It sounds great and well thought through, as always for Signal. I wonder how they handle two potential security holes: 1. Imagine a journalist publicizes a use

It sounds great and well thought through, as always for Signal. I wonder how they handle two potential security holes: 1. Imagine a journalist publicizes a username for a long time, then changes it. The old username would persist in data stores online and in address books, and would be used in error. An attacker could acquire the old username and impersonate the journalist. Perhaps a solution is to make at least some usernames, possibly at the user's option, non-reusable. 2. One strength of Signal is not only do they not collect much user data, they can't. Under some court order, they could retain the hashed usernames.