Inter-CU LTM Security Keys via MAC-CE Counters

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Solution Overview

Problem

Existing New Radio (NR) technologies lack support for inter-centralized unit (CU) lower layer triggered mobility (LTM) in secondary node (SN) changes, necessitating secure exchange of security-related information without relying on Radio Resource Control (RRC) or Medium Access Control (MAC) messages.

Innovation Solution

Utilizing a modified Medium Access Control (MAC) control element (MAC-CE) to convey counter parameters or indices for deriving security keys, enabling secure inter-CU LTM in SN changes without RRC or MAC messages, and employing secure RRC configurations for key derivation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If inter-CU LTM is supported, then security key exchange capability is improved, but signaling overhead increases due to additional security-related information exchange

Engineering Contradiction:

Improvesecurity key exchange capabilityVSAvoidsignaling overhead

Core Design Contradiction:

Adaptability or versatilityVSLoss of information

Solution Approach 1:

The patent combines security key exchange information with existing MAC-CE signaling structures used for LTM operations. By merging security parameters into the MAC-CE format already required for mobility triggering, the patent avoids separate dedicated signaling messages, thereby reducing overall signaling overhead while enabling inter-CU LTM security key exchange capability

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The MAC-CE signaling element is designed to serve multiple functions simultaneously: it triggers LTM operations and conveys security key derivation parameters. This multi-functional approach eliminates the need for separate RRC or MAC messages dedicated solely to security information exchange, reducing signaling overhead while improving adaptability

Inventive Principle:

Principle #6Universality (Multi-functionality)

2Reliability

If security key derivation is enabled for inter-CU LTM, then communication security is improved, but processing complexity increases due to key derivation operations

Engineering Contradiction:

Improvecommunication securityVSAvoidprocessing complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent performs security key derivation operations in advance during RRC configuration phases, preparing key material before LTM operations are triggered. This preliminary key preparation reduces processing complexity during actual LTM execution, as the UE and network nodes already have the necessary key derivation materials ready when mobility events occur

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent introduces counter values and key derivation parameters as intermediary elements that facilitate secure key generation without requiring complex real-time cryptographic operations during LTM. These intermediaries (counter values, pre-configured parameters) simplify the key derivation process while maintaining communication security

Inventive Principle:

Principle #24Intermediary (Mediator)

3Loss of time

If MAC-CE is used for triggering SN change, then latency is reduced, but security information exposure risk increases

Engineering Contradiction:

ImprovelatencyVSAvoidsecurity information exposure risk

Core Design Contradiction:

Loss of timeVSObject-affected harmful factors

Solution Approach 1:

The patent changes the parameter encoding format in MAC-CE from direct security key material to derived counter values and indices. This parameter transformation maintains the low-latency advantage of MAC-CE triggering while reducing security exposure, as the transmitted parameters are mathematical derivatives rather than direct representations of sensitive key material

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent converts the potential security risk of MAC-CE transmission into a benefit by using the MAC-CE's brevity and speed to transmit only essential counter parameters rather than full security keys. The 'harm' of using MAC-CE (potential exposure) is transformed into a 'blessing' by designing the message to contain only minimal, non-sensitive parameters that still enable secure key derivation

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Data Source

PatentUS20250287275A1Handling Security Keys for Inter-CU LTM in a Secondary Node

Publication Date: 2025.09.11 APPLE INC

AI summary

An apparatus configured to process, based on signaling received from a first serving cell, a configuration for New Radio dual connectivity (NR-DC) operation in which the first serving cell is a master node (MN) and a second serving cell is a secondary node (SN), the configuration including at least one candidate SN for lower layer triggered mobility (LTM), process, based on signaling received from the SN, a medium access control (MAC) control element (MAC-CE) that triggers a SN change to a first candidate SN, the MAC-CE including a counter value for deriving a security key of the first candidate SN, derive the security key for the first candidate SN based on the counter value and perform operations associated with the SN change from the SN to the first candidate SN.