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Re: [Cfrg] Adopting "AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption" as a CFRG document ---- Some clarifications

"Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu> Tue, 26 April 2016 19:47 UTC

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From: "Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu>
To: Taylor R Campbell <campbell+cfrg@mumble.net>
Thread-Topic: [Cfrg] Adopting "AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption" as a CFRG document ---- Some clarifications
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Date: Tue, 26 Apr 2016 19:47:21 +0000
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Subject: Re: [Cfrg] Adopting "AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption" as a CFRG document ---- Some clarifications
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On 4/26/16, 15:25 , "Taylor R Campbell" <campbell@mumble.net on behalf of
campbell+cfrg@mumble.net> wrote:

>   Date: Tue, 26 Apr 2016 18:22:16 +0000
>   From: "Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu>
>   AES seems to have resistance to related key attacks. I find this issue
>   (aka [K0, K1] vs [K1, K0]) a far lower risk that having the same key
>used
>   for both messages.
>
>Alex Biryukov and Dmitry Khovratovich, `Related-key Cryptanalysis of
>the Full AES-192 and AES-256', <https://eprint.iacr.org/2009/317>.

Sure, with 2^{99.5} time and data complexity (to be precise, $2^{77.5}$
time and $2^{99.5}$ data). Not my immediate threat risk, what about you?

At least, the authors seem to think so (section 7):

	However, both our attacks are still mainly of theoretical
	interest and do not present a threat to practical applications
	using AES.

				
			
		
	

 

Not to mention that the record keys are produced by a PRP (yes, I call AES
this in this context :), so any meaningful relation between the two
resulting 256-bit record keys is that $K_{1} = K_{2}^{‘}$ and vs. versa.

>Maybe the particular attacks discussed in that paper don't work
>directly in this scenario (they almost certainly don't, since they
>work with four keys), but `AES seems to have resistance to related key
>attacks' is hardly the case.

“MAYBE . . . don’t work DIRECTLY”?


P.S. AES192 seems to fare much better, with the time complexity $2^{176}$
and the data complexity $2^{123}$. I might prefer better numbers, but both
count as “resistant” in my book. ;)

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