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Re: [TLS] drop obsolete SSL 2 backwards compatibility from TLS 1.3 draft

mrex@sap.com (Martin Rex) Wed, 18 February 2015 01:17 UTC

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To: Yuhong Bao <yuhongbao_386@hotmail.com>
Date: Wed, 18 Feb 2015 02:17:46 +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: Re: [TLS] 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.

Speaking of Windows 7 SChannel, just today I enabled TLSv1.2
for the IIS on my Windows 7 test machine (it is disabled by default)
and tried to negotiate TLSv1.2 with it -- but I'm facing handshake failures.

I'm sending a pretty vanilla, extension-less ClientHello

  record=(3,0) and ClientHello.client_version=(3,3)

with the following TLS cipher suites:

  TLS_RSA_WITH_AES128_CBC_SHA
  TLS_RSA_WITH_RC4_128_SHA
  TLS_RSA_WITH_RC4_128_MD5
  TLS_RSA_WITH_3DES_EDE_CBC_SHA

and the Microsoft IIS server on my Win7 machine closes the connection.
The following two Events get written to the Windows "System" Eventlog:

 Source: Schannel   EventID: 36874
  An TLS 1.2 connectin request was received from a remote client application,
  but none of the cipher suites supported by the client application are
  supported by the server.  The SSL connection request has failed.

 Source: Schannel   EventID: 36888
  The following fatal alert was generated: 40.
  The internal error state is 1205.


As some might notice, the ClientHello lists the mandatory-to-implement
TLSv1.2 cipher suite (rfc5246, section 9), so it is extremely unexpected
to see the handshake fail when proposing TLSv1.2, but succeed when
proposing only TLSv1.1.

Since I can connect with a browser with TLSv1.2 (and with openssl-1.0.1j)
the issue might be the absence of the TLS signature algorithm extension.

Per rfc5246, the use of TLS extensions is a MAY for the TLSv1.2 client and
accepting a ClientHello without extensions is a MUST for the TLSv1.2 server:

(rfc 5246, section 7.4.1.2  ClientHello)
https://tools.ietf.org/html/rfc5246#page-42

   extensions
      Clients MAY request extended functionality from servers by sending
      data in the extensions field.  The actual "Extension" format is
      defined in Section 7.4.1.4.


                                       A server MUST accept ClientHello
   messages both with and without the extensions field,


Myself I don't have any Win8 / Win2012 machines, but I assume they're
also both affected.  I only tried this because the customers ADFS server
(2012 I believe) didn't want to interop.


-Martin

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