The open fronthaul interface is a standard protocol for a link between the radio units and the distributed unit in RAN, enabling different vendors interoperable. We study the security requirements of the open fronthaul interface for 5G networks. The O-RAN management plane (M-plane) mandates an end-to-end security using SSHv2, whereas the O-RAN control and user plane (CU-plane) do not support any security measure yet. We investigate MACsec for the CU-plane security, which is recommended as one of security options in the eCPRI specification. Furthermore, we implemented quantum-safe crypto solutions using a hybrid mode key exchange and signature schemes, which can be applied for the post-quantum SSH and MACsec protocols.

The industrial demand on MACsec in Ethernet networks is increasing substantially, in particular for 5G networks, mainly due to its efficiency paired with strong security. MKA (MACsec Key Agreement) is a companion protocol of MACsec that provides methods of authentication and cryptographic key establishment. In this paper, the MACsec and MKA protocol are analysed under a quantum attack scenario. Even though the threat of quantum computers should not be overstated, it is necessary to provide a new countermeasure that is robust against this potential, yet critical risk. Symmetric-key crypto algorithms defined in MACsec and MKA can achieve 128-bit quantum security if 256-bit keys are mandated. However, classical public-key crypto schemes are known to be vulnerable to quantum attacks so that MKA protocol needs to support post-quantum public-key crypto schemes. We implemented a McEliece-based key establishment which is the most conservative post-quantum public-key cryptosystem with a large size of key, yet feasible for MKA. For entity authentication, we implemented a XMSS hash-based signature scheme that is standardised in IETF. We verified by experiments that selected schemes fit well for a MACsec-enabled Ethernet network.

In 5G networks, an optical fronthaul transports massive user data from remote radio heads (RRH) to the core network (CO) with high throughput and low latency. eCPRI is a new standard interface for the Ethernet-based optical fronthaul network to enhance the efficiency and performance. If fronthaul networks are deployed in an unsafe domain, an end-to-end security system should be implemented over the data flow, which usually requires additional overhead and processing time. This redundancy may cause unexpected latency and performance degradation in the data transport for 5G networks. According to the specification of eCPRI, vendors may optionally implement either IPsec or MACsec to ensure the security of transmission. In this paper, we investigate security solutions suitable for the Ethernet-based optical fronthaul network. We analyze the standard security protocols such as IPsec and MACsec for the security of the high speed link. We propose WireGuard as an alternative security solution which may replace IPsec. According to our analysis, the extended overhead for security protocols has negligible impact on the latency. However, eCPRI processing time can be prolonged, which may cause additional latency and eventually affect the maximum transmission distance of fronthaul networks. As an example, we performed a simulation of eCPRI traffic and measured the latency of WireGuard. We also point out that a re-key interval should be carefully chosen not to compromise the security of the high capacity link such as 5G fronthaul networks. Our analysis is further extended to a long-term security solution which is resistant to quantum attacks.

Linear cryptanalysis introduced by Matsui is a statistical attack which exploits a binary linear relation between plaintext, ciphertext and key, either in Algorithm 1 for recovering one bit of information of the secret key of a block cipher, or in Algorithm 2 for ranking candidate values for a part of the key. The statistical model is based on the expected and observed bias of a single binary value. Multiple linear approximations have been used with the goal to make the linear attack more efficient. More bits of information of the key can potentially be recovered possibly using less data. But then also more elaborated statistical models are needed to capture the joint behaviour of several not necessarily independent binary variables. Also more options are available for generalising the statistics of a single variable to several variables. The multidimensional extension of linear cryptanalysis to be introduced in this paper considers using multiple linear approximations that form a linear subspace. Different extensions of Algorithm 1 and Algorithm 2 will be presented and studied. The methods will be based on known statistical tools such as goodness-of-fit test and log-likelihood ratio. The efficiency of the different methods will be measured and compared in theory and experiments using the concept of advantage introduced by Selçuk. The block cipher Serpent with a reduced number of rounds will be used as test bed. The multidimensional linear cryptanalysis will also be compared with previous methods that use biasedness of multiple linear approximations. It will be shown in the simulations that the multidimensional method is potentially more powerful. Its main theoretical advantage is that the statistical model can be given without the assumption about statistical independence of the linear approximations.