# A Class of Perfectly Secret Autonomous Low-Bit-Rate Voice Communication Systems
## 7 Experimental Evaluation  
In the first step of the synthesis of the APS-VCS system, it is necessary to choose operational values for the main system parameters, such as the secret key rate, SNR of synthesized signals, innovation entropy rate, and security margin. Figure 15 shows the interdependencies of the operating ranges of these quantities, obtained on a real APS-VCS system, by averaging 100 values for each SNR value in the range from 10 to 50 dB. A distillation of secret keys was performed using three different PA strategies: mean (blue line), \(3\sigma\) (orange line), and \(3\sigma+s\), \(s=20\) (green line). If the PA strategy \(3\sigma+s\) is taken as a reference, and KR is at least 2.4 kb/s, it is obtained for an SNR [dB] working range [29.4, 47.5], innovative entropy [kb/s] [65; 88], KR = 2.5 kb/s, and security margin [b] [70; 460]. The order of selection is as follows; see Figure 15:  
Figure 14: Empirical distribution of Huffman–Renyi distance \(D_{HR}\) (\(\overline{D}_{HR}=1.84\), \(\hat{\sigma}_{D_{HR}}=2.39\)) for synthesized signals with parameter SNR = 39.9 dB. Figure 15: Interdependencies of operational ranges of main quantities: secret key rate, SNR of synthesized signals, innovation entropy rate, and security margin in the synthesis of APS-VCS system. Security margin dependence on SNR is shown for the secret key rate of 2.5 kb/s. 1. The desired KR is selected. Recall that it must satisfy constraints (5) and (6). 2. The security margin \(s\) is chosen in accordance with the requirements of the overall security of the system. Taking into account (26) and (28), with increasing \(s\), the degree of compression in the PA block increases, and thus, the information [PERSON] can obtain about the generated keys decreases exponentially. 3. The choice of security margin \(s\) uniquely determines the SNR. 4. The obtained value for SNR uniquely determines the innovative entropy. Since for each pre-fixed KR, the security margin can be in a wide range, the system designer has great freedom to easily choose the parameters of the synthesizer that will simultaneously satisfy the requirements for the rate of generating secret keys, their maximum entropy, non-repeatability, and negligible information leakage to [PERSON]. All these elements confirm the basic requirements that must be fulfilled by the secret keys of the Vernam cipher in order to maintain its perfect secrecy. Figure 16 shows the functional description of the operation of the APS-VCS system. The subsystem for generating secret keys starts working after the end of the open communication phase. A necessary condition for the functioning of this subblock is the initial successful filling of the FIFO memories on the side of [PERSON] and [PERSON] with distilled secret keys of at least 160 bits. The first 80 bits will be used as a secret key to encrypt the LSP of the first block of the input speech signal, while the next 80 bits will be used for the LP synthesizer in the SKD block. Algorithm 1 provides a detailed explanation of the process of transforming the binary sequence into LSP parameters, while Algorithm 2 offers a detailed explanation of the transformation of LSP parameters into LP synthesis filter H(z) parameters. Table 2 shows the result of the experimental evaluation of distilled secret keys based on source \(CR\big{(}\hat{S}_{l}\,,\,\hat{S}\big{)}\) during the open phase of communication. Since the average value of KR = 12.88 kb/s, just 1 s of open communication on average fills the FIFO memories with the entire 12.88 kb, which far exceeds the required 160 bits. Note that the Key Acceptance Rate (KAR) does not have to have a maximum value of 100%, which is an important indicator of the efficiency of the SKD protocol in usual applications [11]. The measured value of the exchanged bits on the public channel (average--6175 bits, max--7063 bis) per block of 8640 bits shows a significant information leakage about the input speech, which is not a security treat because it is not encrypted at this stage.