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Re: [Cfrg] (flaws with Curve25519 DH function, if one does not check the output) Re: Elliptic Curves - curve form and coordinate systems

Nico Williams <nico@cryptonector.com> Tue, 17 March 2015 03:33 UTC

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Date: Mon, 16 Mar 2015 22:33:56 -0500
From: Nico Williams <nico@cryptonector.com>
To: David Leon Gil <coruus@gmail.com>
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References: <5501E6A5.5040608@brainhub.org> <A6F30412-8E0A-4D8D-9F26-580307B46874@shiftleft.org> <20150316002255.28855.qmail@cr.yp.to> <20150316044906.GA27479@mournblade.imrryr.org> <5506D5BB.3090700@gmail.com> <20150316135620.GC27479@mournblade.imrryr.org> <5506EF80.7010809@gmail.com> <CACsn0ck6EY1PVB39a6gTxrnxgPTY_quMRGya2jm79CsH4iLC4Q@mail.gmail.com> <CAA7UWsW-NGZ7PEn1iLF9r_rOxuJsntCiKVJjm56JG4yhc4U0PA@mail.gmail.com>
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Subject: Re: [Cfrg] (flaws with Curve25519 DH function, if one does not check the output) Re: Elliptic Curves - curve form and coordinate systems
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On Mon, Mar 16, 2015 at 03:08:38PM -0700, David Leon Gil wrote:
> On Monday, March 16, 2015, Watson Ladd <watsonbladd@gmail.com> wrote:
> > On Mon, Mar 16, 2015 at 7:58 AM, Rene Struik <rstruik.ext@gmail.com
> > <javascript:;>> wrote:
> >
> > But let's assume that we need contributory behavior. Then simply
> > returning an ignorable error code doesn't work. Instead the function
> > should return 32 random bytes if computing 0, so as to avoid ignoring
> > the errors.
> 
> I'm really inclined to think that it is better to use an encoding format
> that rejects points that do not ensure contributory behavior. I don't think
> that it's wise to assume that protocol designers will understand when
> this is required as well as you do.

Don't the alternatives have harder input validation problems?

Having to check for just one weak public key, that seems like a good
deal to me.

Nico
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