## 5 Gaia-X-Med: Our Solutions
### Identity and Its Problems  
In order to establish a solid basis for trust in a federation, Gaia-X prescribes the use of Decentralized Identifiers (DIDs), as previously mentioned [27]. As we will see in detail in the following, these identifiers are used widely throughout the federation, for example in Credentials and thus Contracts. As a trust-enabling mechanism, they can be verified, clarifying which legal person taking part in the federation is behind the identifier. To implement DIDs, basically, two main approaches are available, namely blockchain and the web domain approach As per the W3C DID specification, in both approaches, the controller of a DID, which here is also its subject, specifies the method of verification of the DID and, for practical reasons, ensures the facilities to go through with it are in place [27]. In the blockchain approach, the DID controller and subject take part in a distributed ledger storing all the data, possibly utilizing cryptographic wallets. To sufficiently enable trust and acceptance among participants, however, the specific choice of distributed ledger or blockchain is pivotal, ranging from individual to federation-wide to European to global distribution, and from commercially-focused crypto currencies like Ethereum ([[https://ethereum.org/en/](https://ethereum.org/en/)]([https://ethereum.org/en/](https://ethereum.org/en/)) (accessed on 5 December 2024)) to ones supported by official European institutions like electronic IDentification, Authentication and trust Services (eIDAS) ([[https://digital-strategy.ec.europa.eu/en/policies/eidas-regulation](https://digital-strategy.ec.europa.eu/en/policies/eidas-regulation)]([https://digital-strategy.ec.europa.eu/en/policies/eidas-regulation](https://digital-strategy.ec.europa.eu/en/policies/eidas-regulation)) (accessed on 5 December 2024)), European Blockchain Services Infrastructure (EBSI) ([[https://ec.europa.eu/digital-building-blocks/sites/display/EBSI/What+is+EBSI](https://ec.europa.eu/digital-building-blocks/sites/display/EBSI/What+is+EBSI)]([https://ec.europa.eu/digital-building-blocks/sites/display/EBSI/What+is+EBSI](https://ec.europa.eu/digital-building-blocks/sites/display/EBSI/What+is+EBSI)) (accessed on 5 December 2024)), or the European Self-Sovereign Identity Framework (ESSIF) ([[https://essif-lab.eu/](https://essif-lab.eu/)]([https://essif-lab.eu/](https://essif-lab.eu/)) (accessed on 5 December 2024)), aligning more with Gaia-X as a European initiative. While seemingly feasible, we ultimately decided against this approach, since we determined its integration with the Gaia-X concepts as adapted by us to be too resource-intensive a task for a problem of medium importance. However, due to its potential for solving similar problems encountered by other projects in the health domain and beyond, we encourage the pursuit of this approach by Gaia-X itself, including its conceptual integration as a tech nology into the specifications, alongside our adaptations if needed, and its implementation via XFSC, for example. While potentially useful, the blockchain approach is not without problems, even being mentioned in the abstract of the specifications of the did:web Method [29], the core technology of the alternative web domain approach. As stated there, DIDs that target a distributed ledger often do not become adopted on mass due to not accumulating enough meaningful trusted data around the associated identities. Despite likely being remedied once the mentioned official institutions are involved, this remains true, at least for federation-wide ledgers. Representing an alternative, the web domain approach uses the did:web Method and prescribes the DID controller, and in this case also the subject, as the domain directly associated with the participant behind the identifier. Verifying an identifier in this case then amounts to checking whether the correct domain is at the other end of the communication.