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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> Thu, 21 April 2016 21:41 UTC

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From: "Blumenthal, Uri - 0553 - MITLL" <uri@ll.mit.edu>
To: Adam Langley <agl@imperialviolet.org>
Thread-Topic: [Cfrg] Adopting "AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption" as a CFRG document ---- Some clarifications
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Date: Thu, 21 Apr 2016 21:41:01 +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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And if you're absolutely viscerally opposed to OFB - for the 1st encryption use the nonce itself, and for the 2nd encryption input - XOR the nonce with 0x00..01.

Sent from my BlackBerry 10 smartphone on the Verizon Wireless 4G LTE network.
  Original Message  
From: Blumenthal, Uri - 0553 - MITLL
Sent: Thursday, April 21, 2016 16:35
To: Adam Langley
Cc: Yehuda Lindell; cfrg@irtf.org; Adam Langley
Subject: Re: [Cfrg] Adopting "AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption" as a CFRG document ---- Some clarifications

For the 3rd time: why not generate record key using AES-OFB?

Sent from my BlackBerry 10 smartphone on the Verizon Wireless 4G LTE network.
  Original Message  
From: Adam Langley
Sent: Thursday, April 21, 2016 16:28
To: Blumenthal, Uri - 0553 - MITLL
Cc: Shay Gueron; Andy Lutomirski; Yehuda Lindell; cfrg@irtf.org; Adam Langley
Subject: Re: [Cfrg] Adopting "AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption" as a CFRG document ---- Some clarifications

On Thu, Apr 21, 2016 at 1:11 PM, Blumenthal, Uri - 0553 - MITLL
<uri@ll.mit.edu> wrote:
> I’m afraid Andy is correct. Say one record has its nonce xxxx…xxxx0 (127
> bits plus 0), and another record has its nonce xxx…xxx1. The record key
> produced for both records will be the same, because it clobbers/ignores the
> LSB.

That is what I understood when I wrote those words. I might have
misunderstood Shay however!

This is the reason that the Security Considerations section mentions
that it's only safe to repeat an nonce 2^31 (rather than 2^32) times
with the AES-256 version.


Cheers

AGL

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