[tbb-bugs] #17374 [Tor Browser]: Disable 1024-DH Encryption by default

[Tor Bug Tracker & Wiki]

#17374: Disable 1024-DH Encryption by default
-------------------------+--------------------------
 Reporter:  cypherpunks  |          Owner:  tbb-team
     Type:  defect       |         Status:  new
 Priority:  High         |      Milestone:
Component:  Tor Browser  |        Version:
 Severity:  Normal       |     Resolution:
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
  Sponsor:               |
-------------------------+--------------------------

Comment (by yawning):

 Replying to [comment:4 cypherpunks]:
 > Are there any estimates on how many servers (and which) use the weaker
 encryption by default? If it is only a small portion (and no major sites,
 only obscure rarely visited ones), I'm sure the security tradeoff is very
 much worth it.

 That's a good question, I'd certainly hope that most of the major sites
 would prefer ECDH over DHE.  The change as proposed will also totally
 break sites that use DHE with non-standard DH parameters, which is bad.
 That may be rather uncommon (though it's easy-ish to do).

 If it were up to me, I'd tie this to the security slider (since that's
 where usability/security tradeoffs live), and when set to a sufficiently
 high value, refuse to do DHE depending on the `ServerDHParams` received as
 part of the handshake (Eg: Reject groups that are < 1536 bits in length).

 It's more involved (and may require patching NSS), but it:

  a. Gives users an option that isn't "no crypto" if their obscure site
 doesn't work.
  b. Will function as intended for people that refuse to use ECDH, and
 instead provide a large enough group.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17374#comment:5>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online