## 3 Model and Scheme
### Scheme Implementation
#### 3.2.2 The CCES-Based Scheme  
We propose a CCES-based scheme under the above scenario, taking m-sequence as the pseudo-random sequence \(PRS\). M-Sequence Generation  
As a kind of typical pseudo-random binary sequence used in radio communication, m-sequence has complete characteristics of \(PRS\) and a broad range of applications, like FHSS, code division multiple access (CDMA), encryption, etc. m-sequence represents the longest sequence generated by the linear feedback shift register (LFSR) shown in Figure 4a. In Figure 4a, \(c_{i}\) represents the connection state and \(c_{i}\in\{0,1\}\) where 0 and 1 mean _disconnected_ and _connected_, respectively, and \(a_{i}\) represents the shift register. A necessary and sufficient condition for the sequence generated by an LFSR to be m-sequence is that its \(f(x)\) is a primitive polynomial [32]. Message Preprocessing  
In this section, the original message \(M\) will be converted into \(M^{\prime}\) by the encoding rule \(F_{1}\). Supposing the original message \(M\) is \"hello\", we first gain the decimal value of each character according to the ASCII table, then convert them to binary. Since the printable characters' range in ASCII is [32; 126], the maximum binary is \"0b1111110\" with a length of 7 and the minimum is \"0b100000\" with a length of 6. To unify the format and process conveniently, we fix the binary length as 7 and left-pad it with zeroes. Considering that the primitive polynomials' orders are different, we appropriately merge the converted binary values and right-pad them with zeroes. At this stage, the necessary prerequisite communication data elements, including cryptographic keys, are distributed by a Certificate Authority (CA) or a trusted third party to ensure secure initialization. M-Sequence Modulation  
In this section, m-sequence will be modulated by \(m_{k}^{\prime}\) in \(M^{\prime}\) in turn, which is depicted in Figure 4b. It is worth mentioning that the initial values of \(a_{i}\) in LFSR are used as the PRS generation key shared by \(P_{s}\) and \(P_{r}\), namely \(PRSkey\). For instance, we select \(f(x)=x^{7}+x^{3}+1\) as the primitive polynomials to generate m-sequence and \(PRSkey\) is \"0b1010101\". Thus, the unmodulated m-sequence \(PRS\) is: 0b1010101000010111110011100101010100001101101000010101010100000001001001101101110000111111110001110011100100101010111111  
carrier of \(M_{PRS}\), verification field to identify the legal pieces of end spreading sequences, and framing flag to distinguish different \((m_{prs})_{k}\), respectively. Firstly, \((m_{prs})_{k}\) needs to be divided into \(m\) (\(m=\frac{P}{n}\)) groups where \(p\), \(n\), and \(m\) means the length of \((m_{prs})_{k}\), the unit length (i.e., the number of bits per element), and the number of subgroups in \((m_{prs})_{k}\), respectively. Then, each subgroup needs to be converted from binary to decimal, thereby deriving the \(DstPort_{k}\) sequences corresponding to \((m_{prs})_{k}\). Eventually, based on the pseudocode shown in Algorithm 1, we can also gain the corresponding \(DstIP_{k}\) and \(SrcPort_{k}\) sequences. Following the above assumption, we divide \((m_{prs})_{k}\) into 127 subgroups, respectively, and the results are shown in Table 1. Based on Algorithm 1, we assume that the \(mappingkey\) is 85, \(DstIP_{pool}\) is \(\{192.168.1.91,\cdots,192.168.1.100\}\), and the \(binlen\) is 8; then, the generated \(DstIP_{k}\) sequences are displayed in Figure 5 and \(SrcPort_{k}\) sequences are displayed in Table 2, where the length of \(DstIP_{k}\) and \(SrcPort_{k}\) sequences are both 127. However, the number of corresponding SrcPort-DstIP-DstPort combinations is too large to transmit practically. In the CCES scheme, we merely successively select \(e\) (SrcPort, DstIP, DstPort) pairs to indicate \((m_{prs})_{k}\), which not only greatly reduces the quantity of SrcPort-DstIP-DstPort combinations, but also brings little transmission accuracy disturbance.