Mobile Equipment 128-bit Ciphering Key Storage and Tracking
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Solution Overview
Problem
Current wireless communication systems, particularly in UMTS, face challenges in handling and managing 128-bit ciphering keys for GSM and GPRS services, as existing specifications do not comprehensively address the handling of these keys in mobile equipment, leading to issues with key validity and security context management.
Innovation Solution
A method for obtaining, deriving, and storing 128-bit ciphering keys (Kc-128) in mobile equipment, separate from the USIM, along with their associated ciphering key sequence numbers (CKSN), to track the most current UMTS security context and ensure key validity, including the ability to generate and store multiple Kc-128 keys for circuit-switched and packet-switched domains, and to overwrite or delete keys as necessary based on security context changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Kc-128 is stored only in mobile equipment and deleted when ME is powered down, then security is improved, but key availability and service continuity deteriorate
Solution Approach 1:
The patent applies preliminary action by deriving and storing the 128-bit ciphering key (Kc-128) in the mobile equipment before it is needed for ciphering operations. The key is derived from UMTS security keys (CK and IK) obtained from the USIM during authentication, and is prepared in advance so that when ciphering is required, the key is already available in the ME without needing to be derived at that moment. This resolves the contradiction by maintaining security (key not permanently stored) while ensuring availability (key prepared beforehand).
Solution Approach 2:
The patent implements dynamics by making the Kc-128 key dynamic rather than static. The key is generated on-demand from UMTS security keys when needed for ciphering, and is automatically updated when new authentication occurs. The key exists in the ME only temporarily during its usage window, being deleted when no longer needed. This dynamic approach maintains security while ensuring continuous availability through regeneration from persistent USIM credentials.
2Adaptability or versatility
If multiple Kc-128 keys are stored for different domains, then service versatility is improved, but device complexity increases
Solution Approach 1:
The patent applies segmentation by separating key management for different service domains (circuit-switched and packet-switched). Each domain has its own Kc-128 key derived from domain-specific UMTS security keys (CK_CS/IK_CS for circuit-switched, CK_PS/IK_PS for packet-switched). This segmentation allows the ME to support multiple services with appropriate keys for each domain while keeping the key management structured and manageable through clear domain separation.
Solution Approach 2:
The patent implements universality by creating a unified key derivation mechanism that works across different service domains. The same fundamental process (deriving Kc-128 from CK and IK) is used for both circuit-switched and packet-switched domains, providing a universal approach that simplifies implementation while supporting multiple services. The ME can handle different domains using the same cryptographic principles and procedures.
3Reliability
If Kc-128 is derived from UMTS security keys, then key strength is improved, but operational constraints increase
Solution Approach 1:
The patent uses an intermediary approach by introducing the UMTS security keys (CK and IK) as intermediaries between the USIM and the ciphering algorithms. Instead of using GSM-era keys directly, the system derives stronger 128-bit keys (Kc-128) from the 128-bit UMTS security keys. This intermediary derivation process enhances key strength while maintaining operational flexibility, as the ME can derive Kc-128 on-demand from the persistent USIM credentials when needed for different services and scenarios.
Data Source
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AI summary
Techniques for handling ciphering keys in a mobile station comprising a mobile equipment (ME) and a Universal Subscriber Identity Module (USIM) are disclosed. An example method includes obtaining a UMTS cipher key (CK), integrity key (IK), and ciphering key sequence number (CKSN) from the USIM, deriving a 128-bit ciphering key (Kc-128) from the CK and the IK, and storing the Kc-128 and the CKSN on the mobile equipment, separate from the USIM. The stored CKSN is associated with the stored Kc-128, so that the Kc-128's correspondence to the most current UMTS security context can be tracked. This example method applies to the generation and storage of a 128-bit ciphering key for either the packet-switched or circuit- switched domains. A corresponding user equipment apparatus is also disclosed.