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[TLS] Downgrade Dance steps (Re: drop obsolete SSL 2 backwards compatibility from TLS 1.3 draft)

mrex@sap.com (Martin Rex) Sat, 14 February 2015 08:54 UTC

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To: Yuhong Bao <yuhongbao_386@hotmail.com>
Date: Sat, 14 Feb 2015 09:54:29 +0100
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From: mrex@sap.com
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Cc: Florian Weimer <fweimer@redhat.com>, "tls@ietf.org" <tls@ietf.org>
Subject: [TLS] Downgrade Dance steps (Re: drop obsolete SSL 2 backwards compatibility from TLS 1.3 draft)
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Yuhong Bao wrote:
>
> Just confirmed that IE on Win7 even when misconfigured to enable
> SSLv2 doesn't send a SSLv2 ClientHello with TLS 1.2 on it which
> means that SChannel probably can't do it at all. Andrei Popov
> should be able to confirm this.

IIRC, Win7 shipped with TLSv1.2 *disabled*.

I see the following downgrade dances with MSIE 10
on (a fully patched) 64-bit Win7sp1.

What's a little weird about the MSIE downgrade dances:
  - duplicated attempts with certain Hellos in certain configurations
  - empty connection attempts when SSLv2 is disabled.

The (dummy) server that I had MSIE connect to read the ClientHello
from the network waited a second and then closed the network connection.


-Martin


everything enabled (SSLv2->TLSv1.2):

  TLS record (3,3)  ClientHello.client_version(3,3) with TLS extensions
  SSL2 record  ClientHello.client_version(3,1)
  SSL2 record  ClientHello.client_version(3,0)
  SSL2 record  ClientHello.client_version(0,2)


SSLv2 plus default (SSLv3,TLSv1.0,TLSv1.1):

  SSL2 record  ClientHello.client_version(3,2)
  SSL2 record  ClientHello.client_version(3,1)
  SSL2 record  ClientHello.client_version(3,0)
  SSL2 record  ClientHello.client_version(0,2)

SSLv2 plus TLSv1.0+TLSv1.1 (this looks weird!):

  SSL2 record  ClientHello.client_version(3,2)
  SSL2 record  ClientHello.client_version(3,1)
  SSL2 record  ClientHello.client_version(0,2)
  SSL2 record  ClientHello.client_version(0,2)


For comparison the (hopefully more common scenarios):

SSLv3->TLSv1.2:

  TLS  record (3,3)  ClientHello.client_version(3,3) with TLS extensions
  TLS  record (3,1)  ClientHello.client_version(3,1) with TLS extensions
  SSL3 record (3,0)  ClientHello.client_version(3,0)
  TCP-connect + TCP-close

TLSv1.0+TLSv1.1+TLSv1.2

  TLS  record (3,3)  ClientHello.client_version(3,3) with TLS extensions
  TLS  record (3,1)  ClientHello.client_version(3,1) with TLS extensions
  TCP-connect + TCP-close


And finally the more esoteric configurations:

TLSv1.1+TLSv1.2

  TLS  record (3,3)  ClientHello.client_version(3,3) with TLS extensions
  TCP-connect + TCP-close

TLSv1.2 alone

  TLS  record (3,3)  ClientHello.client_version(3,3) with TLS extensions
  TCP-connect + TCP-close

SSLv3+TLSv1.2

  TLS  record (3,3)  ClientHello.client_version(3,3) with TLS extensions
  SSL3 record (3,0)  ClientHello.client_version(3,0)
  SSL3 record (3,0)  ClientHello.client_version(3,0)
  TCP-connect + TCP-close

TLSv1.0+TLSv1.2

  TLS  record (3,3)  ClientHello.client_version(3,3) with TLS extensions
  TLS  record (3,1)  ClientHello.client_version(3,1)
  TCP-connect + TCP-close

SSLv2+TLSv1.2

  TLS  record (3,3)  ClientHello.client_version(3,3) with TLS extensions
  SSL2 record  ClientHello.client_version(0,2)
  SSL2 record  ClientHello.client_version(0,2)
  SSL2 record  ClientHello.client_version(0,2)

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