# Blockchain-Based Evidence Trustworthiness System in Certification
## 4 Secure Evidence Trustworthiness System
### Sensitive Evidence  
Although the audit trail considered was based on directly recording the raw information on the Blockchain, this is not an option for evidence. Evidence is considered sensitive information, and consequently, it should not be recorded on the Blockchain. Even in permissioned networks with highly secure access solutions, information recorded on the Blockchain can never be removed, which contravenes current data protection laws, such as the General Data Protection Regulation (GDPR) [42]. To ensure data integrity without revealing any input data, evidence is not directly stored in the Blockchain. Instead, evidence remains in a secure local storage and just hashes of the pieces of evidence are stored in the Blockchain. A hash function is a mathematical algorithm that converts any input data into a unique string of characters [43]. It is important to note that even the smallest change in the input data will generate a completely different hash. Moreover, reversing the process to return to the original input data is practically impossible. These features make this method secure for various sensitive applications, such as digital certificates, unique signatures for sensitive or secret documents, digital identification, and key storage. In this sense, hashes help verify that the information has not been tampered with, while keeping the information private by design. SHA2-256 is considered a suitable hashing function due to its wide standardization, robust security, and balanced performance [44]. However, the deterministic nature of hashing increases the likelihood of identifying the original data when it has a low degree of order (it contains little information and the range of possible values is limited), making re-identification easier through brute force and dictionary attacks [45]. This degree of order in the data is known as entropy. Higher  
Figure 2: Evidence trustworthiness system for certification (extended [38]). entropy means a dataset contains more information and the range of possible values is wider, reducing the risk of re-identification. In the context of evidence, high entropy is virtually guaranteed, as it usually contains raw data which do not adhere to any specific format or schema and have no specific size (any kind of evidence could be included), so its entropy is considered high by default. Moreover, details such as timestamps, usually included as information related to the evidence, also have a high variability. If there were some low-entropy pieces of evidence whose hash values could be easily broken, the entropy would have to be increased by adding a random number. This random number needs to be locally stored and associated with the evidence itself in order to allow the recalculation of the correct hash to be compared with the one recorded on the Blockchain and be able to determine if evidence has been tampered with. Thus, the information to be recorded in the Blockchain is, at least, the hash of the raw evidence (considering the random number, if applicable) and an identifier of the piece of evidence. Moreover, any additional non-sensitive contextual information such as evidence origin, evidence gathering tool, timestamp, etc., can be also recorded in the Blockchain to increase the utility of the system (i.e., filters application). The information recorded on the Blockchain is accessible in a user-friendly way through the Blockchain monitor from the considered audit trail solution. It provides dashboards accessible through the browser to obtain the recorded evidence hashes from the Blockchain by the evidence identifier. In contrast, raw evidence remains stored in any existing local storage of the organization as long as the information is accessible by auditors in a manual way to be able to verify its integrity.