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Re: [TLS] client and server verify_data in draft-rescorla-tls-renegotiate.txt

David-Sarah Hopwood <david-sarah@jacaranda.org> Sat, 14 November 2009 02:44 UTC

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Date: Sat, 14 Nov 2009 02:44:41 +0000
From: David-Sarah Hopwood <david-sarah@jacaranda.org>
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Subject: Re: [TLS] client and server verify_data in draft-rescorla-tls-renegotiate.txt
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David-Sarah Hopwood wrote:
> Robert Dugal wrote:
>> A co-worker an I have been discussing the draft and have some concerns
>> about sending client and server verify_data in the extension.
>>
>> In the ClientHello, the client sends the verify_data from the client's
>> previous Finish message as the renegotiated_connection data.
>>
>> In the ServerHello, the server sends both the cient's verify_data and
>> the server's verify_data.
>>
>> This data is not normally sent in the clear and I am wondering if this
>> will provide additional data that can be used to mount some other form
>> of attack .
> 
> I can't see a feasible attack as long as the hash function doesn't
> reveal information about its input. However, I think you're right to
> be concerned; we need input from experienced cryptographers about this.

Actually this is definitely fine, because the verify_data will be
encrypted if the preceding session was not controlled by the attacker
(and if it was, then the attacker knows the verify_data anyway).

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

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