# Trusted Third Party Application in Durable Medium e-Service
## 1 Introduction  
Electronic documents are widely used in many areas, including business transactions, where contracts between various companies often go beyond national borders, and cooperations are concluded around the world. Electronic documents are convenient because they can be easily and cheaply stored, sent, and shared with other people and companies. In addition, there are many tools for their editing and processing. Therefore, paper documents are slowly being replaced by electronic data, and more and more businesses transactions are carried out using only electronic documents, which greatly facilitates conducting economic activity. Many of the processed documents, especially official ones or business documents, often contain information that should be protected, and their publication may be harmful to enterprises or citizens (e.g., by disclosing their data), and even violate their personal and property rights. Users have many concerns about the security of their digital transactions. One of the greatest threats is cybercrime [1]. These may include activities such as computer hacking, theft of personal data, internet fraud, attacks on IT systems, etc. Another threat is the risk of losing access to electronic documents, for example as a result of computer failure, malware infections, or damage to the data medium. Users may also be concerned about the security of personal data that may be misused by third parties. That is why it is important to use secure and trusted tools to protect electronic documents from all these threats. Moreover, there are also situations when the documents (or more broadly: data) should neither be made public nor changed (modified). This is the so-called non-repudiation problem [2], and covers situations when large corporations (e.g., banks, telecommunications companies, etc.) publish documents (e.g., on their websites or internal systems), but then make unauthorized changes to these documents. In such a situation, the user has no control over these changes (and often they are not even aware of them) and--as an individual--cannot prove irregularities in court. The crux of this problem is to assure all parties of the document that the data recorded in the electronic document will not be changed by any of the parties without the knowledge and consent of the others. This is especially important for documents that are used in business transactions. Cryptography methods and blockchain technology can be used to face this problem. In the case of applying cryptography, this mainly involves encrypting the document (using symmetric or asymmetric cryptography). On the one hand, the point is to make the encrypted data unreadable for third parties (without the secure key, the data cannot be decrypted), so even if they are shared or leaked, the risk of their disclosure is very small. On the other hand, by using the hash function, a hash of the document can be generated and shared to all parties. Thanks to that, if a document is changed by one of the parties, the other parties will be able to easily detect it, because the hash of the changed document will not match the original hash. Blockchain is a technology that allows the creation of distributed and immutable records of data. It consists of a network of interconnected nodes (e.g., computers) that store copies of data stored in the blockchain. Each new entry in the registry (so-called block) is confirmed by most network nodes, which ensures that the data are safe and unalterable. Blockchain can be used to solve the non-repudiation problem because it ensures that the data contained in the register are immutable. Thanks to this, even if one of the parties would like to change the data in the electronic document, it will be impossible because this change will be detected by other network nodes and immediately rejected. Therefore, using cryptography and blockchain can ensure that the data contained in the electronic document remain unchanged and indisputable.