## 1 Introduction
Just as the security of data in the cloud is important, the preservation of the privacy of the data such as medical information of people is also very important. However, the preservation of data privacy on the cloud has various challenges. Some of these challenges include:  
1. Cloud service providers may not have privacy protection that is strong enough to protect the data that are put in their care [28];
2. It is possible that cloud service providers change their technology without informing their customers; in such a case, issues related to performance and latency may arise [29; 30];
3. If cryptography algorithms alongside authentication techniques are used to preserve the privacy of data on the cloud, searching for information from the encrypted data becomes an issue, as encryption conceals the relationships among the data [31];* If the secrete key of encryption is released to cloud service provider, the security of the data being protected may be breached [32; 33; 34; 35; 36];
* Apart from breaching the security of the data, performing decryption of the data before searching for particular information in the data stored on the cloud is not efficient, especially when dealing with a huge amount of encrypted data or resource starve devices. Researchers have identified these problems and have come up with different approach to address issues around the privacy preservation of cloud data. However, deficiencies have been found in many of these existing techniques. Searchable encryption (SE) appears a suitable approach to solve the data privacy problem in the cloud setting. An additional challenge is raised by SE in the multiuser setting, whereby each user may have access to a set of encrypted data segments stored by a number of different users. Multiuser searchable encryption schemes allow a user to search through several data segments based on some search rights granted by the owners of those segments. Privacy needs in this setting are many, and not only the confidentiality of the data segments but also the privacy of the queries should be ensured against impostors and potentially malevolent CSP. However, existing searchable encryption techniques such as Fully Homomorphic Encryption (FHE) implementations (e.g., [37; 38; 39; 40]) require computationally intensive operations which make the schemes impractical. Practical FHE is still far from being realized, and improving the performance of FHE is still a very active research area. In this research work, a secure IoMT model with an efficient cloud data privacy-preserving technique is projected for securing and preserving the privacy of sensitive medical information on the cloud. The scheme employed the use of a hybrid-based encryption/decryption scheme named Hybrid Modified Caesar Cryptosystem (HMCC). In HMCC, the existing stream Caesar cipher is modified into block cipher for improved data security, Elliptic Curve Diffie-Hellman (ECDH) is employed for secret key sharing, and an Elliptic Curve Digital Signature Algorithm (ECDSA) is applied to achieve digital signing and signature verification. The scheme proposes the application of B+ file organization for easy file storage and retrieval on the cloud. This research has contributed to the body of knowledge as outlined below:  
* The existing Caesar cipher was modified to improve its security by extending the character set of the existing Caesar cipher from 26 to 256 (0-255), reshuffling the extended character set using an encryption key, and determining the ciphertext characters based on the location of each plaintext character in the plaintext and the location of the character on the character set using modulus 256 addition. The modulus 256 subtraction was used for recovering the plaintext characters from the ciphertext based on the position of each character on the ciphertext and on the character set.