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Re: [CFRG] AES-CTR + HMAC-SHA-256 AEAD

Adam Langley <agl@imperialviolet.org> Mon, 15 March 2021 16:44 UTC

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From: Adam Langley <agl@imperialviolet.org>
Date: Mon, 15 Mar 2021 09:43:58 -0700
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To: John Mattsson <john.mattsson=40ericsson.com@dmarc.ietf.org>
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Subject: Re: [CFRG] AES-CTR + HMAC-SHA-256 AEAD
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On Sat, Mar 13, 2021 at 12:15 AM John Mattsson <john.mattsson=
40ericsson.com@dmarc.ietf.org> wrote:

> Adam Langley wrote:
>
> >I don't know what AES-CM is. Is it AES-CTR?
>
>
>
> Yes, Counter Mode (CM) is just CTR. The terminology CM was used in RFC
> 3711 (SRTP) and McGrew’s paper from the same time.
> https://cr.yp.to/bib/2002/mcgrew.pdf.
>
>
>
>
>
> I wrote:
>
> “It looks like the frame counter CTR and the block counter collide, but
> maybe I miss something.”.
>
>
>
> A better description is probably that it is undefined how the 12 byte
> nonce is mapped to the 16 byte IV in AES counter mode. I would recommend:
>
>
>
> frame_ctr = encode_big_endian(CTR * 2^64, 16)
>

I think it's more than underspecified, I don't think it meets basic AEAD
requirements. I'll contact the authors about that.


Cheers

AGL

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