Secure data storage and data verification using blockchain based smart contract

Hyperledger Fabric and Ethereum blockchain platforms are platforms that use smart contracts that stand out by providing a decentralized, secure and transparent environment and are suitable for different usage scenarios. Hyperledger Fabric, which is especially preferred in corporate applications, draws attention with its modular structure, authorization and identity management mechanisms, channel system and its structure that allows the use of special smart contracts related to the transaction. It is of great importance for some institutions to keep military, and medical images or images/documents with high confidentiality. The HFSecImg study presented within the scope of this thesis was created as a solution proposal to important problems such as data security and data validation. This study offers an effective solution in terms of data security and verification with the use of blockchain technology. This solution is aimed to effectively solve the data validation problem by using a smart contract to securely encrypt the image data, distribute it among the stakeholders and store it reliably, ensuring stakeholder signatures and data verification of the encrypted image in the broadcast. In the first step of this approach, the image is encrypted with the chaotic system-based image encryption method. It is aimed to increase the security of private information between stakeholders by determining the key used in encryption by Shamir's Secret Sharing method. The summary information of the encrypted image and the digital signatures of the stakeholders are stored in the Hyperledger Fabric blockchain network. In addition, the private and public keys in the signatures of the stakeholders were created using the RSA encryption algorithm. The use of smart contracts is emphasized in the study, which is based on data integrity and security. The signatures of the stakeholders and the encrypted image kept in the distributed ledger go through the verification process with the Hyperledger smart contract mechanism. In this way, security and data integrity are ensured by using blockchain technology. It is aimed that the encrypted image is stored in the cloud system, open to everyone on the network, easy to access and low-security risk. The analyzes and the mechanisms used have shown that a successful secure data storage and data validation mechanism has been established. In the second study, called Ethtrace, the use of blockchain technology in transparent donation applications and the advantages of using Ethereum-based smart contracts are discussed. In this study, a sample application has been developed for this issue. It is aimed to automate the donation process, resolve disputes between donors and recipient organizations, and provide secure donation management. When the application is evaluated in terms of transparency, traceability and security, it has been seen that it gives successful results. This study aims to increase the social benefit by making the donation process transparent and secure, and by ensuring that donors' donations are used effectively. At the same time, by offering the opportunity to donate to more than one organization at the same time, it has contributed to the literature in terms of appealing to a wider donor mass and revealing a possible synergy between donation applications and blockchain technology. The common aspect of HFSecImg and Ethtrace blockchain applications implemented within the scope of this thesis is the use of blockchain smart contracts. In addition, the advantages, disadvantages and areas of use of smart contracts belonging to two different applications are examined in detail. The differences between the applications are privacy, security, cost, etc. have been studied from many perspectives. As a result, information about the platform that should be selected according to the application to be developed is given. It has been shown that the choice of blockchain platform is very important depending on the structure of the developed application.