## 2 Preliminaries
Moreover, all initiatives analyzed support educational information to be directly verified except for Nicosia's development [21], which relies on a Portable Document Format (PDF) file to list the hash digests of issued certificates. This contribution also supports a rich variety of academic data types [34, 35, 36, 37, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50], including flexible record formats [34, 46, 47]. Another relevant feature provided by this conceptual framework is the possibility of sharing only specific parts of the educational information of a given holder [32, 45]. To facilitate access and verification of academic information, our solution supports the generation of QR codes or websites [17, 30, 41, 48, 51]. Only two initiatives [38, 52] in addition to this proposal support learning analytics or statistics from the registered information. This feature may provide in the future still unknown information on worldwide educational trends, the most demanded studies, the profile of alumni from the characteristics of diplomas issued, etc. Finally, some initiatives were identified that provide persistent storage even if the issuing academic institution disappears [31, 35, 39, 43, 44, 45, 38]. To address this situation, several approaches are utilized and, in the case of the contribution discussed in this paper, it is addressed in a way that is compatible with the GDPR. After a thorough analysis of the scientific literature about blockchain applications in the field of education including relevant surveys [22, 23, 24, 25, 26, 27], relevant challenges were identified. They are enumerated below, together with the approaches followed in this research to address them. 1. Education is a discipline with a global dimension that encompasses many stakeholders who must exchange information despite the fact that their organizations are independent. Blockchain brings many applications to this field that require a new interconnection model [49] that is difficult to integrate into the proprietary systems of each entity. The system proposed in this research is adaptable and does not require different organizations to modify their information systems. 2. Achieving privacy in blockchain applications is a real challenge when personal information, protected by many regulations in different countries, is processed. Public blockchains should not be chosen to tamperproof store personal information, even encrypted [35], and consequently, private or consortium blockchains should be preferred [1]. However, from a privacy and regulatory point of view, in some countries (e.g., European Union countries under the scope of the GDPR), it may not be a valid solution [4]. Conscious of this issue, some proposals [1, 17, 38, 46] store information offline but register the corresponding hash digests on the blockchain to ensure the integrity and traceability of information. Nonetheless, in some countries with more permissive regulations (e.g., Brazil [39]) personal information can be openly stored if related to public education. On the contrary, as pointed out above, the European GDPR forbids registering any personal data in unmodifiable permanent storage such as a blockchain [53], which must be anonymized. This is a relevant design constraint, and it is important to emphasize that hashing personal data, from the GDPR's point of view, cannot be considered a completely valid anonymization technique [4]. Finally, the immutability feature of blockchain also makes it impossible to modify or delete data, even for legitimate reasons, which, in turn, collides with GDPR's right to be forgotten. The model proposed in this research solves this challenge with a design that preserves the privacy of users and complies with the GDPR. 3. Storage, scalability, and performance of blockchain-based systems to issue, store, and verify academic information are also key challenges to be addressed in order to achieve its general adoption when the number of transactions containing academic information increases.