# On the Security of Offloading Post-Processing
## 3 Outsourcing Information Reconciliation
### Multiparty Computation-Based Outsourcing
This is even more true for high-cost security-certified equipment, because upgrading security-certified equipment is a cumbersome and costly process which typically requires recertification. Being able to update certain non-critical components without needing to exchange or recertify the core QKD device hardware can greatly simplify the upgrade process. To show how the advantages relate to concrete use cases, we will quickly mention three examples. Access networksIn the case of access networks, we find constraint resources (computing energy), and the network units must be low-cost because they are the driving cost factor, especially since only very low key rates are typically required (AES key refreshing). If computational resources are pooled in such a scenario, costs can be substantially reduced not just in case of a reduced user subscription ratio but also due to time sharing of centralized CPU resources. Furthermore, because the optical part is low energy, the dominant cost and energy factor is when a CPU is partially idle, which should be avoided. Satellite communicationSatellites have a particularly long lifetime (20-30 years) and have to be remotely operated and maintained. They also have limited access to energy resources and reducing energy consumption is of paramount interest. This is especially true if low-cost (mobile) earth stations should be supported or even inter-satellite links. Offloading post-processing can make satellite transceivers possible and increase the connectivity for individual satellites. Integrated COTS HardwareFinally, aside from the evident advantages of outsourcing protocols such as REM-IR to data centers, the concept can also be interesting when applied within the device. QKD devices are complex systems [43] and comprise many different components, which makes security auditing and certification very hard. To achieve strong security guarantees, only trustworthy hardware and software can be used to process key material in plaintext [44]. Furthermore, to prevent from side channel attacks and backdoors, it would be desirable to reduce the number of trusted components and the complexity of the secure environment in a device as well as possible. Therefore, if the components processing sensitive key materials can be reduced, this results in a smaller attack surface, simplifies security analysis and helps in the certification process. The MPC-based protocols presented can be used for this purpose (i.e., to reduce the trusted environment on the device architecture level with all its benefits). Within a device, it is also feasible to realize the secure channels required in the MPC model. Thus, it would allow for the integration of COTS hardware in QKD systems only processing keys in encrypted form. ## 6 Conclusions

In this work, we introduced the idea of offloading the information reconciliation and privacy amplification steps of QKD post-processing. These are the two computationally intensive tasks in processing raw key measurements to secure a shared key between two QKD peers. We showed that outsourcing information reconciliation is possible and straightforward for DV-QKD, even in a single-server model and against an active adversary. However, for CV-QKD, which leverages reverse reconciliation to overcome the 3 dB transmission bound, the same is not true. We also gave an intuition that it is not possible in general to achieve positive key rates with a single server and analyze potential performance in a multi-server setting. We also looked into privacy amplification, where we propose a protocol for multiple servers. Finally, we laid out the potential benefits and discussed use cases where this approach is relevant. Proving the impossibility of single-server PA offloading as well as _weak offloading_ is left for future work. Additionally, MPC-optimized versions of sum-product decoders are currently under investigation and will be presented in a follow-up work.