## 2 State of the Art
### Blockchain for Document Verification  
Certificate forgery is a problem that has been previously addressed by different blockchain-based solutions [56, 57, 58, 59, 60]. For instance, [PERSON] and [PERSON] proposed to manage healthcare training certificates through the use of smart contracts and a blockchain called CertificateChain [61]. Specifically, the authors explored the feasibility of storing full files into the blockchain by breaking them into slices. To demonstrate the feasibility of the proposed approach, CertificateChain was evaluated in terms of scalability and performance. The obtained results show that the operation of the system becomes unviable in mainnets (i.e., fully operational blockchain networks) where the size of the transactions is directly proportional to the transaction costs, but it can work well in private blockchains. Regarding academic fraud, there are currently many studies and proofs of concept available, many of which have been analyzed by the literature [7, 62]. For instance, the Massachusetts Institute of Technology (MIT) [8] developed an application where students' diplomas were stored, managed and verified through the Bitcoin blockchain. However, Bitcoin limits significantly the potential use cases of this kind of applications in comparison to other blockchains like Ethereum [13]. For example, [PERSON] et al. [25] took advantage of Ethereum and stored encrypted student diplomas in their blockchain. In such a system, the verification only consists of encrypting the provided diploma and then checking whether the resultant hash matches one of the hashes stored in the blockchain. [PERSON] et al. [63] proposed a more global approach that aims to be used across European universities. The proposed system is based on the use of a token akin to the European credits (European Credit Transfer and Accumulation System, ECTS) and on a verification process that consists in checking the amount of tokens of a student. As the authors point out, the system could be further improved by introducing the standardization of the ARs among universities. [PERSON] et al. [4] proposed to use verifiable credits in cross-university courses. Thus, the credits earned by the students and the hash value of the records would be stored in a blockchain. Moreover, data would be maintained jointly by the universities, and one university would be capable of verifying the final examination of students, thus recognizing the credits from other universities, and at the same time, effectively protecting students' privacy using Elliptic Curve Digital Signature Algorithm (ECDSA). In the described system, the proposed architecture is conceived to make use of a consortium blockchain like Hyperledger Fabric, which can outperform other relevant solutions in terms of cost and security [64, 65, 66]. In [67], the process of fetching the academic certificate is enforced by several smart contracts, and the academic data is stored on a centralized subsystem to comply with the General Data Protection Regulation (GDPR). The smart contract can be used by the holder of the academic certificate to grant or revoke access to third parties. The same authors took a similar approach in [68], using the blockchain smart contract to create Non-Fungible Tokens (NFTs) that, when issued, allow owners (e.g., third-parties) to access student's data. The authors provide a novel approach to GDPR compliance with [67,68], but the main focus of these works is the academic certificate or diploma. It is also worth mentioning that only a few previous works propose fully decentralized approaches. For instance, in the system described in [69], an administrator is responsible for encrypting the student's data, which are then uploaded to IPFS. These data need to be tamper-proof and not modifiable, so cipher texts are uploaded to a blockchain so their authenticity could be verified. The encrypted data can be decrypted and displayed to the student from IPFS using the proper credentials.