[tor-bugs] #17728 [Tor]: Use TLS handshake rather than date header to check time with authorities

[Tor Bug Tracker & Wiki]

#17728: Use TLS handshake rather than date header to check time with authorities
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     Reporter:  teor         |      Owner:
         Type:  enhancement  |     Status:  new
     Priority:  Medium       |  Milestone:  Tor: 0.2.9.x-final
    Component:  Tor          |    Version:
     Severity:  Normal       |   Keywords:
Actual Points:               |  Parent ID:
       Points:               |    Sponsor:
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 tor currently checks its clock against the directory authorities by
 reading the HTTP date header in the directory documents.

 In #15775, we allow clients to bootstrap using fallback directories,
 rather than authorities.

 In #4483, we make multiple connections, and use the first connection that
 starts downloading. If there are multiple connections downloading, we
 favour authority connections, so that tor can still get a clock check.

 But if tor used the date from the TLS handshake, it could abort authority
 connections sooner. This would place less load on the authorities.

 This would be similar to the tlsdate implementation:
 https://github.com/ioerror/tlsdate

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17728>
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