3. A trusted directory for the public verification keys of credential issuers. In order to verify credentials, and to enable anyone to verify the source, integrity, and validity of any data, a globally accessible and trustworthy storage for the public keys of the issuers of credentials is required. In order to ensure the trust in this storage, malicious changing of data should be prevented, the data should be chronologically ordered, so you can retrieve current keys, and no single provider should be trusted with its control. Which is why a distributed ledger, often called a blockchain, is the natural choice as a storage medium.