# Trusted Third Party Application in Durable Medium e-Service
## 2 Related Works
This problem can be resolved with the support of newly evolving trusted hardware technologies like Intel SGX [6] and ARM TrustZone [7], where the contract code might be deployed in trusted environments (so-called trusted world in TrustZone and enclaves in SGX). The trusted hardware stops the TTP from modifying the integrity of the contract program and prevents the investigation of sensitive data. Some solutions apply the TTP in the digital data models of data security. In such techniques, the TTP is responsible for data authentication, confidentiality, and data integrity. For this purpose, one-time passwords (OTP) are used to provide authentication. In addition, data confidentiality is ensured by the cryptography algorithms used for data encryption and decryption, and data integrity is ensured using data hashes [8]. Another interesting application of a semi-trusted third party is presented in [9], where authors introduce a buyer-seller watermarking protocol that can be very useful for copy deterrence and privacy preservation in the cloud environment. The proposed solution combines services of cloud infrastructure as a service (IaaS), a cloud service provider (CSP) considered as a semi-trusted third party, a privacy homomorphism cryptosystem with Diffie-Hellman key exchange algorithm, and robust and fair watermarking techniques. The proposed approach addresses the problems of piracy tracing, anonymity, tamper resistance, or customer rights problems. The cloud plays a crucial role because it reduces communication overhead and supports the watermarking process. In [10], the authors propose a TTP scheme based on signcryption using the session key exchange protocol (symmetric cryptography is used here). Thus, the session key exchange protocol prevents eavesdropping during cyberattacks such as denial of service (DoS) or man-in-the-middle (MITM) attacks. Because the encryption process is performed by the cloud service user (CSU), and the decryption is made by the CSP, the additional overhead of TTP encryption and decryption has been reduced. Moreover, the main role of TTP is to resolve a dispute between CSU and CSP. The proposed solution provides seven security functions, such as data integrity, data confidentiality, authenticity, non-repudiation, transmission secrecy, non-falsification, and non-traceability. Approaches and solutions based on the TTP have been widely known and used over the last several decades. In the 1990s, research was already being conducted around the TTP concept, and a solution was sought for the problem of providing TTP services [11], such as managing cryptographic keys for end-to-end encryption in a way that meets legal requirements. The authors also present other possibilities and problems in the use of TTP; e.g., the need for a certification hierarchy to define a common point of trust for different TTPs, sharing keys between TTPs, and proposing a bidirectional communication scheme in which two keys are used-one for each direction (it is also possible to combine both of these keys into one session key or use only one of them) [11]. The authors in [12] present a key exchange scheme and data encryption between users with the use of TTP. The main aim is to ensure the security of the exchanged data, as well as shift most of the computational load to the TTP. A trusted third party may also be responsible for storing and periodically renewing public key certificates. The proposed solution combines TTP with symmetric and asymmetric cryptography, ensuring a high level of data security. Another interesting solution is a protocol proposed in [13], which combines advantages of security, scalability, simplicity of application, and the possibility of real implementation. The protocol uses a lightweight TTP, available online. Thanks to this, the end user requires only a standard e-mail reader and a web browser. The proposed solution also does not need a public key infrastructure.