Re: [TLS] Comments/Questions on draft-gutmann-tls-encrypt-then-mac-00.txt

"Blumenthal, Uri - 0558 - MITLL" <> Tue, 24 September 2013 16:48 UTC

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From: "Blumenthal, Uri - 0558 - MITLL" <>
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Date: Tue, 24 Sep 2013 12:47:19 -0400
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Subject: Re: [TLS] Comments/Questions on draft-gutmann-tls-encrypt-then-mac-00.txt
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> We've had a lot of discussions about this. Strictly speaking if a non-
> truncated HMac reveals (too much) information of the internal state of
> the hash algorithm something totally went wrong with the hash function.

There are two issues. One is how much info the non-truncated MAC reveals. Another one is that it enables key guessing with just one valid message+MAC pair.

> As a result of this BSI TR-03116-4 chapter 2.3, 1st bullet states
> "do not use truncated_hmac". I'm sorry again for the lack of an english
> translation at the moment. The Federal Office for Information Security
> will probably provide one later.

I wonder what their reasoning is.

> In THEORY a truncated_hmac offers an option for attacks as only a fraction of the original HMac has to match.

Certainly.  It reduces the collision-creating space from square root of full MAC size to square root of truncated MAC size. For the benefit of requiring 2^(# of truncated bits) valid MACs to successfully forge a MAC. 

With modern MACs and keys either attack should not be practical. Still, I personally prefer to truncate, though not necessarily all the way down to 96 bits.