# Integrating Classical Preprocessing into an Optical Encryption Scheme
## 5 Conclusions  
In this paper, we give a definition for a symmetric-key encryption scheme using mesoscopic states, including a security definition for such a scheme. We provide an example of such schemes using AlphaEta in combination with a variant of a classical AONT and error correction. Leveraging both an optical and a classical communication channel, we obtain an efficient construction with an interesting (information-theoretic) security guarantee. A forward security property is from a classical point of view quite remarkable: even after revealing the complete secret key, due to the underlying physical principle, the individual bits of the payload still remain hidden. In combination with a classical authenticated encryption, our design creates a situation where an adversary, based on physical principles, does not have even access to the classical output of a cipher, adding a conceptually interesting layer of security to a classical cipher, this being the main advantage over classical solutions. When using repetition codes in our construction, the correctness parameter is improved, while the security parameter does not change, and only some part of the ciphertext is transmitted through the optical channel, which poses an advantage with respect to purely quantum schemes. From an implementation point of view, it deserves noting that our design integrates naturally with existing experimental setups for AlphaEta. All the steps we add can be seen as pre-processing and post-processing of the payload, cf. Figure 2. Therefore, for potential users, e.g., in industry, the experimentally demanding implementation of the optical channel does not have to be altered to be able to benefit from the security guarantees our approach offers. In addition, if one is willing to work with heuristic security arguments, the integration of classical authenticated encryption as outlined in Section 4.1 appears fairly attractive. \begin{table}
\begin{tabular}{c c c c} \hline \(\boldsymbol{r}\) & \(\boldsymbol{\frac{c-1}{2}}\) & \(\boldsymbol{\delta}\) & \(\boldsymbol{\delta^{\prime}}\) \\ \hline
3 & 1 & \(10^{-9}\) & \(3.84\times 10^{-16}\) \\ \hline  & & \(10^{-5}\) & \(3.84\times 10^{-8}\) \\  & & \(10^{-1}\) & \(0.9736\) \\ \hline
7 & 3 & \(10^{-9}\) & \(4.48\times 10^{-33}\) \\ \hline  & & \(10^{-5}\) & \(4.48\times 10^{-17}\) \\  & & \(10^{-1}\) & \(0.2951\) \\ \hline
101 & 50 & \(10^{-9}\) & \(2.56\times 10^{-428}\) \\ \hline  & & \(10^{-5}\) & \(2.56\times 10^{-224}\) \\  & & \(10^{-1}\) & \(1.47\times 10^{-22}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Probability of error for transmitting 128 bits with a repetition code. In general, symmetric-key encryption schemes using mesoscopic states as defined here, should interface naturally with schemes like AlphaEta, as the cryptographic processing assumes certain (abstract) guarantees provided by the invoked optical channel only. In this paper we leave the integration of more involved error correction techniques as future work; it seems fair to say that already our basic design offers a reasonably efficient combination of classical and physical techniques for securing a data transmission. Individual contributions to this article: conceptualization, H.P., R.S., and A.S.C. ; methodology, H.P., R.S., and A.S.C. ; software, H.P. ; validation, H.P., R.S., and A.S.C. ; formal analysis, H.P. and A.S.C. ; investigation, H.P., R.S., and A.S.C. ; resources, R.S. and A.S.C. ; data curation, H.P. ; writing--original draft preparation, H.P., R.S., and A.S.C. ; writing--review and editing, H.P., R.S., and A.S.C. ; visualization, H.P. ; supervision, R.S. ; project administration, R.S. and A.S.C. ; funding acquisition, R.S. and A.S.C. This research was funded in part by the NATO Science for Peace and Security Programme under grant G5448, through research project MTM2017-83506-C2-2-P by the Spanish MICINN, and through AFRL/RIKF Award No. FA8750-15-2-0047. We would like to thank [PERSON], who brought [15] to our attention, and [PERSON] for helpful discussions.