Re: [TLS] lasgt call comments (st Call: draft-altman-tls-channel-bindings (Channel Bindings for TLS) to Proposed Standard)

Simon Josefsson <> Wed, 28 October 2009 14:11 UTC

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From: Simon Josefsson <>
To: Larry Zhu <>
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Date: Wed, 28 Oct 2009 15:11:28 +0100
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Subject: Re: [TLS] lasgt call comments (st Call: draft-altman-tls-channel-bindings (Channel Bindings for TLS) to Proposed Standard)
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I agree with Larry that the current definition of tls-unique is
impossible to implement in TLS libraries in the way you'd typically
want, i.e., a tls_get_tls_unique_cb() function.

However, there IS a way that TLS libraries can support the current
approach: add a callback interface to provide applications with the TLS
Finished message, and let the application handle the complexities
regarding connections vs session.

That is what I decided to do in GnuTLS, a callback like this:

  typedef void (*gnutls_finished_callback_func) (gnutls_session_t session,
						 const void *finished,
						 size_t len);
  gnutls_session_set_finished_function (gnutls_session_t session,
					gnutls_finished_callback_func func);

Generally, I'm not happy with draft-altman-tls-channel-bindings so if
this is an opportunity to consider alternatives I want to remind you of
this work:

It makes sure to bind the channel binding uniquely to BOTH the current
connection and the current session.  The
draft-altman-tls-channel-bindings-07 document only binds to the current
TLS connection.  So from this perspective, my work has the same issue,
but it is different in other aspects.


Larry Zhu <> writes:

> There is a design issue in tls-unique. For vendors who implement TLS in a separate library, the TLS library does not by itself control the transport therefore it would not know if there is a new connection, so that the current specification is not implementable for these vendors.
> It would be much easier to say the following instead:
> The client's TLS Finished message from the first handshake of the session (note: TLS session, not connection, so that the channel binding is specific to each TLS session regardless of whether session resumption is used).
> And the updated text does reflect what has been deployed for tls-unique.  
> I would like to raise a red flag now. Needless to say that I will start a discussion with the responsible AD and the rest of the editors of this ID to fix this issue, and do so based on consensus. 
> Pasi, please consider this issue blocking for now.
> Thanks,
> --Larry
> -----Original Message-----
> From: [] On Behalf Of The IESG
> Sent: Monday, October 05, 2009 9:27 AM
> To: IETF-Announce
> Cc:;;
> Subject: [TLS] Last Call: draft-altman-tls-channel-bindings (Channel Bindings for TLS) to Proposed Standard
> The IESG has received a request from an individual submitter to consider 
> the following document:
> - 'Channel Bindings for TLS '
>    <draft-altman-tls-channel-bindings-07.txt> as a Proposed Standard
> The IESG plans to make a decision in the next few weeks, and solicits
> final comments on this action.  Please send substantive comments to the
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