# A Class of Perfectly Secret Autonomous Low-Bit-Rate Voice Communication Systems
## 7 Experimental Evaluation
The leakage rate of secret keys used to fill the initial content of the FIFO memories is only 0.0012 b/b. This value can be reduced at the request of the designer by introducing an additional security margin, which can be of the order of several hundred bits; see Figure 15. \begin{table}
\begin{tabular}{c c} \hline \hline
**Performance Measures** & **Mean Value** \\ \hline KR [kb/s] & 12.88 \\ KR [\%] & 9.98 \\ KAR [\%] & 51.93 \\ Mean number of parity bits [per block] & 6175 \\ Max number of parity bits [per block] & 7063 \\ LR [b/b] & 0.0012 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of SKD protocol performed over the 24 speakers in the first open stage of communication, measured on vPCP-V system. After the phase of open communication and successful filling of the FIFO memories with the initial content, the system transitions to secure communication. Simultaneously, synthesis and public channel communication are performed. Secret keys distillation based on source \(CR\big{(}\hat{S}_{IA},\hat{S}_{IB}\big{)}\) is now occurring. In Table 3, the results of this SKD protocol performed over the 1000 blocks and measured on the vPCP-V system are shown. Since the graphical presentation of these results is provided in Figure 15, Table 3 shows numerical results only for five characteristic SNRs, in the range from 10 to 50 dB. It is noted that in the  
Figure 16: APS-VCS system function in transmission mode. The system function in reception mode is fundamentally of the same structure. already mentioned operating range [29.4; 47.5], KAR does not fall below 100%, while LR is in the order of 0.0011-0.0042 b/b. The presented results show that the SKD over the source \(CR\big{(}\hat{S}_{IA},\hat{S}_{IB}\big{)}\) with large security margins ensures stable refreshment of the FIFO memories with newly generated secret keys, which are then used as secret keys of the Vernam cipher in the main channel. As observed in Figure 15, with an increase in the SNR of the synthesizer, the KR increases regardless of the applied PA strategy. This behavior can be easily explained based on the model (14) of synthesized signals, according to which the mutual correlation between \(\hat{S}_{IA}\) and \(\hat{S}_{IB}\) increases with increasing SNR; that is, with the decreasing influence of local noise \(\xi\) relative to the deterministic component \(h_{LSP}*\delta\). The decrease in innovation entropy with increasing SNR follows the same mechanism: as SNR increases, the proportion of noise \(\xi\) in the total synthesized signal decreases, and, consequently, the corresponding innovation entropy declines. The increase in security margin \(s\) with rising SNR is directly related to its definition as the difference \(c-|K_{i}|\), where \(c\) represents the lower bound of [PERSON]'s conditional Renyi entropy of order two, and \(|K_{i}|\) is the actual length of the distilled secret key; see (26). Since \(c\) directly determines the maximum KR of the specific system (a higher \(c\) leads to a higher maximum distilled KR), it is clear that the variation of the security margin will follow the same dependency with changes in SNR, as also observed in Figure 15.  
Therefore, we can conclude that the experimental evaluation confirms an excellent agreement with the theoretically expected results, both in terms of the key rate and the changes in innovation entropy and security margin with respect to the SNR of the model (14). The presented order of selecting key system parameters KR, SNR, and \(s\) can also be interpreted in the following way. The operating point B in Figure 15 is obtained at the intersection of the KR dependence on SNR and the desired value of KR. This point determines the lower bound for SNR. Point A in Figure 15 is obtained by determining the maximum allowable SNR based on the minimum permitted innovation entropy. The allowable SNR variation interval directly dictates the range of possible values for KR, innovative entropy, and security margin \(s\). **Remark 8**.