Combined Mode Algorithm

Description

A combined mode algorithm in 3GPP security architecture refers to a cryptographic algorithm that performs both ciphering (encryption) and integrity protection (message authentication) in a single operation. This is in contrast to separate algorithms for ciphering and integrity, which require two distinct cryptographic processes. The concept is defined in 3GPP TS 33.771 (Study on security for 5G) and is exemplified by the NCA4/5/6 algorithms specified in TS 35.240, TS 35.243, and TS 35.246. These algorithms are part of the 5G security framework and are designed to provide efficient and robust security for control plane and user plane data.

In traditional 3GPP systems (e.g., LTE), ciphering and integrity protection were handled by separate algorithms: a ciphering algorithm (e.g., 128-EEA1/2/3) and an integrity algorithm (e.g., 128-EIA1/2/3). While effective, this approach requires two separate cryptographic operations, which can increase computational overhead and latency. Combined mode algorithms address this by integrating both functions into a single algorithm, leveraging modes of operation such as AES-GCM (Galois/Counter Mode) or similar authenticated encryption with associated data (AEAD) schemes. This not only simplifies the security architecture but also improves performance by reducing the number of cryptographic primitives needed.

The NCA4/5/6 algorithms are based on the 3GPP specification for the 5G Authentication and Key Agreement (AKA) and are designed to be used with the 5G security context. They operate on the packet data convergence protocol (PDCP) layer, where they secure user plane and control plane data. The combined mode algorithm takes as input the plaintext, a secret key, and additional authenticated data (AAD) such as the bearer identity and direction, and outputs ciphertext and an integrity tag. The receiver verifies the integrity tag and decrypts the ciphertext. This ensures both confidentiality and integrity in one pass.

The introduction of combined mode algorithms in Release 20 reflects the ongoing evolution of 5G security to meet the demands of ultra-reliable low-latency communication (URLLC) and enhanced mobile broadband (eMBB). By reducing the processing overhead, these algorithms help achieve lower latency and higher throughput, which are critical for 5G use cases. They also simplify key management and reduce the risk of misconfiguration by using a single algorithm identifier.

Overall, combined mode algorithms represent a significant advancement in 3GPP security, providing a more efficient and streamlined approach to protecting user and control plane data. They are expected to be widely adopted in 5G networks and beyond.

Purpose & Motivation

The combined mode algorithm was created to address the inefficiencies and complexities of using separate ciphering and integrity algorithms in mobile networks. In earlier generations, separate algorithms were used, which required two cryptographic operations per data packet, increasing computational load and latency. As 5G networks aim for higher data rates and lower latency, the overhead of separate algorithms became a bottleneck, especially for URLLC services.

Additionally, managing separate algorithms increases the complexity of security configuration and key management. A combined mode algorithm simplifies this by providing a single algorithm that handles both functions, reducing the number of security parameters and potential points of failure. It also aligns with modern cryptographic practices, such as AEAD, which are designed to provide both confidentiality and integrity in a single, efficient operation.

The motivation also includes the need for stronger security guarantees. Combined mode algorithms like AES-GCM are well-analyzed and provide robust security, including resistance to certain attacks that separate algorithms might be vulnerable to. By standardizing these algorithms in 3GPP, vendors and operators can ensure interoperability and consistent security across 5G networks. The introduction in Release 20 reflects the maturity of 5G security and the continuous effort to optimize performance without compromising security.