## 3 The Proposed IoMT Model
### Secure Data Transmission and Storage
These results show that HMCC has similar behaviour to the standard AES algorithm in terms of autocorrelation analysis. * **Comparative Analysis of HMCC, AES, and DES based on SKAC and SPAC**

SKAC and SPAC analyses were conducted for HMCC, AES, and DES. A summary of the results for the three ciphers for SKAC and SPAC results are respectively given in Tables 12 and 13, where the mean, standard deviation (STD), and coefficient of variance

Figure 10: **(a)**: Autocorrelation of plaintext. **(b)**: Comparative analysis of autocorrelation of ciphertexts produced by HMCC, AES, and DES. (CV) of the SKAC and SPAC values are calculated. The coefficient of variance measures the amount of deviation of data points from the mean. It is calculated by dividing the standard deviation by the mean of the dataset. As the CV is the ratio of STD and means, it provides a means of comparing variations in different datasets even when the means are different. It can be seen in Tables 12 and 13 the AES has the lowest CV values, while DES has the highest values of CV. These results imply that AES provides better performance in terms of SKAC and SPAC than HMCC and DES. However, HMCC performs better than DES in terms of SKAC and SPAC. It can, therefore, be said that HMCC performance in terms of SKAC and SPAC is comparable to that of the standard AES. ## 5 Limitations and Areas of Future Research

The proposed IoMT in this paper promises to be efficient and secure. However, there is always rooms for improvement. The value that represents the interpretation of the measured value from the sensor is arbitrarily assigned. There should be a standardized method to achieve this feat. Security and storage aspects could be improved by employing a heterogenous and compact way of representing data, such as the technique proposed in [69]. Multiple IoT Environment (MIE) [70] and the Multiple Internets of Things (MIoT) [71; 72; 73; 74] address data-driven and semantics-based aspects because they consider the contents exchanged by smart objects during their transactions; consequently, the extension of the concept in the proposed IoMT model to both MIE and MIoT will play a significant role. Future research should also consider the implementation of the proposed IoMT model in a real CSP platform. ## 6 Conclusions

In this contribution, a secure IoMT model that is resistant to various security attacks and preserves the privacy of patient data is proposed. In the design, an existing stream Caesar cipher was modified into a symmetric block cipher and an efficient technique that searches through encrypted files stored on the cloud without decrypting the file was developed. The Elliptic Curve Diffie-Hellman technique was employed to exchange shared secret keys to form the Hybrid Modified Caesar Cryptosystem (HMCC). A complete security analysis of the proposed HMCC was carried out based on python simulations. Generally, results show that the HMCC is resistant to brute-force attacks, sensitive to any change in secret key and plain text, can resist an entropy attack, generates a uniform histogram with low autocorrelation, is robust against differential, dictionary, and frequency analysis and classic attacks, and strict key avalanche criterion and strict plaintext avalanche criterion are satisfied within very minimal errors. Additionally, analysis of the IoMT-based model reveals its resistance to all forms of Man-in-the-Middle attacks. A comparative security analysis of the projected HMCC with existing ciphers also revealed that the HMCC security is comparable to that of standard AES and more secure than the DES, and the encryption time is very fast. HMCC also uses variable key length, which makes it \begin{table}
\begin{tabular}{c c c c} \hline \hline
**CIPHER** & **MEAN** & **STD** & **CV** \\ \hline MCC & 0.498977 & 0.004795 & 0.009609 \\ AES & 0.499663 & 0.00397 & 0.007945 \\ DES & 0.438242 & 0.005367 & 0.012248 \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Comparative Analysis of SKAC for HMCC, AES, and DES.