# Trusted Third Party Application in Durable Medium e-Service
## 5 Conclusions  
The paper proposes a novel concept of TTP application in the form of a durable medium electronic service. The main research problem relates to the design and implementation of a trusted and secure e-service for processing and managing sensitive electronic documents. The presented solution applies blockchain technology combined with cryptographic methods. Moreover, an independent company playing the role of a TTP is considered. The application of blockchain technology ensures that once stored, documents cannot be changed nor deleted in the future. In the presented solution, the TTP acts as a trusted, impartial arbitrator who monitors the process of storing and processing documents, and in the case of disputes, makes final and binding decisions. TTP is also a provider of a safe and trusted durable medium e-service, thanks to which documents are stored in a server infrastructure independent of other parties (e.g., a bank and its customers). Due to the nature of this work, the presented research combines both scientific and implementation aspects. The possibility of the application of a trusted third party (combined with blockchain technology) is also analyzed. The experimental verification results confirmed the correctness of the developed e-service. The key feature of the presented approach relies on a unique combination of decentralized blockchain with TTP, which increases the security and reliability of electronic document storage and management. Moreover, the proposed e-service meets all the requirements and law regulations regarding durable media, and was developed based on real market needs. Finally, it was implemented in the real company \"Perceptus sp. z o. o.\" from Zielona Gora (Poland), expanding its product offer. Despite the above-presented advantages, the proposed solution also has limitations. One of them is the fact that the presented e-service requires continuous operation. This means that the servers on which the e-service is implemented require continuous availability, and therefore may generate significant costs. Moreover, the need to ensure the availability of nodes in various locations (decentralization of the service) may also enlarge maintenance expenses. Another disadvantage of the developed solution can also be the large number of generated cryptographic keys. In the proposed approach, a symmetric AES key is generated for each processed document, which--given a large number of documents (numbered in millions)--means the need to store and manage a very large amount of critical cryptographic material in the HSM, whose capacity is also limited. On the other hand, it is worth emphasizing that such a large number of symmetric keys positively impacts the security aspect of the proposed solution (e.g., interception or leakage of one AES key will result in access to only one document, while the rest are safe). Further work will focus on improving the efficiency of the developed solution, reducing transaction costs and increasing both the scalability and the bandwidth of the e-service (i.e., communication with the blockchain, IPFS, and HSM). Moreover, the performance analysis of the blockchain is planned to be carried out to observe how the transaction execution time changes under various conditions. Based on the performance analysis results, there is a plan to compare with other existing approaches and products; however, access to the performance data of other solutions (especially commercial ones) is very difficult. Research plans also involve developing a new consensus protocol that would be more effective and use fewer resources. Additionally, the possibility of using blockchain technology in other areas of the industry will also be investigated. Conceptualization, G.B., K.K. and R.W. ; methodology, G.B., K.K. and R.W. ; validation, R.W. and A.B. ; formal analysis, G.B., R.W. and A.B. ; investigation, G.B., K.K. and R.W. ; resources, K.K. ; data curation, G.B. and K.K. ; implementation, K.K. ; experimental verification, G.B., K.K. and R.W.