1 Introduction

The deployment of decentralized software applications for higher education institutions (HEIs) has the potential to establish a decentralized network among various departments within HEIs. The contemporary blockchain-based systems offer a solution for these networks to facilitate the management, verification, and generation of data in a manner that is autonomous, transparent, and secure [1]. Decentralized application systems play a very important role in the operation of blockchain-based systems, as they allow the management of data and the preservation of privacy by transferring the data to different storage, be it cloud storage, a blockchain database, or different servers. In this way, they offer real-time communication between users and external devices and the realization of transactions based on predefined parameters. The concept of implementing blockchain technology in higher education institutions has been discussed for some time, with initial proposals originating from the European Commission [2]. The objective behind these proposals was to enhance the quality of education. However, to date, there is a lack of a comprehensive blockchain system that can effectively measure student progress and validate academic credentials, such as diplomas. While there have been some limited solutions and research conducted in this area, a comprehensive system is yet to be developed [3].

Despite protocols and cryptographic security mechanisms for the protection of privacy, integrity, and real-time access to data, there are still many limitations and challenges in terms of data storage and secure client–server communication. The goal is to get around these problems by building decentralized architectures for blockchain-based systems that work, especially for keeping data safe and accessible [4]. The safety and reliability of blockchain technology is seen in the way it operates through nodes. Each node in the blockchain network retains the data of the preceding node, together with the whole transaction history of the network it belongs to. Therefore, the authentication of ownership, history and execution of transactions is done automatically, and it is impossible to change the data in such a structure since each node contains a copy of the entire ledger [5].

The process of digital transformation and the conversion of centralized application systems into decentralized systems pose significant challenges, particularly within the context of HEI. The intricate nature of private and sensitive data is involved in these institutions. Such a process is not only the adaptation of contemporary technologies, but it is also a process when all current data must be transferred, all current systems must be modified, all databases must be adapted up to the change of architectures and the creation of new models of operation and management of systems in the HEIs [6]. The integration of artificial intelligence (AI) plays a crucial role in the development of blockchain systems for higher education institutions. This is particularly significant in ensuring security measures, establishing smart contracts, and addressing concerns related to privacy and ownership. The incorporation of AI in these areas represents the most recent advancements in implementing blockchain systems, specifically within the context of higher education institutions [7]. Without going into details, AI in the blockchain system for document management in higher education institutions would facilitate the detection of potential threats to the system and automate various tasks. This is made possible by the system's ability to learn from human actions and subsequently replicate them without human intervention [8].

It is worth noting that the combination of artificial intelligence with blockchain has led to the introduction of the concept of decentralized AI, which means the processing, analysis and transmission of data that have previously been processed by a blockchain infrastructure. The implementation of decentralized AI is of great importance, especially for increasing data security and intelligent processing of data through machine learning, respectively deep learning. In terms of security, AI can be used in the blockchain system to detect system anomalies and efforts to prevent any misuse, the use of biometric devices to identify users in the system [7]. The Homomorphic encryption enables AI to perform calculations on cryptographically protected data (processing encrypted data without the need to previously decrypting it). Our system DIAR (Diploma Integrity Authentication Record) can leverage AI capabilities in order to create a user-friendly interface for all users, enabling the inclusion of chat bots for communication with users and help in various aspects, voice recognition, and intelligent searches using photo recognition. AI can also be used in the process of generating and verifying academic documents, to verify ownership of documents and guarantee authorship, but above all the automatic generation, evaluation and verification of documents using intelligent mechanisms and in smart contract programming [8]. Above all, AI enables continuous auditing and monitoring of the system, always warning of risks, and enables the system to learn more and more from human actions, to perform the same automatically in the future. Also it can influence the preservation of data privacy using techniques such as federated learning, or machine learning, which enable, among other things, the reduction of data transfer all the time, and the creation of secure models for data transmission between all nodes in the blockchain network [9].

Prior to the development of such a decentralized system using blockchain and AI for the generation and verification of academic credentials, in this paper we develop a conceptual model that outlines the system's functionality and the processes it will undertake. The objective of conceptual modeling is to establish the fundamental framework of a system, encompassing its behavior within its operational environment [10]. Typically, this process precedes the programming of smart contracts. During the development of a conceptual model, all entities that will participate in the system, as well as their interconnections, concepts, operations, communications, and collaborations, are incorporated [11].

Through the paper, we first describe the concept of conceptual modeling, the systematic architecture of some processes in higher education institutions, up to the concrete design of the diagram, where we describe in detail the communication infrastructure between the main actors in these institutions. The proposed blockchain system aims to offer the following features:

  • Data authentication, ensuring the protection of data integrity;

  • Data transparency, as certain public data will be visible within the system. Consequently, each user within the blockchain network will have the ability to verify or authenticate any information pertaining to any student;

  • Stability, in the context of system functioning, refers to the capacity of the system to continue operating seamlessly even in the event of an assault on any of its nodes;

  • User-friendly interface, facilitating seamless communication between system actors without compromising the overall design;

  • Detecting any misuse in terms of generating the diploma in electronic form;

  • Generation and verification in real time;

We believe that the application of blockchain technology for the creation of digital systems for the generation and verification of documents is the future that should be applied in higher education institutions and other institutions of the same nature. The rest of the article is organized as follows. In the next section, we provide details of blockchain solutions that have to do with institutions of higher education, giving the main characteristics for some of them, and at the same time the challenges of implementation in institutions of higher education. Section III is the most important section in our work, where, among other things, we present in detail the conceptual model of the blockchain system, followed by system architecture and the architecture of the process in the generation and verification of diplomas in higher education institutions. Also, part of the third section is the design of the smart contract architecture, graphically presenting the connection and the most important smart contract of the blockchain system. In section IV, we discussed the desirable properties of our system, including its advantages and limitations in relation to state-of-the-art. Finally, Section V concludes this research, as the last stage for practical implementation of the system.

2 Background and related works

Before proceeding with the design phase of the conceptual model and the detailed architecture of these processes, in this section we provide background information and conduct a literature review pertaining to the associated problems in the domain of blockchain and HEI.

Although various prototypes have been developed to generate and authenticate academic credentials, as summarized in Table 1, there is currently no research demonstrating their successful implementation and positive outcomes. This research aims to highlight the disparities between various blockchain solutions. Additionally, we will outline the distinctions between our approach and existing ones, explaining the fact that the proposed architecture and its detailed structure have the potential to be practically implemented in the near future. Compared to CERTbchain, a system that visually represents the verification of academic credentials until it is practically implemented, the DIAR system is meticulously designed, featuring a comprehensive architecture and conceptual model. Notably, it provides a detailed account of the generation and verification processes of diplomas, which are crucial in higher education institutions. The DIAR system is compatible with the Ethereum platform and offers more than just a smart contract. It provides a comprehensive solution that goes beyond verifying the unique hash values [12]. In addition to this, the DIAR system will use IPFS (InterPlanetary File System) as file storage in the first phase of implementation to test validation and performance, unlike CERTbchain, which did not use any database, but only Metamask to confirm the completion of the transaction for verification, and service manager as an interface between the user and the blockchain network. Docschain is another blockchain solution [13], to overcome some limitations of the Blockcerts platform, introducing optical use character recognition, and these data are stored together with all other student data. Among other things, Docschain offers solutions for the three limitations of Blockcerts. To fix the flow of document generation, where in Blockcerts the flow is changed and the same has not been shown to be successful for the generation of diplomas. Docschain offers solutions for overcoming the problem of issuing individual diplomas, that is, one by one, as well as offers solutions for overcoming the problem with students who have graduated, and how to enter them into the system.The DIAR system, unlike Docschain which offers only verification, offers generation and verification of diplomas. DIAR is based on the most sophisticated identification methods, the most secure privacy protection mechanisms, and above all offers the processing and verification in real time of more diplomas in parallel form.

Table 1 A summary of state-of-the-art blockchain solutions characteristics
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Curmi et al. [14] has presented research on the benefits of using blockchain, some methodologies for the development of a document verification system, without giving details of the implementation, proposing any conceptual model or even the architecture of any system. It is a description in short points, but useful when starting the design phase of a blockchain system for the generation and verification of diplomas.

In contrast to the systematic literature review, which discussed the blockchain implementation for the generation and verification of diplomas in higher education institutions [15], Raimundo et al. [16], has conducted a systematic bibliometric literature review regarding the applications of blockchain in the field of education in general. There is nothing specific regarding the application of blockchain in higher education institutions with respect to diploma generation and verification, much less any model or architecture proposal, or any theoretical solution to this problem.

2.1 Blockchain solution for higher education settings

To date, a considerable number of blockchain solutions related to HEIs have been created, where some of them are well documented through modeling, detailed architectures and even real implementations through interfaces; however, there is currently a lack of contemporary research that demonstrates the use and effectiveness of the same practices at HEIs [16]. Among other things, there are a large number of challenges that make blockchain technology limited in higher education institutions, but above all, blockchain programming is dependent on smart contracts, while smart contracts have their own challenges, which in this case make the blockchain system dependent and limited in many aspects. Table 1 presents a comprehensive overview of various blockchain solutions that relate to the generation and verification of academic credentials. The table shows the most prominent solutions, their practical implementation status, the storage approach/technology, the real-world scalability capabilities, the availability of documentation and codes, and the utilization of smart contracts. Additionally, the table assesses the stability of these solutions by examining their current existence and maintenance status up until the present day.

It is important to note that there is still no development of blockchain databases that will cooperate with decentralized blockchain systems, even though there are efforts in the direction of creating decentralized databases. However, there are limitations in terms of data access and management of big data [17]. Blockchain solutions related to data management often use either file storage or centralized databases. However, in some instances, blockchain databases may also be utilized for testing purposes. This constraint serves as one of the factors hindering the continuous growth of blockchain programming in parallel with the advancement of blockchain technology in the realm of data management [18]. Recently, developers have managed to create a hybrid framework for the operation of blockchain systems, using centralized databases but connecting it to the mechanisms of the blockchain network for data encryption, alluding in this way to the operation of a blockchain system. Nevertheless, the problem lies in the operation of off-chain and on-chain storage since the primary concern in this regard is on the real-time transmission and manipulation of data utilizing on-chain blockchain storage [19].

The implementation of decentralized systems in higher education institutions is also characterized by automatic generation of diplomas, the classification of public, private and hybrid services, the reuse and adaptation of data in different platforms, security and transparency, auxiliary tools that help the execution of smart contract in the blockchain network as well as architectures and conceptual models of decentralized systems. The implementation of processes in an automatic form without the intervention of a third party is made possible by the smart contract, which executes the code only after the prerequisites have been met [20]. In higher education institutions, due to the nature of the services, where some are private, some public or even hybrid, it is preferable to use consortium or hybrid blockchain, where it is easier to classify the services depending on the need [21]. All services in the blockchain are transparent, which means that in the blockchain system, students, teachers, and the entire administration can access at any time to get the information they need. Security mechanisms provide data security through authentication and data identity protection, and unlike centralized systems, security is guaranteed in blockchain systems. [22]. The reuse of data in the blockchain system depends on the possible reuse of the smart contract, which in fact represents the standardization process of the smart contract, while the adaptation of the system and its characteristics to different platforms is undoubtedly challenging and depends on the maintainers of the system how they will be able to adapt the services to the newest platforms [23]. The blockchain system cannot function independently, without having auxiliary technologies that offer the realization of services. Undoubtedly, their use is in direct proportion to costs and energy consumption [24].

More about the challenges and limitations of blockchain in data management, and in particular for higher education institutions, we describe in the upcoming section.

The DIAR system is in the phase of a prototype implementation, which includes testing the validity, measuring the efficiency and performance in real time. The Ethereum platform offers many tools that enable, among other things, the programming, execution and testing of smart contracts without additional costs, until their real implementation. The other reason why we consider Ethereum and IPFS is the fact that Ethereum supports the tokenization process, offering standard tokens, fungible and non-fungible tokens, while diplomas are classified as non-fungible tokens [30]. Blockchain file storage is also preferred when it comes to prototypes, because there is still no decentralized database which in practice has shown good results in terms of variable data management. IPFS also enables the storage of data based on its unique value, generated through the Ethereum account, while the storage of data based on the generated hash value makes it easier to verify the same. Among other things, the Ethereum platform is an open platform for anyone who wants to join the blockchain network, while other platforms such as Hyperledger fabric have limitations, and prior warnings are needed to access the network [16].

2.2 Challenges of blockchain in higher education institutions

In the context of higher education institutions, a diverse range of services are provided, resulting in a notable heterogeneity of offerings. These services encompass financial services, data management, real-time processing, transparency, identity protection, and privacy preservation. The presence of these various characteristics further complicates the establishment of a comprehensive system that would facilitate the management and real-time processing of the substantial amount of data generated within these institutions. In blockchain systems, the facilitation of service execution would be significantly enhanced through the establishment of standardized protocols. Each service would adhere to specific standards, thereby streamlining the execution of corresponding smart contracts.

The cost of services and their long-term maintenance in blockchain systems has significant importance. In relation to the generation of diplomas, the cost is relatively low when compared to physical production. This is due to the use of digital generation methods, whereby diplomas are created in a digital format and signed with a digital signature, often represented as a PDF (Portable Document File) file. Therefore, the process of verifying degrees entails invoking a smart contract, wherein the associated expenses are contingent upon the execution costs of the smart contract and the chosen deployment platform. Regarding the implementation and adaptation of the blockchain system on existing platforms, the training of academic staff, students, and all other actors in the use of the system is much more expensive than in centralized systems. At the same time, the cost depends directly on the size of the blockchain network, the amount of data that will be stored in the blockchain database and their processing in real time. That is, the more diplomas are processed, the more the real cost of the system increases, and at the same time the maintenance of the same [19]. Conversely, real-world implementations of blockchain systems across various domains have demonstrated a reduction in annual system maintenance costs. This is due to the elimination of intermediaries, particularly in financial applications like money transfer companies, which greatly benefit from the adoption of blockchain systems [20]. Scalability and the limitation of computing equipment for processing transactions is one of the reasons that can lead to high costs in blockchain systems [18]. Above all, the cost of implementing the blockchain system in higher education institutions depends on many factors, such as the selected platform, the blockchain database, security mechanisms, integration into existing systems, and the training required to use the system [19]. Given the fact that we have chosen Ethereum as a platform, every transaction that is carried out is calculated with Ethereum Gas, and for every transaction we must necessarily have a maximum gas limit. During the practical implementation, we will also consider providing the implementation details of Transaction Gas Cost, Intrinsic Gas Cost, Execution Gas Cost, Deploy Gas Cost, Memory Expansion Cost [20].

The stability of the system is primarily reliant on the maintenance practices implemented within the system's operating platform. The stability of the blockchain system obviously depends on more factors, among which we mention the costs of maintenance and processing of transactions in real time, the chosen platform, database, consensus mechanisms, scalability, interoperability, security, smart contract [31]. The analysis of all security vulnerabilities and analysis tools for Ethereum smart contracts is very important for the system to be stable. Considering the fact that in the proposed architecture we use more sophisticated mechanisms for security, privacy protection and identification, and simultaneously using schemes and mechanisms for cost efficiency, which is one of the main reasons for the system to be long-term and stable, we think that the DIAR system will be sustainable, stable and will offer all the mechanisms for fast, safe, and transparent processing of all the services it will offer.

Smart contracts must be created in a standardized form for the generation, verification and revocation of diplomas. Such a thing would enable easier implementation, increased efficiency, faster data processing and the mitigation of potential errors during the deployment phase [32]. Real-time processing and data communication between blockchain nodes is also very important. For faster processing of data, there are different techniques and mechanisms, but the processing of smaller volumes is one of the options that has practically shown results [33]. Preservation of identity and protection of data privacy is a challenge in itself, where different AI techniques are used. Among others, we can single out self-identification techniques, which provide identification without the intervention of third parties [34]. The adaptation and use of auxiliary tools are important for the stability of the system. Also, the reuse of different structures, architectures, conceptual models, frameworks would reduce additional costs and time for the implementation of a blockchain system for higher education institutions [35]. The use of distributed databases and more nodes within them is one of the best methods for managing big data, a problem that is in every sphere of life, including higher education institutions [36]. Data transfer to off-chain storage, respectively to cloud blockchain storage is possible using smart agents, cloud computing and other AI methods [32]. Maintenance for a longer time, electricity consumption, and storing electronic diplomas in blockchain storage for an indefinite time is a challenge. Therefore, one of the best solutions in this direction is to define legal regulations for the storage of data in these systems [37].

The focus of our work is on defining a conceptual model, a detailed architecture of the diploma generation and verification process, and a smart contract architecture. In Table 2, we present the functional and non-functional requirements, as well as the practical challenges, associated with implementing the DIAR system.

Table 2 Functional and non-functional requirements, practical challenges of DIAR system
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These requirements in detailed form, including use case diagrams, use case scenarios for the main actors included in the system, can also be found in [38]

3 Proposed blockchain system for academic diplomas generation and verification

One of the limitations associated with the use of blockchain technology in higher education institutions relates to the scarcity of scholarly research, literature, manuals, and thorough descriptions detailing the practical implementation of blockchain solutions. According to a comprehensive systematic literature review pertaining to the generation and verification of diplomas, specifically focusing on the period from 2018 to 2022, it is evident that only a limited number of studies have provided a thorough explanation of this process. Obviously, the number of papers related to the application of blockchain for higher education institutions is much higher [16], however, in our analysis, we wanted to be as close as possible to the practical solutions of blockchain systems for higher education institutions, with particular emphasis on diploma generation and verification.

The present study introduces a blockchain system that places significant emphasis on the generation, verification, and revocation processes, which are specific to the regeneration of academic credentials, with a particular focus on diplomas as crucial documents within higher education institutions. Through this paper, we provide a detailed overview of a blockchain system for higher education institutions, first describing the conceptual model, the sophisticated architecture, which consists of multiple layers of transaction processing, up to the systematic architecture of the generation process. And verification of diplomas, and the smart contract architecture that is in the process of programming. From our analysis, it turns out that most of the works either have practical results of their prototypes without providing the details of the implementation of the architecture they have implemented or have only a superficial design of the architecture without any detailed analysis of the practical implementation. The closest research, which resembles the nature of our system, to the best of our knowledge, is Verde [30], which in a detailed form, have implemented the entire system they propose. The Verde platform is a blockchain solution that deals with the decentralized way of generating and verifying diplomas. Business Process Model & Notation (BPMN) is used for designing the processes of registration, verification and nostrification of academic qualifications. The authors support the fact that BPMN helps in the more efficient design of the processes of a blockchain system, and provides efficient methods in addressing issues related to the complexity and use of blockchain systems. Verde has a clearly defined, not too complicated architecture which is mainly based on smart contract deployment to process requests related to registration and verification of academic qualifications between academic units and third-party actors. The main basis of operation of the Verde platform is the creation of the Verde token, as a result of the modification of the ERC-20 token, to emulate student credits in tokens.

Although tokenization as a process is a novelty in the process of generating and verifying academic credentials, in the DIAR system we have made modifications to the tokenization process while generating academic credentials. On the Verde approach, if the student has accumulated 240 ECTS (European Credit Transfer System) or credits, the platform will generate 240 tokens which will be stored in the student's EOA (Externally Owned Account) address, but also in blockchain storage. On the other hand, through the DIAR system we try to reduce the tokenization of the processes, and the diploma is stored as a whole, after it is signed in digital form by the student and the management of the institution. And at the end of the process, a unique value hash is placed and processed as land in the EOA address and blockchain storage. This would in fact greatly reduce the time, but also the other expenses related to the processing, storage and further verification of the same diploma. Also, the restriction on the ERC-20 token is another limitation of the Verde platform. In contrast, DIAR will focus on non-fungible tokens as they are not interchangeable and unique.

Next, the DIAR system offers a clearly defined conceptual model, describing in a clear form the relationship between the main actors who will use the system, as well as a detailed architecture of seven layers of data processing, offering stability, security, and identity protection. The system is clearly described, with a detailed description of the process of generation and verification of diplomas and giving importance to the process of tokenization of diplomas, which is easier to process diplomas and store in blockchain storage. The significance of prioritizing diplomas is rooted in the digitization of numerous processes within contemporary centralized systems employed by higher education institutions. Despite the electronic generation of various documents, diplomas, being crucial credentials, have not yet been generated in electronic format. This is primarily due to the inability of centralized systems to ensure the security and privacy required for the generation and verification of such documents. The absence of security and traceability mechanisms within these systems further exacerbates the potential for misuse by users involved in the generation and verification processes.

It is a well-established fact that, upon completion of their studies, students proceed to submit an application for their diploma, a process that has been more time-consuming in recent times. This delay is mostly attributed to the thorough verification of the student's academic records. However, this prolonged verification period raises concerns over potential instances of fraudulent activity. Specifically, there is a possibility that the data included within the diplomas may be altered during the generating process, hence facilitating various forms of abuse. Tariq et al. [29] clarify four types of possible misuse in higher education institutions regarding academic credentials, including diplomas. The first one has to do with forgery of the diploma, including diploma details, marks, credits, signatures, seals, etc. The second type of forgery has to do with misuse by staff within the institution, who create duplicates of the stamps or even change the students' files from the beginning so as not to leave traces. The third type of misuse has to do with the generation of fictitious diplomas, where the institutions do not exist or are non-higher education institutions but generate diplomas with fake stamps and signatures. And the fourth type of misuse has to do with a wider network of falsification, when a higher education institution is given permission for accreditation for different courses without having the academic staff and not fulfilling the necessary conditions for the operation of that institution.

The blockchain system can contribute here by the very fact that the data inserted in the blockchain storage cannot be changed, but even if they are, they alert the system. The immutability of data is closely related to the nature of blockchain, where in addition to the generalized ledger, where records of all blocks and every action on the network are kept, each block also keeps records of this ledger as well as the actions of neighboring blocks. Changing data is almost impossible. To the second type of misuse, blockchain contributes with the characteristic of traceability, where everyone in the system is responsible for the actions they perform, as well as depending on the privileges they have. Not everyone can access private data, except for those who are responsible. For any eventual change in the system the blockchain alerts the management of the higher education institution. Regarding the third and fourth types of various misuses, the fact that diplomas are digitized, the same can be easily verified through this transparent system for all users. In this context, the creation of a national platform would help the selection process of higher education institutions. In such a case, it would be even easier, especially for the verification of diplomas, because it is very easy to check whether the diploma presented by the respective person belongs to that higher education institution or not, through the online Blockchain system that offers services smoothly, and each institution of higher education will be certified by the respective state institutions [16].

The implementation of blockchain technology in educational institutions has the potential to enhance various processes, such as student registration, semester tracking, and payment transactions. By utilizing the blockchain system, these tasks can be automated, ensuring greater efficiency and security. Additionally, the blockchain system can generate grades based on professor evaluations, replacing the need for manual grading. Overall, the adoption of blockchain technology in educational settings offers significant advantages in terms of efficiency and prevention of misuse. Nevertheless, the implementation of such a procedure takes substantial effort and dedication. Consequently, we suggest that existing systems within higher education institutions include the generation and verification of diplomas, or alternatively establish a distinct system to fulfill this purpose.

The conceptual model of the proposed blockchain-based system is presented in Fig. 1, which includes the main entities and actors as well as their roles. As can be seen, after completion of all academic requirements, the institution of higher education issues a digital diploma to the student. This diploma is digitally signed by the institution and requires the student to generate their own digital signature. Prior to this, the student must verify the accuracy of the relevant data. Subsequently, the diploma is accredited by the system maintainer, which can be either the institution itself or a private company with whom the institution has entered into an agreement. Next, the accreditation process converts the diploma into an immutable token, which is then stored in the blockchain network and eventually in blockchain storage for long-term preservation. The use of the private key, whether the digital signature or any other type of identification of the students, is the only method by which they in the future identify the ownership of the diploma, but also the immutability of the same. In order to facilitate the process of verification, it is recommended that each diploma generated be equipped with a QR code that contains relevant information. Hence, when an employer seeks to authenticate a degree through an accredited verifier, it is possible to promptly verify its validity by utilizing the blockchain system and examining the QR code. This process also involves confirming whether any modifications have been made to the blockchain network's address, thereby ensuring the authenticity of the degree. The revocation of the degree is done at the request of the higher education institution, for various reasons, however the student is also informed, and the same is then regenerated with another address in the blockchain network.

Fig. 1
figure 1

Conceptual model of proposed blockchain system

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The use of intelligent agents is crucial for enhancing the efficiency of our system by mitigating abuses, issuing warnings, and expediting procedures. These intelligent agents acquire information via human interactions and possess the capability to use that knowledge in problem-solving scenarios that extend beyond their pre-existing knowledge [35]. The use of intelligent chatbots is one possible use in the proposed system. These chatbots serve as a means for students, management, and all system users to get simplified assistance in navigating the system. Another implementation is the creation of different virtual spaces and virtual simulations that allow students to think about different real cases, to develop intelligence in solving problems, or in other words by making learning more attractive through concrete examples in virtual spaces [39]. Furthermore, the use of AI may be implemented in the processes of user identification and authentication. This presents the potential for users to choose from a range of identification methods, such as facial recognition, fingerprint scanning, or other self-identifying techniques [40]. Protection from various cyber-attacks that can happen to the blockchain system, and warning in time, through intelligent agents, is also a good opportunity for the application of intelligence in the blockchain system [41]. Figure 2 serves as a continuation of Fig. 1, illustrating the sequential representation of processes with the corresponding processes taking place in each actor of the blockchain system.

Fig. 2
figure 2

Flowchart of issuing and verifying diploma

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3.1 System architecture

The selection of the issuer and accreditor of degrees has significant importance. The entity referred to might be either the institution of higher education itself or a licensed commercial enterprise that has a formal arrangement with the institution of higher education. Figure 3 illustrates the many levels of the architecture of a blockchain system specifically designed for higher education organizations. The blockchain system comprises the main actors, namely the higher education institution, students, employers, and verifiers. In this context, the higher education institution assumes the dual function of certificate generator and accreditor since it is responsible for issuing and validating the degrees.

Fig. 3
figure 3

Blockchain system architecture

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Next, we have the user interface layer, which serves as the visual component of the system, facilitating communication between the primary actors and the system. The aspect of identity and access pertains to the use of techniques employed by actors for identification, namely self-sovereign or Decentralized Identity methods. The management component of the blockchain system encompasses the stage whereby the credentials of the actors are authenticated, followed by the subsequent processing of data based on the specific request. The component of the Ethereum blockchain pertains to the variety of development tools included in the Ethereum network. These tools facilitate various functionalities such as the deployment of smart contracts, the creation of distinctive Ethereum addresses, hash values, and tokenization, among others. The network layer relates to the arrangement of nodes inside the blockchain network, including peers, communication protocols, and consensus mechanisms.

Prior to transmitting the data to the storage, it is essential that the data undergo encryption and successfully pass the established security protocols. In terms of data storage, a hybrid data management system is often used, including IPFS, cloud storage, and centralized databases. The proposed system would use the IPFS in conjunction with a centralized database. Lastly, we have also highlighted the part of the on-chain blockchain that refers to the execution of transactions in the context of higher education institutions, including mining processes, validators, and HEI blocks.

The security aspect and the mechanisms that will be used for data protection are very important, since the same mechanisms must ensure that the privacy of the data of students and other users in the system will be protected. Undoubtedly, in the system we must have data classification, both for students and other actors in the system [39], therefore we envision the usage of public, private and consortium blockchain in the system, depending on the nature of the data [16]. From the different methods used to protect privacy [18], we, among others, propose a combination of Zero knowledge proof, AES, SHA256, Homomorphic encryption and Ring signature, depending on the nature of the services offered, which are presented in the security layer in the framework of the architecture of the DIAR system. Zk-Snark is a specific method of Zero knowledge proof, with guarantees that no information is shared between the recipient and the sender; identity, privacy and authentication are preserved respectively, and that only that information that should be public are shared, despite the long the process of verification, identification of users or even during the generation of various academic documents [42]. Because of their openness, blockchain systems are vulnerable to a variety of threats. Nevertheless, blockchain account models and certificate issuance systems provide a greater challenge, as it requires careful classification of services into appropriate blockchain categories based on their characteristics. The combination of security mechanisms, with the corresponding more specified techniques, help to create a more secure scheme to preserve identity, privacy and provide secure communication between the main actors of the system, always considering the blockchain platform, and auxiliary equipment used for the execution of transactions [43].

3.2 Issue process of the diploma

The proposed approach represents a modified architecture based on the VerDe blockchain platform [30]. The new architecture has a strong focus on the internal processes of the blockchain system, especially the processes of generation and verification of diplomas [44]. The complete procedure for the development of diplomas or other academic certificates is shown in Fig. 4. The student's first action is to submit a formal request for the issuance of a diploma. Subsequently, the system starts the process of compiling the student's records at the conclusion of their studies. This entails gathering information relating to the student's payments, earned credits, and any other relevant facts associated with their academic journey. Once the data has undergone verification inside the blockchain system, it is then entered into the external account of the Ethereum platform prior to being sent for further processing within the blockchain system. The act of creating or inserting data into an externally owned account on the Ethereum blockchain network pertains to:

  • Generation of the private key, which is usually a hexadecimal number associated with the Ethereum address, and must be kept secret since access is made through this number,

  • Derivation of the corresponding public key from the private key,

  • Calculation of the Ethereum address after it is derived from the corresponding public key,

  • Storing the Ethereum address and the private key in a safe form, the private key must not be shared while the Ethereum address can be public because it actually identifies the student's account,

  • Using the Ethereum account, after verification, transactions can be carried out or even smart contracts can be accessed in the blockchain network [45, 46].

Fig. 4
figure 4

Detailed process of diploma issuance

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Only after the transaction is verified that everything is in order, then the student's credentials are encrypted through cryptographic mechanisms using algorithms such as AES or SHA256. After the data is encrypted, then the smart contract is called to record the data in the blockchain storage, and then another smart contract is invoked to generate the student's data. The use of tokens actually represents a guarantee for the storage of the student's earned credits and the generation of ID during storage in blockchain. Upon invocation of the smart contract responsible for diploma generation, the execution process commences, taking into account the provided parameters. A crucial step within this process involves comparing the token with the student's identification. If the token matches the student's ID, the diploma is generated for the corresponding student. The success of the whole procedure depends on the verification of the student's identification via the use of the token that was produced earlier.

The process of generating tokens and sending them to the externally owned account (EOA) address inside the Ethereum blockchain is intricately linked to the execution of a smart contract. This involves the development, execution, generation, transfer, and ultimately, the delivery of tokens to the private EOA address associated with the student. It is noteworthy to mention that apart from the execution of the smart contract, there are additional costs associated with the processing and generation of tokens until they are transferred to the student's Ethereum address. These costs serve as remuneration for the miners involved in processing the transaction, regardless of its outcome.

3.3 Verification process of the diploma

The verification process is the opposite of generation. The verifier, after logging into his external Ethereum account, searches for the student through the interfaces in the system. The search is conducted using certain criteria, followed by the initiation of the smart contract for verification. The subsequent procedures include validating the student's data, decrypting the data, and ultimately displaying the results to the verifier, if successful. Clearly, this process incurs its own cost, which the verifier must pay through the EOA account.. Figure 5 illustrates the process of diploma verification, depicting the key entities involved, namely the verifier, the blockchain system, and the Ethereum platform. The diagram provides an overview of the sequential steps involved in the verification process.

Fig. 5
figure 5

Detailed process of diploma verification

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3.4 Smart contract architecture

Decentralized applications, commonly referred to as DApps, function by utilizing the Ethereum Virtual Machine (EVM), which represents a virtual machine. These applications operate by executing smart contracts, which are integral components of the Solidity programming code. Smart contracts facilitate the interaction between blockchain systems and the underlying blockchain network [47]. When a smart contract is put on a blockchain network, its immutability feature ensures that the contract remains unalterable, even by the contract itself. Hence, it is readily apparent that a smart contract has undergone modifications in order to provide evidence of an attempted entry into the blockchain network. In the event that a smart contract encounters faults throughout its execution, it is essential to terminate the contract since it lacks the capability to undergo modifications during this phase [48].

Smart contracts have emerged as a practical solution in the realm of finance, facilitating the execution of transactions without the need to rely on traditional financial intermediaries such as banks [49]. These services are sometimes referred to as DeFi (Decentralized Finance) services. Digital identity is an area where smart contracts may be used. This technology enables users and students to establish their own autonomous systems for identification. The use of smart contracts is also seen in the administration of decentralized systems, whereby their automated functionality enables the execution of predetermined services without the need for external involvement [50]. Moreover, smart contract technology is increasingly being used in the field of medicine, particularly in the areas of patient data management, interclinical connectivity, patient history management, and pharmaceutical applications [51]. Also, smart contracts are being used in the development of exchangeable and non-exchangeable tokens, serving as practical examples for real-world scenarios [52].

Efforts are underway to develop automated mechanisms capable of safeguarding smart contracts from diverse attacks [53]. Additionally, these mechanisms aim to automatically troubleshoot smart contracts in instances where errors arise without requiring any modifications to the internal structure of the smart contract. Simultaneously, it is important to safeguard the smart contract from different external threats throughout its whole execution process [54]. Nevertheless, the use of blockchain technology, particularly smart contracts, remains limited in several domains, including institutions of higher education. There are several factors contributing to this, including the limited adoption of Solidity as a programming language for smart contracts. Firstly, Solidity is a relatively new language and thus possesses certain limitations. Secondly, a significant number of developers have little interest in Solidity and smart contract development due to a perceived lack of profitability compared to other cryptocurrency-related activities. Lastly, the immutability of smart contracts poses challenges in various domains, as any errors identified during execution cannot be rectified but instead necessitate the destruction of the contract and a complete restart of the process. In such a process, the cost is substantial [55]. The overcoming of these shortcomings can be seen in the standardization of the smart contract. However, the concept of standardization encompasses more than just the development of a standard smart contract for certain activities. It also involves establishing standard conditions for smart contracts, including requirements, characteristics, and their use in various contexts. A thorough investigation of the platform and its intended use is necessary to achieve the standardization of smart contracts [56].

The proposed architecture of the smart contracts used in the blockchain system for the generation and verification of degrees is shown in Fig. 6. Along with other elements, the figure includes an additional six significant smart contracts. The User Interface serves as a channel via which all users are linked to the blockchain system. The Student Registration encompasses the comprehensive collection of relevant data required for the issuance of diplomas. All financial transactions inside the blockchain system, including the issuance and verification of diplomas, as well as other transactions, will be facilitated exclusively by the smart contract Payment. The concept of Identification and Digital Signature is a smart contract that focuses on the use of identification mechanisms by users inside a system. It also emphasizes the usage of private keys or private digital signatures for the purpose of signing electronic documents. The application Generate Diploma is designed to facilitate the creation of diplomas, while the application Verify Diploma serves the purpose of validating the diplomas earned by students upon completion of their academic programs.

Fig. 6
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Smart contract architecture

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4 Discussion

In this section, the desirable properties of the system are discussed, including its advantages and limitations in relation to state-of-the-art. Comprehensive scientific studies are scarce and do not emphasize the intricacies of the process, architecture, conceptual model, and programming components employed in the practical implementation of smart contracts. It is noteworthy that in this particular context, the VerDe [30] platform was used as sources of motivation in our work. This platform has been described in detail and provide tangible prospects for practical application. The proposed system characterizes itself with a multi-layered network architecture, where each tier integrates distinct data processing techniques. This design enhances the security of data encryption, decryption, and transmission in off-chain storage for extended periods. The proposed solution thoroughly considers the necessary steps for data insertion in blockchain storage, including legal and software protocols. Additionally, the paper is meticulously examining the supporting environments necessary for the successful execution of the smart contracts. Table 3 displays the attributes of the blockchain systems, along with their respective explanations, and the proposed architecture for our system, indicating the layers that include each feature. The full development and implementation of the system involves creating the smart contract and making sure it works seamlessly with the blockchain storage system for the data that will undergo testing. This process will be part of our next study.

Table 3 Description of the features of blockchain-based systems including the proposed system
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The aim of this work is to explain the systematic framework of the blockchain system. This entails delineating the essential levels comprising the blockchain system, along with the relevant parts that may be integrated inside each layer. The blockchain system enhances security and data privacy by eliminating intermediaries during data transfers and preventing unauthorized access to the database. In contrast to centralized systems, where the administrator holds privileged access to all services, the blockchain system will ensure a higher level of security and validation. The blockchain system for data encryption will use the AES algorithm with a combination of SHA256, although other alternatives foreseen in the architecture are not excluded [57]. System security also depends on consensus mechanisms, validation and verification of transactions, smart contract security and their safe execution, privacy, auditing, and monitoring [58]. The process of validation has significant importance in ensuring the security of any given process. Hence, the concept of validation will encompass several aspects, such as network block validation, transaction validation, token validation, and smart contract validation. Additionally, the consensus mechanism plays a special role in validation, representing the agreement that nodes reach among themselves on how they will cooperate with each other [59]. Otherwise, validators are part of the on-chain blockchain, since they are present in every process that is carried out within the blockchain network.

In terms of traceability and identity, the blockchain system offers the possibility that every service that is performed is known to the author of the service; even attempts are made to interrupt the execution of the smart contract or to alert the system during unauthorized access. Blockchain technology offers mechanisms for creating methods for generating the identity itself without the need for third-party intervention, but the user himself chooses the method of identification, which he stores in the blockchain storage [60]. Blockchain systems in higher education institutions have the potential to uphold integrity through immutability and decentralization. Blockchain technology ensures that smart contracts cannot be altered, thereby fostering the standardization of these contracts. Decentralization is also very important for data security because it allows independent processes and limits access only to blockchain storage [47].

The idea being put forward offers many benefits concerning security, consensus mechanisms, and the use of on-chain features, which are crucial for enhancing the system's operational efficiency. One benefit of our service is the ability for students to customize their way of identification inside the system for signing documents, particularly diplomas, in addition to the option of using a digital signature. This includes the use of intelligent agents. The VerDe system incorporates tokenization as a crucial procedure within its consensus mechanisms and security layer. Similarly, the Cerberus system has commonalities with regards to its design and modular concept.

Even though the blockchain-system implementation in HEIs is innovative approach, offering services previously never applied in digital form to date, still they have limitations. The provided solution is not an exception. In the context of blockchain, one challenge relates to the relatively slower transaction speed compared to existing systems that use highly efficient data processing mechanisms. Generating results in blockchain networks takes significantly longer due to the inherent time-consuming nature of data processing within the network. Maintenance is another concern, as it can be handled either by the same administrator as the centralized system or by a specialized private company. In some cases, the monthly costs for maintenance may even exceed those of a centralized system.

5 Conclusion

This study proposes DIAR, a novel blockchain-based system for the generation and verification of academic credentials, namely degrees. At first, the paper presented a comprehensive overview of several blockchain solutions that share similar characteristics. Subsequently, it highlighted the associated issues, outlining their constraints and proposing potential strategies to overcome them. Furthermore, the paper underlined the areas in which further research should be conducted, drawing upon the most recent scholarly research. The conceptual model of the blockchain system was developed, including a flowchart diagram illustrating the process of diploma generation. Additionally, the system's detailed architecture was constructed, incorporating crucial processes. The study also elaborated upon and devised a comprehensive framework outlining the intricate structure of the diploma generation process as well as the corresponding architecture for its verification. Lastly, the paper provides an analysis of the significance of smart contracts inside the blockchain framework and the design of a pivotal smart contract architecture specific to the suggested blockchain system for higher education institutions.

The proposed blockchain system aims to address a significant societal issue, namely the prevention of credential abuse. Diplomas have substantial importance since they are used for employment opportunities and career advancements. The implementation of this system aims to streamline the process of diploma generation by digitizing it. It will provide students with the opportunity to expedite the signing and request for diploma generation through their preferred identification methods. Consequently, they will be able to promptly obtain an electronic copy of their diploma, thereby eliminating the need for prolonged waiting periods and unnecessary administrative costs.

The future work will involve the ongoing development of the Solidity smart contract, the implementation of user interfaces utilizing modern technologies, the deployment of the Ethereum Virtual Machine, and the incorporation of various development tools that have been thoroughly examined as essential components of the blockchain system creation process. Additionally, future research endeavors will also aim to comprehensively outline the sequential progression of challenges that may encounter throughout the development journey. Moreover, an empirical validation and functionality testing to provide a rigorous assessment of the system will be conducted.