# Trusted Third Party Application in Durable Medium e-Service
## 4 Experimental Verification of the Proposed e-Service--A Case Study  
The presented solution was experimentally verified using a few test scenarios. Two of them are presented in the paper. In the presented scenarios, the e-service of a durable medium was implemented for the ISP, which plays a dominant role in the process (it means that only the ISP can initiate document processing), and the role of the ISP clients is limited only to accepting or rejecting the document. TTP plays the role of the e-service provider and a supervisor, which monitors the flow of document storage and processing (including management of the blockchain and IPFS network). The blockchain was implemented using the Hyperledger Besu [44] platform. Besu is an Ethereum client designed to be enterprise-friendly for both public and private permissioned blockchains. Moreover, it includes several consensus algorithms including PoS, PoW, and PoA (IBFT 2.0, QBFT, and Clique). This platform also supports smart contracts, which have a significant impact on the security of the entire solution [45]. Table 1 presents the selected configuration parameters of the blockchain implementation (please note that some of them are specific only for the Besu environment). \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline blockchain type & private with IPFS \\ consensus algorithm & IBFT \\ number of nodes allowed to approval new blocks & 4 \\ block period seconds (in seconds) & 2 \\ request timeout seconds (in seconds) & 4 \\ gas limit & 9,007,199,254,740,991 \\ contract size limit & 2,147,483,647 \\ minimum gas price & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The selected configuration parameters of the blockchain implementation. In the first test scenario, it was assumed that the ISP wanted to introduce a new document (e.g., an annex to the contract) for each of its 10,000 customers. Each of these documents are personalized and dedicated only to one of the ISP clients and must be agreed with the client (accepted or rejected). As a result, the e-service must process 10,000 various documents that need to be agreed with 10,000 ISP customers. In the first step, the ISP selects the 10,000 documents and customers to which this document concerns and sends the documents and 10,000 identifiers of the ISP clients to whom these documents refer to the TTP. TTP first signs all the documents with an electronic TTP signature and generates 256-bit symmetric AES keys-one for each document. Then using the AES keys, each of the 10,000 documents is encrypted (each document has a corresponding different AES key). Then, TTP sends the encrypted (signed) documents to the IPFS server, which generates a hash using the SHA-256 algorithm for each document and stores it in the IPFS network together with the encrypted document. For each stored document, IPFS also generates a link which, together with the hash, is sent back to the TTP. In the next step, TTP adds 10,000 transactions to the blockchain (one transaction for each document). Each transaction contains a hash of the document (DID field in the blockchain structure), the identifier of the entity that sent the document (this is a numerical value identifying the ISP-CID field), and the identifier of the document addition operation (OID field). Due to the limitations of the data limit that can be stored in the blockchain, the document itself is not saved in the block. The right, encrypted documents along with their hashes are stored in the IPFS network. In the next step, TTP encrypts each of the AES keys with a 2048-bit ISP public key using the asymmetric RSA cryptographic algorithm. Then, a list containing 10,000 items is sent back by TTP to the ISP. Each item in the list contains the following information: a hash of the document, a link to the encrypted document in the IPFS network, an encrypted AES key, and the client identifier to which the document relates. Then, the ISP decrypts all the AES keys using his private RSA key.