Re: [Cfrg] I-D Action: draft-irtf-cfrg-gcmsiv-03.txt

Shay Gueron <shay.gueron@gmail.com> Thu, 19 January 2017 01:05 UTC

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From: Shay Gueron <shay.gueron@gmail.com>
Date: Wed, 18 Jan 2017 17:05:05 -0800
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To: Adam Langley <agl@imperialviolet.org>
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Subject: Re: [Cfrg] I-D Action: draft-irtf-cfrg-gcmsiv-03.txt
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Hello everyone,


I will try to address, really briefly, two of the points raised here and
deferred to me.

The definition of POLYVAL: there is an inherent discrepancy in the
definition of AES-GCM. 128-bit blocks need to be viewed as polynomials, and
also as 16 bytes for input/output for AES (AES is defined over bytes). The
way that the polynomials are defined leads to bytes that have the reverse
order of bits, compared to how AES views a bytes. I will publish a
 detailed paper on this issue. For now, there is some explanation in my
talk at RWC 2011 [1].
POLYVAL is defined in a way that is consistent with AES.

The other topic is why truncate the AES (throw away half of the bits) when
generating per-nonce keys. The idea is to get indistinguishabilityle bounds
that do not have a term that is quadratic in the number of queries (like
the ones we would get if we used all of the bits). Roughly speaking the
truncation gets us a term that looks like q/2^(96) instead of q^2/2^{129}.
The bounds and discussion can be seen at  [2].

Anyway an organized paper with a statement on the security bounds, is going
to come out in a paper that we are working on these very days. We will post
it soon.

Thank you, Shay

[1] https://crypto.stanford.edu/RealWorldCrypto/slides/gueron.pdf

[2] S. Gilboa, S. Gueron, “The Advantage of Truncated Permutations”,
https://arxiv.org/abs/1610.02518   (submitted on 8 Oct 2016).

2017-01-18 16:13 GMT-08:00 Adam Langley <agl@imperialviolet.org>rg>:

> On Wed, Jan 18, 2017 at 1:05 PM, John Mattsson
> <john.mattsson@ericsson.com> wrote:
> > - In addition to listing the performance penalty compared to GCM. The
> >   draft should also mention that compared to GCM, some nice properties
> >   disappear:
> >   - Neither Encryption nor Decryption is online as encryption/decryption
> >     cannot start before the whole plaintext/ciphertext is known.
>
> I agree that this is true for encryption, but I don't believe that
> AES-GCM should be *de*crypted in a streaming fashion, but rather that
> records should be sized so that this isn't a problem. (At which point
> the benefit for streaming encryption becomes small or moot.)
>
> I've a long spiel about the dangers of processing unauthenticated
> ciphertext which I'll spare you :) But, even for small machines, the
> memory needed to safely buffer the decrypted plaintext to avoid
> releasing it before it's authenticated is equal to the memory to
> buffer the ciphertext followed by decrypting in-place.
>
> So I actually quite like that the tag is at the end of the message
> with AES-GCM-SIV because it makes it harder to do what I think is the
> "wrong" thing.
>
> >   - GCM-SIV removes the possibility to preprocess static headers (AAD).
>
> Indeed.
>
> (I wasn't sure where to put these points in the spec so, for the
> moment, I've added an appendix for "Additional comparisons with
> AES-GCM". I'm collecting changes in GitHub before making a new
> version. For this message, see
> https://github.com/agl/gcmsiv/commit/c6c7fd388dd122251264222b7491c6
> 212e818319.)
>
> > - “The result of the encryption is the resulting ciphertext (truncated
> >    to the length of the plaintext) followed by the tag."
> >
> >   I suggest that the tag is placed first instead of last in the
> >   ciphertext. This makes decryption online, which makes a large
> >   difference. Suggestion:
> >
> >   “The result of the encryption is the tag followed by the ciphertext
> >    (truncated to the length of the plaintext)"
>
> (See above.)
>
> >
> >
> > - "within 5% of the speed of AES-GCM."
> >   Should state when this is the case, e.g. long plaintext/aad.
>
> Done.
>
> >
> > - I think the draft should give performance data also for short
> >   plaintexts/aad or even better list the performance in number of
> >   operations:
> >
> >   GCM:
> >     Block Cipher Operations = p + 1
> >     GF(2^128) Multiplications = p + a + 1
> >
> >   GCM-SIV-128
> >     Block Cipher Operations = p + 5
> >     GF(2^128) Multiplications = p + a + 1
> >
> >   GCM-SIV-256
> >     Block Cipher Operations = p + 7
> >     GF(2^128) Multiplications = p + a + 1
> >
> >   (if I got it right...)
>
> I think that's correct and I've added that to the new appendix.
>
> >   Where p is the block length of the plaintext and a is the block length
> >   of the additional authenticated data,
> >
> >   I doubt that encryption of short messages are anywhere near 5% of GCM.
> >
> > - The "++" and "[:8]" operation should probably be defined.
>
> Done.
>
> >
> > - What it the security/performance tradeoff with truncation in the key
> >   derivation? What would the security properties be if "[:8]" was
> >   removed?
>
> I'll have to let Shay answer this, but the rough idea is that, since
> AES is a permutation, not two ciphertexts can be equal given that
> we're encrypting different plaintexts using the KDF phase. However,
> ideally we would want a PRF where outputs can be the same. By taking
> only the first eight bytes of each ciphertext block, we better
> approximate a PRF.
>
> > - The definition of U32LE seems unnecessary and only adds complexity.
> >   I suggest:
> >     OLD "U32LE(3) ++ nonce"
> >     NEW "03 ++ 000000 ++ nonce
>
> Good point.
>
> >
> > - The term K1 is only used in Test Vectors. I guess it is an old term
> >   that should be removed.
>
> Done.
>
> > Some editorials:
>
> Thank you for all these. They should be taken care of.
>
> > - OLD "The record-authentication key is 128-bit and the
> >        record-authentication key"
> >   NEW "The record-authentication key is 128-bit and the
> >        record-encryption key"
> >
> > - "} else if bytelen(key-generating-key) == 32 {
> >      record-encryption-key = AES128(key = key-generating-key,"
> >
> >   Should be AES256
>
> Indeed, and record-authentication-key is wrong too!
>
> > - Spacing around "+" and "*" are not consistent.
> >
> > - "the the"
> >
> > - yeilds
> >
> > - remainding
> >
> > - RFC7322 says "A comma is used before the last item of a series"
>
> I think I've leave this one to the RFC Editor!
>
>
>
> Cheers
>
> AGL
>
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