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Re: [TLS] Encrypt-then-MAC again (was Re: padding bug)

mrex@sap.com (Martin Rex) Mon, 11 November 2013 23:34 UTC

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To: Michael D'Errico <mike-list@pobox.com>
Date: Tue, 12 Nov 2013 00:33:49 +0100
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From: mrex@sap.com
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Subject: Re: [TLS] Encrypt-then-MAC again (was Re: padding bug)
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Michael D'Errico wrote:
>
> In trying to figure out what's stalling the encrypt-then-mac draft
> I came across this old message (below).  The complaint against the
> draft appears to be that since the MAC is no longer encrypted, there
> may be a way to recover the secret HMAC key.  HMAC-MD5 is mentioned
> as the problem, so I checked to see which cipher suites use it.

Personally, I would really prefer to _not_ move the MAC outside
of the encryption, i.e. leave it as pad-mac-encrypt like Serge Vaudenay
originally recommended.  In particular, keeping the hands off the
GenericStreamCipher PDU processing.

And in order to minimize the code changes and maximize the benefits,
I would use the same _fixed_ GenericBlockCipher PDU for all protocol
versions SSLv3->TLSv1.2 (i.e. including the random IV from TLSv1.1)
and with similar protection from downgrade attacks (for browsers doing
fallback reconnects) as rfc5746 renegotiation by defining another SCSV.


-Martin

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