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It seems some in the GA4GH community view passports as being provided to the server by the client, rather than the server retrieving them from clearing houses Brokers as proposed above. See:
https://docs.google.com/document/d/1T3uYGS2yZflDbLRbG4uxhi8ICqk9C9xWPmJ0DQpFvDU/edit#heading=h.5atd0vqkj5vq
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