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

"Blumenthal, Uri - 0662 - MITLL" <uri@ll.mit.edu> Thu, 26 November 2009 16:09 UTC

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From: "Blumenthal, Uri - 0662 - MITLL" <uri@ll.mit.edu>
To: "tls@ietf.org" <tls@ietf.org>
Date: Thu, 26 Nov 2009 11:04:35 -0500
Thread-Topic: [TLS] Quest for Unified Solution to TLS Renegotiation
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Subject: Re: [TLS] Quest for Unified Solution to TLS Renegotiation
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Yes, previous_verify_data MUST be a part of the input to the PRF. What Yoav
Nir said (XOR-ing it with the master_secret), should work (out of hand -
anybody sees a problem with it?).

    What David-Sarah said, but you could XOR it with
    the master_secret as input to the PRF, or even better,
    use it instead of the finished_label.


On 11/26/09  00:41 , "David-Sarah Hopwood" <david-sarah@jacaranda.org>
wrote:
> Michael D'Errico wrote:
>>> Eric Rescorla wrote:
>>> 
>>>>>    3) Incorporate previous verify_data into Finished calc.
>>>> 
>>>> I'm not happy with this, actually.
>> 
>> Would this work:
>> 
>>      Finished  =  verify_data  XOR  previous_verify_data
> 
> No. The previous_verify_data has to be in the input to the PRF
> that also depends on the master_secret. If you just XOR it to
> the output, then the attacker doesn't need to know the master_secret
> in order to change the unmodified Finished to the modified one.

-- 
Regards,
Uri
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