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> There have been efforts to augment NTP with authentication, but they still assume a world where each client trusts one or more time servers absolutely.

OpenNTPD has "constraints" where it makes HTTP requests (using TLS) to webservers and checks that the time provided by the NTP server is within a certain threshold of the time returned in the HTTP Date header.

Much simpler and doesn't require dedicated servers.

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tlsdate is a much cleaner implementation of this idea, taking the time from the handshake. TLS 1.3 as it stands makes sending the server time optional.

The 'Date' header is tricky because it is a timestamp of when the document was generated, not when it was served. Caching proxies have no obligation to (and in most cases shouldn't) update the value.

Some TLS implementations return a randomised date for the handshake anyway, which is why constraints works the way it does. TLS 1.3 killing it is just gravy.

If you're worried about a caching proxy you can set the constraint to a URL that returns something dynamic. Although it would be interesting to see what % of the top TLS-enabled webservers don't return something recent for HEAD / HTTP/1.1

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