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Re: [TLS] Quest for Unified Solution to TLS Renegotiation

David-Sarah Hopwood <david-sarah@jacaranda.org> Thu, 26 November 2009 05:35 UTC

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Date: Thu, 26 Nov 2009 05:35:21 +0000
From: David-Sarah Hopwood <david-sarah@jacaranda.org>
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Subject: Re: [TLS] Quest for Unified Solution to TLS Renegotiation
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Eric Rescorla wrote:
> At Wed, 25 Nov 2009 13:03:16 -0800,
> Michael D'Errico wrote:
>> I think that it would be good to find a unified solution that
>> everybody is happy with, and believe that this may be it:
> 
> This was actually suggested by Bodo Moeller last week.
> 
>>    1) Client-to-Server signalling can be done in either of
>>       two ways:
>>
>>       A) an empty Secure_Renegotiation (SR) extension, or
>>       B) a "magic" cipher suite
>>
>>    2) Server-to-Client signal is always an empty SR extension
> 
> I just sent a message suggesting 1 and almost 2.
> 
>>    3) Incorporate previous verify_data into Finished calc.
> 
> I'm not happy with this, actually.
> 
> This requires breaking the current clean definition of handshake
> hashes as the hash of all the handshake messages and simply adding
> some synthetic message in the middle. I don't think this is anywhere
> near as clean, as evidenced by the ongoing debate about whether to put
> it the synthetic data in the front, the back, incorporate it into the
> PRF, or chain the pre-existing handshake messages. [FWIW, I think I
> prefer the last of these implicit versions.]
> 
> By contrast, RI allows that part of the system (which is well
> understood) to remain the same and in fact when RI is offered on the
> first handshake, there is no change to the TLS core at all. It's all
> just hashed into Finished as part of the ordinary TLS procedures,
> and everything is in fact compliant TLS, which I think is desirable.

This approach and RI have different security properties: because RI
does not change the Finished computation, it relies on the recipient
of the extension checking that its contents are correct.

OTOH, I think it's only necessary for the previous verify_data to
be sent once. So, how about sending that only in the ServerHello, i.e.

  1) Client-to-Server signalling:
       MUST send a "magic" cipher suite
       MAY send an empty Secure_Renegotiation (SR) extension (and
         SHOULD do so if it is sending other extensions).

  2) Server-to-Client signal MUST be a Secure_Renegotiation extension
     containing the previous client and server verify_data.
     A patched client MUST check this value.

  3) No change to Finished computation.

  4) A server that is doing a renegotiation MUST reject connections
     from clients that don't include the magic cipher suite.

- --
David-Sarah Hopwood  ⚥  http://davidsarah.livejournal.com
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