Re: [Crypto-panel] Review of AES-GCM-SIV

Bjoern Tackmann <bjoern.tackmann@ieee.org> Tue, 04 July 2017 12:27 UTC

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From: Bjoern Tackmann <bjoern.tackmann@ieee.org>
Date: Tue, 4 Jul 2017 14:27:52 +0200
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To: Tibor Jager <tibor.jager@upb.de>
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Subject: Re: [Crypto-panel] Review of AES-GCM-SIV
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Hi all,

On Sun, Jul 2, 2017 at 2:37 PM, Tibor Jager <tibor.jager@upb.de> wrote:

>
> On 01/07/2017 20:34, Bjoern Tackmann wrote:
> > Please find my review below. It's a nice piece of work and overall in
> > quite good shape.
> >
> > After looking at the other reviews: I do not quite understand Tibor's
> > comment on the bit-length vs. byte-length, given that the draft states
> > that the scheme takes "arbitrary-length plaintext & additional data
> > byte-strings" -- and for me the term "byte-strings" means that the
> > byte-length of the strings is an integer.
>
> Indeed, this is one of the sections that suggests that it is implicitly
> assumed that "valid" plaintexts and AD have always a byte-length which
> is an integer.
>
> What I found *potentially* confusing is:
>
> - Then it would also be somewhat more intuitive/consistent to include
> the byte-length of plaintext and AD in the length block. The current
> draft includes the bit-length. (This is of course technically fine and
> essentially just a different notation, but *could* be confusing.)
>
> - Also the example in Section 8 mentions the bit-length.
>

I fully agree that it would be less ambiguous to do these computations in
terms of byte-length. I do not see any advantage of having the scheme
operate internally in terms of bit-length, when only byte-length strings
are allowed.



> - It would also make sense to let the encryption algorithm abort, if the
> lengths of plaintexts and AD are not a multiple of 8 bits (and one could
> ignore this check in applications where this is guaranteed by the
> environment - but this is of course something that only the application
> developer can decide).
>

Agreed.


Best,
Björn