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

David-Sarah Hopwood <david-sarah@jacaranda.org> Thu, 26 November 2009 17:24 UTC

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Date: Thu, 26 Nov 2009 17:23:06 +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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Blumenthal, Uri - 0662 - MITLL wrote:
> 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?).

The previous_verify_data is not necessarily secret, so this requires the
PRF to be secure against related-key attack. The particular PRFs used in
SSLv3 and TLS 1.0 to 1.2 should be secure in that sense, but (if we
decide to use this approach) I still prefer what I suggested before:

    verify_data =
      PRF(master_secret, finished_label, Hash(handshake_messages)
                                         + previous_verify_data)
        [0..verify_data_length-1];

which uses the PRF in a more conventional way.

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

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