC) to bind the security to the specific message instance. This prevents an attacker from replaying a previously captured message, as the Serial Number would be different. The Serial Number is typically managed by the Sending Entity, which increments or changes it whenever the message content is modified. The Receiving Entity maintains a record of the last received Serial Number or a window of valid Serial Numbers to detect replays. The two-octet length allows for 65536 possible values, which is sufficient for most applications, especially when combined with other freshness mechanisms such as counters. The Serial Number is part of the secured packet structure, which includes other fields such as the Message Identifier, the MAC, and the encrypted payload. The exact format and usage are defined in the specifications. In the context of OTA (Over-The-Air) management of (U)SIM cards, the Serial Number ensures that configuration updates are applied in the correct order and that old updates cannot be replayed. It is a simple yet effective mechanism for message uniqueness and replay protection. The evolution of the Serial Number definition shows that in Rel-5 it was simply 'two octet field which identifies a particular message.' In Rel-8, the definition was expanded to include the linkage to the Message Identifier and the fact that it is altered every time the message is changed. This clarification emphasizes the dynamic nature of the Serial Number and its role in message freshness. Subsequent releases have retained this definition without change, indicating its stability and continued relevance. The Serial Number is a fundamental building block for secure messaging in 3GPP networks, enabling trust in remote management and other sensitive operations.

Purpose & Motivation

The Serial Number exists to provide a unique identifier for each instance of a message, enabling the receiving entity to distinguish between new messages and replays. It solves the problem of replay attacks, where an attacker could capture a legitimate message and retransmit it to cause unauthorized actions. By altering the Serial Number every time the message changes, the system ensures that each message is unique and can be validated for freshness. This is particularly important in remote management of (U)SIM cards, where commands such as enabling or disabling services, updating parameters, or installing applications must be protected against replay. The Serial Number also aids in message sequencing and integrity, as it is included in the MAC calculation. The concept was introduced in Release 5 as part of the secured packet framework and has remained a cornerstone of 3GPP security. Its simplicity and effectiveness have made it a lasting component of the specifications.