# Trusted Third Party Application in Durable Medium e-Service
## 3 The Proposed TTP Application  
The first step in the design process of the durable medium e-service was to develop functional requirements based on the state-of-the-art analysis (presented \"related works\" section), as well as on the technical capabilities of the real company. The designed e-service should meet the following functional requirements:  
* data of one entity (e.g., bank, internet service provider-ISP) should be stored in a non-public database limited only to the data of a given entity,
* data should be stored in a way that prevents their modification (once saved in the database, it should not be possible to delete or modify them),
* data (including sensitive electronic documents) should be stored in an encrypted form so that in the event of a possible leak, they (and the personal data contained in them) are useless to cybercriminals or third parties,
* access to data should be, on the one hand, convenient (especially for users/customers), and on the other hand, secure and controlled,
* a fast symmetric encryption algorithm (e.g., AES) should be used to encrypt documents,
* the e-service should be developed for entities (e.g., financial) with a large number of clients (the entity plays a dominant role) that need to store and process documents, meeting the requirements of a durable medium (by Polish law regulations),
* data should be saved in a decentralized database, allowing data to be distributed between nodes;
* the service should offer the ability to generate an AES symmetric key and encrypt/decrypt it using an asymmetric key pair generated by the Rivest-Shamir-Adleman (RSA) algorithm.  
To meet all the above-mentioned functional requirements, in the proposed approach, a TTP plays the role of a trusted intermediary in the process of transmitting, exchanging, and processing electronic documents between several parties. Thanks to this, electronic documents will be processed securely, and their integrity and immutability will be maintained. Additionally, the presented electronic service (e-service) uses immutable and secure blockchain technology [34].  
The next stage in the design process was to model the e-service. The main idea of the proposed e-service on a high level of abstraction is presented in Figure 1. The approach assumes that two business sides and one TTP participate in the process. Moreover, one of the business parties is the dominant entity over the other (e.g., a bank towards its clients) due to the fact that the proposed solution is dedicated to large business entities or institutions that offer services to a large number (e.g., tens of thousands) of clients. Nevertheless, the technique can be easily adapted to other business scenarios where more than two business parties are involved, and they may even be equivalent to each other (e.g., three entrepreneurs processing a common, tripartite business agreement).  
The proposed approach is mainly focused on two important aspects. The first one is related to security, implemented through the use of blockchain technology and cryptographic methods securing the process on the TTP side. Blockchain ensures that data is immutable and cannot be modified by any party, which also ensures data integrity and reliability. The second aspect refers to trust, which is achieved by using a TTP as an impartial arbiter independent of any parties, managing the electronic document processing. Moreover, the TTP stores the blockchain database in its infrastructure. This means that only the TTP can perform block addition operations, as well as encrypt, sign, and share documents. This approach significantly reduces the possibility of interference by one party in the document flow, even if it is the dominant entity in the business relationship (e.g., a financial institution towards its customers).