## 3 QualiChain Architecture, Pilot Concept, Research Questions, and Execution Methodology
### QualiChain Architecture
#### 3.1.3 Semantic Blockchain for the Issuance and Validation of Accreditations  
The Semantic Web is a family of technologies designed to enable interoperable and easy integration of data on the Web and to support a decentralized network of machine-readable data connected by links. As such, the concept of Linked Data was integral for the semantification of the QualiChain blockchain. Linked Data are RDF (Resource Description Framework) representations of data, in which data are represented as semantic triples that also create links between different datasets that are also machine-readable.  
In QualiChain, the LinkChain platform was utilized to support the publication, sharing, and validation of Linked Data using a blockchain-backed verification model. By verification, we mean that, given a data item (or data items), a user can determine, in a trustworthy manner, when it was published, by whom, and whether or not it has been altered since publication. One of the key requirements for LinkChains is that it is designed around individual privacy and control of data. A user in LinkChains ought to be able to control their own data fully, but, where data have external value of some form, it should be possible to demonstrate their integrity. Qualifications are an ideal example of such valuable data, being both personal, and also, given their relation to jobs and income, with an incentive for users to modify and embellish them. LinkChains provides guarantees to data consumers that such modifications have not taken place. The implementation of LinkChains in QualiChain supports the publication of verifiable data using the Ethereum blockchain, making use of ERC721 tokens. ERC721 [37] is a specific Ethereum standard for non-fungible tokens, meaning tokens that cannot be spent. By making ERC721 tokens that can neither be spent nor traded, we achieved the equivalent of a badge, which can be given to a user as a permanent asset. More specific information about LinkChains can be found in [38; 39].  
Furthermore, the OpenBadges specification [40] was employed in QualiChain as a method of representing machine-readable educational qualifications with blockchain-backed verification. In fact, most of the ongoing initiatives into the blockchain validation of qualifications use OpenBadges as a standard format. Conceptually, a badge consists of a set of linked objects, typically represented in JSON-LD (the Linked Data extension of the widely used JSON format). As JSON-LD, a badge is a piece of semantic data in the Semantic Web's Resource Description Framework (RDF) data model and can therefore be connected to any relevant data in the wider Semantic Web. In QualiChain, a badge consists of the following attributes.  
* Badge Class: a general description of the qualification it represents.
* Issuer: the individual or institution giving the badge to a learner.
* Recipient: the individual receiving the badge
* An object representing an assertion that a badge of the given Badge Class has been issued to that recipient (by the issuer).  
To summarize, in QualiChain, OpenBadges are used to represent the QualiChain smart badges, and LinkChains is used to verify them. Badge-specific wrappers around LinkChains were created to give developers APIs to create, issue, and verify a badge, and to input the information into a PNG file (i.e., OpenBadge JSON-LD is embedded into the image as metadata). This service also uses the platform authorization system, except for badge verification, as we wanted anyone to have the capability to verify badge data, and because verification should not rely on trusting a centralized service. Badge issuing specifically within QualiChain is kept as an access-controlled web service, because this anchors a badge to the blockchain, and it is important to ensure that we can control access to the ability to issue badges anchored with the QualiChain private key.