# Blockchain-Based Evidence Trustworthiness System in Certification
## 5 Validation and Results  
### Cybersecurity Certification  
The evidence trustworthiness system was included and validated within the MEDINA framework developed within the H2020 MEDINA project [46] for the cybersecurity certification of cloud services. The MEDINA framework is currently being evolved within the EMERALD Horizon Europe project, currently under development [47]. EMERALD is a security framework for continuous audit-based certification in compliance with European cloud security certification schemes. It is based on a continuous and automatic gathering and evaluating process of evidence in order to have a secure operational service certifiable through a given conformity assessment method. EMERALD includes a local evidence storage where raw evidence is saved. However, this storage cannot be considered secure since, as introduced in Section 3, the stored information could be modified. In this context, the trustworthiness system improves the trustworthiness of the evidence collected, providing a reliable and transparent source of evidence for the auditors. In EMERALD, not only evidence but also conformity assessment results are stored in the trustworthiness system. The EMERALD framework is designed so that its assessment results are considered valid evidence for the auditors of cloud services security certification processes. Therefore, it was decided to prepare the trustworthiness system to store the assessment results as well. Figure 6 shows the data model of information to be recorded in the trustworthiness system. Figure 5: Automatic integrity verification in the evidence trustworthiness system. As mentioned in Section 4.1, the evidence identifier (id), the timestamp, and the hash of the raw evidence (valueHash) are always stored. Raw evidence does not conform to any specific format or schema and has no fixed size (any type of evidence can be included), thus its entropy is inherently high. Additionally, EMERALD adds some contextual details such as the cloud service (cspId) and the resource (e.g., compute instances, databases, storage, networking, and auxiliary services) (resourceld) that the evidence refers to; it also adds the tool with which the evidence has been automatically gathered (toolld). In the case of the assessment results, the assessment id (id), the timestamp, and two hashes are recorded. These hashes refer to the security assessment result from the EMERALD framework, as well as the compliance result. Similarly to the raw evidence, the assessment results do not follow any particular format or schema, and thus their entropy is considered high by default. However, the compliance result has a very low entropy. It refers to the positive or negative result of compliance with the assessment process. It is a Boolean value, so it can only have two possible values: true or false. This makes it relatively easy to compromise security. To mitigate this, the entropy is increased by appending a random 256-bit value to the positive or negative compliance result, as introduced in Section 4.1. As a result, entropy increases and security improves. Furthermore, the contextual details in this case are the metric against which the conformity assessment is executed (metricld) and the evidence that has been considered for the assessment (associatedEvidenced). All evidence values and evaluation results are recorded in the evidence trustworthiness system following the data models in Figure 6 to guarantee their integrity and transparency. The Blockchain monitor provides a graphical dashboard to access the information recorded on the system through a browser. The dashboard adapted to the context of EMERALD is shown in Figure 7, where the details of the recorded values of different pieces of evidence and assessment results are shown. Filters for different contextual details have been implemented to improve the usability of the tool for auditors.