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

mrex@sap.com (Martin Rex) Thu, 14 November 2013 15:05 UTC

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To: "Bryan C. Geraghty" <bryan@ravensight.org>
Date: Thu, 14 Nov 2013 16:05:31 +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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Bryan C. Geraghty wrote:
> Martin Rex wrote:
> 
> > Ralf Skyper Kaiser wrote:
> >> 
> >> Encrypt-then-mac has some other security benefits. (Among others that 
> >> the decrypt routine never has to process bogus/bad messages [e.g. 
> >> those where the MAC would fail in current encrypt-mac].).
> > 
> > Which benefits are this?
> > 
> > I am not aware of any security benefit for the real world from doing EtA
> over AtE.
> 
> Look up "padding oracle attack". Lucky13 is a fairly recent example of a
> real-world attack against AtE.

You're mistaken.

The "padding oracle attacks" against CBC, both originally described
by Serge Vaudenay, and the attack described by Hugo, are against
"authenticate-extend-encrypt" schemes one of which being the
"mac-pad-encrypt" used by TLS.

I was asking for security benefits of a EtA over a _real_ AtE scheme, and
I am not aware of any.

-Martin

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