Re: [PATCH v3 4/4] fscrypt: Add LEA-256-XTS, LEA-256-CTS support

[Eric Biggers]

On Thu, Jun 29, 2023 at 07:59:14PM -0700, Eric Biggers wrote:
> On Thu, Jun 29, 2023 at 07:01:11PM +0900, Dongsoo Lee wrote:
> > On Tue, Jun 27, 2023 at 23:38:30 -0700, Eric Biggers wrote:
> > >On Mon, Jun 26, 2023 at 05:47:03PM +0900, Dongsoo Lee wrote:
> > >> when SIMD instructions are available, it performs even faster.
> > >
> > >This will only be true once there is actually an applicable implementation
> > of
> > >LEA-XTS and LEA-CTS using SIMD instructions included in the kernel.
> > >
> > >Perhaps it is your plan to go through and accelerate LEA-XTS and LEA-CTS
> > for the
> > >common CPU architectures.  However, it is not included in this patchset
> > yet, so
> > >it should not be claimed in the documentation yet.
> > >
> > >> Particularly, it outperforms AES when the dedicated crypto
> > >> +instructions for AES are unavailable, regardless of the presence of SIMD
> > >> +instructions. However, it is not recommended to use LEA unless there is
> > >> +a clear reason (such as the absence of dedicated crypto instructions for
> > >> +AES or a mandatory requirement) to do so. Also, to enable LEA support,
> > >> +it needs to be enabled in the kernel crypto API.
> > >
> > >I think I'd prefer that you omit the mention of the "absence of dedicated
> > crypto
> > >instructions" use case for now.  fscrypt already supports another algorithm
> > that
> > >fulfills exactly that use case (Adiantum), and that algorithm already has
> > >optimized implementations for arm32, arm64, and x86_64.  LEA does not have
> > that
> > >yet.  So it does not really bring anything new to the table.  I'm also
> > unsure it
> > >would be appropriate to recommend a "lightweight" cipher at this point...
> > >
> > >That would leave "mandatory requirement" as the rationale, at least for
> > now,
> > >similar to SM4.
> > >
> > >- Eric
> > 
> > As you might expect, we are working on a SIMD implementation of LEA in a
> > general-purpose CPU environment. However, since no such implementation has
> > been submitted yet, we agree that it's right to leave it out for now.
> > 
> > In the next version, we would like to change the description to the
> > following:
> > 
> > LEA is a South Korean 128-bit block cipher (with 128/192/256-bit keys)
> > included in the ISO/IEC 29192-2:2019 standard (Information security -
> > Lightweight cryptography - Part 2: Block ciphers). If dedicated cipher
> > instructions are available, or other options with performance benefits
> > are available, using LEA is likely not a suitable choice. Therefore,
> > it is not recommended to use LEA-256-XTS unless there is a clear reason
> > to do so, such as if there is a mandate. Also, to enable LEA support,
> > it needs to be enabled in the kernel crypto API.
> 
> I don't think that really addresses my comment, due to the second sentence.  I
> understand that you would like to advertise the performance of LEA.  But as I
> mentioned, it's not yet realized in the kernel crypto API, and in the context of
> fscrypt it won't really bring anything new to the table anyway.  For now I think
> LEA is best described as a "national pride cipher" alongside SM4...  Keep in
> mind, it can always be changed later if new use cases come up.
> 
> Could you just omit the documentation update from your patch?  I actually need
> to rework the whole "Encryption modes and usage" section anyway since it's
> growing a bit unwieldy, with 6 different combinations of encryption modes now
> supported.  The information needs to be organized better.  It currently reads
> like a list, and it might be hard for users to understand which setting to use.
> 
> I'll add on a patch that does that and adds the mention of LEA support.
> 
> - Eric

I've sent out
https://lore.kernel.org/linux-fscrypt/20230630064811.22569-1-ebiggers@xxxxxxxxxx/T/#u.
One of the things it does is add a nice list for the "national pride ciphers".
So we can just add ciphers like SM4 and LEA, and any that people might insist on
adding in the future like Camellia, Kuznyechik, SEED, ARIA, etc., to that list,
and not have to waste time with each one individually...

- Eric