# Trusted Third Party Application in Durable Medium e-Service
## 2 Related Works  
The main aim of this section is to present selected solutions using TTP (very often combined with blockchain technology). In the beginning, it is worth explaining what a trusted third party is. TTP is an institution or organization that acts as a trusted intermediary in the process of exchanging data or information between (two or more) parties. TTP can be used in various solutions, for example, to ensure the security of financial transactions, to authenticate users' identities, or to ensure the immutability and integrity of data. Many solutions use TTP, which differs in terms of the scale of operation, method of implementation, and competencies. The most important TTP solutions include:  
* Financial institutions, such as banks or insurance companies, which act as trusted intermediaries in the process of exchanging data and information related to financial transactions;
* Certification systems such as Public Key Infrastructure (PKI), which verify the identity of users and ensure the security of transmitted data;
* Electronic document management platforms that allow the storing, processing, and exchanging of electronic documents securely. It is worth noting that when a service is provided involving conflicting parties, consensus can be achieved by including TTP in digital transactions. On the other hand, if the problem concerns the need to trust one selected party (here TTP), the blockchain network can decentralize trust (trust is spread over many nodes). However, the problem resulting from the use of blockchain technology is that there is no native mechanism to securely manage the identity of the actors involved [5]. In our deliberations, a blockchain is considered a section of middleware that comprises built-in services. Such services are used for the expansion of distributed applications deprived of the participation of a component (like, an authentication or a database server) performing as a central control. The outstanding facilities given by a blockchain include ineffaceable append-only storage, public key-based access control for the carrying out of operations, unrestricted auditability of records, and remarkably, consensus services of a few disciplines. Many varieties of applications are being created based on blockchain; smart contracts are the most identified. During the implementation stage, a blockchain can be well-defined as a distributed data structure (also called the ledger) comprised of an ordered, back-linked list (chain) of blocks. Particular blocks are added to the list based on an append-only model, and only when several nodes holding local copies of the present state of the blockchain attain consensus about its afterward global state. Furthermore, blocks are customarily used for storing records of transactions completed by parties that may not trust each other and reinforce the peer-to-peer (P2P) execution prototype of the transactions. The transactions are actions intended to shift the current state of the blockchain and are not inevitably bank transactions. Smart contracts can be set up on TTP. In particular, a TTP is considered an institution that--along with having the technical structure for accommodating smart contracts--has received trust, reputation, and authority. Time-stamping services, certification authorities, payment gateways, settlement services, custodian services, and various brokerage services are examples of e-commerce applications that customarily use TTP services. For smart contract applications, a TTP is logically positioned in the middle of the two business associates, and it is trusted for hosting and running a single instance of the smart contract. A smart contract witnesses all the actions that associates perform, and retains undisputable records about them. In our assumptions, a TTP-based structure is trustworthy, preventing the modification of smart contracts and securing sensitive data that the corresponding contractual parties uncover.