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[TLS] Removing restriction on cross-domain resumption

Victor Vasiliev <vasilvv@google.com> Wed, 13 September 2017 21:57 UTC

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From: Victor Vasiliev <vasilvv@google.com>
Date: Wed, 13 Sep 2017 17:57:21 -0400
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Subject: [TLS] Removing restriction on cross-domain resumption
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Currently, TLS 1.3 specification forbids resuming the session if SNI values
do not match.  This is inefficient in multiple cases, for example, if you
have a wildcard domain cert, and the user is likely to visit multiple
subdomains over a longer timespan, so there is no existing connection to
pool on (or it's impossible to pool because of different IP addresses).

Last time we discussed this,
  https://www.ietf.org/mail-archive/web/tls/current/msg21655.html
no one has pointed out a good security reason why this should be
forbidden.  Also, the requirement as stated requires the server to enforce
it, while in reality, most implementations I am familiar with offload the
burden to the clients.

I wrote the following PR to remove the requirement:
  https://github.com/tlswg/tls13-spec/pull/1080
The PR still discourages clients to resume across domains by default (due
to likelihood of wasting a ticket which could have came in handy later), so
I'm currently writing a draft for an extension to inform clients that it's
okay to do that (it will be in spirit of PR #777
<https://github.com/tlswg/tls13-spec/pull/777>).

  -- Victor.

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